Handbook Corrosion (1992) WW Part 9 pptx

Because corrosion fatigue cracks may be difficult to detect until they have reached dangerous proportions, a practical safeguard is to protect steel components throughout their service l

However, corrosion engineers have determined that the best measure of the potential corrosivity of a given soil is its electrical resistance measured in ohms per cubic centimeter The electrical resistivity of the soils is determined by the nature and concentration of the ions formed by the chemical salts dissolved in the soil moisture These ions may also determine the progress of corrosion If the primary product of corrosion is relatively insoluble and deposits as a film on the metal surface, further corrosion may be reduced or completely stifled Numerous inspections of existing structures have established that in sandy and loamy soils with resistivity of 4500 · cm and higher the engineer need not be concerned with the corrosion on the soil side of galvanized steel pipe (Table 13)

Table 13 Relationship of soil corrosion to electrical resistivity

Soil Class Corrosion

resistance

Electrical resistivity, ·cm

Fig 4 Method of estimating service life as developed by the California Division of Highways This correlates pH

with the electrical resistivity of soils to determine years to perforate a steel sheet Local durability records are used for confirmation or control Multiply years to perforation by factor for increase in metal gage This figure is based on 16-gage galvanized steel pipe with a coating thickness of 1.6 mm (0.064 in.) Source: Ref 12

The engineer can determine the years to perforation for other gages of pipe by using the appropriate multiplying factor For example, years to perforation for 8-gage ( 4.2-mm, or 0.16-in.) pipe would be 40 times 2.8, or 112 years Moreover, perforations resulting from the pitting action of galvanic cells will not affect the useful service of a storm drainage structure, as compared to a pressure conduit from which a valuable or dangerous product would be lost With proper maintenance, the life of corrugated steel pipe can be extended indefinitely

Except in the most corrosive soils, the maximum depth of pitting in steel specimens exposed for approximately 12 years was more than 11 times that in zinc specimens, although the ratio for the rates of corrosion was only approximately one-half that figure (Ref 13) This resistance to pitting, combined with the fact that rusting does not appear to start until nearly all of the zinc and zinc-alloy layers have corroded away, reduces the risk of premature failure in galvanized piping It cannot be overemphasized that, although these results serve as a useful guide to the performance of zinc coatings in particular soils, local experience should always be sought

The Corrosion of Zinc in Chemical Environments

As indicated previously, the use of zinc for corrosion-resistant applications accounts for about one-half of the total consumption of the metal However, the available information reveals that in most applications the resistance of zinc to the atmosphere and in various waters is the primary corrosion criterion In one sense, these may be considered chemical environments, because the chemical reactions between zinc and the constituents of pure or polluted atmospheres and waters determine the life of the zinc exposed to air or water

Zinc is usually not considered to be a useful metal in the acidic or strongly alkaline chemical environments encountered, for example, in the chemical-processing industries (see the article "Corrosion in the Chemical Processing Industry" in this Volume) The corrosion of zinc increases in aqueous chemical solutions on either side of the 6 to 12 pH range This should not be considered a fixed rule, because many other factors, such as agitation, aeration, temperature, polarization, and, in some cases, the presence of inhibitors, may have considerable influence on the corrosion

There is considerable interest in the use of zinc in milder chemical environments For example, zinc in used in contact with many organic chemical and chemical specialties, such as detergents, insecticides, and agricultural chemicals In most cases, zinc comes in contact with such chemicals during the handling, packaging, and storage of the commercial products

The corrosion resistance of zinc to the chemicals is usually the primary consideration, but in some cases, the effect of zinc corrosion on a consumer product or chemical is of greater concern than the actual corrosion rate of zinc For example, zinc in contact with certain organic chemicals may in rare cases cause polymerization or catalyze some other undesirable change that would alter the original composition of properties of the product In other cases, some change in the appearance or texture of a consumer product may be caused by the relatively slight corrosion of zinc Thus, zinc would be considered incompatible, although it does not corrode excessively

There are many situations in the chemical industry in which zinc serves a useful purpose Zinc-coated tanks and cylinder are widely used in oil refineries and other plants for storing oil and petroleum products, chlorine, CO2, and other industrial gases (see the articles "Corrosion in Petroleum Production Operations" and "Corrosion in Petroleum Refining and Petrochemical Operations" in this Volume) Refrigerating plants and cooling equipment, as well as degreasing plants, are almost universally protected by zinc coatings Zinc-coated steel is also used on structural steelwork around chemical plants, where it is exposed to high humidity and a variety of chemical fumes Galvanized steel is extensively used for roofing and siding on pulp and paper processing buildings (see the article "Corrosion in the Pulp and Paper Industry" in this Volume)

Other uses of zinc-coated steel in the chemical industry include applications in floating roof type storage tanks for volatile liquids as well as galvanized wire cloth and mesh belts for the movement of chemicals through various production stages Galvanized steel containers are used to store strategic chemicals in outdoor locations From these representative examples

it is apparent that zinc and zinc-coated steel are definitely useful in the chemical industry

Corrosion in Dissolved Salts, Acids, and Bases

Zinc is not used in contact with acid and strong alkaline solutions, because it corrodes rapidly in such media The section

"Corrosion of Zinc in Water" in this article indicates the safe range in which it may be used

Very dilute concentrations of acids accelerate corrosion rates beyond the limits of usefulness Alkaline solutions of moderate strength are much less corrosive than corresponding concentrations of acid, but are still corrosive enough to impair the usefulness of zinc

Zinc-coated steel is used handling refrigeration brines that may contain calcium chloride (CaCl2) In this case, the corrosion rate is kept under control by adding sufficient alkali to bring the pH into the mildly alkaline range and by the addition of inhibitors, such as sodium chromate (Na2CrO4) Certain salts, such as the dichromates, borates, and silicates, act as inhibitors to the aqueous corrosion of zinc

Nonaqueous Corrosion

Organic Compounds. Many organic liquids that are nearly neutral in pH and substantially free from water do not attack zinc Therefore, zinc and zinc-coated products are commonly used with gasoline, glycerine, and inhibited trichlorethylene The presence of free water may cause local corrosion because of the lack of access to oxygen When water is present, zinc may function as a catalyst in the decomposition of such solutions as trichlorethylene, with acid attack as the result Some organic compounds that contain acidic impurities, such as low-grade glycerine, attack zinc Although neutral soaps do not attack zinc, there may be some formation of zinc soaps in dilute soap solutions

Gases. Zinc may be safely used in contact with most common gases at normal temperatures if water is absent Moisture content stimulates attack Dry chlorine does not affect zinc Hydrogen sulfide (H2S) is also harmless because insoluble zinc sulfide (ZnS) is formed On the other hand, SO2 and chlorides have a corrosive action because water-soluble and hygroscopic salts are formed

Indoor Exposure. Zinc corrodes very little in ordinary indoor atmospheres of moderate relative humidity In general, a tarnish film begins to form at spots where dust particles are present on the surface; the film then develops slowly This attack may be a function of the percentage of relative humidity at which the particles absorb moisture from the air However, moisture has little effect on the tarnish formation up to 70% relative humidity The degree of corrosion is related to the relative humidity at and above this point because the zinc corrosion products absorb enough moisture or stimulate the attack to a perceptible rate

Rapid corrosion can occur where the temperature decreases and where visible moisture that condenses on the metal dries slowly This is related to the ease with which such thin moisture films maintain a high oxygen content because of the small volume of water and large water/air interface area Considerably accelerated corrosion can then take place with the formation of a film that is too thick Chromate protective films are used to a considerable extent to prevent attack where accidental or limited contact with water is expected Atmospheres inside industrial buildings can be corrosive, particularly where heated moisture and gases, such as SO2, condense near a cool room

Contact With Food Products. Zinc should not be used in contact with acidic foodstuffs unless they can be expected

to remain dry Otherwise, the zinc must be adequately protected by copper-nickel-chromium plating or another satisfactory impervious coating The slight acidity present in many foodstuffs can attack the zinc, and this may give the food a metallic taste For the same reason, the zinc die casting used in any equipment to hold or dispense drinks should also be plated or otherwise protected Consumption of food contaminated with zinc may cause nausea but is not dangerous, and this rarely arises because of the taste A summary of the compatibility of untreated zinc with various media is presented in Table 14

Table 14 Compatibility of untreated zinc with various media Medium Media descriptor Compatibility

Aerosol propellants Excellent

Weak, cold, quiescent Fair

Carbon tetrachloride Excellent

Cleaning solvents Chlorofluorocarbon Excellent

Diesel oil Sulfur free Excellent

Gas (a) Towns, natural, propane, butane Excellent

Mineral, acid free Excellent

Lubricants

Perchlorethylene Excellent

Refrigerants Chlorofluorocarbon Excellent

Trichloroethylene Excellent

(a) Chromate passivation treatment recommended because of the possible

presence of moisture traces

Stress-Corrosion Cracking and Corrosion Fatigue

Stress-Corrosion Cracking (SCC). The effects of corrosion and stress on the performance of a material are often treated as separate concerns However, in conjunction, the two can cause the phenomenon of SCC, which can destroy a metallic component faster than either stress or corrosion separately In essence, SCC is a process in which cracks in the metal grow under the combined effects of tensile stress and a corrosive environment When the cracks become sufficiently large, the component fractures Often, but not always, the stress-corrosion resistance of an alloy is very dependent on its microstructure Information on the mechanisms of SCC is provided in the article "Environmentally Induced Cracking" in this Volume Testing and interpretation of SCC data can be found in the article "Evaluation of Stress-Corrosion Cracking" in this Volume

Corrosion Fatigue. When metal is subjected to vibration or cyclic stress, fatigue strength is more important than ultimate tensile strength Under corrosive conditions, even the advantages of alloy additions and heat treatments that considerably increase the tensile strength and fatigue limit of steel have only a minor effect on corrosion fatigue behavior Corrosion fatigue is the weakening that occurs in a metal after it has been stressed repeatedly in the presence of corrosive agents Generally, the corrosion fatigue characteristics of low-alloy steels are little or no better than those of ordinary low-carbon steel unless the metal has a protective coating

The plastic deformation that occurs in the metal at the bottom of a crack during repeated stressing raises its energy and thus increases its susceptibility to chemical attack The strengthened material that would otherwise prevent the cracks from spreading is therefore destroyed by corrosion in preference to the surrounding metal, and the crack continues to grow This process continues by further plastic deformation of metal, which is in turn destroyed

In addition to this basic mechanism of corrosion fatigue, certain subsidiary effects also play a part For example, the strains occurring in the surface of the stressed metal tend to disrupt such protective films as would otherwise be formed

by the corrosion products, with the damage to the protective film often taking the form of small cracks or crevices The small areas of metal exposed by these cracks are anodic to the surrounding film and corrode at an accelerated rate with the formation of pits; this sets the mechanism of corrosion fatigue in motion The total amount of corrosion involved in corrosion fatigue is often extremely small It is important to prevent corrosion from the start because once it has begun cyclic stressing may lead to early failure even though further attack is prevented

These facts clearly point to the necessity of sacrificial, electrochemical protection as a means of preventing corrosion fatigue Steel components can be protected throughout their life by a sacrificial zinc coating to give them complete protection If a coating with no sacrificial properties is used, a surface fault may well lead to fatal pitting

Steel and Corrosion Fatigue. Detailed investigations into the corrosion fatigue resistance of steels in moist air and saline solutions have give some surprising results The ultimate tensile strength and indeed the pure fatigue limits of steels are increased considerably by alloying or special heat treatments, but these have only a minor effect on corrosion fatigue behavior

Thus, under practical conditions the behavior of a special steel may be little better than that of ordinary carbon steel unless it has a protective zinc coating These are significant considerations in determining what preventive measures to adopt in practice, and that explain why sacrificial or electrochemical protection is of major importance Because corrosion fatigue cracks may be difficult to detect until they have reached dangerous proportions, a practical safeguard is to protect steel components throughout their service lives by a sacrificial coating of zinc, which gives them complete cathodic protection When zinc coatings are used, small to medium-sized imperfections in the coating are of relatively little importance, and corrosion is prevented even at the growing points of the microscopically small crevices that cause corrosion fatigue

Any type of coating that covers the surface completely and retards the onset of corrosion may help, but if it is not anodic

to steel, it cannot exert cathodic protection at coating defects In many general uses, the fact that nonanodic coatings cannot prevent corrosion at small defects may be unimportant When fatigue failure is not a consideration, a very small amount of corrosion has no significant effect

Wherever corrosion fatigue can occur, however, the situation is very different Coatings that cannot protect sacrificially may, as a result of discontinues, cause serious pitting, and trouble may be accentuated by concentrating the corrosion in small areas, thus stimulating the growth of fatigue cracks Additional information on corrosion fatigue can be found in the articles "Mechanically Assisted Degradation" and "Evaluation of Corrosion Fatigue" in this Volume

Corrosion Protection With Zinc Anodes

Historical Development of Cathodic Protection. Historically, zinc was one of the very first metal to be used as a galvanic anode; early in the 19th century, Sir Humphrey Davy secured pieces of zinc to the copper sheathing on wood hulls of British Navy vessels to prevent severe underwater corrosion of the copper The latter was used as a barrier shield

to stop penetration and destruction of the wood hulls by marine borers and to prevent attachment of barnacles to hulls

Sir Humphrey thus became the first practitioner of cathodic protection and is credited with developing the concept of the electrochemical series of the elements He concluded from actual sea trials aboard ship that corrosion of the copper sheathing could be arrested by using zinc protectors, but the fouling problems by marine organisms were not entirely resolved Further experiments demonstrated that by varying the area of the protector metal in relation to the copper the latter would corrode at a low, tolerable rate that was insufficient to cause perforation but adequate to exhibit antifouling characteristics The electrolytic protection of metals from corrosion in a bulk electrolyte, as demonstrated by Sir Humphrey, has developed into a universally accepted method of corrosion control known as cathodic protection

Cathodic Protection Systems. In practice, cathodic protection is applied to potentially corrodible metals underground or underwater by galvanic anodes (self-generated current) or power-impressed systems Galvanic anode systems do not require an external source of current, because the protective current is self-generated when the galvanic anode is electrically connected to the structure to be protected in a bulk electrolyte

The most commonly used power-impressed systems employ anodes with low anodic corrosion rates that are electrically connected to a rectifier that converts alternating current (ac) to direct current (dc) The negative terminal of the rectifier is grounded to the structure to be cathodically protected, and the positive terminal is connected to the relatively inert anodes When the system is activated, current flows from the rectifier to the anodes, through the electrolyte to the structure (cathode) to be protected, and back to the rectifier through the return path The anodes commonly used in impressed-current systems include, but are not limited to, high-silicon cast iron, graphite, platinized titanium, and lead-silver alloys (seawater only) The cathode, or structure to be protected, does not recognize the source of protective current, because it

is possible to polarize most structures by using galvanic anodes or impressed-current systems

Other power sources used in impressed-current systems include, but are not limited to, the following: engine generator sets, wind-powered generators, thermoelectric generators, gas turbines, solar cells and fuel cells Although the sources of power for supplying protective current in cathodic protection systems are numerous, zinc is the one that is most frequently used; therefore, the rest of this section will discuss the details relating to the function of zinc as a galvanic anode

Applications of Zinc Anodes. The use of zinc in commercial cathodic protection systems involves environments as diverse as seawater, brackish water, freshwater, and a wide variety of soils Because the environment plays a significant role in determining the success or failure of zinc galvanic anode systems, seawater and soils will be treated separately

The application of cathodic protection to reduce or prevent corrosion damage occurring on the steels hulls of marine craft, such as ships, launches, barges, floating docks, buoys, and pontoons, makes it possible to increase substantially the interval between dry dockings and to reduce the amount of maintenance work to be accomplished during each dry docking period The material and labor costs of installing a zinc anode cathodic protection system are usually small compared to such maintenance work as chipping, painting, and replacing hull plates

The economic benefits that accrue from marine cathodic protection depend to a large extent on the duty cycle of the craft under consideration Maximum savings are obtained when cathodic protection is applied to vessels that are dry-docked only for reasons of anticorrosion maintenance Included in this category would be most barges, dredges, buoys, pontoons, and stored ships It is not unusual under these conditions for a cathodic protection system, designed for a 3-year life, to pay for itself within 1 year Even in the case of merchant ships, which must be dry-docked annually for removal of marine growths and reapplication of antifouling paint, cathodic protection, if properly applied, can prove economical because it reduces or eliminates corrosion damage to exposed steel propeller shafts and to areas on the hull where the paint has become damaged

Zinc is an ideal metal for cathodic protection in seawater because it does not subject the adjacent painted surfaces of the hull to high potentials, which are injurious to many commonly used paints Nevertheless, good marine paints, application practice, and reasonable maintenance are of course necessary Zinc has a high ampere-hour capability per unit volume (530 A · yr/m3, or 15 A · yr/ft3) This means that the total volume of zinc required is not large; therefore, the effect of the installation on the speed of the ship is generally very small if the anodes are correctly installed The installation of zinc anodes on marine craft is straightforward, particularly if it is accomplished when the vessel is in dry dock

Table 15 lists the chemical composition and impurity limits of the current United States Government (MIL-A-18001H) and ASTM (B 418) zinc anode specifications The basic zinc anode specification containing aluminum and cadmium as alloying elements has become the worldwide standard for zinc anodes used in cathodic protection systems in seawater and brackish water at ambient temperatures Committee B-2 of ASTM cautioned that the threshold level for intergranular corrosion of type I composition zinc-aluminum-cadmium alloy is particularly severe above a temperature of about 50 °C (120 °F) For additional information on anode selection, see the articles "Marine Corrosion," "Corrosion of Magnesium and Magnesium Alloys," "Corrosion of Aluminum and Aluminum Alloys," and "Cathodic Protection" in this Volume

Table 15 Zinc anode composition specifications for seawater use

Table 16 Zinc anode composition specification for elevated-temperature exposure

Element ASTM B 418 Type II

Underground Zinc Anodes. The cost of installing cathodic protection as a means of stopping the corrosion of coated steel distribution piping is usually small when compared to the cost of repairing leaks and making replacements The perforation of the pipe wall as a result of corrosion may occur sooner on a well-coated pipe than on a bare pipe, because corrosion current is concentrated at holidays or damaged areas in the coating The total metal loss, however, of a coated pipe will usually be negligible compared to that which would occur on a bare pipe, and the danger of developing general structural weakness is therefore greatly reduced

Fortunately, it is relatively easy and inexpensive to apply cathodic protection to a coated pipeline as compared to a bare pipeline, because the current requirements are only a small fraction of that required for the bare pipeline The combination

of a good pipe coating plus cathodic protection has proved to be both economical and successful to such an extent that the current practice is to provide both a coating and cathodic protection on virtually all new transmission pipelines This practice is often carried out even in areas where there is no certainty that severe corrosion would occur

Because the environment around an anode is significant when determining the type and nature of anodic films or coatings, successful anode performance is related to the presence of a friendly environment in contact with the zinc Soils containing significantly more dissolved sulfates and chlorides than carbonates, bicarbonates, nitrates, and phosphates are often compatible with zinc anodes However, by packing a prepared backfill consisting of hydrated gypsum, bentonite clay, and sodium sulfate (Na2SO4) around an anode, a friendly environment is created, and such a packaged anode can be used in almost all soils The most popular consists of 75% gypsum, 20% bentonite clay, and 5% Na2SO4; this environment provides a relatively low resistivity and thus permits high current output levels In time, however, the

Na2SO4 tends to leach out of the backfill, and higher resistivity occurs

The principal zinc specification (ASTM B 418 Type II) used for underground cathodic protection systems given in Table

16 is the unalloyed high-purity zinc with iron controlled to 0.0014% maximum The literature does not show any term field tests comparing the unalloyed Type II materials, with the zinc-aluminum-cadmium composition of MIL-A-18001H, or the ASTM B 418 Type I given in Table 15 Although the latter alloy anodes prepackaged in prepared backfill have been installed, comparative results are lacking

long-Zinc Coating Processes

Seven methods of applying a zinc coating to iron and steel are in general use: hot dip galvanizing, continuous-line galvanizing, electrogalvanizing, zinc plating, mechanical plating, zinc spraying, and painting with zinc-bearing paints This section and the section "Painting With Zinc-Bearing Paints" in this article contain brief descriptions of each process, the nature of the coating formed, and the practical advantages and limitations of each method There is usually at least one process that is applicable to any specific purpose Because the processes are complementary, there are rarely more than two processes to be seriously considered as the best choice for a particular application Additional information on zinc coating processes can be found in the articles "Hot Dip Coatings" (continuous and batch processes are described),

"Electroplated Coatings," "Thermal Spray Coatings," "Corrosion of Carbon Steels," and "Corrosion in the Automotive

Industry" in this Volume Reference to Surface Engineering, Volume 5 of the ASM Handbook is also recommended

Hot Dip Galvanizing

In hot dip galvanizing, the steel or iron to be zinc coated is usually completely immersed in a bath of molten zinc It is by far the most widely used of the zinc coating processes and has been practiced commercially for almost two centuries The modern hot dip galvanizing process is conducted in carefully controlled plants by applying the results of scientific research, and it is far removed from that of years ago, although it is still dependent on the same basic principles

The process is primarily applied to finished parts and to semifabricated materials, such as sheet, strip, wire, and tube, on the continuous automated lines of the steel producers There is an obvious advantage in galvanizing after fabrication in that the zinc completely seals edges, rivets, and welds so that there are no uncovered parts at which rusting can begin

Continuous Galvanizing. In 1936, a revolutionary new process for continuously coating coils of sheet steel by hot dipping was introduced in the United States This process, known as the Sendzimir process, uses a small amount of aluminum in the zinc bath and produces a coating with essentially no iron-zinc alloy and with sufficient ductility to permit deep drawing and folding without damage to the coating Other processes for continuous zinc coating of sheet steel without alloy layer formation were later developed and joined the Sendzimir process Today, nearly all hot dip galvanized sheet steel is produced by continuous methods

Machines measuring 150 m (500 ft) or more in length that galvanize the sheet at speeds frequently exceeding 90 m/min (300 ft/min) are producing at the rate of more than 9,000,000 Mg/yr (10,000,000 tons/yr) in the United States A variety

of coating weights and types are produced that vary from as little as 0.5 oz/ft2 (150 g/m2) of sheet to a maximum of 2.75 oz/ft2 (840 g/m2) of sheet The standard product is 1.25 oz/ft2 (380 g/m2) of sheet The most dramatic advance in the use

of galvanized steel sheet in the automotive industry Corrosion problems, intensified by accelerated usage of salt on the highways in the winter months, have resulted in an increase to current levels of over 900,000 Mg (1,000,000 tons)

The annual production of galvanized pipe is being increasingly mechanized and should in the near future approximate the speeds of, and apply coatings comparable to, those of the continuous lines for sheet The product is broadly used for fencing, sign poles, playground equipment, plumbing, and other construction purposes

The steel industry also galvanizes steel wire or bridge cables, fencing, armored conductor cable, and general steel wire strand for hundreds of uses The product is woven into wire used in armored cable with aluminum or copper conductors and manufactured into many products

In addition, continuous lines were developed to apply heat-cured organic finishes to metal strips Prepainted galvanized strip products are now used for roofing and siding, interior partitions, rain goods, furniture, appliances, and other applications

The Galvanizing Process. Before the iron or steel parts are dipped in the molten zinc, it is necessary to remove all scale and rust This is usually done by pickling in an inhibited acid To remove molding sand and surface graphite from iron castings, shot- or gritblasting is generally used, usually followed by a brief pickling operation

In the dry galvanizing process, the work is prefluxed by dipping in a flux solution of zinc ammonium chloride, then passed to a low-temperature drying oven It is then ready for dipping into the molten zinc bath, the surface of which is kept relatively clear of flux In contrast, in wet galvanizing, the pickled articles are dipped into the molten zinc bath through a substantial flux blanket

In all processes, as the work enters the bath, the protective layer that has prevented oxidation of the freshly pickled surface peels off, and the work is immediately wetted by the molten zinc This involves interpenetration of the iron and zinc with the formation of the alloy layers

Current galvanizing baths are adapted to close thermostatic control In some of these, hot gases are circulated around the sides of the bath; in others, direct gas or electric heating systems are used The usual operating temperature is 445 to 460

°C (830 to 860 °F)

Developments in galvanizing resulted in improved quality through better fluxes, better temperatures control, and better cleaning practices Larger kettles and improved materials made it possible to galvanize large structural pieces Methods were developed for continuously coating wire and pipe in semicontinuous lines

Nature of the Hot Dip Galvanized Coating. Figure 5 shows a photomicrograph of a typical hot dip galvanized

coating consisting of a series of layers Starting from the base steel at the bottom of the section, each successive layer contains a higher proportion of zinc until the outer layer, which is relatively pure zinc, is reached Therefore, there is no real line of demarcation between the iron and the zinc; instead, there is a gradual transition through the series of iron-zinc intermetallics, which provide a powerful bond between the base metal and the coating

Fig 5 Typical hot dip galvanized coating Note the gradual transition from layer to layer, which results in a

strong bond between base metal and coating

The structure of the coating (that is, the number and extent of the alloy layers) and its thickness depend on the composition and physical condition of the steel being treated and on a number of factors within the control of the galvanizer For example, heavier coatings tend to be deposited on rough-surface and coarse-grain steel, and the total thickness of the alloy layer tends to be slightly greater at corners than at hollows, or shallow areas

The total thickness of the coating may be controlled by varying the time for which the work is immersed in the molten zinc and the speed at which it is removed If only a thin coating is required, as is sometimes the case in sheet or wire galvanizing, the work is mechanically wiped upon being withdrawn from the bath in order to remove excess zinc The temperature of the bath has little effect on the nature of the coating if it is kept between 430 and 470 °C (805 and 880 °F) Small or threaded parts are often centrifuged after being hot dipped galvanized in order to remove excess zinc and to produce a more uniform coating

The ratio of the total thickness of the alloy layers to that of the outer zinc coating is also affected by varying the time of immersion and the speed of withdrawal of the work from the molten zinc bath; the rate of cooling of the steel after withdrawal is another factor to be taken into account Sheet galvanizers operating continuous-strip processes usually suppress the formation of alloy layers by adding 0.1 to 0.2% Al to the bath; this increases the ductility of the coating and makes the sheet more amenable to fabrication

Other elements may be added to galvanizing baths to improve the characteristics and appearance of the coating Tin and antimony give rise to well-defined spangle effects, and the presence of some lead in the bath is generally considered desirable The addition of 1% Pb reduces the surface tension by more than 40% when compared with pure zinc This reduction in surface tension tends to help the drainage of the bath metal as the workpiece exits the bath Aluminum also improves the appearance of the coating and the corrosion resistance

Advantages and Limitations. An important advantage of hot dip galvanizing is that, unless zinc is removed by mechanical devices (a practice confined to the automated galvanizing of sheet, strip, wire, and tube) or by centrifuging, the work will probably be thoroughly covered and will carry a thick coating usually weighing 1.8 to 2.2 oz/ft2 (550 to 670 g/m2) All edges, rivets, seams, and welds are thus sealed by the hot dip process Furthermore, it will often be found to be the most economical process when large amounts of steel are to be treated The size of galvanizing baths does of course limit the size of articles that may be treated, but large parts can be protected by suitable double dipping (one end at a time)

Heating fabricated parts to 450 °C (840 °F) by immersion in molten zinc baths occasionally has undesirable effects, but these can usually be overcome For example, warping can be eliminated by paying careful attention to welding techniques

so as to balance stresses Possible embrittlement of malleable cast iron, which is likely only if its phosphorus content exceeds 0.07%, can be avoided by quenching in water from 650 °C (1200 °F) before galvanizing The danger of

embrittlement in galvanizing articles that have previously undergone severe localized cold working can be overcome by suitable stress relieving

Electrogalvanizing

The additional development of continuous electrogalvanizing lines added another dimension to zinc-coated steel, that is, very thin, formable coatings ideally suited to deep drawing or painting Zinc is electrodeposited on a variety of mill products by the steel industry: sheet, wire, and, in some cases, pipe Electrogalvanizing at the mill produces a thin, uniform coat of pure zinc with excellent adherence The coating is smooth, readily prepared for painting by phosphatizing, and free of the spangle that is characteristic of some other zinc coatings

Electrogalvanized steel is produced by electrodepositing an adhering zinc film on the surface of sheet steel or wire These coatings are not as thick as those produced by hot dip galvanizing and are mainly used as a base for paint

The coating produced on strip coils or sheets has a coating weight in the range of less than about 0.06 to 0.2 oz/ft2 (18 to

60 g/m2), or 1.3 to 4.3 m (0.05 to 0.17 mil) thick on each side A small amount carries considerably less approximately 0.025 oz/ft2 (7.6 g/m2), or 0.5 m (0.21 mil) on each side

Zinc is usually electrodeposited on steel wire in the range of 0.3 to 3 oz/ft2 (90 to 915 g/m2) The diameter of plated wire (including wire that is cold drawn after plating usually ranges from 0.23 to 4.9 mm (0.009 to 0.192 in.) Steel carbon contents range from 0.08 to 0.85% Tensile strengths range from 345 to 2070 MPa (50 to 300 ksi) Heat-treated and coated wire can be cold drawn to approximately 95% reduction in area, depending on chemical composition, heat treatment, and diameter

Nature of the Electrogalvanized Coating. The pure zinc coating deposited is highly ductile Because of its excellent adhesion, electrogalvanized steel strip and wire have good working properties, and the coating remains intact after severe deformation

Electrodeposited zinc coatings are simpler in structure than hot dip galvanized coatings They are composed of pure zinc and have a homogeneous structure Surfaces have a smooth texture whose appearance can be varied by additives and special treatments in the plating bath They can be used where a fine finish is needed

Electrogalvanizing provides adequate protection for many types of mild exposures These coatings are frequently treated with chromate conversion solutions to improve appearance, reduce staining, and retard the formation of white corrosion products under high-humidity conditions

Advantages and Limitations. In electrogalvanizing, steel strip or wire is continuously fed through suitable entry equipment, a series of washes and rinses, and a plating bath Either an acid sulfate zinc or cyanide zinc bath is used as the plating bath Both produce even, adhering zinc deposits Although brighteners are not used for electrogalvanizing, grain refiners are usually added to help produce a fine, tightknit zinc surface on the steel

Zinc electrodeposits are considered to have the best adhesion of any metallic coating Good adhesion depends on very close physical conformity of the coating with the basis metal Therefore, particular care must be taken during initial cleaning Electrodeposition affords a continuous process for applying zinc coatings to parts that cannot be hot dipped They are especially useful where a high processing temperature could damage a part

Applications. Electrogalvanized sheets are produced in various tempers suitable for simple bending or forming, for curving, and for rolling into cylinders without fluting Spot welding is easily accomplished if care is taken

Electrogalvanized steel is easily prepared to receive decorative finishes Much of it is produced with a phosphate treatment or an organic coating The phosphate treatment provides an adequate surface for a good bond with organic finishing materials Organic coating applied over electrozinc thus treated maintains good adhesion in adverse conditions, such as sudden changes in temperature and high humidity Phosphated electrogalvanized steel is used for parts subject to atmospheric corrosion or salt spray and for parts that will be lacquered or painted Phosphate treatment increases corrosion resistance markedly, particularly in atmospheres with a high sulfur content

Electrogalvanized sheet is used for manufacturing water cooler housings, exterior panels of ranges, freezers, dryers, washers, air conditioners, and other major appliances It is used for deep-drawn parts for kitchen cabinets, refrigerators,

and allied products instead of plain cold-rolled sheet because zinc holds better in this dies and reduces breakage significantly Bakery goods and other merchandizing display cases, stud systems for steel building construction, acoustical ceiling members, and television antennas are also made of electrogalvanized steel

A new application that should greatly benefit the automotive industry is one-side electrogalvanizing The galvanized side protects against corrosion, and the bare side can take the baked enamel finish required by the outer automobile surface (see the article "Corrosion in the Automotive Industry" in this Volume)

Electrogalvanized wire is especially useful in applications in which the wire must be bent, twisted, or wrapped around its own diameter When formed, the coating does not rack, peel, or flake Many chain link fences are made from zinc-electrocoated wire because it is not rough and therefore is safe to handle The wire is used for conveyor belts, twisted wire brushes, chains, baskets, kitchen utensils, staples, cages, bobby pins, clotheslines, and telephone and transmission wire

Zinc Plating

Coatings of zinc may also be applied to iron and steel surfaces by electroplating The article to be plated is made the negative electrode in an electrolytic cell containing a solution of a zinc salt through which an externally generated dc current is passed Zinc is supplied to the cell as expendable positive electrodes or as zinc salts added directly to the plating solution

Surface Preparation. The adherence of electrodeposited zinc coatings depends on the metal-to-metal bond between the plated coating and the underlying steel surface Therefore, particular attention must be given to the preparation of the surface before plating to obtain a coating in true physical contact with the entire steel surface

The usual method of removing all rust, scale, and grease from the steel surface involves cleaning the surface thoroughly

in a hot alkaline bath by soaking the parts for a short period of time This is often followed by use of an electrolytic alkaline cleaner and a spray alkaline cleaner An acid dip is then carried out to remove oxides and scale There must be adequate rinsing between the alkaline/acid baths and the acid/plating baths to avoid contamination of the plating bath by carryover from the cleaning baths

Zinc Plating Baths. Zinc plating is done in an acid or an alkaline bath Although the alkaline-cyanide zinc baths are the most efficient and have the best throwing power, they do create a serious pollution problem Therefore, more alkaline, hydrochloric acid (HCl), and H2SO4 zinc baths are being used The H2SO4 baths are primarily used in the tubing, wire, and sheet electrogalvanized areas All of the other baths are used for barrel plating Barrel plating is used to plate a large volume of small articles These are placed in a suitably constructed rotating barrel that is immersed in the plating solution

Various brightening agents may be added to the baths to give a deposit that is more lustrous than that obtained from normal zinc plating baths The amount of brightening agent requires very careful control, and the bath and the zinc anode must both be kept particularly pure when brighteners are used

Nature of the Electroplated Zinc Coating. The normal electroplated zinc coating is dull gray with a matte finish, but whiter and more lustrous deposits an be produced by adding special agents The coating consists of pure zinc and is of uniform composition throughout It adheres by metal-to-metal bonds

Advantages and Limitations. Electroplating is the most precise of all zinc coating processes and therefore is particularly suitable for coating delicate articles, such as instrument parts, on which a fine finish is essential Furthermore, the parts need never be heated to a temperature above the boiling point of water

Because the pure zinc coating is extremely ductile, it is quite easy to form zinc-plated sheets without damaging the coating On simply shaped articles, it is possible to control the thickness of electroplated zinc coatings within fine limits Barrel-plated articles have coatings that are more uniform in thickness than those normally obtained

Zinc electrodeposits expand very slightly during plating; therefore, if the deposit does not adhere properly, blistering may develop The effect develops slowly and takes several hours or even days for completion; it is most marked in acid-sulfate deposits

Mechanical Plating

This process is used to deposit a specified thickness of zinc on a steel surface in a uniform manner, even in threaded areas Parts plated by the process do not encounter hydrogen embrittlement The plate thickness can be varied from 2.5 to

125 m (0.0001 to 0.005 in.)

In the plating of fasteners, a large, lined, steeled tumbling container is used for the complete process A predetermined amount of parts, glass beads, and water are added to the tumbler, and the following system is used First, an inhibited acidic powder cleaner and an acid copper salt are added and maintained at a pH to 1.8 The parts are then tumbled to achieve proper cleaning and an immersion coating of copper Second, the catalyst (promoter chemical) and zinc powder (8 to 20 m) are added and tumbled

The resulting zinc deposit is cold welded to the copper by the peening action of the glass beads By regulating the amount

of zinc, it is possible to predetermine the film thickness of zinc that will result

Zinc Thermal Spraying

Zinc thermal spraying consists of projecting atomized particles of molten zinc onto a prepared surface Two types of spray guns are in commercial use today: the powder gun and the wire gun

Surface Preparation. The surface preparation of the work is the same for each process and involves cleaning and then roughening the surface to be sprayed The usual method of roughening is coarse gritblasting

Whatever method of surface preparation is used, the sprayed zinc coating should be applied as soon as possible after the surface has been prepared in order to reduce the possibility of oxidation and thus increase the effectiveness of the metal-to-metal bond The time lag is reduced to a minimum in mechanized plants that have been built for production line work and for treating construction steelwork before assembly

Two Methods of Spraying Zinc. Two methods of spraying zinc produce coatings of comparable quality They are the powder process and the wire process

stream of powder-laden gas is heated by the flame surrounding the nozzle, and outside this, a cone of compressed air gives impetus to the stream of molten droplets

There are four feeds to the gun: combustible gas, oxygen, compressed air, and zinc powder suspended in air or a gas A wide choice of fuel gases is available because the gas does not have to function as an atomizer

stream of compressed air disintegrates the film of molten metal as it forms and sprays it out of the nozzle The oxygen and fuel gas supplied for the flame are at the same pressure so that they may be simply mixed in a small chamber

In hand tools, such as those used for zinc coatings, a compressed air turbine is used to drive the wire feed through worm reduction gearing Nozzles often consist of a sleeve of hard steel surrounded by copper to disperse the heat generated by the blowpipe flame Various types of wire-spraying guns have been developed in which electricity is used in place of gas for heating

The Nature of the Thermally Sprayed Zinc Coating. The sprayed coating is slightly rough and slightly porous The specific gravity of a typical zinc coating is about 6.35 as compared to 7.1 for cast zinc This slight porosity does not affect the protective value of the coating, because the zinc is anodic to steel The zinc corrosion products that form when the coating is in service fill up the pores to produce a solid coating The slight roughness of the surface makes it an ideal base for paint

The mechanism by which the zinc adheres to the underlying surface has been the subject of many experiments and much speculation Although no alloy layer is formed and the bond is purely mechanical, the adhesion of zinc to a properly prepared iron or steel surface is excellent Recently, it has been suggested that pre-heating the base metal gives even greater values for adhesion In the ordinary process, the impact of the particles of molten zinc on the surface causes only a very slight increase in temperature of the base metal

Advantages and Limitations. Zinc spraying has a significant advantage over most other methods of zinc coating in that it can be applied to work of almost any shape or size on the site When applied to finished parts, the welds, ends, and rivets receive adequate coverage Moreover, it is the only satisfactory method of depositing unusually heavy zinc coatings with thicknesses of 0.25 mm (0.01 in.) and greater

Zinc spraying is usually not suitable for depositing coatings inside cavities, although special types of nozzles are available for applying coatings inside short lengths of tube The process is seldom economical for treating open structures, such as wire mesh, because of the large loss of metal that would result

The mechanized plants, such as those used for treating construction steelwork before assembly, make it possible to deposit completely uniform coatings In hand spraying, the degree of uniformity achieved depends entirely on the skill of the operator

Painting With Zinc-Bearing Paints

Although zinc coatings can be applied in several ways, the size of a structure or piece of equipment places limitations on the method used On very large structures, painting is often the only practical method Two types of zinc pigment coatings are available for corrosion control: zinc dust/zinc oxide and zinc-rich coatings

Zinc Dust/Zinc Oxide Coatings

Zinc dust/zinc oxide paints, also known as metallic zinc paints, contain a pigmentation of approximately 80% zinc dust, 20% ZnO, and approximately 80% pigment by weight These paints offer excellent rust-inhibitive properties, adhesion, film distensibility, and abrasion resistance Because they adhere tightly to zinc and other metals, they are ideal for prime and finish coat applications and may be used as a primary first coat even over primarily rusted surfaces

Zinc dust/zinc oxide paints are used for the protection of many types of steel structures under a variety of service conditions They are particularly well suited to use on galvanized steel; are highly satisfactory for priming steel for atmospheric and underwater exposure; and can be used on many outdoor structures, such as bridges, water tanks, and dams, where rusting must be prevented

In accordance with Federal Specification TT-P-641 which covers primer paints and zinc dust/zinc oxides for galvanized surfaces, there are three types of zinc dust/zinc oxide paints:

• Type I: zinc dust/zinc oxide linseed oil for outdoor exposure, recommended as a primer or finish coat

for broad, general use, especially when there is widespread rusting of the steel surfaces; should be air dried only

• Type II: zinc dust/zinc oxide alkyd resin paint, a heat-resistant paint sometimes sold as a stack paint,

may also be used for outdoor exposures where rust is not severe; quick drying; can be air dried or baked

at temperatures to 150 °C (300 °F)

• Type III: zinc dust/zinc oxide phenolic resin paint; used for water immersion and other severe moisture

conditions; may be air dried or baked at temperatures to 150 °C (300 °F)

These paints, when properly formulated and prepared, can be applied by brushing, dipping, or spraying Although the presence of ZnO prevents rapid or hand settling, adequate agitation of the paint in the dip tank is necessary to ensure the coating homogeneity necessary for maximum metal protection Pressure equipment should be used when spraying, and the distance between the paint reservoir and the spray gun should be as short as possible to ensure the proper rate of feed

to the nozzle Again, some agitation of the paint in the reservoir is recommended

Zinc dust/zinc oxide paints possess high covering power and can hide backgrounds of almost any color when spread at the rate of approximately 20 m2/L (800 ft2/gal) However, because the protection afforded by a paint coating is directly related to its thickness, the necessary protection cannot be guaranteed unless the dry film is thick enough for the specific environmental conditions Therefore, care must be taken to avoid spreading the paints too thin The natural blue-gray color of zinc dust/zinc oxide paints provides an aesthetic appearance, but if another color is desired, red, buff (orange-yellow), and green can be obtained by varying the pigment

To prepare surfaces for zinc dust/zinc oxide paints, rust (or scale) and any accumulation of leaves, dirt or other foreign materials should be removed This may be accomplished on large structures by sandblasting and on small structures or areas with a deck or wire brush

Zinc-Rich Coatings

In recent years, a number of paints have been developed that will deposit a film of metallic zinc having many properties

in common with zinc coatings applied by hot-dip galvanizing, electro-plating, metal spraying, and mechanical plating Such paint films will protect the underlying steel sacrificially if they contain 92 to 95% metallic zinc in the dry film and if the film is in electrical contact with the steel surface at a sufficient number of points They are effective where steel is subjected to high humidity and water immersion Under normal conditions, zinc-rich coatings are long lasting and most effective where a regular maintenance program may be difficult In applications in which steel is immersed in brackish or salt water, zinc-rich coatings, along with a suitable top coat, should be used Most zinc-rich paints are of the air-drying type, although oven-cured primers containing a high content of zinc dust are available

The type of zinc dust used is a heavy powder, light blue-grey in color, with spherically shaped particles having an average diameter of approximately 4 m Such powder normally contains 95 to 97% free metallic zinc with a total zinc content exceeding 99%

Surface Preparation. Zinc-rich primers must be applied over clean steel surfaces to provide the metal-to-metal contact essential to successful performance of the coating Abrasive blasting is the most effective method of cleaning steel Although white metal blast-cleaning (NACE No 1) is preferred, near-white SSPC-SP-10 or Commercial Blast Cleaning SSPC-SP-6 is acceptable (Ref 14)

Where the zinc is supplied as a separate component, it should be added slowly to the vehicle with constant agitation After

a homogeneous mix is obtained, the primer may be applied with air spray Airless spray may also be used, but the nozzles may wear quickly Because zinc settles rapidly, continuous agitation of the paint is essential during application, and fluid lines should be kept as short as possible

To obtain a wet coat, the gun should be kept within 30 cm (1 ft) of the surface Uneven film thickness due to brushing or rolling may result in mudcracking in the thick portions Zinc-rich primers should be applied at a dry film thickness of 0.06

to 0.08 mm (2.5 to 3.5 mils)

The Nature of the Zinc-Rich Coating. Depending on the binder, zinc-rich coatings fall into two classes: organic and inorganic The inorganic solvent-base types are derived from organic alkyl silicates, which become totally inorganic upon curing Each offers particular protection characteristics, and each requires different preparation of the steel surface The following comparisons should be helpful in selecting the most useful binder system

The organic zinc-rich coatings are formed by using zinc dust as a pigment in an organic binder This binder may be any of the well-known coating vehicles, such as chlorinated rubber and epoxy The zinc dust must be in sufficient concentration so that the zinc particles are in particle-to-particle contact throughout the film Thus, zinc provides cathodic protection In the case of the organic binder, there is no reaction with the underlying surface other than for the organic vehicle to wet the steel surface thoroughly and to obtain mechanical adhesion

Organic zinc-rich coatings do not require a white blast preparation of the steel surface, although a commercial blast should be included if the application is heavy service For mild-service applications, the organic coating can be applied to

a well-hand-cleaned surface, even if traces of rust are present

Some proponents feel that maintaining proper humidity during surface preparation, application, and curing is not necessary Because this type of coating is more flexible than inorganic coatings, exacting surface preparation for bonding

to a substrate is not required Finally, although organic coatings are more compatible with top coats, they are somewhat less abrasion resistant than the inorganic types

As to the advantages of these coatings, organic zinc-rich coatings require less critical surface preparation, allow greater variation in application techniques, are less sensitive to varying climatic conditions during application and curing, and are more flexible and more resistant to chemical environments Their disadvantages include flammability, blistering, harmful solvent effects, sensitivity to atmospheric influences, and relatively low heat resistance

For better resistance against continuous exposure to salt water and to acid or alkali chemical fumes, zinc-rich coatings should be top coated with organic topcoats to provide a totally organic system, with optimum intercoat compatibility A top coating may also be applied to provide color or to prevent gradual erosion of the zinc coating Although zinc-rich coatings vary in application characteristics, they can be applied by brush or spray, and depending on the specific formulation, one coat can vary in thickness from 2 to 7 mils

Inorganic Zinc-Rich Coatings. Many inorganic zinc-rich coatings use water solutions of alkali silicates as vehicles Others use phosphates, silicones, and modifications of these groups

Self-cured coatings are two-package materials consisting of zinc dust and a vehicle; they are mixed immediately before application Postcured coatings are three-package materials that consist of zinc dust, the vehicle to be mixed with it before application, and a curing agent that is applied on top of the coating

The inorganic zinc coating forms its film and its adhesion to the steel surface methods quite different from those of the organics The coating system is a chemically reactive system, and the chemical activity is similar for either the water-or the solvent-base inorganic Zinc is the principal reactive element in the inorganic coating systems and is primarily responsible for the development of initial insolubility Depending on the formulation, other metal ions may be present in the system that also react and aid in the insolubilization of the coating The silicate vehicle can also react with underlying iron surface to form a chemical bond with the iron or steel substrate

Inorganic zinc-rich coatings commonly require a white metal blast as preparation for the steel surface Because inorganic coatings generally have limited flexibility and tend to break or crack upon bending or impact, careful preparation of the steel surface is required to ensure a good bond between the coating and the steel However, despite the difficulties of preparation, these inorganic coatings are unaffected by solvents, oils, petroleum products, aliphatics, aromatics, ketones, and alcohols They do not chalk, peel, or lose thickness over long periods of time Also, they are easier to weld through and have excellent abrasion resistance and surface hardness

Inorganic zinc-rich coatings offer good conductivity; good adhesion to clean steel; excellent resistance to weather, sunlight, and variations in temperature; resistance to radiation, heat, and abrasion; and reduced undercutting Conversely, these coatings require unusually good surface preparation, display a lack of distensibility and adhesion to some metals other than steel and zinc, require moderate temperatures and atmospheric humidity for cure, and exhibit unsatisfactory durability under conditions of continuous immersion in electrolytes and a lack of resistance to strong acids and alkalies

Zinc Dust/Zinc Oxide Paint Versus Zinc-Rich Coating

Whether to use a zinc dust/zinc oxide paint or a zinc-rich coating depends on a number of factor, including cost of surface preparation, paint application, and anticipated length of surface Zinc dust/zinc oxide coatings are ideal for rural or semi-industrial atmospheres They are particularly effective on galvanized surfaces

The widely used zinc dust/zinc oxide primers based on ordinary drying oil media do not give general electrolytic protection against corrosion and therefore do not fall in the category of zinc-rich paints

Zinc-rich coatings are preferred for the protection of steel or galvanized steel structures exposed to marine environments

or immersed in seawater Applications include interiors of floating roof tanks, cooling tower piping, pipe racks and exterior piping in refineries, chemical plant maintenance, offshore drilling platforms, aboveground pipelines, structural steel before erection, exterior of pressure vessels, ammonia tanks, ship holds, and air conditioning equipment

A top coat finish may be necessary in aggressive atmosphere The top coat must adapt to the environment and must guarantee compatibility with, and adhesion to, the zinc-rich primer

Advantages and Limitations of Zinc-Rich Paints. Zinc-rich primers offer a more versatile form for applying zinc

to steel galvanization; large, continuous complex shapes and fabricated new or existing structures can be easily coated at manufacturing shops or in the field Their performance has earned them a prominent place in the field or corrosion protection coatings However, the limitations of zinc-rich paints include cost, difficulty in applying, and the requirement

of clean steel surfaces They must be top coated in severe environments (pH under 6.0 and over 10.5)

Zinc Chromate Paints

Zinc chromate pigments are unique in that they are useful as corrosion inhibitors for ferrous and nonferrous metals Zinc chromate was developed just before World War II and was the major pigment in marine primers It is yellow in color, but can be tinted green Zinc chromate paints are used as an after-pickling coating on steel and as a primer for steel and aluminum Federal Specification TT-P-645 covers zinc chromate paints

References

1 R.M Burns and W.W Bradley, Protective Coatings for Metals, 2nd ed., Reinhold, 1955, p 66-147

2 Zinc Coatings for Corrosion Protection, Zinc Institute, 1978, p 20

3 E.A Anderson The Atmospheric Corrosion of Rolled Zinc in Symposium on Atmospheric Corrosion of

Nonferrous Metals, STP 175, American Society for Testing and Materials, 1955, p 126-134

4 W Machu, Corrosion of Metals and Metal Coatings in Tropical and Sub-Tropical Climates, Werkst

Korros., Vol 5, 1954, p 395-398

5 Report of Subcommittee V on Atmospheric Exposure Tests of Zinc Alloy Die Castings, Committee B-6,

in Proceedings ASTM, Vol 61, American Society for Testing and Materials, 1961, p 273-281

6 "Specification for Zinc (Slab Zinc)," B 6, Annual Book of ASTM Standards, American Society for Testing

and Materials

7 "Specification for Zinc Alloy Die Castings," B 86, Annual Book of ASTM Standards, American Society for

Testing and Materials

8 G.L Cox, Effect of Temperature on the Corrosion of Zinc, Ind Eng Chem., Vol 23 1931, p 902-904

9 H Grubitsch and O Illi, The Hot Water Corrosion of Zinc II, Korros Metall., Vol 16, 1940, p 197

10 H Grubitsch and H Huemer, Scanning Electron Microscope Studies of the Hot Water Corrosion of Zinc,

Werkst Korros., Vol 24 (No 1), 1973, p 1-7

11 R Rosset and A Jardy, Protection of Galvanized Steel Against Cold and Hot Water Corrosion by a

Pyrophosphate Coating, in Proceedings of Intergalva/82, May 1982; French patent 7402178, 1974; French

patent 7637425, 1976

12 Handbook of Steel Drainage and Highway Construction Products, American Iron and Steel Institute,

1971, p 214-215

13 Zinc Coatings for Corrosion Protection, Zinc Institute, 1978, p 25

14 Zinc Coatings for Corrosion Protection, Zinc Institute, 1978, p 14

Corrosion of Tin and Tin Alloys

Daniel J Maykuth and William B Hampshire, Tin Research Institute, Inc

Introduction

TIN is a soft, brilliant white, low-melting metal that is most widely known and characterized in the form of coating for steel, that is, tinplate In the molten state, it reacts with and readily wets most of the common metals and their alloys Because of its low strength, the pure metal is not regarded as a structural material and is rarely used in monolithic form Rather, the metal is most frequently used as coating for other metals and in alloys to impart corrosion resistance, enhance appearance, or improve solderability It also finds wide use in alloys, the most important of which are tin-base soft solders and bearing alloys and copper-base bronzes

Pure Tin

Pure tin is subject to two phenomena that are sometimes confused with the corrosion process in the ordinary atmosphere These are its low-temperature allotropic modification and its susceptibility to whisker growth To avoid this confusion, these processes are discussed below

Allotropic Modification. At temperatures from 13.2 °C (55.8 °F) to its melting point of 232 °C (449.6 °F), tin exists in

a body-centered tetragonal (bct) structure commonly known as -tin Below 13.2 °C (55.8 °F), the form can change to

a diamond cubic structure known as -tin, which lacks cohesion and appears as a friable gray powder This is sometimes called the tin pest This transformation does not occur spontaneously unless the tin is of extremely high purity and is exposed to subzero temperatures The transformation can be accelerated by inoculating the with crystals or by deforming the -tin at low temperatures (Fig 1) Some details of the mechanisms and kinetics of this process are discussed in Ref 1

Fig 1 Gray tin transformation on pure tin Both samples were stored at -20 °C (-4 °F), but the sample on the

left was bent at this temperature and the other was left undisturbed

The transformation is inhibited by the presence of small amounts of bismuth, antimony, or lead Hot-dipped tin coatings and most electrodeposited coatings seem to be immune to this phenomenon, probably because of impurity effects Thus,

no traces of transformation were evident on hot-dipped tinplate cans after burial for 46 years in arctic snow or on electroplated tin coatings on refrigerator parts (Ref 2) However, transformation has occurred with thicker deposits; when such low-temperature exposure is anticipated, the incorporation of about 0.1% Bi is recommended to avoid the problem (Ref 3)

Tin Whiskers. Tin is subject to a form of recrystallization at room temperature that manifests itself as a growth of thin (1- to 2- m, or 0.04- to 0.08-mils, diam) single-crystal filaments from the surface of tin coatings These can begin to form

in as little as 5 weeks and may grow at a rate up to 1 mm/mo (0.04 in./mo) Although the mechanism is not clearly understood, formation of tin whiskers appears to be favored by residual or applied stress, by the presence of a brass substrate, and by high-purity electrodeposited tin (Ref 4, 5, and 6) The potential for whisker growth can be minimized if not completely eliminated by reflowing the tin coating or by incorporating 2 to 10% Pb into the electrodeposited tin

Atmospheric Corrosion. In clean dry air, tin retains a bright appearance for many days In one study, a light dulling was observed after 100 days, and noticeable, faint yellow-gray tarnish film was seen after 150 days (Ref 7) However, it was also reported that the reflectivity of tin remains practically unchanged over long periods when the tin is washed with soap and water (Ref 8) Thus, at ordinary temperatures, the surface oxide film on tin is very thin and exhibits a very slow rate of growth The rate of oxidation increases with temperature Above 190 °C (375 °F), a film thickness sufficient to produce interference colors is reportedly produced in a few hours; at 210 °C (410 °F), this film thickness is produced in

20 min (Ref 2)

The results of a comprehensive 20-year study of the atmospheric corrosion resistance of bulk tin were reported by an ASTM Committee (Ref 9-13) Sheets of commercial 99.85% purity tin, measuring 230 × 300 mm (9 × 12 in.) were exposed at seven sites in the United States, including industrial, seacoast, and rural atmospheres Results are listed in Table 1

Table 1 Corrosion of tin exposed in different environments for 10 and 20 years

Average corrosion rate(a)

10 years 20 years

Sample location

mm/yr mils/yr mm/yr mils/yr Heavy industrial 0.0017 0.067

Marine heavy industrial 0.0013 0.051

Marine (New Jersey) 0.0019 0.075

and eighteenth century sarcophagi in Vienna revealed some evidence of deterioration that was suspected to be the tin pest

It was found, however, that the casting was porous and that air and moisture produced corrosion products of stannous oxide (SnO) and SnO2, causing the observed swelling, blistering, and cracking

Oxidation. At extremely low temperatures, the oxidation of tin is very superficial In one investigation, resistivity measurements were used on tin condensation films formed at 1.5 to 300 K; in all cases, a step function indicating that the growth of tin oxide first began at 23 K was found (Ref 15) No further growth of the oxide was detected at 50 to 150 K

The most comprehensive studies of interactions between tin and oxygen were those discussed in Ref 16, 17, 18, and 19, in which 99.994% pure foils and a vacuum microbalance were used to measure oxidation rates at oxygen pressures between

10-3 and 500 torr (0.13 and 6.7 × 104 Pa) and temperatures from 150 to 220 °C (300 to 430 °F) The essential features of oxidation behavior were found to be explainable in terms of the microstructure of the oxide With oxygen pressure under

1 torr (133 Pa), dendritic -SnO crystallites grew at an increasing rate, with the rate-determining factor apparently being the dissociation of oxygen Above 1 torr (133 Pa), the oxidation rate curves had a characteristic sigmoid shape, in which the initial stages corresponded to the lateral spread of oxide from numerous nuclei to form -SnO platelets Subsequent growth followed a logarithmic law and was consistent with control by tin diffusion through an oxide film under a parabolic or cubic law, while the formation of cavities in the oxide film progressively reduced the area through which diffusion could take place For long oxidation times, the thick oxide film was subject to random fracture, leading to erratic results

The oxidation of tin containing 0.17% Pb and 0.024% Sb was examined at 168 to 211.5 °C (335 to 413 °F) and oxygen pressures of 4 to 9 torr (533 to 1200 Pa) (Ref 20) These oxidation rate data were not significantly different from those given in Ref 16, 17, 18, and 19

The effects of impurities on the oxidation rate of tin were also studied by using microbalance techniques under conditions similar to those described in Ref 16, 17, 18, and 19 (Ref 21) The results are summarized in Table 2 These results were later rationalized in terms of the relative thermodynamic stability of the oxides formed, as follows If the oxide of the alloying element is less thermodynamically stable than SnO, the oxidation rate of the alloy remains unchanged for additions whose ions have the same valence as the tin However, when the formal ionic charge of the alloying element exceeds that of the tin for example, antimony, bismuth, iron, and titanium then the oxidation rate of the tin increases Those alloying elements forming an oxide more stable than SnO for example, zinc, indium, phosphorus, and germanium undergo preferential oxidation at the surface, thus inhibiting the oxidation of tin (Ref 22)

Table 2 Effect of alloy additions of 0.1 at.% on the oxidation rate of tin at 190 °C (375 °F) and an oxygen pressure of 10 torr (1330 Pa)

Alloying element Increase in weight after

1000 min, g/cm2 Manganese 2.7

Another study investigated the effects of alloying additions at levels of 0.01, 0.1, and 1% on the oxidation of molten tin (Ref 26) Antimony, lead, bismuth, and copper had negligible effects, while higher concentrations of lead increased the temperature at which significant oxidation occurs Magnesium, lithium, and sodium significantly increased the oxidation rate, but zinc, phosphorus, indium, and aluminum decreased the rate The oxidation of an alloy containing 0.01% Al was about the same as that of pure tin at 425 °C (795 °F)

Other laboratory oxidation studies were concerned with tin in contact with air The formation of an oxide film was shown

in Ref 27 and 28, and weight increment curves were developed in Ref 29 In another study, the oxidation rate was determined to be linear after the first few days and was nonprotective (Ref 6) Lastly, the oxidation of tin and tinplate was investigated by using coulometric and x-ray techniques (Ref 30, 31) Up to 130 °C (265 °F), the oxidation followed a

logarithmic rate law that tended to become parabolic at higher temperatures At room temperature, the oxide film appeared to be amorphous, but at higher temperatures, -SnO was detected, possibly with some SnO2

One study found that SnO forms on tin immediately above its melting point and that SnO2 forms at higher temperatures (Ref 32) This effect was demonstrated by spot heating a piece of tinfoil (Ref 33) Stannic oxide was found at the center and was surrounded by SnO, which was in turn ringed with an amorphous oxide According to other researchers, the disproportionation of SnO to tin and SnO2 is a slow process, even at 300 °C (570 °F) (Ref 34) The need for extreme care

in oxidation studies, especially with regard to surface preparation, was emphasized in Ref 22 This was demonstrated by using cathodic cleaning to show the effects of humidity (Ref 35)

Minor impurities in tin also affect its oxidation behavior in air Small amounts of indium, phosphorus, or zinc were found

to slow the oxidation (Ref 30) In addition, traces of aluminum were shown to cause embrittlement as a result of intercrystalline attack (Ref 36) Antimony additions, however, counteracted this effect

Reaction With Other Gases. Tin does not react with hydrogen or nitrogen below its melting point, nor is it reactive with dry ammonia (NH3) Molten tin reacts with carbon dioxide (CO2) according to:

Above 650 °C (1200 °F), molten tin reacts with water vapor to form SnO2 and hydrogen

From 25 to 100 °C (75 to 212 °F), hydrogen sulfide (H2S) has little apparent effect on tin, but above 100 °C (212 °F), stannous sulfide (SnS) forms Stannous sulfide and stannic sulfide (SnS2) are also formed by reacting tin with sulfur at high temperatures Tin also reacts readily with SCl2, S2Cl2, NOF, and hydrofluoric acid (HF) Tin is readily attacked by chlorine, bromine, and iodine at room temperature, but fluorine reactions become significant only above 100 °C (212 °F)

Water. In hot or cold distilled water, the only action of tin is the slow growth of an oxide film, with a negligible amount

of metal entering solution Water that was freshly distilled in a tin was found to have less than 1 ppb Sn in solution (Ref 2) Storage in tin-lined or tinned copper tanks for 24 h produced, in the worst instances, only a few ppb, but in some cases, the tin content remained below 1 ppb

In tap water of 7.2 pH at 25 °C (75 °F), specimens of 99.99% cold-rolled tin showed a weight gain of 0.023 mg/dm2/d (1.2 × 10-4 mm/yr, or 0.04 mils/yr) in 50 days and the formation of an insoluble film (Ref 37) With harder tap waters of 7.4 and 8.6 pH, weight losses of the order 0.046 and 0.01 mg/dm2/d (2.3 × 10-4 and 5 × 10-5 mm/yr, or 0.09 and 0.02 mils/yr), respectively, were incurred in 50 days Precipitated carbonate was mainly responsible for localized water line attack with hot and cold hard waters because no attack occurred without the precipitate Addition of 5% Sb to the tin prevented localized attack by hard water

The results of corrosion test data on tin and several tin alloys in seawater under conditions of total immersion are shown

in Table 3 It was also observed that application of a fairly thick 60Pb-40Sn alloy coating over copper will protect it from erosion by seawater at high velocity (Ref 38)

Table 3 Corrosion of tin and tin alloys totally immersed in seawater

Penetration rate((a)

years

mm/yr mils/yr

Test location

99.75 tin Cast bar 4 0.0022 0.087 Bristol Channel

99.2 tin Cast bar 4 0.0008 0.03 Bristol Channel

Babbitt alloy (Sn-7.4Sb-3.7Cu) Cast plate 1.4 0.060 2.4 Kure Beach, NC

Solder (Sn-50Pb) Sheet 0.5 0.075 2.95 Bogue Inlet, NC

Solder (Sn-60Pb on copper) Plate 2.1 0.011 0.43 Kure Beach, NC

Table 4 compares the corrosion rates for tin samples exposed vertically in various acids open to the air at 30 °C (85 °F) The greater weight loss over the 96-h period was largely attributed to the access of oxygen to the solutions

Table 4 Corrosion rate of tin in 0.1 N acids at 30 °C (85 °F) exposed vertically in solutions open to air

Average penetration rate(a) 24-h test 96-h test

(a) Converted from weight loss data, assuming a tin density of 7.29 g/cm3

The following general comments concern the effects of other acids (Ref 25) Hot hydrobromic (HBr) and hydroiodic (HI) acids rapidly attack tin, but the rate of attack is slow with HF Tin is slowly attacked by HClO2 and is readily attacked by HClO3 Sulfurous acid (H2SO3) attacks tin, but sodium acid sulfite (NaHSO3) is noncorrosive Pyrosulfuric acid (H2S2O7) and chlorosulfonic acid (SO2ClOH) react rapidly with tin; nitric acid (HNO3) reacts rapidly with tin over a wide range of concentrations, and the reaction is complex

Bases. Tin may be dissolved by alkaline solutions, with the production of soluble stannates or stannites Corrosion will usually follow if the surface oxide layer can be dissolved; this will occur with pH greater than 12 and may occur at pH values down to 10 When corrosion is possible, its rate is governed by the temperature and the rate of arrival of oxygen or other oxidizing agents to the initial surface and is not greatly affected by the character of the alkali in long periods of immersion However, in intermittent immersion, the corrosion rate is affected by the nature of the alkali and its concentration because these affect the time for removal of the oxide film The corrosion rates of tin in various alkaline solutions exposed to air at 60 °C (140 °F) are summarized in Table 5

Table 5 Corrosion rate of tin in alkaline solutions exposed to air at 60 °C (140 °F)

Penetration rate(a)

(a) Converted from weight loss data, assuming a tin density of 7.29 g/cm3

Hydrogen evolution does not occur on a tin surface in alkaline solutions Thus, exclusion of oxidizing agents, including air, can provide complete protection unless the tin is in contact with another metal on which hydrogen evolution can occur Additions of oxygen absorber can prevent corrosion even without the exclusion of air, but they must be replenished Small additions of oxidizing agents to alkalies stimulate corrosion, but sufficiently large additions can be completely effective Soluble chromates are particularly effective in this way Saturated NH3 solutions do not attack tin, but more dilute solutions behave like other alkaline solutions of comparable pH

Other Liquid Media. Milk and milk products are usually nonreactive with tin, although a long period of stagnant contact may produce local corrosion (Ref 41) Sulfide solutions and materials containing sulfur dioxide (SO2) as a preservative produce sulfide stains, but the rate of metal loss in low Beer dissolves a trace of tin from freshly exposed metal Although this may cause an objectionable haze in the beverage, the action usually ceases within a short period To avoid this effect, the tin surfaces can be passivated by using alkaline chromate solutions

Most organic liquids, including ethers, alcohols, ketones, esters, hydrocarbons, and chlorinated hydrocarbons, are inert toward tin in the absence of water (Ref 25) However, a reaction was reported between tin and lower alcohols at elevated temperatures, and when mineral acidity can arise, as with chlorinated hydrocarbons containing water, there may be some corrosion (Ref 42) Animal, vegetable, or mineral oils and fatty acids are also essentially inactive, and the absence of any catalytic action of tin on their oxidation makes tin or tin-coated vessels suitable for these products

Galvanic Behavior. When immersed in electrical contact with a more noble metal, such as copper or nickel, tin is much more likely to be corroded, and any loss of metal will be faster, with an increase in the number of locally corroded spots in conditions favorable to local corrosion However, contact with such metals as aluminum or zinc can prevent corrosion of tin entirely, and a tin coil or vessel can be protected by joining it to a strip of one of these metals The

galvanic-corrosion behavior of tin and tin-lead alloys in contact in seawater with numerous alloy steels and other structural materials is summarized in Table 6

Table 6 Seawater corrosion of galvanic couples

Source: Ref 38

Passivation of Tin. Tin can be readily passivated with or without an applied potential The solutions most frequently used are the strongly oxidizing chromate solutions, which produce a thin, tenacious oxide layer that is quite protective This film is 40 to 50 (16 × 10-8 to 20 × 10-8 in.) thick when prepared by immersion in an alkaline chromate solution at

80 to 90 °C (175 to 195 °F) for 15 min (Ref 43) Anodic passivation with a current density of 500 A/dm2 (32 A/in.2) for 5

s in 0.5% sodium hydroxide (NaOH) forms a 300- (12 × 10-7-in.) thick film In 0.005 M potassium chromate (K2CrO4) solution, SnO is oxidized to SnO2 above a potential of 0.2 V versus an Ag/AgCl electrode, and the oxide continues to thicken even after the oxygen evolution potential is reached The passivation behavior of tin in solutions of phosphoric acid (H3PO4) (Ref 44), NaOH (Ref 45), sodium borate (NaBO2), and sodium carbonate (Na2CO3) (Ref 46) has also been studied, and is reviewed in detail in Ref 25

Soft Solders

Most soft solders contain from 2 to 100% Sn, with the balance consisting of lead, although some special-purpose solders substitute silver or antimony for some or all of the lead Two features are particularly relevant to the corrosion behavior of solders with regard to their function as a joining material First, fluxes are usually used, and, second, the solder exposure areas are usually much smaller than the area of the materials being joined

By nature, fluxes function as oxide removers and may contain hygroscopic products that, if not removed, will promote corrosion A mild flux, such as pure natural resin, is inactive at normal (room) temperatures and therefore has a harmless residue

More powerful fluxes may consist of natural resin with additions for example, chlorides and bromides or mixtures of chlorides, H3PO4, and derivatives Residues from such fluxes must usually be completely removed by mechanical wiping

or with solvents

The area effect can be minimized by coating the joined metals with tin or tin-lead alloys However, the suitable design of joints and the formation of protective corrosion products over the solder often permit the satisfactory use of soldered joints in conditions that may at first appear hostile

Simple binary tin-lead solders consist essentially of eutectic mixtures, and their corrosion behavior is similar to that for either metal, with the overall behavior similar to that of the predominant metal Both metals are attacked by acids and alkalies, but the presence of lead, which forms many more insoluble compounds than tin, creates further possibilities for the formation of protective layers in near-neutral aqueous media The addition of other elements has not been found to affect the corrosion resistance of tin-lead alloys appreciably (Ref 2) Also, the behavior of lead-free solders containing silver or antimony with tin does not differ greatly from that of pure tin

Atmospheric Corrosion. Even small additions of lead to tin impair the retention of its bright reflective surface in common atmospheres With increasing lead content, the appearance of soldered joints becomes increasingly dull, like that

of lead However, destructive corrosion (except effects from flux residues) is highly unusual On rare occasions, within enclosed spaces, condensed pure water may extract lead, but more common causes of trouble are volatile organic acids Acetic acid (CH3COOH) vapors from wood or insulating materials and formic acid (HCOOH) or other acids that may come from insulating materials may attack lead-containing solders to produce a white incrustation and cause serious destruction of metal Where such attack occurs, substitution of a solder with a higher tin content may eliminate the problem

Contact of solder with other metals can impose a serious risk in conditions of exposure to sea spray or where pockets or crevices can trap moisture or flux residues In most atmospheric conditions, the formation of lead sulfate (PbSO4) protects the solder However, in chloride pollution conditions, nickel, copper, and their alloys are likely to be cathodic to solder Zinc tends to be strongly anodic to soft solders, but correctly designed zinc roof coverings appear to suffer no deterioration at the soldered joints (Ref 2)

Immersion. Natural waters and commercial treated waters that are aggressive to lead are likely to corrode solder at a rate that increases slowly, in proportion to its lead content, up to about 70% Pb, then more rapidly at higher lead contents Selective dissolution of lead can also occur in distilled, demineralized, or naturally soft waters, causing serious weakening

of joints (Ref 2) In the general run of commercial waters, the ability of lead to form insoluble oxides, sulfates, and carbonates usually protects solders against serious attack Although rare, selective dissolution of tin has been reported during prolonged contact of solders with solutions of anionic surface-active agents

When freshly exposed to water, solders are anodic to copper, but soldered joints in copper pipes are widely used without trouble in conventional commercial and domestic cold- and hot-water systems Despite this generally good corrosion resistance, it has been demonstrated that, under adverse conditions, lead may be leached from the commonly used 50Sn-50Pb plumbing solder into water traveling through the pipe; this is a cause of increasing concern (Ref 47, 48) The lead content of water passing through soldered copper pipes is usually less than that recommended by various regulatory authorities, although higher values may be found in new installations and in some soft water areas (Ref 48) Public concern about all sources of lead in the human diet is well documented in numerous publications, and in some countries, including the United States, legislative action has been undertaken to prohibit the use of lead-containing solders and to tighten existing water quality standards (Ref 49)

Soldered joints in brass usually perform well in domestic waters, but good joint design is imperative In automobile radiators in which there are no inhibitors, ethylene glycol, although not directly aggressive, does appear able to detach protective deposits that may form on soldered joints Properly tested and approved inhibitors avoid this problem Sodium nitrite (NaNO2), which is used as an inhibitor for some metals, will attack solders and must be used in conjunction with sodium benzoate (NaC7H5O2)

In seawater or uninhibited brines, the high conductivity and predominance of chloride makes galvanic action at a soldered joint more likely to continue destructively, and soldered joints in copper, nickel, and their alloys may need protection by coatings Although tin or tin-coated metals can be used in contact with aluminum alloys even in salt water, the soldering process introduces sufficient aluminum to the solder to render it susceptible to intergranular corrosion If tin-zinc solders are used, the zinc can prevent the serious embrittling action, although some corrosion will still occur under moist conditions

Pewter

By definition, modern pewter is an alloy that contains 90 to 98% Sn, 1 to 8% Sb 0.25 to 3% Cu, and a maximum of 0.05% Pb and As (Ref 50) Material that conforms to these standards has about the same degree of corrosion resistance to ordinary atmospheres as pure tin Alloys within this range are widely used for decorative items, containers, and flatware Indoors, they retain a bright, white luster in the same manner as pure tin Because contamination from fabrication residues can deteriorate the protective oxide, care should be exercised in finishing to remove residues from soldering fluxes and cleaning solutions Regular, simple washing with a mild soap solution will ensure that the surface remains in good condition

Pewter tankards and plates also have about the same degree of corrosion resistance to foods and drinks as tin does With the normal contact time, the amount of tin dissolved by beer is insufficient to cause a haze However, citrus juices or vinegar will etch a pewter surface if contact is maintained for more than an hour Undisturbed neutral salt solutions may produce black spots and, later, local pitting Strong alkaline cleaning agents may also etch the surface

In years past, pewter alloys contained lead in sufficient quantities to affect its corrosion resistance significantly, for example, by producing a dark patina during atmospheric exposure Modern pewter can be chemically treated to reproduce this patina Several proprietary processes are available, including those based on immersion in iron chloride (FeCl3) or sodium nitrate (Na2NO3) solutions or acidic solutions of copper and arsenic (Ref 51, 52)

Bearing Alloys

The most widely used babbitt bearing alloys are usually classified as tin- or lead-base and have composition ranges within the following limits:

Composition, % Alloy addition

Tin-base Lead-base Tin 65-91 0-20

Lead 0.35-18 63 (min)

Antimony 4.5-15 10-15

Copper 2-8 1.5 (max)

The tin-base alloys are much more corrosion resistant against the action of the acids contained or formed in lubricating

oils (see the article "Tin and Tin Alloys" in Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, Volume 2, ASM Handbook) An addition of as little as 3% Sn in lead appears to prevent corrosion from the development

of oil acidity (Ref 53)

In some instances of marine use, the formation of a hard, crusty oxidation product has been observed on tin-rich bearings (Ref 54) When free access of salt water to a bearing is possible, the cathodic relationship of the babbitt alloys to steel renders them unsuitable, and bearing alloys such as Zn-70Sn-1.5Cu are preferred (Ref 2)

Some aluminum-base alloys containing 5 to 40% Sn and 0.7 to 1.3% Cu have also found use as bearing alloys in automobiles These alloys are manufactured using thermal treatments designed to produce structures that avoid a continuous network of the tin in order to obviate the risk of susceptibility to corrosion by the presence of moisture (Ref 55) With normal lubrication, the aluminum-tin alloys appear to be as fully resistant to corrosion as the tin-base babbitt alloys The aluminum-tin alloys, however, are not suitable for exposure to wet conditions

Other Tin Alloys

Tin-Copper. Alloys in this group are all copper-base and consist mainly of bronzes, gunmetal, and brass that contains tin additions Understandably, their corrosion behavior in air is based on the behavior of copper, which tends to develop a layer of basic green salts (mainly sulfates), that is adherent, protective, and has a pleasing appearance More information

on the corrosion resistance of copper alloys is available in the article "Corrosion of Copper and Copper Alloys" in this Volume

rural, suburban, urban, industrial, and marine environments for 1 year (Ref 56) Evaluations included weight gains as well

as changes in tensile strength and electrical resistance The bronze samples ranked consistently high among the materials tested, as indicated by the tensile strength data shown in Table 7

Table 7 Tensile strength loss in copper alloys after exposure for 1 year in various environments

Environments included industrial, marine, rural, suburban, and urban locations; data are averages for all five environments

to brass

Table 8 Tensile strength loss in copper alloys after exposure for 10 years at four sites

Strength loss, % Exposure site

Copper Tin

bronze (8% Sn)

70-30 copper- zinc

70Cu- 29Zn- 1Sn Heavy industrial 5.9 7.2 30.9 9.0

Marine, heavy industrial 6.3 8.0 28.2 7.9

Severe marine 7.6 5.7 8.0 2.5

A similar study involved exposure of screen wire cloth at four sites for up to 9 years (Ref 59) A Cu-2Sn bronze was found to exhibit the lowest strength losses at all sites from a group of alloys that included brasses, aluminum bronze, and nickel-copper Outstanding corrosion resistance of a Cu-2Sn bronze exposed to sulfur-bearing gases in railway tunnels was also reported (Ref 60.)

Another investigation compared the behavior of five stainless steels and a low-alloy steel with that of a Cu-4.38Sn-0.36P bronze exposed at tropical inland and seacoast sites for 8 years (Ref 61) The coastal site was more aggressive toward the bronze, which showed higher weight losses at both sites than the stainless, but the low-alloy steel was more severely attacked However, the bronze was free of pitting and suffered no loss in strength, which was not the case with some of the stainless steels In Ref 62, these researchers summarized the results of 16-year exposures on three tin-containing alloys (Cu-4.38Sn-0.36P, Cu-39Zn-0.84Sn, and Cu-40Zn-1Fe-0.65Sn) exposed at marine, inland semirural, and two tropical sites In general, the copper alloys resisted corrosion in the tropical zones, although less so at coastal sites as compared to inland sites The tin-containing alloys were as good as, or slightly superior to, the other alloys

More recent work by the same investigators included previous data plus additional information on the following cast bronzes: Cu-5Sn-5Pb-5Zn, Cu-6Sn-2Pb-3Zn-1Ni, Cu-9Sn-3Zn-1Ni, and Cu-3Sn-2Zn-6Ni (Ref 63) The conclusions were much the same as before The later work included a study of the effect of coupling phosphor bronze to equal areas of numerous other metals, and this work indicated that the coastal sites were 4 to 8 times more aggressive than the inland sites Evaluation of the effect of corrosion on the solderability of a Cu-2Sn-9Ni alloy was reported by workers at Bell Telephone, who found this material to be superior to both nickel-silver and an 8% Sn phosphor bronze (Ref 64)

Alloys in the Cu-Sn-Al system were evaluated, and those alloys containing at least 5% each of tin and aluminum were found to have good corrosion resistance in rural, urban, and industrial environments (Ref 65) The most promising material was Cu-5Sn-7Al Another researcher noted that such alloys could be brittle, but that the addition of 1% Fe and 1% Mn overcame this difficulty without detracting from the corrosion resistance of the alloy (Ref 66)

Tin-Silver. In the mid-1930s, tin-silver alloys were assessed as potential replacements for sterling silver (silver-copper alloy) in decorative applications (Ref 67) In this work, an Ag-7.5Sn alloy was found to show improved corrosion resistance over pure silver in several environments In a later extension of this work, alloys with up to 10% Sn were tested

in atmospheres containing H2S, SO2, and indoor air as well as for resistance to salt and oxidation upon heating in air (Ref 68) Comparison to sterling silver showed the tin-silver alloys to be at least as good as the sterling alloys, and in some cases even better Specifically, their resistance to chloride attack was considerably better, and less discoloration occurred upon heating in air Also, preoxidation of the tin-silver alloys improved resistance to attack by sulfur-containing atmospheres

Tin and Tin-Alloy Coatings

Tin coatings can be applied by various processes, including hot dipping, electrodeposition, spraying, and chemical displacement Electrodeposits can be matte or bright as plated, and matte deposits less than 8 m (0.3 mils) thick can be brightened by momentary fusion The latter can be effected by conductive or resistive heating in air or by immersion in a suitable oil

In the standard electrodeposition process, alkaline stannate, acid sulfate, or fluoborate solutions are all widely used The alkaline solutions give smooth, matte deposits, but the acid solutions usually require organic addition agents to produce smooth, coherent coatings If improperly controlled, these agents can increase the risk of dewetting during soldering or flow melting

The ranges of coating thicknesses that are practical for the various processes are as follows (Ref 2):

Thickness Process

m mils Chemical replacement Trace-2.5 Trace-0.1

Because deposits less than about 12 m (0.5 mils) thick are not likely to be pore free, the heaviest practical deposits should be used when tin is specified for corrosion resistance Table 9 lists recommended tin coating thicknesses for quality tin coatings for various service conditions

Table 9 Recommended tin coating thicknesses for typical applications

Thickness

range Service condition

1.3- 0.1

0.05-Insulated copper wire; pistons and other lubricated machine components

Mild (exposure to relatively clean indoor

atmospheres)

5.0

2.5- 0.2

0.1-Connectors, wires, etc., plated primarily for immediate solderability

or where storage periods are short

7.6

3.8- 0.3

0.15-This range is considered best for parts that must be reflowed:

connectors, circuit boards, wire, busbars; deposits heavier than 7.5

m (0.3 mil) may dewet

Moderate (exposure to average shop and

warehouse atmospheres)

12.7

7.6- 0.5

0.3-Connectors, fasteners, busbars, wire, transformer cans, chassis frames; adequate for good shelf life and in service

Severe (exposure to humid air, mildly

corrosive industrial environments)

25.4

12.7- 1.0

0.5-Connectors, wire, gas meter components, automotive air cleaners; adequate as a nitride stop-off

Very severe (exposure to seacoast

atmospheres; contact with certain chemical

corrosives)

25.4-127

5.0

1.0-Water containers; oil-drilling pipe couplings

Source: Ref 69

Tests conducted by the Metal Finishing Supplies Association (MFSA) showed that bright acid tin deposits generally perform better than the matte tin deposits in salt spray corrosion tests (Ref 69) However, no published specifications recognize any difference between the corrosion performance of these processes Similarly, because tin is cathodic to almost all of the commonly used base metals and undercoating metals, the MSFA recommends that the same in tin coating thicknesses be applied to any of the common base metals Also, the use of a copper or nickel undercoating does not justify the use of thinner tin deposits (Ref 69)

Tin Coatings on Steel. Tin on steel is widely used in packaging The single most important product of this type is tinplate Modern tinplate is a highly developed, sophisticated product that is produced at high speeds to yield a coiled, thin, low-carbon steel strip carrying a very thin (0.1 to 2 m, or 0.004 to 0.08 mils) tin coating on each side Because of the importance of tinplate, its preparation and properties as well as its performance as a container for food and food products will be discussed in the section "Tinplate" in this article

This section will primarily deal with heavier tin coatings that are usually applied to individual components by batch processing for nonpackaging applications, such as food-processing equipment, electrical and electronic components, wire, and fasteners Unless otherwise stated, these coatings, unlike tinplate, have not been subjected to fusion or reflow treatments and are therefore free of the iron-tin intermetallic layer, which can exert profound effects

One study compared the behavior of 25- m (1-mil) thick plated layers of tin, 80Sn-20Zn, and zinc on steel at three sites

in Nigeria for 2 years (Ref 70) Samples were exposed at 30° to the horizontal, about 1.2 m (47 in.) above ground, facing south and in sheltered exposure where they were supported vertically inside ventilated box The test results are given in Table 10

Table 10 Corrosion rate of 25- m (1-mil) thick coatings on steel at three tropical sites after 2 years of exposure

Material loss, full exposure test

Weight loss, sheltered exposure test mg/dm2

The Protective Coatings (Corrosion) Subcommittee of the Corrosion Committee of the British Iron and Steel Research Association reported test results after 12 years of exposure in an industrial area (Sheffield), two marine atmospheres (Colshot and Congella, South Africa), and a rural area with heavy rainfall (Flanwryted Falls) (Ref 72, 73) These data, listed in Table 11, indicated that tin deposited by any of several methods appeared more protective in the industrial area than at the other sites This behavior was attributed to the production of protective corrosion products in the pores of the coatings Similar observations have been reported (Ref 74), and similar evaluations have been conducted using accelerated corrosion tests and outdoor exposure in urban Berlin (Ref 75) One conclusion, based on 1 year of exposure, was that reflowing of tin coatings improved their corrosion resistance, except in salt spray exposure Also, deposits from

an acid electrolyte were said to be better than those from a stannate bath

Table 11 Summary of atmospheric corrosion tests on tin-coated steel at four exposure sites

Sheffield Flanwryted Falls Colshot Congella Coating method

(a) T, Coating thickness in mm

(b) L, Lifetime in years of coating as determined by rust appearing on more than 5% of

the specimen

(c) Average of duplicate results that did not agree well

Additional atmospheric corrosion test results have been reported (Ref 76, 77, 78, 79) In one study, data were summarized from 10 years of exposure for tin-plated steel in industrial, marine, and rural atmospheres that included estimates of the added cost of SO2 pollution Another study included tropical exposures of samples in China in two environments In the first environment, samples were mounted at 45° outdoors facing south In the second, the racks were sheltered from solar radiation, wind, and rain Recommendations based on 58 months of testing were that matte tin coatings 25 m (1 mil) thick should not be exposed to either environment for more than 1 year and that the life of similar coatings 32 m (1.2 mils) thick would be less than 2 years (Ref 79)

The general conclusion, based on results of most of the above outdoor studies, was that the corrosion resistance of tin coatings, that is, their protection of steel, was not very high (Ref 22) In addition, this is reflected in the international standard ISO 2093-1973 covering tin coatings, which carries the following tin coating thickness recommendations:

Minimum tin thickness

On steel On

nonferrous metals (a)

Type of service

m mil m mil Exceptionally severe 30 1.2 30 1.2

Tin Coatings on Nonferrous Metals. Tin coatings are widely used on nonferrous substrates, usually for one or more

of the following reasons:

• Improvement and retention of solderability

• Excellent compatibility (low toxicity) with foods

• Prevention of galvanic effects between dissimilar metals

• Low electrical resistance

Not surprisingly, copper and copper-base alloys are the most frequently tinned nonferrous materials Tin tends to be anodic to copper and copper alloys, including the intermetallic tin-copper compounds Therefore, accelerated corrosion of the tin coating might be expected in aqueous environments Indeed, this is sometimes evidenced by black spots on a tin coating that result from localized corrosion around discontinuities Although normally associated with total aqueous immersion, these black spots can also appear on outdoor exposure involving cyclic condensation (Ref 22)

Deterioration of the solderability of tinned copper during aging has been studied by many researchers, and accelerated test procedures have been devised to simulate the effect (Ref 80, 81, 82) Similarly, changes in the contact resistance of tin coatings have been related to increases in the thickness of the oxide film on its surface (Ref 83)

Special mention should be made of the corrosion behavior of tin coatings on brass in ordinary atmospheres Zinc diffuses through tin coatings fairly rapidly; significant zinc levels are reached on the surface of a coating thickness of 7.5 m (0.3 mil) in about 1 year (Ref 34) Zinc at the surface oxidizes readily to form white corrosion products that adversely affect its solderability and contact resistance To avoid such problems, a 2.5- m (0.1-mil) thick barrier layer of either copper or nickel is recommended over the brass (Ref 84)

Immersion Tin Coating. Contrary to the standard electromotive force (emf) series of metals, tin can be applied by immersion (chemical displacement) on copper This is done by using a cyanide or a thiourea type of solution

An outstanding application is tinning of the inside of copper tubing Such tubing in coil form is used in water coolers The tin prevents delivery of greenish water from new coolers and eventually disappears By then, the copper surface has become conditioned to deliver water appearing as it did when it entered the cooler

Tin-cadmium alloy coatings for the corrosion protection of tin were first studied by plating duplex coatings of tin on cadmium and then heat treating These and later electrodeposited surfaces (Ref 85) were found to have phenomenal resistance to salt spray tests, and they were successfully used for some time to protect the engine components of naval aircraft (Ref 86)

Tin-cadmium coatings resemble tin-zinc coatings in appearance and behavior Because cadmium is less effective at sacrificially protecting steel exposed at pores, the optimum cadmium content in the coating ranges from 25 to 50% The initial electrolyte development discussed in Ref 85 was followed by an investigation of a range of alloys; it was concluded that the alloys performed better than cadmium alone in marine environments (Ref 87) Another study found that the attack

on tin-cadmium coatings by organic vapors was less than for pure cadmium (Ref 88) In addition, tin-cadmium was found

to be superior to tin-zinc when in contact with jet fuels or in hot synthetic oils This work was supplemented by that described in Ref 89, which suggests that tin-cadmium alloys, particularly with a chromate surface treatment, performed better than cadmium coatings of the same thickness

More recently, tin-cadmium alloy coatings were shown to provide better corrosion resistance to steel than duplex coatings

of tin and cadmium (Ref 90) Lastly, zinc or tin-zinc coatings were found to be more protective to steel in industrial atmospheres than tin on cadmium or cadmium on tin, but this behavior was reversed in a marine environment (Ref 91)

Tin-Cobalt Coatings. As expected, the properties of tin-cobalt electrodeposits are similar to those for tin-nickel Intermetallic deposits of SnCo (Ref 92, 93) or SnCo mixed with Sn2Co (Ref 94) have been produced, and proprietary plating systems have been patented These deposits are bright and are similar to chromium plate; most studies of their performance have concerned systems of steel coated by nickel, with a thin film of tin-cobalt applied to obtain a bright finish

An evaluation of tin-cobalt coatings for their resistance to salt spray, NH3, and in copper-accelerated salt spray (CASS) tests revealed that the deposit was resistant to all of these environments and was more ductile than tin-nickel electrodeposits (Ref 95)

Two researchers also tested systems of nickel plus tin-cobalt in CASS and outdoor exposure tests (Ref 96, 97) Their conclusions were similar even though different baths were used and minor differences in the deposits were obtained Thus their corrosion resistance was comparable to that for a nickel-chromium system in all but the more severe conditions

Corrosion tests on coatings of 0.2- m (0.8-mil) tin-cobalt over duplex bright nickel were compared with the same thickness of chromium (Ref 98) The tin-cobalt appeared markedly inferior to chromium in outdoor exposure and wear resistance, but was reasonably satisfactory as a substitute for decorative chromium for indoor use

Tin-Copper Coatings. Tin alloys close to the Cu3Sn intermetallic composition (40 to 45% Sn) were once used as a material for mirrors; hence the name speculum These alloys resemble silver in brightness and appearance; they find some use as tableware and on bathroom fixtures, but are not used outdoors, where they rapidly turn dull and gray However, even the indoor corrosion resistance of the alloy is seriously impaired if the composition is not optimum ( 42% Sn), and the subsequent need for close control of plating conditions has prevented large-scale development of the coating

Tin-bronze deposits containing about 12% Sn were reported to be superior to copper as an undercoat for nickel-chromium coatings with regard to weathering behavior (Ref 99, 100) Some results were also reported with tin-copper coatings over steel in industrial and marine environments (Ref 101)

Tin-lead coatings with a wide range of composition are applied by hot dipping or electrodeposition Steel strip coated with tin-lead alloys by hot dipping and sold as sheet or coil carries the general designation of terneplate The tin content varies from 2 to 20% In general, the higher the tin content, the lower the porosity of the terneplate and therefore the greater the protection afforded to the substrate Like tin coatings, tin-lead does not offer any galvanic protection to steel in the atmosphere; protection against rusting depends on coating continuity and on the formation of protective corrosion products A comparison of the behavior of a Pb-12Sn coating with pure tin and lead revealed that both lead-containing coatings developed white films believed to be PbSO (Ref 72)

In a more comprehensive study, a range of electrodeposited tin-lead coatings obtained with different bath additives was evaluated (Ref 57) The performance of a Pb-5.5Sn coating in salt spray and outdoor testing was found to be superior to pure lead and lead-tin alloys containing tin additions of 7, 10, or 15% The Pb-10Sn and Pb-15Sn alloys were comparable

in behavior to pure tin These results were partially supported by those of another study, which consisted of atmospheric exposures at three sites on electrodeposited lead and coatings of Pb-5Sn and Pb-14Sn (Ref 102) Superior protection was achieved with the tin-containing alloys at all sites, which included severe industrial, rural, and marine environments

The results of tests on a number of commercial terneplate compositions in accelerated corrosion tests, SO2, humidity, and salt spray as well as outdoor exposure in both industrial and marine environments are given in Ref 103 Performance was assessed largely on the degree of rusting of the underlying steel after 12 months of exposure Lead-tin alloys showed greater resistance to chloride attack than lead-antimony alloys It was also noted that coverage of the steel increased with the tin content of the alloy and that resistance of the coating to attack appeared to increase in both chloride-rich and humid conditions

Tin-Nickel Coatings. Alloys containing 18 to 25% Ni can be deposited from a cyanide-stannate bath to give bright coatings with good resistance to HNO3 (Ref 104) However, because of their high hardness and brittleness, no interest has been shown in these coatings Similar results have been reported with a complex pyrophosphate bath (Ref 105) Primary commercial interest has centered on the intermetallic compound NiSn (containing about 67% Sn), which is readily deposited from mixed chloride/fluoride electrolytes (Ref 106, 107)

The NiSn intermetallic is metastable and does not transform to a mixture of other intermetallics unless it is heated (Ref 108) The deposit is hard, bright, and has reasonable solderability It also has good wear resistance and remarkable resistance to attack by a wide range of solutions For these reasons, tin-nickel coatings have found use as decorative corrosion-resistant finishes for balance weights, drawing instruments, pistons in automobile braking systems, and some food contact applications Recommended coating thicknesses for this alloy coating have been specified in ISO 2179 1972

as follows:

Thickness Intended duty

m mils Severe environments 25 1

Within the past 10 years, studies of the corrosion resistance of tin-nickel deposits have centered on their effect on the electrical contact resistance of this alloy, either alone or with a thin coating of gold The contact resistance of tin-nickel is sufficiently low to merit consideration for moderate-voltage applications (about 50 V), but too high for low-voltage uses Several extensive studies have been reported in this field (Ref 115, 116) This effect on contact resistance is largely a

result of the insulating passive film that forms on SnNi and the high hardness of the material An excellent review of the work in this field is available in Ref 22 It is generally agreed that tin tends to concentrate at the surface of tin-nickel electrodeposits, but no adequate explanation of the oxidation behavior of this alloy is currently available

The resistance to attack of the coating by various acids and chemicals has been studied, and the results are given in Table

12, which also compares these with coatings of tin and nickel (Ref 117) Generally, the results show that tin-nickel has a high resistance to attack by acids, alkalies, and several neutral salt solutions This behavior is attributed to the presence of

a passive air-formed film

Table 12 Corrosion resistance in various media of tin, nickel, and tin-nickel alloy

1 M hydrochloric acid 61.5 41.5 24.8 Covered by adherent brown film

0.5 M sulfuric acid 19.5 25.6 14.5 Slightly darkened

0.05 M sulfurous acid 0.2 725.0 0.5 None

1 M formic acid (pH 1.8) 22.1 35.5 nil None

1 M acetic acid (pH 2.4) 22.2 43.6 0.6 Very slightly darkened

0.5 M oxalic acid (pH 1.1) 12.3 16.4 12.0 Etched on immersed area; dark stain at waterline

1 M lactic acid (pH 1.9) 18.0 17.8 2.1 None

0.5 M tartaric acid (pH 1.7) 10.6 10.0 0.5 None

0.3 M citric acid (pH 1.9) 12.0 19.2 0.4 None

0.3 M ferric chloride (pH 1.5) 290.0 303.0 2.3 and 6.2 None, except slight local action on edge

Sodium hypochlorite (40 g/L available chlorine) 1.3 625.0 22.0 and 67.0 Bottom edge badly etched; none elsewhere