Handbook Corrosion (1992) WW Part 14 pdf

1 Uniform corrosion of ACI CD-4MCu cast stainless steel pump impeller after 1 year in an environment containing 50% H3PO4 and 10% gypsum.. The reaction of CaOH2 with atmospheric carbon

Classifiers blades Ni-Hard type 4

Ore chutes Impact, gouging,

abrasion, pH 6-8

Ni-Hard, nickel-containing manganese steel

Scrapers Impact loading, gouging,

abrasion

Cast ASTM A579 steel, Ni-Hard cast iron, hard cast irons

Wire rope Corrosive-abrasive, pH

2-12

Kevlar, steel wire rope

Piping Corrosive-abrasive Type 316 stainless steel, CN-7M, Ni-Hard cast irons, rubber covered

fiberglass-reinforced plastic

Scrubbers Off-gas products High-grade nickel alloys

Chain conveyors Corrosive-abrasive Plated (nickel, cadmium, or zinc) steels

References

1 G.R Hoey and W Dingley, Corrosion Control in Canadian Sulfide Ore Mines and Mills, Can Min Metall

Bull., Vol 64, May 1971, p 1-8

2 G.J Biefer, Corrosion Fatigue of Structural Metals in Mine Shaft Waters, Can Min Metall Bull., Vol 58,

June 1967, p 675-681

3 N.S Rawat, Corrosivity of Underground Mine Atmospheres and Mine Waters: A Review and Preliminary

Study, Br Corros J., Vol 11 (No 2), 1976, p 86-91

4 I Iwasaki, K.A Natarajan, S.C Riemer, and J.N Orlich, Corrosion and Abrasive Wear in Ore Grinding, in

Wear of Materials 1985, American Society of Mechanical Engineers, 1985, p 509-518

5 T.P Beckwith, Jr., The Bacterial Corrosion of Iron and Steel, J Am Water Works Assoc., Vol 33 (No 1),

June 1941, p 147-165

6 B Intorre, E Kaup, J Hardman, P Lanik, H Feiler, S Zostak, and W.E Rinne, Complete Water Reuse

Industrial Opportunity, in Proceedings of the National Conference, American Institute of Chemical

Engineers, 1973, p 88

7 F.N Speller, Corrosion: Causes and Prevention, McGraw-Hill, 1951, p 208

8 S.L Pohlman and R.V Olson, "Corrosion and Material Problem in the Copper Production Industry," Paper

229, presented at Corrosion/84, National Association of Corrosion Engineers, 1984

9 K Adam, K.A Natarajan, S.C Riemer, and I Iwasaki, Electrochemical Aspects of Grinding Media Mineral

Interaction in Sulfide Ore Grinding, Corrosion, Vol 42 (No 8), 1980, p 440-446

Wire Rope

Mine shaft depths of 1830 m (6000 ft) are not uncommon; gold mines in South Africa approach depths of 2285 m (7500 ft) (Ref 10) The hoisting equipment used in these mines, especially ropes, on which lives of personnel depend, is subjected to the corrosive environment of the mines Although wear is also a factor, corrosion is perhaps the most serious aspect of mine safety Corrosion is difficult to evaluate and is a more serious cause of degradation than abrasion (Ref 11)

If corrosion is evident, the remaining strength cannot be calculated with safety, nor is there any reasonable way to determine whether or not the rope is safe except by the judgement of the inspector

Statistical analysis from the results of rope tests on mine-hoist wire ropes has shown that 66% of the ropes exhibited the greatest strength loss in the half of the ropes nearest the conveyance (Ref 12) This is the portion of the rope that is in contact with the shaft environment during most of its service life

Replacement of hoist rope is a routine procedure in most mines and is suggested every 18 to 36 months, depending on the mine environment and use (Ref 1, 13) Some regulatory agencies will not allow the use of a shaft rope on which marked corrosion is evident (Ref 11)

Adequate service life of hoist rope is economically desirable Therefore, the composition of present-day hoist rope has been extensively studied Carbon steel strand wire has competition from such substitute materials as stainless steel (Ref 14) and synthetic fibers (Ref 15) Austenitic stainless steel rope (15.5 to 18.5% Cr and 11 to 13% Ni) is available and has endurance strength of 72 to 83% of that of carbon steel wire Much can be said for synthetic fiber rope construction For example, Aramid fiber rope has exceptional strength-to-weight ratios, outstanding tension-tension-fatigue performance (Ref 15), and excellent corrosion resistance

Roof Bolts

Roof bolts are extensively used for roof support in underground mines More than 120 million roof bolts are used per year

in the United States mining industry (Ref 11) Roof bolts made of low-carbon steel in a number of design variations are subject to corrosion attack in the mine environment In sulfide mines, the roof bolts have been reported to fail within 1 year by breaking at a distance of approximately 355 mm (14 in.) inside the drill hole (Ref 1) This roof bolt failure has been related to stress-corrosion cracking Roof falls are associated with such roof bolt failures

Pump and Piping Systems

Corrosion in pump and piping systems is well known in the mining and mineral-processing industries The first indication

of pump corrosion is that the pump no longer meets the flow demands of capacity and head requirements Also, the external surfaces are corroded and encrusted with corrosion product

Because recognition of corrosion type is so important in diagnosing corrosion problems and their prevention and because published information on case histories is scarce, corrosion types will be discussed and illustrated in the following sections in this article

Uniform Corrosion. The most common form of pump corrosion is characterized by uniform attack on the entire exposed surface Figure 1 shows uniform corrosion on a stainless steel pump impeller that was exposed to 50% phosphoric acid (H3PO4) and 10% gypsum pumping fluid for approximately 1 year Figure 2 shows uniform corrosion due to potash brine on a cast (hard) iron pump runner

Fig 1 Uniform corrosion of ACI CD-4MCu cast stainless steel pump impeller after 1 year in an environment

containing 50% H3PO4 and 10% gypsum Courtesy of A.R Wilfley & Sons, Inc., Pump Division

Fig 2 Uniform corrosion of an abrasion-resistant iron pump runner that contacted potash brine slurry (a) End

view (b) Side view Courtesy of A.R Wilfley & Sons, Inc., Pump Division

Pitting corrosion on an Alloy Casting Institute (ACI) CF-8M stainless steel casting pump case is illustrated in Fig 3 This pump case, which was exposed to low pH and high Cl- concentration, failed after approximately 3 years of service

Fig 3 Pitting corrosion of an ACI CF-8M stainless steel pump case used to pump a nickel plating solution with a

high concentration of Cl - and a high operating temperature This damage occurred during 3 years of service Courtesy of A.R Wilfley & Sons, Inc., Pump Division

Erosion-corrosion of an ACI CN-7M stainless steel cast impeller after exposure to hot concentrated H2SO4 with solids present is shown in Fig 4 Erosion-corrosion is evident in Fig 5, which shows that the erosion-corrosion damage increased on the portion of the impeller that had the greatest fluid velocity This impeller, cast from an abrasion-resistant while iron, was used to pump fluids containing 30% solid (iron ore tailings) at a pH of 11.2

Fig 4 Erosion-corrosion of ACI CN-7M stainless steel pump components that pumped hot H2SO 4 with some solids present Note the grooves, gullies, waves, and valleys common to erosion-corrosion damage Courtesy of A.R Wilfley & Sons, Inc., Pump Division

Fig 5 Erosion-corrosion of an abrasion-resistant iron pump runner used to pump 30% iron tailings in a fluid

with a pH of 11.2 This runner had a service life of approximately 3 months Note that most of the damage is on the outer peripheral area of the runner where fluid velocity is the highest Courtesy of A.R Wilfley & Sons, Inc., Pump Division

Crevice Corrosion. Figure 6 shows crevice corrosion on an ACI CF-8M cast stainless steel pump case that was gasket sealed on the discharge flange Attack is evident in the region where the gasket was placed It is important to design mining and milling equipment for easy drainage and cleaning in order to prevent the buildup of stagnant water that will produce concentration cells and lead to crevice corrosion and pitting Figure 7 illustrates both poor and improved design

for avoidance of localized attack More information on proper design is available in the article “Designing to Minimize

Corrosion” in Corrosion: Fundamentals, Testing, and Protection, Volume 13A of ASM Handbook

Fig 6 Crevice corrosion at the intake flange of an ACI CF-8M stainless steel pump case Notice that the

corrosion damage occurred under the gasket Courtesy of A.R Wilfley & Sons, Inc., Pump Division

Fig 7 Poor and improved engineering design to avoid crevice corrosion Source: Ref 16

Intergranular corrosion is common to stainless steel pump castings Figure 8 shows the excessively attacked grain boundaries A quick test to identify this type of corrosion is to peen the pump casting with a small hammer Loss of acoustical properties is evidence of intergranular grain-boundary attack, especially in sensitized stainless steel castings A form of intergranular corrosion associated with weld deposits that is commonly called weld decay is shown in Fig 9, which illustrates a field-weld repair of an ACI CN-7M stainless steel impeller that was not postweld heat treated (solution annealed and quenched) to restore corrosion resistance This weld-repaired casting was exposed to a phosphoric anhydride (P2O5) solution at 80 °C (175 °F) It is evident that the weld decay occurred in the heat-affected zone of the weld deposit

Fig 8 Intergranular corrosion of an ACI CN-7M stainless steel pump component that contacted HCl-Cl2 gas fumes Note the grain-boundary attack Courtesy of A.R Wilfley & Sons, Inc., Pump Division

Fig 9 Weld decay of an ACI CN-7M stainless steel pump impeller that was field weld repaired with no postweld

heat treatment The pump service was P2O5 solution at 80 °C (175 °F) (a) Overall view of impeller (b) Closeup view of the weld repair and the associated weld decay, which occurred adjacent to the weld deposit Courtesy

of A.R Wilfley & Sons, Inc., Pump Division

Dealloying. Figure 10 shows selective leaching (dealloying) of a high-nickel cast iron The physical dimension of the pump component remained constant, while porous, selectively leached regions grew into the casting This porous layer consists of residual graphite contained in the cast iron and corrosion product This pump component was exposed to fluosilicic acid (H2SiF6) and failed after 12 days of service

Fig 10 Selective leaching of a cast iron pump impeller after 12 days of service in H2SiF6 Section through the impeller shows the selectively leached layer, which contains graphite and corrosion product Courtesy of A.R

Wilfley & Sons, Inc., Pump Division

Galvanic corrosion between two stainless steels is illustrated in Fig 11 These AISI type 304 stainless steel stud bolts held together an Alloy 20 (ACI CN-7M) pump housing The bolts became anodic to the housing in 45% H2SO4 and subsequently failed Table 4 lists various combinations of pump an valve trim materials and indicates the combination that may be susceptible to galvanic attack

Table 4 Galvanic compatibility of materials used for pump components

Trim Body material

Brass or bronze

Nickel- copper alloy

Type 316

Cast iron Protected Protected Protected

Austenitic nickel cast iron Protected Protected Protected

M or G bronze 70-30 copper nickel May vary(a) Protected Protected

Nickel-copper alloy Unsatisfactory Neutral May vary (b)

Alloy 20 Unsatisfactory Neutral May vary (b)

(a) Bronze trim commonly used Trim may become anodic to body if

velocity and turbulence keep stable protective film from forming

on seat

(b) Type 316 is so close to nickel-copper alloy in potential that it does

not receive enough cathodic protection to protect it from pitting under low-velocity and crevice conditions

Fig 11 Galvanic corrosion of AISI type 304 stainless steel stud bolts that fastened two Alloy 20 (ACI CN-7M)

pump components The pump was pumping 45% H 2 SO 4 at 95 °C (200 °F) The stud bolts were anodic to the Alloy 20 pump housings Courtesy of A.R Wilfley & Sons, Inc., Pump Division

Cavitation is a familiar term within the industry The causes are also familiar; principally, there is a lack of net positive suction head, which is the suction pressure that should be available to the pump for correct performance Cavitation usually manifest itself by another familiar pump characteristic noise The buildup and subsequent collapse of bubbles on the impeller create the familiar popcorn noise

The effects of cavitation (the violent collapse of bubbles) are illustrated in Fig 12 and 13 Plastic and metal impellers have been found to be susceptible to cavitation damage A contributing cause of cavitation has been found to be plugged filters on the intake (suction)side of the pump, coupled with the marginal net positive suction head available to the pump Table 5 lists engineering materials based on their resistance to cavitation damage

Table 5 Rating of materials for cavitation resistance

Most resistant

Stellites

17Cr-7Ni stainless steel welding rod

18-8 stainless steel welding rod

Bronze welding rod (Cu-10Al-1.5Fe)

25Cr-20Ni weld

Eutectic-Xyron 2-24 weld

Ampco bronze casting

18-8 cast stainless steel

Nickel-aluminum bronze, cast

13% Cr cast iron

Manganese bronze, cast

18-8 stainless steel spray metallizing

Fig 12 Cavitation damage of phenolic plastic pump impeller Note the craterlike depression on the damaged

surface caused by the collapse of bubbles on the impeller surface Courtesy of A.R Wilfley & Sons, Inc., Pump Division

Fig 13 Cavitation damage of an ACI CN-7M stainless steel pump impeller that pumped NH4NO 3 solution at 140

°C (280 °F) Courtesy of A.R Wilfley & Sons, Inc., Pump Division

Figure 12 shows the craterlike damage caused by cavitation on a plastic impeller Figure 13 shows cavitation damage on

an ACI CN-7M stainless steel impeller that pumped hot ammonium nitrate (NH4NO3); the pumping installation had a total lack of a net positive suction head

Erosion-Corrosion. During mill processing, fluids are pumped containing particulates that are usually carried in a corrosive medium Pumping this slurry promotes erosion-corrosion in piping, tanks, and pumps Erosion-corrosion is a

function of the fluid velocity and the nature of the particulates and fluid The following procedures can be used to reduce erosion-corrosion or to increase the service lives of piping and pumping systems:

• Increase the thickness of pipes

• Use larger inside diameter pipes to reduce fluid velocity for the transport of a specific fluid volume

• Streamline bends in piping to ensure laminar flow

• Use nonmetallic ferrules inserted in the inlet ends of pipes

• Design for easy replacement of parts that experience severe erosion-corrosion

• Use coatings that produce an erosion-corrosion resistant barrier, such as rubber coatings (Ref 18)

Material selection is an important consideration for erosion-corrosion resistance Alloy hardness has also been shown to

be a factor in erosion-corrosion resistance Generally, soft alloys are more susceptible to erosion-corrosion than their harder counterparts, but the relative hardness properties of the alloy can be misleading, because the hardening mechanism affects resistance to erosion-corrosion (Ref 8) For example solid-solution hardening has been found to offer greater resistance than that provided by conventional heat treatment One example of this is the cast precipitation-hardening alloy ACI CD-4MCu, which outperforms Alloy 20 (CN-7M) and austenitic stainless steels in many applications

Economics often enters into material selection Cast iron is relatively more economical and frequently exhibits better erosion-corrosion resistance than cast steel High-silicon cast iron (14.5% Si) has been found to be an economical selection

Active-passive materials, such as stainless steels and titanium, owe their corrosion resistance to their developing a protective passive oxide film This protective film, however, can be continuously damaged by erosive-abrasive processes Selection of passive alloys should be based only on experience and/or laboratory test results

Joints must be reliable Welded pipe, such as carbon steel or stainless steel, is free of flanges but is costly to install Nonwelded joints are susceptible to crevice corrosion; therefore, stainless steel, in particular, will not attain its expected service life

References cited in this section

8 S.L Pohlman and R.V Olson, "Corrosion and Material Problem in the Copper Production Industry," Paper

229, presented at Corrosion/84, National Association of Corrosion Engineers, 1984

16 R.F Steigerwald, Corrosion Principles for the Mining Engineer, in Symposium Materials for Mining

Industry, AMAX Molybdenum, Inc 1974

17 M.G Fontana and N.D Greene, Corrosion Engineering, McGraw-Hill, 1967

18 L.D Eccleston, Protective Coatings in the Mining Industry, Can Min Metall Bull., Vol 72 (No 3), 1979, p

170-173

Tanks

Most tanks are made from low-carbon steel for economic considerations The most common corrosion protection for these tanks is the use of coatings and linings Cathodic protection can also be used and in commonly employed in conjunction with a coating (Ref 18) Coating materials can be classified as cement, epoxy, epoxy-phenolic, neoprene, latex, sprayed polyresin coating, polyesters and vinyl esters (heavy coatings), and baked phenolic Steel tanks are also lined with natural rubber, synthetic elastomers, rubber-backed polypropylene, and glass Glass lining would require an oven bake

Stainless steel and titanium alloys have also been used for tanks Their use depends on their specific corrosion resistance

to the solution Selection of an alloy type becomes a question of economics

Reactor Vessels

A variety of materials are used, depending on the corrosivity of the media being contained With neutral or alkaline pH, carbon steels are often used With increasing corrosivity, consideration is first given the austenitic stainless steels, then iron-nickel-chromium superalloys, and finally nickel-base superalloys For special environments, copper, copper-nickel, and nickel-copper (Monel-type) alloys are used (Ref 19) Titanium is known to have excellent corrosion resistance in some of the most aggressive solutions

Cyclic Loading Machinery

The mining and mineral industry use large numbers of rotary and cyclic loaded equipment This equipment is subject to fatigue and corrosion fatigue Table 6 illustrates the significant reduction of fatigue strength of various materials that were tested in mine water and compared to fatigue strength in air This problem can be reduced by designing heavier sections into the part to reduce load and by applying protective coatings (Ref 18)

Table 6 Fatigue and corrosion fatigue strengths of various alloys at 10 7 cycles

Corrosion fatigue strength

1 G.R Hoey and W Dingley, Corrosion Control in Canadian Sulfide Ore Mines and Mills, Can Min Metall

Bull., Vol 64, May 1971, p 1-8

2 G.J Biefer, Corrosion Fatigue of Structural Metals in Mine Shaft Waters, Can Min Metall Bull., Vol 58,

June 1967, p 675-681

3 N.S Rawat, Corrosivity of Underground Mine Atmospheres and Mine Waters: A Review and Preliminary

Study, Br Corros J., Vol 11 (No 2), 1976, p 86-91

4 I Iwasaki, K.A Natarajan, S.C Riemer, and J.N Orlich, Corrosion and Abrasive Wear in Ore Grinding, in

Wear of Materials 1985, American Society of Mechanical Engineers, 1985, p 509-518

5 T.P Beckwith, Jr., The Bacterial Corrosion of Iron and Steel, J Am Water Works Assoc., Vol 33 (No 1),

June 1941, p 147-165

6 B Intorre, E Kaup, J Hardman, P Lanik, H Feiler, S Zostak, and W.E Rinne, Complete Water Reuse

Industrial Opportunity, in Proceedings of the National Conference, American Institute of Chemical

Engineers, 1973, p 88

7 F.N Speller, Corrosion: Causes and Prevention, McGraw-Hill, 1951, p 208

8 S.L Pohlman and R.V Olson, "Corrosion and Material Problem in the Copper Production Industry," Paper

229, presented at Corrosion/84, National Association of Corrosion Engineers, 1984

9 K Adam, K.A Natarajan, S.C Riemer, and I Iwasaki, Electrochemical Aspects of Grinding

Media Mineral Interaction in Sulfide Ore Grinding, Corrosion, Vol 42 (No 8), 1980, p 440-446

10 S.A Bryson, Repair Work and Fabrication in Gold Mining Environments, FWP J., Vol 24 (No 2), 1984, p

Rope Tests on Mine-Hoist Wire Ropes, Can Min Metall Bull., Vol 77 (No 11), 1984, p 50-54

13 H Precek and J Zeigler, Ropes for Use at Great Depths in Mining, Wire Ind., Vol 52 (No 8), 1985, p

486-487

14 H Hartmann, Hauling Ropes for Shaft Installations Under Extreme Corrosive Conditions, Wire Ind., Vol 46

(No 3), 1979, p 179

15 N O'Hear, Developments in Aramid Fibre Ropes, Wire Ind., Vol 49 (No 11), 1982, p 845-850

16 R.F Steigerwald, Corrosion Principles for the Mining Engineer, in Symposium Materials for Mining

Industry, AMAX Molybdenum, Inc 1974

17 M.G Fontana and N.D Greene, Corrosion Engineering, McGraw-Hill, 1967

18 L.D Eccleston, Protective Coatings in the Mining Industry, Can Min Metall Bull., Vol 72 (No 3), 1979, p

In considering the ramifications of corrosion and its prevention in different types of structures, it is useful to group such structures into various categories Thus, the modern high rise whether an office building, an apartment building, a condominium structure, or a special-purpose building such as hospital poses particular problems related to the corrosion

of major structural components (which may be either totally metallic or may contain metallic material) and creates the necessity for corrosion considerations in any connector assembly used to tie the curtain-wall system on the building back

to the structural frame or to the backup wall system In low-rise structures, similar concerns exist, but may be less critical when the structure is only a few stories high

Under such circumstances, the possibility of danger to people and property resulting from failure of, for example, wall tie systems may be less Nevertheless, failure of metallic materials within structure may lead to unsightliness or to possible lack of weathertight behavior Both may require extensive reworking of the structure to reestablish adequate building performance

curtain-Parking structures are frequently of conventionally reinforced concrete or prestressed/posttensioned construction Where such structures are in the snow belt areas of the country or where chloride may intrude because of the proximity of marine environments, the reinforcement may be at risk from corrosion Stadiums are another example in which either steel-frame

or concrete-frame approaches can be used In these cases, certain approaches may be needed to ensure structural integrity, particularly in view of the safety of the many thousands of people who may visit the structure and fill it to capacity

Bridges have received much attention with regard to corrosion This has been particularly prevalent in the snow belt states, where deicing salt application has led to significant premature deterioration of decks and supporting structures However, supporting structures have also suffered damage because of saline water intrusion, particularly in the splash zone of reinforced concrete structures Also, the use of the weathering steels can be a problem in such structures, especially where design or construction practice does not adequately account for the particular limitations in the use of this type of steel

Other specialized structures in which specific corrosion conditions may be a concern include sewage plants, which may have isolated situations related to contaminated water and the use of treatment chemicals Other buildings or structures can be used to house or support chemical plants and/or utility plants In the latter case, specific aspects of corrosion prevention in nuclear plants especially containment buildings have been the subject of critical review Because many nuclear containment vessels are designed with posttensioned concrete, the longevity of any corrosion protection system applied to the posttensioning tendons is critical This article will not address specific interactions with manufactured industrial chemicals, because this area is more reasonably the province of general or specific knowledge related to the particular chemical to which the building material will be exposed

Metal/Environment Interactions

The most common metallic material used in structures is low- or medium-carbon steel Specialized or strengthened materials, including heat-treated steels, stainless steels, and nonferrous metals, are sometimes employed; use of these materials is the exception rather than the rule (they will be discussed when appropriate in this article) The specific interactions that will be considered are the reactions of steel with the atmosphere in all of its forms including polluted atmospheres, interior atmospheres, and internal atmospheres (for example, in a cavity wall) in which the major criteria determining the performance of unprotected metal include the corrosivity of the atmosphere, the temperature, and the time of wetness of the metal

A major research effort over the past 20 years has been expended in determining the reaction of steel with cementitious materials, which include mortar, concrete, and their variants The chemical that typically controls corrosion behavior under these circumstances is the calcium hydroxide (Ca(OH)2) introduced into the cementitious material by the portland cement or, in the case of mortars, lime The reaction of Ca(OH)2) with atmospheric carbon dioxide (CO2) is an important degradation mechanism that will radically change the behavior of steel that is in contact with or embedded in the cementitious material The other major factor that influences the corrosion of steel in this environment is the introduction

of chloride ion (Cl-) into the cementitious material Chloride may play a decisive role in causing the cementitious material

to change from a protective to a nonprotective environment with regard to embedded steel

This article will discuss the generic situation of metallic materials specifically steel in reacting with the environments found in structures These environments will be discussed in specific terms, particularly as related to atmospheric conditions and cementitious environments The utility of different corrosion protection methods will be described in relationship to particular structures and to the different environments, either atmospheric or cementitious, encountered by the steel Finally, examples of problems that have arisen in the corrosion performance of metallic materials in these environments will be delineated, with particular attention paid to the different problems as they relate to different structures in which the metallic material may exist

General Considerations in the Corrosion of Structures

Corrosion of Steel in the Atmosphere. Atmospheric corrosion is discussed at length elsewhere (see the article

“Corrosion of Carbon Steels” in this Volume and “Atmospheric Corrosion” in Corrosion: Fundamentals, Testing, and Protection, Volume 13A of ASM Handbook) However, a brief discussion of factors affecting such behavior in structures

is necessary Steel corrosion in the atmosphere is typically a function of temperature, humidity, and the presence in the atmosphere of components that increase the corrosivity of the environment The temperature/humidity ratio is particularly important, because the interaction among these factors leads to a function known as the time of wetness, during which a film of liquid is present on the surface of the steel Corrosion occurs by a typical aqueous corrosion mechanism during the time that this film is present on the steel surface The thinner the film, the easier the diffusion of oxygen through the film that drives the corrosion reaction

The presence of agents in the atmosphere that can dissolve in the liquid film and promote or inhibit its production by changing the dew point can markedly influence the corrosion behavior of the steel Thus, the presence of pollutants in the atmosphere particularly sulfur dioxide (SO2) and related compounds, which can lead to so-called acid rain will influence the corrosion behavior of the steel by acting several ways, such as:

• An increase in the conductivity, and therefore the corrosivity, of the liquid film

• Changes in the relative humidity at which the film may form

• Influence of the dissolved constituent on the formation and protectiveness of any corrosion product

films that may exist on the steel

Therefore, the corrosion behavior of steel used in structures can be expected to be a function of the geographical location, which in turn will influence the temperature, humidity, and degree of pollution that exists This behavior is indicated in Table 1

Table 1 Corrosion rates of carbon steel calibrating specimens at various locations

Corrosion rate

environment m/yr mils/yr

Cape Canaveral, FL (18-m, or 60-ft, elevation, 55 mm, or 60 yd, from ocean) Marine 132 5.2

Cape Canaveral, FL (9-m, or 30-ft, elevation, 55 m, or 60 yd, from ocean) Marine 165 6.5

Cape Canaveral, FL (ground level, 55 m, or 60 yd, from ocean) Marine 442 17.4

Source: Ref 1

Corrosion in marine environments is a specialized subset of atmospheric corrosion This is because of the influence of wind-blown or particulate sea salt that may contact exposed steel Sea salt is particularly aggressive to steel, possibly because of the concentration of magnesium chloride (MgCl2) it contains Therefore, chloride from the major constituent

of sea salt, sodium chloride (NaCl), is deleterious to the corrosion behavior of steel because of its effect on the conductivity of the liquid film and its destruction of protective corrosion product films, but MgCl2 acts to acidify the liquid film and, by its deliquescent action, to increase the time of wetness This is the primary reason why the corrosion rates of structures in marine environments are considerably higher than in any other type of location Of course, the possibility of splash zone action on the corrosion of structures is beyond the scope of this article (see the article "Marine Corrosion" in this Volume) The dramatic increase in the corrosion rates of steel in marine locations is indicated in Table

1

Specialized environments exist in which specific pollutants are present as a function of the particular use or location of the structure Therefore, chemical plant and refinery buildings and structures may be particularly vulnerable to certain types of atmospheric pollutants produced by the processes occurring within the plant and may require particular protective measures Again, this is beyond the scope of this article, and the reaction of steel to such environments is best determined by noting the effect of the pollutant on the corrosion of steel in such publications as Ref 2

The atmosphere within closed structures will usually be very different from that on the external surfaces: In general, the presence of pollutants may be significantly reduced, and the control of temperature and humidity may mean that the overall environmental corrosivity is significantly lower than that on the outside surfaces However, there can be action that affects this circumstance For example, the well-known barrier effect of internal walls, which is related to their containment of the internal environment, may be breached at certain locations, with the result that air may escape from the interior of the building into the cavity If structural or other steel is present in this cavity, the interaction of the steel with air, which may be deliberately humidified, in contact with a possibly cold surface can lead to condensation and subsequent corrosion problems that would otherwise be unanticipated

Such problems have been noted in the past not only from the viewpoint of structural steel framing but also from the possibility of corrosion of elements in the system that tie the external cladding back to the frame of the building This will

be discussed in more detail later in this article Therefore, concern is necessary not only for the influence of the external environment on the corrosion of steel but also for the effects of the internal environment, particularly the lack of containment

Fireproofing is often mandated by fire codes for structural steelwork within buildings In the past, fireproofing was used that contained corrosive constituents Under circumstances in which moisture levels at the structural steel may increase (because of the exfiltration of humid air or the infiltration of water), the corrosive constituent in the fireproofing can lead

to unacceptable corrosion of the steel This potential problem is addressed in ASTM E 937, which is a standardized test for determining the corrosivity of such materials under standardized conditions (Ref 3) The use of coatings that meet conditions related to this test is required by many codes and should be mandated by the specifying architect or engineer

Whenever steel is used in buildings, there is always the possibility that galvanic contact may occur Therefore, galvanized conduit may contact bare steel Copper-base alloys are used as wiring for example, lightning conduction rods In general, the relative positions in the galvanic series of metals in seawater will normally dictate whether or not there is a potentially significant problem with galvanic corrosion when structural elements are in contact The environment surrounding the potential galvanic couple will have a significant effect; more severely corrosive environments allow more galvanic activity to occur Therefore, unanticipated contact between different metals and the impact of such contact on corrosion behavior must be an ever-present concern

Although the use of the lower-strength steels generally indicates that environmental/mechanical interactions leading to unexpected brittle failure are very unlikely to occur in the normal environments used in structures, the use of higher-strength steels (particularly those of the quenched-and-tempered variety) may increase the likelihood of such stress-corrosion or hydrogen embrittlement failures, especially in more aggressive environments such as marine conditions Compounding the problem is the welding that may be performed for example, on bridge structures Therefore, quenched-and-tempered steels of the ASTM A709, grade 100, type, which have yield strengths in the 690-MPa (100-ksi) range, must be welded carefully so as, not to create hard regions in heat-affected zones that may be susceptible to hydrogen embrittlement in corrosive environments

Similar considerations pertain to the area of embedded posttensioning steels These steels are typically of the cold-drawn type (not quenched and tempered), corresponding to such standards as A416, grade 270 (1860 MPa, or 270 ksi, minimum ultimate tensile strength for seven-wire strand), and A421 (1655 MPa, or 240 ksi, minimum ultimate tensile strength for 6.4-mm, or -in., diam wire) Although laboratory test data indicate that cold-drawn steels are considerably less susceptible to failure by hydrogen embrittlement than their quenched-and-tempered counterparts, care must be taken to minimize the aqueous and/or atmospheric corrosion of such steels before and after concrete placement

With regard to this aspect, the atmospheric corrosion of other metallic materials commonly used in structures is of less concern Aluminum can be used for a variety of building components, including windows and door frames If the aluminum is protected from direct contact with uncured mortar or concrete and if a commonly used anodized coating is specified, little atmospheric corrosion is noted except under severe marine conditions Zinc is typically used only as a protective coating for steel, and as such, the corrosion behavior will be covered in a later section in this article

Steel in Cementitious Materials. The behavior of steel in contact with cementitious materials (concrete and mortar)

is generally governed by the properties of the portland cement that is a constituent of the mortar or concrete Portland cement is an alkaline material; the alkalinity results from the presence of Ca(OH)2 and other soluble alkali salts The pH

of a saturated Ca(OH)2 solution is approximately 12.5 Reference to the Pourbaix (potential-pH) diagram for iron (Fig 1) shows that, under these circumstances and in the presence of moisture and oxygen, the steel will be in a passivated condition because of the formation of a thin film of oxide generally considered to be FeOOH

Fig 1 Pourbaix (potential-pH) diagram for the system iron-water at 25 °C (75 °F) Source: Ref 4

The effect of pH on the corrosion rate of steel in aerated water is shown in Fig 2 If the oxygen is depleted of if the cementitious material is allowed to dry significantly, then the passive film may well be disrupted However, under these conditions, the corrosion rate is expected to be extremely low because of the high resistivity and low driving force for the reaction As will be discussed later in this article, there are various pollutants to the cementitious environment that can modify this behavior, leading to significant corrosion of steel in contact with cementitious materials

Fig 2 Effect of pH on the corrosion rate of iron in aerated soft water at room temperature

Concrete is a hard, dense material that consists of cement paste surrounding aggregate particles The major difference between mortar and concrete is in the size of the aggregate Concrete contains a high proportion of large aggregate, such

as gravel and crushed rock Steel that is embedded in concrete, generally as reinforcement, can be of two main types In conventional reinforced concrete, reinforcing bars are used to support the tensile loading, because of the relative weakness of unreinforced concrete in tension versus its significant strength in compression Such reinforcement will be sized and placed according to requirements determined by the structural engineer Bar sizes are usually stated as number

X, where X represents the numerator in a fraction of eighths

Other types of reinforcement in concrete are deliberately stressed in tension to place the concrete into residual compression For prestressed concrete, this is typically accomplished by placing the stressed steel into the concrete form and pouring the concrete around the steel Once the concrete has cured, then the external tension on the steel is released The bond between the steel and the concrete, however, now places the surrounding concrete into compression, leading to the well-known concept of prefabricated, prestressed concrete

Another method of accomplishing this same effect is posttensioning, in which the unstressed steel is placed in the concrete form before the concrete is poured The steel is typically prevented from developing a bond with the concrete during the pouring process After pouring and curing of the concrete, the steel now embedded in (but separated from) the concrete is tensioned by a variety of mechanisms, and the tensioned steel is then anchored to the ends of the concrete structure In this way, the concrete around the steel is placed into tension

Other steel components that may be embedded in concrete include such items as water lines (which may typically be galvanized steel) and the forms used as containment during the concrete pouring process In many cases, these forms can

be removed; in others, the forms will remain in place, As such, the forms are exposed to the atmosphere on their outer surface and to the concrete environment on their inner surface The problems that have occurred during the use of such forms, particularly where additives to the concrete have caused a corrosive situation to occur, will be discussed later in this article

In other cases, unanticipated contact may occur between different materials and steel embedded in concrete Aluminum can be used for various articles embedded in or located on concrete for example, balustrades and lampposts If contact can occur between embedded steel and such aluminum in environments that increase the corrosivity of the steel, as will

be determined later in this article, accelerated corrosion of the aluminum can occur, with corresponding galvanic protection of the steel Although it is conceivable that, for example, copper water pipes may contact embedded steel reinforcement, there are no recognized instances of problems resulting from such contact

The use of mortar, which is a cementitious material, is a necessary factor in the construction of brick masonry, and steel is used in such masonry in a variety of applications The use of steel joint reinforcement to enhance the strength of the masonry by placing a mesh or ladder-type system within the bed joint is a common occurrence Such a system can also be

used to connect two wythes of masonry The use of ties or anchors to anchor masonry used as a curtain-wall system back

to the building frame or to a supporting structure is also necessary Typical reinforcements, ties, and anchors are shown in Fig 3 In these cases, although bare steel would suffice in uncontaminated high-pH mortar, it is common practice to use a more corrosion-resistant material for reasons that will be discussed later Typically, galvanizing is recommended and used either hot-dip or electroplated

Fig 3 Typical metallic anchors (a to g) and ties (h to o) used in masonry walls

Other metallic components that may contact mortar include window frames and doorways These may be steel or aluminum, if metallic Such steel components can be protected against atmospheric corrosion with organic coatings, but

in other cases, metallic coatings, such as galvanizing, can be used Aluminum components can corrode in the uncured or green mortar environment, but in the absence of further corrosion enhancers, the corrosion rate normally drops to a very low level once curing has occurred and the mortar dries

The behavior of steel in concrete and mortar as it is influenced by high pH can be drastically altered by two primary factors The first is carbonation, or the reaction of the Ca(OH)2 with CO2 from the air The reaction to form calcium carbonate (CaCO3) lowers the pH of the cementitious material; under these conditions, the steel is no longer passive, but can become active and corrode significantly This process of carbonation is a function of the porosity of the concrete or mortar, which is a function of such factors as the water/cement ratio, placement adequacy, and vibration/consolidation In general, however, mortar is much more permeable than concrete and carbonates much more rapidly Under these circumstances, the mortar in masonry structures must be considered to undergo carbonation at a relatively early point in its life To prevent the corrosion of embedded steel due to carbonation, it is common practice to use protective coatings on steels (as has already been discussed, galvanizing is common) or to use inherently corrosion-resistant materials, such as stainless steels

The second major contributor to the corrosion of embedded steel in cementitious materials is the influence of chloride Chloride in sufficient quantities prevents the formation of the initial protective oxide film on steel (if the chloride is present in the material mix during the curing process) or, if added later, breaks down the passive film and allows corrosion to proceed by a mechanism similar to the pitting of stainless steel passive films by chloride

Possible chloride sources in the mixing process are the aggregate, chloride introduced with the mixing water (a problem frequently encountered in the Middle East, where brackish water is often the only water available), and chloride-containing admixtures deliberately added to the mix as a set accelerant or so-called antifreeze most commonly calcium chloride (CaCl2) Chloride can also find its way into the cementitious material after placement and curing, because of the application of chloride to the outer surface Sources of such chloride are marine environments Deicing salt placed onto concrete bridge decks, parking structures, and so on, may also find its way onto buildings as a result of splash from

roadways Another possible source for chloride in mortar is the absorption of hydrochloric acid washing solutions applied

to the masonry surface

The quantity of chloride necessary to cause corrosion of bare steel in uncarbonated cementitious systems depends initially

on the stage at which the chloride is introduced For bare steel in initially chloride-free concrete that is subjected to chloride infiltration from the external surface, corrosion has been found to initiate on the steel when the chloride level reaches between 0.02 and 0.04% by weight of concrete at the steel interface When the chloride is present in the mix, the tricalcium aluminate of the concrete can insolubilize a certain portion of the chloride during the curing process Recognition of this fact initially led to the allowance of additions of 2% CaCl2·H2O by weight of cement to concrete to accelerate set More recent work, together with the release of bound chloride as concrete ages, has led to a recognition that maximum allowable chlorides in concrete should be a function of both the nature of the steel and the expected service environment of the concrete The recommendations of American Concrete Institute Committee 201, which are being revised at this time, are given in Table 2

Table 2 Recommended maximum water-soluble chloride levels in concrete mix (prior to service) for different reinforced concrete exposures

recommended chloride level,

wt% of concrete

Conventionally reinforced concrete in a moist environment and exposed to chloride 0.10

Conventionally reinforced concrete in a moist environment but not exposed to chloride (includes locations

where the concrete will be occasionally wetted, such as parking garages, waterfront structures, and areas with

potential moisturecondensation)

0.15

Aboveground building construction where the concrete will stay dry No limit

A compounding factor in the chloride-induced corrosion of reinforcement is the action of macrocells Because of the electrically continuous and extensive nature of the typical reinforcement in concrete, differences in chloride concentration and oxygen level at different locations on the steel can be anticipated Under these conditions, separated anodic (high-chloride, low-oxygen) and cathodic (low-chloride, high-oxygen) areas can easily develop along the reinforcement, particularly where chloride enters the concrete from an exterior surface This separate, macrocell action frequently leads

to severe corrosion at the anodic areas Macrocell action has been worsened unintentionally during concrete repair Therefore, the replacement of deteriorated, spalled, chloride-contaminated concrete adjacent to reinforcing steel with fresh, chloride-free concrete can introduce a potent cathode into the system, with the result being accelerated corrosion of the surrounding reinforcement This can lead to the well-known spall around a spall deterioration phenomenon

The effect of corrosion induced by such a mechanism is somewhat dependent on the nature of the steel For reinforcing steel, the major problem arising from an increased corrosion rate is the production of a voluminous oxide, which occupies

a greater volume than the steel from which it was produced Under these circumstances, significant tensile stresses can be introduced into the surrounding cementitious material, leading to cracking of the brittle cementitious material Calculation

of the Pilling-Bedworth ratio (see the article “Gaseous Corrosion Mechanisms” in Corrosion: Fundamentals, Testing, and Protection, Volume 13A of ASM Handbook) allows the magnitude of the volume expansion to be assessed, but this is

typically found to be a factor of between 2 and 10, depending on the exact nature of the corrosion product

The effect on high-strength prestressing steel may not be the same Such steel typically has an ultimate tensile strength exceeding 1380 MPa (200 ksi) that is produced by cold forming As discussed previously, steels of this hardness may be susceptible to stress-corrosion cracking or, more probably, hydrogen embrittlement In this context, then, corrosion processes that may produce hydrogen can be very deleterious to the structure containing the posttensioning or prestressing This is due to the possibility of hydrogen embrittlement leading to brittle fracture of this steel and therefore

to the loss of compression of the surrounding concrete Examples of this problem will be discussed later in this article

Protection Methods

Atmospheric Corrosion. One of the most important aspects of corrosion protection is corrosion prevention through the consideration of the fundamental aspects of good design practice Such considerations include:

• Avoidance of upturned angles, channels, and so on, that can collect moisture

• Avoidance of pockets within welded structures

• Grinding welds flush

• Elimination of crevices that can lead to accelerated corrosion

Examples of details to avoid, as well as more corrosion-resistant details, are shown in Fig 4 Although some of these discrepancies can be mitigated by other corrosion protection methods, the selection of weathering steels makes adherence

to good design and fabrication detailing mandatory

Fig 4 Design and fabrication details to be considered in corrosion prevention (a) Constructional members,

sills, etc (b) Joints (c) Crevices (d) Air circulation (e) Corners, edges, and surfaces

Various methods are available for protecting steel against corrosion in so-called atmospheric conditions The protection system is generally a barrier coating or a metal alloying of the steel that effectively introduces a barrier coating by a normal corrosion process

The barrier coatings used to protect steel from atmospheric corrosion are of three main types: organic coatings, inorganic coatings, and metallic coatings The selection of one of these coatings is a function of the expected environmental severity

in which the structure is to perform, the corrosivity of that environment, the expected lifetime of the structure, and the possibility of further maintenance coating In other words, coating selection is a combination of both technical and economic considerations

Organic coatings are perhaps the most frequently used coatings for protecting steel from atmospheric corrosion This

is true with regard to bulk structural steel However, such components as fasteners rarely have organic coatings, because

of the difficulties related to the fit of fastener systems incorporating such coatings and the poor durability of such coatings under abrasion and tightening conditions A review of the different types of organic coatings that are available is included

in the article “Organic Coatings and Linings” in Corrosion: Fundamentals, Testing, and Protection, Volume 13A of ASM Handbook Nevertheless, it is important to highlight in this article certain aspects of coatings procedures that are of vital

importance in the longevity of the coating

Although some coatings have recently become available that are less susceptible to the problems of incomplete surface preparation, it is generally recognized that the quality of the surface preparation is a major factor in the service life of the coating Perhaps the most extensive evaluation of paint behavior as a function of substrate preparation has been conducted

by the Steel Structures Painting Council, which has issued a series of standards on different degrees of cleaning prior to the application of paints

The degree of cleanliness to be attained depends on the sensitivity of the chosen paint system to the level of surface cleanliness and on the required longevity of the system In this regard, it is important to recognize that different methods

of surface preparation are available, including centrifugal blast cleaning, abrasive air-blast cleaning, water-blast cleaning, and hand and power tool cleaning Hand tool and power tool cleaning are typically employed only where small areas must

be cleaned of preexisting scale and paint It is also important to recognize that contaminants on steel surfaces that are not visible to the naked eye may influence the longevity of the subsequent coating Therefore, inorganic salts present on the surface may dissolve in water that permeates through the coating, leading to significant corrosion of the substrate Standard blast-cleaning techniques typically do not remove such contaminants, but water-blast cleaning does However, this again becomes a matter of economics and the sensitivity of the coating system to such surface contamination

The selection of a coating system is influenced by consideration of the likely corrosive environment and the consequences

of coating failure Figure 5 shows some typical preparation and coating formulations Therefore, the need for increased durability of the coating increases with the corrosivity of the environment In this regard, much steel is enclosed within building walls, and such steel is frequently only lightly shop primed Indeed, it has been questioned whether such coating

is in fact necessary, based on the environment in which the steel is to perform

Fig 5 Typical coating systems used in various environments (a) Adduct-cured epoxy for use in exposure to

solvent spillage and alkaline dust (b) Inorganic zinc-epoxy used for severe marine exposures (c) Inorganic zinc/vinyl coating system for use in mild industrial environments (d) Alkyd-base coating for mild inland atmospheric exposure Source: Ref 5

One of the problems in making decisions of this type is that the severity of the environment may not be adequately documented Therefore, if there is exfiltration from the building, then condensation of the steel members can occur, resulting in an extended time of wetness and a corrosion rate that is significantly greater than might otherwise be anticipated Under these circumstances, the thin shop primer may be much less than adequate, and a more rigorous corrosion prevention system should be considered Adverse effects on such steel may include the pick-up of corrosive constituents (particularly chloride) from other building materials for example, from mortar during passage of water through masonry walls If flashing systems are inadequate, then such water may reach structural steel and cause

significant corrosion damage This is another factor that needs to be considered whenever the so-called benign environment within, for example, the walls of a building is considered

Inorganic Coatings Other forms of coating that are midway between the use of a metallic coating and an organic

coating are the inorganic zinc-rich coatings The ultimate life expectancy of this type of material in severe weathering service has not yet been established, but over 20 years of experience has shown that complete protection of the steel substrate is still being provided Protection in this case is largely the result of the good corrosion resistance of zinc itself Indeed, it has been pointed out that the cured inorganic zinc-silicate films can be thought of as a cross between hot-dip galvanizing and a fused ceramic Because of the absence of organics, inorganic coatings are considered to have the best solvent resistance of any type of protective coating This makes them very useful for structures in which vapors are present, such as chemical plants, petroleum refineries, and production facilities

The metallic coating system most commonly used for steel in structures is zinc coating These coatings can be

applied by hot-dip galvanizing or by electroplating for smaller components, such as bolts Electroplating generally lays down a considerably thinner zinc layer than hot-dip galvanizing Therefore, the service life of this layer can be considered

to be shorter The results of numerous investigations of the service lives of galvanized coatings show that the life of the coating is a function of the coating thickness and the environment in which it will operate This is illustrated schematically in Fig 6 Therefore, the thickness of galvanizing to be placed on a piece of structural steel will depend on

an assessment of the nature of the environment and the required longevity In this regard, the severity of the environment within a cavity wall where a zinc coating material may frequently be used has recently become a significant concern; this issue will be discussed later in this article Additional information on zinc coatings is available in the articles “Continuous

Hot Dip Coatings” and “Batch Process Hot Dip Galvanizing” in Corrosion: Fundamentals, Testing, and Protection, Volume 13A of ASM Handbook and “Corrosion of Zinc” in this Volume

Fig 6 Effect of zinc coating weight on service life of galvanized steel sheet in various environments Service life

is measured in years to the first appearance of significant rusting

For small components, other protective systems are available These include cadmium plating, which is considered to be more corrosion-resistant than zinc in marine environments, Cadmium plating is currently viewed with caution because of the toxicity of the plating solutions and the difficulty of disposal Nevertheless, the general corrosion resistance of cadmium plate makes the process worthy of consideration where the small steel components used in structures must be protected

Weathering Steels The final method of protection against atmospheric corrosion is the use of weathering steels In

this class of materials (typified by ASTM specifications A 588 for buildings and A 709 for bridges), small amounts of alloying elements typically nickel, chromium, and copper are added to the steel (Ref 6, 7) Under certain fairly specific situations, these alloying elements are incorporated into the oxide layer that forms on the steel, leading to the formation of

a dense, more protective oxide This oxide then serves as a barrier to further penetration of moisture and effectively acts almost as a "self-painted" coating, lessening the need for any other coating protection An example of the improvement in corrosion behavior of such steels in an industrial environment is shown in Fig 7

Fig 7 Corrosion of three types of steels in an industrial atmosphere Shaded areas indicate range for individual

specimens

Although such behavior is valuable, it must be recognized that it will be observed only under certain circumstances Interestingly, the improvement is generally best in an industrially polluted environment; less improvement is noted in environments containing chloride for example, marine environments Also, the film or patina must undergo repeated wetting and drying to develop Any use of this type of steel in a structure in which this type of exposure is not available will not allow development of the protection In this context, it is vital to adhere to good corrosion-resistant design techniques when using weathering steels to ensure that the steel performs adequately Any design or fabrication detail that incorporates crevices, improperly bolted connections, and so on, will lead to behavior in which weathering steels exhibit

no advantage over other steels Examples of problems related to the use of weathering steels will be discussed later in this article

The ultimate situation in using alloying elements to promote a barrier layer on the surface of steel involves the introduction of sufficient chromium into the steel to allow the formation of stable chromic oxide The stainless steels contain a minimum of 12% Cr and are highly resistant to normal atmospheric conditions More highly alloyed steels may

be required for resistance to severe marine environments However, the high cost of these materials precludes their use except in critical situations Such critical situations may, for example, arise in connection bolts, ties, and other components that must perform for the expected service life of the building and are usually not located where inspection, maintenance, or replacement can be easily carried out Under these circumstances the use of stainless steel becomes a highly advantageous proposition

Cementitious Systems. As discussed previously in this article, cementitious systems normally provide a protective environment to steel Indeed, the longevity of reinforced steel structures supports this situation The major factors that cause steel embedded in cementitious systems to corrode and lead to significant problems are the influences of carbonation and chloride infiltration Under both of these circumstances, the normally protective oxide film on steel

breaks down, and corrosion will proceed at a rate that is sufficient to cause loss of cross section, buildup of voluminous corrosion product with resultant spalling of overlying material, or, in specific cases, hydrogen embrittlement

One of the principal methods of preventing this problem in construction has been to place over the steel a concrete or cementitious material of sufficient quality and thickness to reduce dramatically any infiltration of chlorine or CO2 to the steel surface Because of the diffusion-controlled nature of this infiltration, the depth and permeability of the material have a significant effect on the time at which sufficient chloride or CO2 reaches the steel surface to allow corrosion to initiate and propagate The obvious first step in preventing such corrosion is to ensure adequate concrete depth (typically taken as 50 mm, or 2 in., of clear cover over reinforcing steel) and to use as low a water-to-cement ratio as feasible to promote impermeability within the concrete

Although this approach may be considered adequate for reducing carbonation problems within the expected service lives

of structures, it may not be sufficient to mitigate chloride-induced corrosion The reasons are twofold First, the chloride may have been added to the mix; therefore, it would already be present at the level of the reinforcing steel, irrespective of the type, quality, or depth of concrete cover used Such problems can be prevented at the specification stage; the use of any chloride-containing admixtures for any purpose whatsoever should be prohibited Also, research has shown that prestressing steel may be even more deleteriously affected by chloride-induced corrosion than conventional reinforcing steel; limits are currently being considered on the amount of chloride permissible within the concrete from any source Therefore, chloride-bearing aggregate could have a considerable influence on this situation

Second, in the case of marine environments, and particularly in the application of deicing salt, it becomes difficult, if not impossible, to ensure that the depth of cover and the concrete quality will protect the steel against chloride-induced corrosion attack over the expected service life of the structure In this case, alternative measures become appropriate Such measures involve the application of a barrier coating to the steel or to the cementitious surface or the application of cathodic protection to bare steel, generally at a point in the service life of structure at which corrosion damage has already occurred

Various coatings for steel in reinforced concrete structures have been evaluated These can generally be grouped into either organic coatings and inorganic or metallic coatings Extensive work has demonstrated that the alkaline nature of concrete and the intrusion of chloride into the concrete are best resisted by an epoxy coating on the steel Moreover, the reinforcing steel must be carefully prepared prior to application of the coating, and the application method is critical typically, an electrostatic powder spray, followed by baking to fuse the coating Bars coated in accordance with this technique have been successfully used in bridge decks since 1973 As necessary, repair of such coatings in the field at bar ends and holidays, and so on, can be undertaken using repair materials

Epoxy coatings have been viewed with some disfavor on the basis of bond strength, that is, the possible tendency of the coating to prevent adequate bond development by lessening adhesion at the deformation/concrete interface However, it would appear that recommended coating thicknesses, which are typically 0.18 ± 0.05 mm (7 ± 2 mils), mitigate this problem and do not lead to a significant reduction in bond strength

Additional concerns when using epoxy coatings include the possibility that small holidays in the coating may be subject

to accelerated attack, particularly if epoxy-coated steel is used only in the top mat of the structure, and that bare steel is used in, for example, the bottom mat This is a traditional construction method when deicing salt is applied to the top surface; the bare steel at the bottom is rarely subjected to any chlorine levels, because of the large distance (up to 250

mm, or 10 in.) from the upper surface of the concrete to the level of the lower mat Although it is not clear that this concern has been completely eliminated, testing has indicated that it may not be a major problem In any case, such a problem could of course be largely eliminated through the use of epoxy-coated reinforcing steel throughout the entire structure

Metallic coatings for steel in cementitious materials are typically limited to zinc and nickel As previously discussed, small items, such as threaded fasteners, can be cadmium plated Cadmium plate is apparently slightly more resistant to chloride attack in cementitious materials than zinc because of the apparent formation of a basic cadmium chloride Furthermore, metallic cadmium does not appear to form expansive corrosion products However, the problems with continued cadmium coating related to environmental concerns have already been addressed

By far the most common coating for steel in contact with cementitious materials is zinc either applied by hot-dip galvanizing or plating The factors that determine the rate and which zinc and zinc-coated steel corrode in cementitious

materials are directly linked to the formation and preservation of protective films A stable film, and therefore low corrosion rates, occurs on the zinc in the pH range of about 6 to 12.5

Figure 8 shows the influence of pH on the corrosion rate of zinc It is interesting that the pH of concrete or mortar in the noncarbonated state is typically about 12.5, which is the minimum on the pH versus corrosion rate curve for zinc This observation is supported by laboratory studies evaluating the corrosion rate of zinc Therefore, for a noncarbonated mortar, the corrosion rate of zinc in contact with this mortar is effectively zero Once carbonation occurs, and the pH falls below 10, then the corrosion rate increases typically to a level of approximately 0.5 to 0.8 m/yr (0.02 to 0.03 mil/yr) Therefore the typical use of hot-dip galvanizing as a protective measure for steel exposed to carbonated concrete or, more particularly mortar, is well founded A typical 64- m (2.5-mil) galvanized coating would be expected to last over 80 years under these circumstances

Zinc does not perform as well when chloride is present in the cementitious material either as a result of the original additions or the infiltration from the exterior The use of zinc coatings under these circumstances has been the subject of much debate Indeed, the results of laboratory testing including experiments related to the use of simulated environments such as Ca(OH)2 solution with and without added quantities of chloride as well as laboratory-prepared specimens of zinc-coated steel in contact with chloride-contaminated concrete may be in disagreement with studies of large-scale structures that have been fabricated with galvanized steel Some of these structures are bridge decks; those that are most commonly used to promote galvanized reinforcement are structures in the Caribbean, where no cracking of the structure was noted even after chloride up to a level of approximately 0.26% by weight of the concrete was present at the rebar interface In this case, however, approximately one-tenth of the original galvanized coating and been removed, and further corrosion could be anticipated In other locations where galvanized steel has been used in bridge decks, no reports are currently known in the open literature regarding the performance of such steel

as compared to bare steel in the same type of environment

Studies on reinforced concrete specimens containing galvanized steel, exposed to either marine environments or to artificial ponding in NaCl solution, have led to a variety of conclusions related to the effectiveness of the galvanizing Therefore, this range is all the way from a supposedly accelerated cracking of concrete containing galvanized steel to a retardation of such cracking as compared with bare steel However, it should be emphasized that in most of these studies cracking of the overlying concrete has occurred, indicating that galvanizing is at best a palliative coating for steel in chloride-contaminated cementitious materials

As far as is known, there is no direct evidence regarding the ability of galvanized steel to withstand a certain level of chloride before significant corrosion can occur In studies using electrical resistance probes, it was found that for chloride added to mortar during the mixing phase the corrosion rate of zinc was of the order of 0.5 m/yr (0.02 mils/yr) for a chloride content of approximately 0.15% by weight of the mortar However, in the presence of carbonation, this corrosion

Fig 8 Corrosion of zinc in aqueous solutions as a function of pH

rate increased to 10 m/yr (0.4 mils/yr) Therefore, for a standard galvanized coating approximately 64 m (2.5 mils) thick, the coating would survive for only 6.5 years under this latter condition This is notwithstanding possible problems related to the production of nonprotective and expansive corrosion products from the zinc layer

Zinc hydroxychloride has been found to form during the corrosion of zinc in chloride-contaminated cementitious materials This component is significantly expansive compared to the zinc from which it was produced and can therefore cause cracking of overlying cementitious materials even before corrosion of substrate steel has occurred

Therefore, in general, although zinc-coated steel is somewhat more resistant to chloride-induced corrosion than bare steel, the zinc cannot be relied on to provide protection indefinitely Indeed, the products of corrosion of the zinc itself may lead

to cracking of the overlying material

In addition to applying barrier coatings on the embedded steel to resist corrosion, a similar barrier surface may be applied

to the concrete or masonry surface to resist the penetration of constituents contributing to corrosion These generally take the form of physical barriers, including:

• Organic membranes applied to the concrete surface (often used on parking structures)

• Low-permeability cementitious-base overlays and repairs (incorporating latex)

• Polymer impregnation of the concrete (rarely used)

• Waterproofing agents, for example, drying oils, stearates, or silanes

In using one of these systems, care must be taken not to trap the corrosive environment within the structure Silanes applied to surfaces are effective in preventing this action; they allow water vapor transmission out of the structure but no liquid penetration into the structure This prevents oxygen transport to the steel while allowing the structure to dry

The final method of protection for steel in cementitious materials that may be subject to corrosion is cathodic protection Cathodic protection is a well-established means of preventing corrosion in a variety of environments, particularly for pipelines buried in soil or immersed in seawater It has also been successfully used to ensure the protection of steel pipelines that are buried or encased in concrete

It was first applied to the problem of deterioration of conventionally reinforced concrete structures in California During early experiments, it was necessary to use a total surface anode over the entire structure to be protected in this case, the top surface of the bridge deck because of the low throwing power of the anode and general high electrical resistance of concrete Using this technique, large decks could be protected with only 10 W of power The surface anode method did, however, require the embedment of current supply anodes in a relatively soft overlay composed of coke-asphalt mix, which itself could be subject to fairly rapid deterioration by traffic This technique has been used by others with some success, and it continues to be improved

One of the more important developments in the cathodic protection of structures is the use of different anode systems that allow horizontal surfaces other than upward facing to be protected Therefore, the development of anode systems incorporating conductive polymers, conductive paints, and conductive concrete (containing typically coke or graphite) has made possible the cathodic protection of the underside of vertical surfaces of reinforced concrete structures An example

of such an application will be discussed later in this article

Impressed-current systems using platinized wires in slots in the concrete surface, surrounded by conductive grout, have been used Experience with this system has been clouded because of the deterioration of the grout and surrounding concrete by the low pH developed at the anode and by the poor throwing power of the anode Studies have also been conducted on galvanic (sacrificial anode) systems, which are to be distinguished from the impressed-current systems previously discussed In this type of system, for example, zinc wires are placed in slots above reinforcing steel, and the sacrificial action of the zinc is then transferred to the steel directly below

Some additional observations can be made with regard to the cathodic protection of reinforced steel in concrete First, care must be exercised whenever cathodic protection systems are considered for application to high-strength steels, such

as those used in posttensioning and prestressing systems This is because of the previously discussed possible susceptibility of these steels to hydrogen embrittlement, which may result from the overprotection of the steel and subsequent hydrogen evolution at the steel/concrete or mortar interface

Furthermore, unbonded systems are expected to be difficult to protect because of the possible insulating effects of the sheath that is present around the steel Indeed, even if such unbonded systems could be protected, the poor throwing power due to the lack of contact between the posttensioning steel and the surrounding concrete is probably an insurmountable problem

Finally, considerable attention is currently being given to the appropriate criteria for cathodic protection of steel in cementitious materials Although a transfer of the usual protection criteria from, for example, pipe in soil systems has been made, there is some doubt as to the reasonability or, in some cases, the adequacy of such protection criteria This is due to the different nature of the environment between concrete and soil, particularly as it relates to pH Therefore, rather than relying on a -0.85-V copper sulfate electrode as a standard criteria for protection, there is more interest in using a

potential shift mechanisms and in using E-log i plots for establishing the adequacy of the protection More information on

cathodic protection is available in the article "Cathodic Protection" in this Volume

Case Histories

This section will cite specific examples of failures and problems that have occurred within structures to illustrate the general principles discussed previously, to point out those factors that bear most heavily ont he development of a particular problem or failure, and to determined how protective or preventive measures can be implemented to repair the structure or to prevent the occurrence of failure in the future The case history data to be discussed have been primarily taken from investigations of buildings or structures conducted by the author Other examples of failures are available in the literature

Failures Involving Corrosion of Structural Steel

In general, failures related to the corrosion of conventional structural steel (that which has been painted or otherwise protected) are, in the author's experience, rare Cases do exist in which excessive humidity or chloride-laden water has contacted the metal The use of weathering steels, however, has caused significant problems, particularly where the necessary design features and environment have not been carefully considered during selection of the material Two examples of problems involving weathering steels will be discussed

Example 1: Weathering Steel Corrosion in a Stadium

A large sports stadium situated about 300 m (1000 ft) from the ocean was built with weathering steel in the major structural members The steel was used not only for the exposed portions of the structures but also beneath the stands Significant corrosion and rust flaking were noted on this steel at several locations:

• Underneath the stands, where air circulation was poor and no standard wetting/drying could be anticipated (Fig 9)

• At sheltered locations on the exterior, again where standard wetting/drying was not possible (Fig 10)

• At joint details, where the important concepts of removal of crevices and pockets to retain water had not been practiced in the design of joints for the structure (Fig 11)

Fig 9 Heavy buildup of corrosion scale on weathering steel structural members in conditions of poor air

circulation, high humidity, and no wetting/drying

Fig 10 Corrosion scale buildup on weathering steel structural members, which were in a sheltered area on a

building exterior where wetting and drying did not occur

Fig 11 Heavy corrosion scale buildup on structural members of weathering steel at a pocket where water could

collect and stand

The major problem associated with this structure was the amount of corrosion occurring on the structural steel beneath the stands Although the loads in these structures were determined to be low, it was apparent that corrosion protection would

be necessary A series of tests was conducted to determine the optimum coating protocol, including surface preparation, type of coating, thickness, and number of coats

This example serves to point out the important factors to be considered by the designer when weathering steel is selected

In particular, the protective patina can be developed adequately only with exposure to weather Furthermore, marine environments are not conducive to the formation of such a patina, which apparently develops best in an industrial environment with relatively high sulfur levels in the atmosphere

Example 2: Corrosion of Weathering Steel in a Hotel Parking Garage

A hotel parking garage in the Northeast was constructed with weathering steel in the columns and beams, along with conventional reinforced concrete slabs placed as the decks The hotel and garage were situated in an area that experienced considerable amounts of snow and freezing temperatures Deicing salt was commonly applied to roadways adjacent to the structure and was also probably applied to the reinforced concrete slabs themselves Severe deterioration was noted in the weathering steel beams and columns, particularly those adjacent to leakage points of water, and in expansion joints This corrosion was caused by contact with the deicing salt laden water, effectively destroying any patina that may have been expected to develop on the steel and leading to the production of voluminous, nonprotective oxides

The failure in this case caused by a lack of appreciation of the environment to which the weathering steel was to be exposed Possible solutions to the problem include the use of coatings on the steel to protect it from further contact with chloride-laden water and the correction of water paths that lead to contact with the deicing salt The sheltered location of most of the steel, however, would not allow effective patina development Prohibiting the use of chloride deicing salt on the garage decks would probably reduce the problem somewhat, although pickup of deicing salt from roadways and track-

in into the garage is a perpetually deleterious condition that is unavoidable

Corrosion of Conventional Reinforcement

Corrosion of steel in conventionally reinforced concrete in deicing salt application areas and in marine areas is a known phenomenon on such structures as bridge decks and bridge support structures This corrosion phenomenon has received sufficient illustration in the literature and will not be repeated here An example of a similar problem that has occurred in a building not subjected to deicing salt or ambient marine environments will be used as an illustrative example

well-Example 3: Corrosion of Reinforcing Steel in Building Columns

Figure 12 shows an example of a supporting column in a dormitory building on a Midwestern campus The columns are

of conventionally reinforced concrete, with a spiral of reinforcement that closely approaches the surface of the concrete column The concrete showed cracking and spalling within a few years after its installation Laboratory examination indicated that the concrete cover was low, that the concrete over the steel was carbonated close to the outer surface, and that the concrete contained a significant amount of chloride, apparently added during construction, that was due to either the presence of chloride-bearing aggregate or the deliberate addition of admixtures such as CaCl2 Corrosion of embedded steel was severe in places (Fig 13)

Fig 12 Corrosion-induced spalling of overlying concrete on reinforced columns See also Fig 13

Because of the likelihood that carbonation might progress into the concrete and, together with the chloride, affect the more deeply embedded steel, the following repair plan was devised First, loose and spalled concrete was chipped out of the columns Second, the columns were shotcreted to a depth of approximately 25 mm (1 in.) above the topmost reinforcement Finally, an impressed-current cathodic protection system was placed on the columns, using conductive polymer anodes with integral lead wires This system was positioned above the surface of the concrete with stands, and the entire system was finally covered with an additional layer of concrete This minimal concrete removal, along with the subsequent buildup and installation of

a cathodic protection system, was far more cost effective than the alternatives, which included complete demolition of the column or removal of sufficient overlying concrete cover to necessitate shoring of the column to support the building above

Corrosion of Ties and Anchors

Fig 13 Severe corrosion on reinforcing

steel from the column shown in Fig 12

The corrosion of steel ties and anchors used to attach masonry walls to the building frame, to the backup (usually concrete block) wall, or, more recently, to steel studs has been a source of significant concern Cases are known in which such ties

or anchors have corroded, either in the mortar or in the airspace Two examples will be given

Example 4: Corrosion of Nonstandard Wall Ties

In a hospital in the Midwest, cracking of masonry walls at attachment points to a steel stud backup was observed When the interior wallboard was removed, the tie between the brick wall and the steel stud was found to be in the form of a light-weight C-channel that had been flattened and bent to form a 90° angle (Fig 14) The flattened portion of the anchor was inserted into the masonry bed joint and down into the core of the brick The remaining, intact portion of the channel was attached to the backup steel stud by a self-tapping screw No effective corrosion protection had been applied to the angles; they exhibited remnants of shop primer at certain locations, but the efficacy of any coating had been destroyed during the bending and flattening process

Fig 14 Unconventional masonry tie constructed from flattened C-channel and bent to enter the brick core

Severe corrosion of these angles had occurred in the airspace adjacent to the brick masonry This was apparently caused

by water running down the interior wall and becoming trapped Also, the water would become trapped in the crevices formed by the flattened channel

One effect of this corrosion was the interaction with low cyclic stresses imposed on the tie due to movements between the stud and the wall, leading to cracking of the tie This was particularly prevalent where corrosion had reduced the tie thickness to a fraction of its original dimension (Fig 15)

Fig 15 Severe corrosion and cracking (arrows) on wall tie

This problem could have been prevented through the use of several alternative techniques, including:

• The use of more conventional ties incorporating corrosion-resistant coatings such as galvanizing

• The use of heavy organic coatings after bending to prevent corrosion If this type of tie was to be mandated, then heavy organic coatings applied after bending would have been appropriate

The solution to this problem could involve the use of alternative supplementary anchors or the dismantling of portions of the wall containing such ties and replacement with masonry containing more appropriate anchors to ensure continued support of the masonry wall

In addition to the corrosion of steel in the wall cavity itself, which can generally be lessened by the use of galvanizing, there is also concern regarding the corrosion that may occur in the portion of the tie within the mortar This is the case where significant chloride is present in the mortar, particularly where chloride and carbonation can interest to produce a significantly corrosive environment In these cases, the commonly used galvanized thicknesses may not be sufficient to protect the steel over the expected service life of the building

Example 5: Cracking of Masonry Caused by Corrosion of Ties and Anchors

In many structures that have incorporated high-bond masonry mortar additives, the release of Cl- due to alkaline hydrolysis has significantly corroded uncoated and coated (zinc and cadmium) steel This has been the case for laid-in-place buildings (utilizing conventional ties) and for penalized buildings (in which embedment of connection devices in the mortar is used to affix the panels to the building frame) In this situation, connection devices may corrode within the mortar, with subsequent cracking of overlying masonry due to the buildup of corrosion product

Examples of masonry exhibiting such cracking, which radiates from corroded embedded ties and anchors, are shown in Fig 16 and 17 Under these circumstances, the integrity of the anchor becomes extremely suspect No effective method is known for alleviating this problem once it has occurred Alternative approaches have involved the use of heavily organic coated anchors (for example, epoxy coatings) or the use of austenitic, molybdenum-containing stainless steels, which should resist the onslaught of the Cl-

Fig 16 Corrosion and resulting masonry cracking on anchor embedded in high-bond mortar See also Fig 17

Fig 17 Corrosion and resulting masonry cracking on an anchor embedded in high-bond mortar See also Fig

16

Corrosion of Posttensioning and Prestressing Structures

Unlike other cases of corrosion of steel in structures, the corrosion of posttensioning structures can be troublesome from two viewpoints First, the possibility exists that substantial corrosion can lead to a loss of cross section and therefore failure of the gripping mechanism for the posttensioning strand or of the posttensioning steel itself Second, corrosion products on the surface of the material may release sufficient hydrogen to cause hydrogen embrittlement Two examples will be given

Example 6: Corrosion of Posttensioning Anchorages

A posttensioned garage in the snow belt area of the United States exhibited significant corrosion of conventional reinforcement, as noted by the presence of cracking and spalling of overlying concrete On one occasion, a posttensioning tendon failed This led to a large-scale investigation of posttensioning members, particularly the anchorages The anchorages were found to be significantly corroded because of their location adjacent to leaking expansion joints and because they were surrounded by poor, badly consolidated concrete This had allowed deicing salt to penetrate to the level

of the anchorages, leading to some serve corrosion This was particularly true on the gripping wedges, as illustrated in Fig 18

Fig 18 Corrosion of posttensioning anchorage Note severe corrosion at the two wedge halves

Several of the anchorages were sufficiently corroded such that alternative anchorages had to be installed In others, the corrosion was slowed by the injection of water-displacing grease into the anchorage through a grease fitting This case history shows how poor-quality concrete can significantly affect the performance of metals embedded within it, particularly when the metals are of vital importance to the longevity and safety of the structure

Example 7: Hydrogen Embrittlement of Posttensioning Wires

Single 6.4-mm (0.25-in) posttensioning wires failed in a parking garage in the southern portion of the United States A typical anchorage with a broken buttonhead wire is shown in Fig 19 Several such wires were removed, and the lengths were examined for signs of corrosion Localized shallow pitting was common, as illustrated in Fig 20 Chloride was detected in some of these pits

Fig 19 Posttensioning anchorage with broken wire extending from anchor plate

Fig 20 Pitting corrosion adjacent to fracture on failed posttensioning wire

Scanning electron microscopy metallography of the ends of the fractures revealed an initial crack that was probably caused by hydrogen embrittlement (Fig 21) Apparently, this hydrogen embrittlement had occurred because of the presence of the corrosion on the external surface No chloride was detected on the fracture surface, and none was detected

in the overlying concrete This suggested that the corrosion observed may have initiated before placement of the tendons within the concrete There is no known method of mitigating this type of problem once it has occurred, although it can be prevented through the use of judicious and careful corrosion-preventive techniques during the storage of the tendons before placement

Fig 21 Metallographic cross section through the fracture initiation region of posttensioning wire Note

secondary cracks Etched with 2% nital 55×

3 "Standard Test Method for Corrosion of Steel by Sprayed Fire-Resistant Material Applied to Structural

Members," E 937, Annual Book of ASTM Standards, American Society for Testing and Materials

4 M Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, 1966

5 R Zidell, Coatings for Steel, in Paint Handbook, G.E Weismantel, Ed., McGraw-Hill, 1981

6 "Standard Specification for High-Strength Low-Alloy Structural Steel With 50 ksi (345 MPa) Minimum

Yield Point to 4 in (100 mm) Thick," A 588, Annual Book of ASTM Standards, American Society for

Testing and Materials

7 "Standard Specification for Structural Steel for Bridges," A 709, Annual Book of ASTM Standards, American

Society for Testing and Materials

Corrosion of Metal Processing Equipment

Introduction

METAL-PROCESSING EQUIPMENT is exposed to numerous corrosive environments and corrosion mechanisms treating equipment is subject to high-temperature oxidation, carburization, and sulfidation Corrosion by molten salts and molten metals is also of concern for heat-treating furnaces and accessories (see the section "Corrosion of Heat-Treating Furnace Accessories" in this article) Equipment for plating, pickling, and anodizing is exposed to acid and alkali solutions at temperatures up to or higher than 100 °C (212 °F) (see the section "Corrosion of Plating, Anodizing, and Pickling Equipment" in this article) Information on materials for and prevention of corrosion in these applications is also

Heat-available in Volumes 4 and 5 of ASM Handbook

Corrosion of Heat-Treating Furnace Accessories

G.Y Lai and C.R Patriarca, Haynes International, Inc

Heat-treating furnace accessories include a wide variety of components, such as trays, baskets, pots, blowers, thermowells, belts, hangers, bellows, and dampers Typical heat treatments include annealing, normalizing, hardening, carburizing, nitriding, carbonitriding, brazing, galvanizing, and sintering

The medium or environment used for heat treating varies from process to process The high-temperature corrosion of furnace components depends heavily on the environment (or atmosphere) involved in the operation Typical environments are air, combustion atmospheres, carburizing and nitriding atmospheres, molten salts, and protective atmospheres (such as endothermic atmospheres, nitrogen, argon, hydrogen, and vacuum) Protective atmospheres are used to prevent metallic parts to be heat treated from forming heavy oxide scales during heat treatment The environment can often be contaminated by impurities, which can greatly accelerate corrosion These contaminants (such as sulfur, vanadium, and sodium) generally come from fuels used for combustion, from fluxes used for specific operations, and from drawing compounds, lubricants, and other substances that are left on the parts to be heat treated

The modes of high-temperature corrosion that are most frequently responsible for the degradation of furnace accessories are oxidation, carburization, sulfidation, molten-salt corrosion, and molten-metal corrosion Each mode of corrosion, along with the corrosion behavior of important engineering alloys, will be discussed in detail in this section The compositions of the alloys under discussion are given in Table 1

Table 1 Nominal chemical compositions of high-temperature alloys

Composition, % Alloy

C Fe Ni Co Cr Mo W Si Mn Other AISI type 304 stainless steel 0.08(a) bal 8 18 1.0(a) 2.0(a)

AISI type 309 stainless steel 0.20(a) bal 12 23 1.0(a) 2.0(a)

253MA 0.08 bal 11 21 1.7 0.8(a) 0.17N, 0.05Ce

AISI type 310 stainless steel 0.25(a) bal 20 25 1.5(a) 2.0(a)

AISI type 316 stainless steel 0.08(a) bal 10 17 2.5 1.0(a) 2.0(a)

AISI type 446 stainless steel 0.20(a) bal 25 1.0(a) 1.5(a) 0.25N

E-Brite 0.002 bal 0.15 26 1.0 0.2 0.1

Incoloy alloy 800H 0.08 bal 33 21 1.0(a) 1.5(a) 0.38Al, 0.38Ti

RA330 0.05 bal 35 19 1.3 1.5

Multimet 0.10 bal 20 20 21 3 2.5 1.0(a) 1.5(a) 1.0Nb + Ta, 0.5Cu, 0.15N

Haynes alloy 556 0.10 bal 20 18 22 3 2.5 0.4 1.0 0.2Al, 0.8Ta, 0.02La, 0.2N, 0.02Zr

Incoloy alloy 825 0.05(a) 29 bal 22 3 0.5(a) 1.0(a) 2Cu, 1Ti

Inconel alloy 600 0.08(a) 8 bal 16 0.5(a) 1.0(a) 0.35Al(a), 0.3Ti(a), 0.5Cu(a)

Haynes alloy 214 0.04 2.5 bal 16 4.5Al, Y

Inconel alloy 601 0.10(a) 14.1 bal 23 0.5(a) 1.0(a) 1.35Al, 1Cu(a)

Inconel alloy 617 0.07 1.5 bal 12.5 22 9 0.5 0.5 1.2Al, 0.3Ti, 0.2Cu

Hastelloy alloy S 0.02 3(a) bal 2.0(a) 15.5 14.5 1.0(a) 0.4 0.5 0.2Al, 0.02La, 0.009B

Hastelloy alloy X 0.10 18.5 bal 1.5 22 9 0.6 1.0(a) 1.0(a)

Inconel alloy 625 0.10(a) 5(a) bal 21.5 9 0.5(a) 0.5(a) 0.4Al(a), 0.4Ti(a), 3.5Nb + Ta

Haynes alloy 230 0.10 3(a) bal 3(a) 22 2 14 0.4 0.5 0.3Al, 0.005B, 0.03La

RA333 0.05 18 bal 3 25 3 3 1.25 1.5

Hastelloy alloy N 0.06 5(a) bal 7 16.5 0.5(a) 1.0(a) 0.8(a) 0.35Cu(a)

Haynes alloy 188 0.10 3(a) 22 bal 22 14 0.35 1.25(a) 0.04La

Haynes alloy 25 0.10 3(a) 10 bal 20 15 1.0(a) 1.5

Oxidation

Oxidation is probably the predominant mode of high-temperature corrosion encountered in the heat-treating industry The oxidation discussed in this section involves air or combustion atmospheres with little or no contaminants, such as sulfur, chlorine, alkali metals, and salt

Carbon steel and alloy steels generally have adequate oxidation resistance for reasonable service lives for furnace accessories at temperatures to 540 °C (1000 °F) (Ref 1) At intermediate temperatures of 540 to 870 °C (1000 to 1600 °F) heat-resistant stainless steels, such as AISI types 304, 316, 309, and 446, generally exhibit good oxidation resistance (Ref 1) Very few oxidation data have been reported in this temperature range As the temperature increases above 870 °C (1600 °F), many stainless steels begin to suffer rapid oxidation Better heat-resistant materials, such as the nickel-base high-performance alloys, are needed for furnace components in order to combat oxidation at these high temperatures

Numerous oxidation tests on commercial alloys have been performed at 980 °C (1800 °F) or higher For example, in one investigation, cyclic oxidation tests were conducted in air, with each cycle consisting of exposing the samples at 980 °C (1800 °F) for 15 min, followed by a 5-min air cooling (Ref 2) The performance ranking, in order of decreasing performance, was found to be as follows: Inconel alloy 600, Incoloy alloy 800, type 310 stainless steel, type 309 stainless steel, type 347 stainless steel, and type 304 stainless steel Similar cyclic oxidation tests performed in air at 1150, 1205, and 1260 °C (2100, 2200, and 2300 °F), cycling to room temperature by air cooling after every 50 h at temperature, showed Inconel alloy 601 to be the best performer, followed by Inconel alloy 600 and Incoloy alloy 800 (Ref 3)

In another study, ferritic stainless steels such as E-Brite and type 446 were shown to be significantly better than type 310 and Incoloy alloy 800H in terms of cyclic oxidation resistance in air (Ref 4) These test results showed weight change data of 2.2 mg/cm2 for E-Brite, 10.0 mg/cm2 for type 446 stainless steel, -83.2 mg/cm2 for alloy 800, and -90.3 mg/cm2for type 310 stainless steel after exposure of the samples for a total of 1000 h with 15 min at 980 °C (1800 °F) and 5 min

at room temperature A separate test was also conducted This test involved exposure of the samples at 980 °C (1800 °F) for 1000 h in air with interruptions after 1, 20, 40, 60, 80, 100, 220, 364, and 512 h for cooling to room temperature The weight change results of these four alloys were -12.9, 9.2, 1.7, and 3.0 mg/cm2 for E-Brite, type 446 stainless steel, type

310 stainless steel, and alloy 800, respectively (Ref 4)

An oxidation data base for a wide variety of commercial alloys, including stainless steels, iron-nickel-chromium alloys, nickel-chromium-iron alloys, and high-performance alloys, was recently generated (Ref 5) Tests were conducted in air at

980, 1095, 1150, and 1205 °C (1800, 2000, 2100, and 2200 °F) for 1008 h The samples were cooled to room temperature once a week (each 168 h) for visual inspection The results are summarized in Table 2

Table 2 Results of 1008-h cyclic oxidation test in flowing air at temperatures indicated

Specimens were cycled to room temperature once a week

Oxidation rate at temperature

980 °C (1800 °F) 1095 °C (2000 °F) 1150 °C (2100 °F) 1205 °C (2200 °F)

Metal

loss

Average metal affected (a)

Metal loss

Average metal affected

Metal loss

Average metal affected

Metal loss

Average metal affected Alloy

RA333 0.007

5

0.3 0.02

5 1.0 0.025 1.0 0.058 2.3 0.05 2.0 0.1 4.0 0.18 7.1 0.45 17.7

(a) Average metal affected = metal loss + internal penetration

(b) All figures shown as greater than stated value represent extrapolation of tests in which samples were

consumed in less than 1008 h

Type 304 stainless steel and type 316 stainless steel both exhibited severe oxidation attack at 980 °C (1800 °F), while type 446 showed relatively mild attack Many higher alloys, such as Incoloy alloy 800 and the nickel- and cobalt-base alloys, showed little attack At 1095 °C (2000 °F), type 446 stainless steel suffered severe oxidation Iron-nickel-chromium alloys, such as Incoloy alloy 800H and RA330, also suffered significant oxidation Many nickel-base alloys, however, still exhibited little oxidation At 1150 °C (2100 °F), most alloys suffered unacceptable oxidation, with the exception of only a few nickel-base alloys At 1205 °C (2200 °F), all alloys except Haynes alloy 214 suffered severe attack Alloy 214 showed negligible oxidation at all the test temperatures This alloy is different from all of the other alloys tested in that it forms an aluminum oxide (Al2O3) scale when heated to elevated temperatures Other alloys tested form chromium oxide (Cr2O3) scales when heated to elevated temperatures

The alloy performance rankings (Ref 6) generated from the field in the furnace atmosphere produced by the combustion

of natural gas were found to correspond closely to the air oxidation data presented in Table 2 The alumina-forming alloy

214 was found to be the best performer (Ref 6)

Carburization

Materials problems due to carburization are quite common in heat-treating components associated with carburizing furnaces The environment in the carburizing furnace typically has a carbon activity that is significantly higher than that

in the alloy of the furnace component Therefore, carbon is transferred from the environment to the alloy This results in the carburization of the alloy, and the carburized alloy becomes embrittled

Nickel-base alloys are generally considered to be more resistant to carburization than stainless steels The results of 25-h carburization tests performed at 1095 °C (2000 °F) in a gas mixture consisting of 2% methane (CH4) and 98% hydrogen revealed the weight gain data of 2.78, 5.33, 18.35, and 18.91 mg/cm2 for Inconel alloy 600, Incoloy alloy 800, type 310 stainless steel and type 309 stainless steel, respectively (Ref 7) Extensive carburization tests were recently performed to investigate 22 commercial alloys, including stainless steels, iron-chromium-nickel alloys, nickel-chromium-iron alloys, and nickel- and cobalt-base alloys (Ref 8) Tests were performed for 215 h at 870 and 925 °C (1600 and 1700 °F) and for

55 h at 980 °C (1800 °F) in a gas mixture consisting of 5 vol% hydrogen, 5 vol% CH4, 5 vol% carbon monoxide (CO), and the balance argon The results failed to reveal any correlation between carburization resistance and the alloy base Nevertheless, it was found that the alumina-forming Haynes alloy 214 was the most resistant to carburization among all

of the alloys tested

These findings were confirmed in 24-h tests performed at 1095 °C (2000 °F) in the same gas mixture The carburization data are summarized in Table 3 In this study, alloy 214 (an alumina former) was found to be significantly better than the chromia formers tested Among the chromia formers, however, there is some question regarding the significance of the differences within the carbon absorption range of 9.9 to 14.4 mg/cm2 Perhaps less severe environments are required to separate the capabilities of these alloys Field testing will be an excellent way of determining alloy performance ranking However, few data are available Field tests were recently conducted in a heat-treating furnace used for carburizing, carbonitriding, and neutral hardening operations (Ref 9) Both RA333 and Inconel alloy 601 were found to exhibit better carburization resistance than any of the alloys tested, which included RA330, Incoloy alloy 800, and alloy DS

Table 3 Results of 24-h carburization tests performed at 1095 °C (2000 °F) in Ar-5H2-5CO-5CH4

Alloy Carbon absorption,

mg/cm2 Haynes alloy 214 3.4

Fig 1 Metal dusting of a Multimet alloy component at the refractory interface in a carburizing furnace (a)

Perforation of the component (arrows) (b) Cross section of the sample showing severe pitting (c) Severe carburization beneath the pitted area

Metal dusting has been encountered with straight chromium steels, austenitic stainless steels, and nickel- and cobalt-base alloys All of these alloys are chromia formers; that is, they form Cr2O3 scales when heated to elevated temperatures No work has been reported on the alloy systems that form a much more stable oxide scale, such as Al2O3 The Al2O3 scale was found to be much more resistant to carburization attack than the Cr2O3 scale (Ref 8) Because metal dusting is a form

of carburization, it would appear that alumina formers, such as Haynes alloy 214, would also be more resistant to metal dusting

Sulfidation

Furnace environments can sometimes be contaminated with sulfur Sulfur can come from fuels, fluxes used for specific operations, and cutting oil left on the parts to be heat treated, among other sources Sulfur in the furnace environment could greatly reduce the service lives of components through sulfidation attack

It is well known that base alloys are highly susceptible to catastrophic sulfidation due to the formation of rich sulfides, which melt at about 650 °C (1200 °F) Figure 2 illustrates catastrophic failure of a nickel-chromium-iron alloy tube due to sulfidation attack in a heat-treating furnace The liquid-appearing nickel-rich sulfide phases are clearly visible

nickel-Fig 2 Catastrophic sulfidation of an Inconel 601 furnace tube The furnace atmosphere was contaminated with

sulfur; the component failed after less than 1 month at 925 °C (1700 °F) (a) General view, (b) Cross section of the perforated area showing liquid-appearing nickel-rich sulfides (c) Higher-magnification view of nickel-rich sulfides

The sulfidation of metals and alloys has been the subject of numerous investigations However, the investigations involving commercial alloys examined only a limited number of alloys in each case This frequently does not provide designers or engineers with a sufficient number of alloys to make an informed materials selection A comprehensive sulfidation study was recently undertaken to determine the relative alloy rankings of base alloys (Ref 11) Tests were performed at 760, 870, and 980 °C (1400, 1600, and 1800 °F) for 215 h in a gas mixture consisting of 5% hydrogen, 5%

CO, 1% carbon dioxide (CO2), 0.15% hydrogen sulfide (H2S), 0.1% H2O, and the balance argon The cobalt-base alloys were found to be the best performers, followed by iron-base alloys, and then nickel-base alloys, which, as a group, were generally the worst performers Among iron-base alloys, the iron-nickel-cobalt-chromium alloy 556 was better than iron-nickel-chromium alloys such as Incoloy alloy 800H and type 310 stainless steel The test results of representative alloys from each alloy base group are summarized in Table 4