Volume 13 - Corrosion Part 13 pdf

Zinc, aluminum, and zinc-aluminum alloy hot dip coatings are also used for the corrosion protection of steel in marine environments.. On the other hand, carefully prepared thermal spraye

Fig 26 Pacific Ocean pH at a depth of 500 m (1640 ft) Source: Ref 5

Fig 27 Pacific Ocean pH at a depth of 1000 m (3280 ft) Source: Ref 5

Profiles of pH with depth for the two open ocean locations are shown in Fig 28 A comparison of the corresponding pH and oxygen profiles from Fig 16 and 28 reveals the closely coupled nature of their relationship through the carbon dioxide system, as discussed above The oxygen and pH minima are reached at the same depth for a given location, as was predicted The deep north Pacific water is from 0.15 to 0.40 pH units more acidic than that in the North Atlantic, primarily because of the increased oxidation of organic matter in the North Pacific

Fig 28 Comparison of pH-depth profiles for open ocean sites 2 and 6 (see Fig 5) Note that the data for the

south Pacific are highest at the surface, but are intermediate at depths greater than 500 m (1640 ft) Source: Ref 5

Profiles of pH for the coastal waters off Oregon and New Jersey are shown in Fig 11 and 13, respectively The close correlation between the shapes of the oxygen and pH profiles in both winter and summer for the Oregon data in Fig 11 is particularly striking Upon close examination, the oxygen and pH profiles in Fig 13 do not appear to be closely related in the manner seen earlier In March, the water column is well mixed down to the bottom, and the changes with depth of all four variables are small In August, however, the dissolved oxygen profile is nearly independent of depth, while the pH and temperature profiles show substantial changes Based on salinity and temperature, the oxygen saturation levels during August are about 5.2 mL/L in the surface waters and 6.5 mL/L in the deep water The oxygen profile for August shows that the surface waters are nearly saturated, while in the deep waters, biological activity has used up enough oxygen and produced enough CO2 to decrease the pH but not enough to produce a strong oxygen minimum This indicates the danger inherent in assuming that a pH minimum will always correspond to a similar minimum in oxygen The two profiles may not correspond closely in shape when the biological demand for oxygen is not sufficiently intense to produce

a strong oxygen minimum or when there is a strong temperature gradient

Effects of pH on Corrosion and Calcareous Deposition. The pH of open ocean seawater ranges from about 7.5

to 8.3 Changes within this range have no direct effect on the corrosion of most structural metals and alloys The one exception to this general statement is the effect of pH on aluminum alloys A decrease in pH from the surface water value

of 8.2 to a deep water value of 7.5 to 7.7 causes a marked acceleration in the initiation of both pitting and crevice corrosion This effect accounts for the reported increase in corrosion of aluminum alloys in the deep ocean (see the article

"Corrosion of Aluminum and Aluminum Alloys" in this Volume)

Although variations in seawater pH have little direct effect on corrosion, they do have an indirect effect through their influence on calcareous deposition The surface waters of most of the oceans of the world are 200 to 500% supersaturated with respect to the calcium carbonate species calcite and aragonite This means that precipitation of carbonate-type scales

is likely to be an important part of any corrosion reaction in surface water at most locations The predominant species precipitated in warm surface waters are aragonite and, at interface pH values above 9.3 as experienced in cathodic protection, brucite (Mg(OH)2)

Scale precipitation is most likely to occur in the elevated-pH regime adjacent to cathodically protected surfaces, where

OH- ions are produced during reduction of dissolved oxygen For many years, the corrosion protection industry has relied

on the buildup of calcareous scales to make cathodic protection more economical The higher the pH at the water/metal interface, the more brucite is favored and the lower the calcium-magnesium ratio of the deposit will be A lower calcium-magnesium ratio, in turn, makes the scale less dense and less protective Thus, a high level of cathodic protection applied

in the early stages of immersion, as is sometimes done to accelerate scale buildup, can be counterproductive in terms of scale quality

In deep waters, where the temperature and pH are both lower than at the surface, calcareous deposits do not form spontaneously under ambient conditions, and it has often been difficult to form deposits even under cathodic protection conditions This is partly because the deep waters below 300 m (985 ft) in the Atlantic and 200 m (655 ft) in the North Pacific are undersaturated in carbonates because of low pH and high pressure At the low temperatures of the deep water, calcite is the predominant calcium carbonate phase At first, this would seem to be beneficial because calcite forms a dense, protective film However, calcite formation is strongly inhibited by the free magnesium ions that are abundant in seawater Therefore, only brucite, which is much less protective, tends to form in deep water, and even brucite forms only under cathodic protection conditions when the interface pH is greater than 9.7

In the laboratory, fine-grain, dense, and protective deposits can be formed in cold water with elevated calcium and bicarbonate concentrations and decreased magnesium An economical way to achieve these conditions on a large structure in the real environment does not yet exist

Influence of Biological Organisms

Seawater is a biologically active medium that contains a large number of microscopic and macroscopic organisms Many

of these organisms are commonly observed in association with solid surfaces in seawater, where they form biofouling films Because this subject has been dealt with in detail in the sections on biological corrosion in the articles "General Corrosion" and "Localized Corrosion" in this Volume, only a brief description will be given here

Immersion of any solid surface in seawater initiates a continuous and dynamic process, beginning with adsorption of nonliving, dissolved organic material and continuing through the formation of bacterial and algal slime films and the settlement and growth of various macroscopic plants and animals This process, by which the surfaces of all structural materials immersed in seawater become colonized, adds to the variability of the ocean environment in which corrosion occurs

Bacterial Films. The process of colonization begins immediately upon immersion with the adsorption of a nonliving orgnanic conditioning film This conditioning film is nearly complete within the first 2 h of immersion, at which time the initially colonizing bacteria begin to attach in substantial numbers The bacterial, or primary, slime film develops over a period of 24 to 48 h in most natural seawaters, although further changes in the film can often be observed over more than

a 2-week period Additional information is available in the Selected References at the end of this article

The bacterial film changes the chemistry at the metal/liquid interface in a number of ways that have an important bearing

on corrosion As the biofilm grows, the bacteria in the film produce a number of by-products Among these are organic acids, hydrogen sulfide, and protein-rich polymeric materials commonly called slime The first effect of the composite film of bacteria and associated polymer, an example of which is shown in Fig 30, is to create a diffusion barrier between the metal/liquid interface and the bulk seawater The barrier itself is over 90% water, so it does not truly isolate the interface; instead, it supports strong concentration gradients for various chemical species Thus, the water chemistry at the interface may be different from that in the bulk water, although the two are closely coupled through diffusive processes

Fig 30 Rod-shaped marine bacteria embedded in slime film growing on the surface of a copper-base

antifouling paint after immersion in natural seawater at Woods Hole, MA, for 7 days Depth of immersion was 5

of both copper alloys and steels

Under anaerobic (no oxygen) conditions, such as those found in marshy coastal areas, in which all the dissolved oxygen

in the mud is used in the decay of organic matter, the corrosion rate of steel is expected to be very low Under these

conditions, however, the sulfate-reducing bacteria of the genus desulfovibrio utilize the hydrogen produced at the metal

surface in reducing sulfates from the decaying organic material to sulfides, including H2S The sulfides combine with iron from the steel to produce an iron sulfide (FeS) film, which is itself corrosive The bacteria thus transform a benign environment into an aggressive one in which steel corrodes quite rapidly

Even under open ocean conditions at air saturation, the presence of a bacterial slime film can result in anaerobic conditions at the metal surface Oxygen-utilizing bacteria in the initial film may eventually increase sufficiently in numbers that they use all the oxygen diffusing through the film before it can reach the metal surface This creates an anaerobic layer right next to the metal surface and provides a place where the sulfate-reducing organisms can flourish In all of these examples, the biofilm is able to change the chemistry of the electrolyte substantially at the water/metal interface Thus, the corrosion rate may depend as much on the details of the electrolyte chemistry at the interface as it does on the ambient bulk seawater chemistry Additional details about many aspects of biological corrosion can be found

in the Selected References at the end of this article and in the articles "General Corrosion," "Localized Corrosion," and

"Evaluation of Microbiological Corrosion" in this Volume

Macrofouling Films. Within the first 2 or 3 days of immersion, the solid surface, already having acquired both conditioning and bacterial films, begins to be colonized by the macrofouling organisms A heavy encrustation of these organisms can have a number of undesirable effects on marine structures Both weight and hydrodynamic drag on the structure will be increased by the fouling layer Interference with the functioning of moving parts may also occur

In terms of corrosion, the effects of the macrofouling layer are similar to those of the microfouling layer If the macrofoulers form a continuous layer, they decrease the availability of dissolved oxygen at the metal/water interface and can reduce the corrosion rate If the layer is discontinuous, they may induce oxygen or chemical concentration cells; this leads to various types of localized corrosion Fouling films may also break down protective paint coatings by a combination of chemical and mechanical action Additional information is available in the Selected References at the end

of this article

Marine Atmospheres

Richard B Griffin, Department of Mechanical Engineering, Texas A&M University

The annual cost of corrosion in the United States has been estimated at $167 billion A reasonable fraction of this amount

is the result of atmospheric corrosion The buildings, automobiles, bridges, storage tanks, ships, and other items that must

be coated, repaired, or replaced represent only some of the problem areas of corrosion to the U.S economy

Typically, atmospheric corrosion is broken down into the types listed in Table 9 A variety of factors affect the atmospheric corrosion behavior of materials These include the time of wetness, temperature, material, air contaminants, solar radiation, biological species, and the composition of the corrosion products The particular location of a composed is also important with respect to its corrosion behavior

Table 9 Types of atmospheres and corrosion rates of low-carbon steel

Test duration: 2 years

Corrosion rate

mm/yr mils/yr

Severe 25-m (80-ft) lot, Kure Beach, NC 0.53 21.00

Industrial Brazos River, TX 0.093 3.67

Mild 250-m (800-ft) lot, Kure Beach, NC 0.146 5.73

Rural Esquimalt, BC, Canada 0.013 0.53

Suburban (semi-industrial) Middletown, OH 0.029 1.13

Marine Esquimalt, BC, Canada 0.013 0.53

The marine or marine-industrial type is generally considered to be the most aggressive environment This discussion of marine atmospheric corrosion will include atmospheric corrosion (zone 1, Fig 2) and the splash zone above high tide (zone 2, Fig 2) For carbon steels in marine exposure, the maximum corrosion rate occurs in the splash zone, in which the alloy is wet almost continually with well-aerated seawater The atmospheric corrosion of low-carbon steel is in the range

of 0.025 to 0.75 mm/yr (1 to 30 mils/yr) This section will discuss the specific details associated with the rates of corrosion in marine atmospheres

Important Variables

A number of factors, such as moisture, temperature, winds, airborne contaminants, alloy content, location, and biological organisms, contribute to atmospheric corrosion Each of these factors will be discussed with regard to its contribution to corrosion in the marine atmosphere

Moisture. For corrosion to occur by an electrochemical process, there must be an electrolyte present An electrolyte is a solution that will allow a current to pass through it by the diffusion of anions (negatively charged ions) and cations (positively charged ions) Water that contains ions is a very good electrolyte Therefore, the amount and availability of moisture present is an important factor in atmospheric corrosion For steel beyond a certain critical humidity, there will be

an acceleration in the rate of corrosion in the atmosphere An example of this is shown in Fig 31, in which the critical humidity is 60% for iron in an atmosphere free of sulfur dioxide (Fig 31a) For magnesium under similar conditions, the critical relative humidity is 90% (Fig 31b) The critical relative humidity is not a constant value; it depends on the hygroscopicity (tendency to absorb moisture) of the corrosion products and the contaminants

Fig 31 Corrosion rates of iron and magnesium as a function of relative humidity (a) For iron, the critical

relative humidity is 60% (b) For magnesium, corrosion rate increases significantly at a critical relative humidity

of about 90% Source: Ref 10

One of the measures of the effects of moisture is the time of wetness As Fig 32 shows, corrosion rate increases as time

of wetness increases In addition Fig 32 shows the importance of a contaminant When the sulfur dioxide level increases, there is a corresponding increase in the overall corrosion rate However, the severity of the marine environment is related

to the salt content of the sea spray or dew that contacts the material surface, which is usually more corrosive than rainfall

Fig 32 The increase in corrosion rate of zinc as a function of time of wetness and SO2 concentration Source: Ref 10

For acid rain conditions, there appears to be no significant increase in corrosion rate A study conducted in Sweden from October 1974 to November 1976 for carbon steel showed an increase in corrosion rates with increasing sulfur dioxide; however, the incidences were relatively infrequent The study also showed that the corrosion rates measured for a longer time do not seem to be influenced by the incidences of acid rain Similar results were obtained in a British study on the atmospheric-corrosion rate of zinc

Airborne Contaminants. The second most important factor in atmospheric corrosion is the contaminants found in the air These can be manmade or natural, such as airborne moisture carrying salt from the sea or sulfur dioxide put into the atmosphere by a coal-burning utility plant Figure 32 illustrates the importance of the atmospheric sulfur dioxide level on the corrosion rate of zinc The important contaminants are chlorides, sulfur dioxide, carbon dioxide, nitrogen oxides, and hard dust particles (for example, sand or minerals)

Chlorides. There is a direct relationship between atmospheric salt content and measured corrosion rates The amount of sea salts measured off the coast of Nigeria illustrate this relationship between the salinity and the corrosion rate This is shown in Fig 33, in which salinity of 10 mg/m2/d results in a corrosion rate of less than 0.1 g/dm2/mo, while a salinity of

1000 mg/m2/d results in a corrosion rate of almost 10 g/dm2/mo At the LaQue Center for Corrosion Technology test site

at Kure Beach, NC, a similar effect has been observed for carbon steel The corrosion rate at the site 25 m (80 ft) from the mean tide line was 1.19 mm/yr (47 mils/yr), while at the 250-m (800-ft) site, the corrosion rate for the same material was 0.04 mm/yr (1.6 mils/yr)

Fig 33 Atmospheric corrosion as a function of salinity at various sites in Nigeria Source: Ref 11

The average atmospheric chloride levels, as collected in rainwater for the United States, are shown in Fig 34 The highest levels occur along the coast of the Atlantic Ocean, Pacific Ocean, the Gulf of Mexico The maximum corrosion rate is related to the maximum chloride in the atmosphere This will of course be related to the distance inland, the height above sea level, and the prevailing winds The chlorides of calcium and magnesium are hygroscopic and have a tendency to form liquid films on metal surfaces

Fig 34 Average chloride concentration (mg/L) in rainwater in the U.S Source: Ref 10

Sulfur Dioxide. The presence of SO2 in the atmosphere lowers the critical relative humidity while increasing the thickness of the electrolyte film and increasing the aggressiveness of the environment For carbon steel, the effect of SO2levels is shown in Fig 35, in which data are plotted from three Norwegian test sites The data show that as SO2concentrations are increased the corrosion rate, measured as weight loss, increases For example, at an SO2 concentration

of 25 g/m3, the corrosion rate is approximately 55 g/m2/mo, while for an SO2 concentration of 100 g/m3 the corresponding corrosion rate is approximately 170 g/m2/mo A summary of Scandinavian data for carbon steel and zinc showed the following relationships between corrosion rate and SO2 concentration:

and

where is the atmospheric corrosion rate in g/m2/yr, and [SO2] represents the concentration of SO2 in g/m3 Similar types of relationships have been shown for other alloy systems and locations

Fig 35 Effect of SO2 concentration on the corrosion rate of carbon steel at three Norwegian sites Source: Ref

10

Carbon Dioxide. The opinion of the majority of investigators is that carbon dioxide (CO2) has an effect on the corrosion of metals Carbon dioxide in the presence of water forms carbonic acid (Eq 4); a pH of 5.6 could be obtained with atmospheric CO2 in equilibrium with pure water Carbonates are found in the corrosion products on a number of metals It is possible that for steel and copper the presence of CO2 might lessen the corrosion effects of SO2, because of the nature of the corrosion products formed Carbon dioxide does not have nearly the same level of importance in atmospheric corrosion as SO2 and chlorides do

Location is a very important variable The distance from the sea and the height above the ground are both significant

Distance. Figure 33 shows the effect of moving inland along the coast of Nigeria from the 45-, 365-, and 1190-m (50-, 400-, and 1300-yd) sites at Lagos From studies done on Barbados, the effect of distance is confirmed by the map of the island shown in Fig 36 As can be seen, this represents one of the worst conditions: tropical beach, on-shore winds, and facing a large, uninterrupted stretch of ocean Similarly, at a site in Aracaju, Brazil, low-carbon steel samples were tested

at five sites from about 0.1 km (0.06 miles) to almost 4 km (2.5 miles) from the sea There was, as Fig 37 shows, a rapid falloff in the corrosion rate as the testing site was moved inland By about 1.5 km (0.9 miles) inland, the corrosion rate had reached a value that shows it is basically independent of the marine atmosphere

Fig 36 Estimates of marine atmosphere corrosivity at various locations on the island of Barbados in the West

Indies Based on CLIMAT data Source: Ref 10

Fig 37 Corrosion rate of carbon steel as a function of distance from the sea at Aracaju, Brazil Source: Ref 10

The height of the specimens above seawater is also important This is illustrated in Fig 38(a) in which the corrosion rate of the carbon steel specimens in the 25-m (80-ft) lot at Kure Beach, NC, varies from less than 400 m/yr (16 mils/yr)

at a height of 5 m (16.5 ft) to 600 m/yr (24 mils/yr) at a height of about 8 m (26 ft) The corrosion rate is in direct proportion to the amount of chloride in the atmosphere There is considerably less corrosion for the carbon steel at the Kure Beach, NC, 250-m (800-ft) test site (Fig 38b) The 25-m (80-ft) lot has an average chloride content of approximately 400 mg/m2/d, while the 250-m (800-ft) lot has an average chloride value of approximately 100 mg/m2/d

Fig 38 Effect of elevation above sea level for carbon and HSLA steels at Kure Beach, NC (a) 25-m (80-ft) lot;

(b) 250-m (800-ft) lot Source: Ref 12

In the splash zone, the effect of height above the sea is illustrated in Fig 2, which shows that the corrosion rate is highest slightly above mean high tide This zone would not only have a high chloride content, but would also be alternately wet and dry As the height above the sea increases, the corrosion rate decreases because the specimen is not wet as often

Orientation. Another important factor with regard to the atmospheric corrosion of a material is its orientation with respect to the earth's surface Results for a 1-year exposure of iron specimens placed vertically and inclined at an angle of 30° with respect to the ground are shown in Fig 39 The spread in the data is much greater for the Kure Beach 25-m (80-ft) test lot than for the 250-m (800-ft) test lot In both cases, the vertical specimens showed a higher corrosion rate This was attributed to the formation of a nonuniform, less protective oxide in the vertical position than in the 30° position It is also possible that the 30° samples have the chloride deposits cleaned from their surfaces more easily than the vertical specimens Ratios of the corrosion rate in the vertical position to that in the 30° position are given in Table 10 for five sites In the vertical position, the corrosion rate is greater on the side facing the sea than on the side facing land At the 25-

m (80-ft) lot at Kure Beach, steel pipe specimens corroded at the rate of 850 m/yr (33.5 mils/yr) facing the ocean, as compared to 50 m/yr (2 mils/yr) facing away from the ocean, over a 4.5 year period

Table 10 Comparison of atmospheric-corrosion rates for specimens held vertically and inclined at 30° to the horizontal

vertical/30°

Vandergrift, PA 1.26

25-m (80-ft) lot, Kure Beach, NC 1.41

250-m (800-ft) lot, Kure Beach, NC 1.25

Source: Ref 12

Fig 39 Effect of specimen orientation on corrosion rates of iron specimens exposed vertically and at an angle

of 30° to the horizontal Results of 1-year test at Kure Beach, NC Source: Ref 12

The corrosion rate can also be measured on the skyward or groundward side of specimens that are parallel to the earth's surface Tests conducted at Kure Beach showed that the skyward side corroded at a greater rate after 3 months However, after 6 months of testing, the rates were identical Similarly, for an AZ31 B magnesium alloy in a 30-day test, the skyward-facing specimens lost more material than the groundward-facing ones

The temperature affects the relative humidity, the dew point, the time of wetness, and the kinetics of the corrosion process For atmospheric corrosion, the presence of moisture, as determined by the time of wetness, is probably the most important role of temperature Figure 40 illustrates the effect of temperature on iron, zinc, and copper There are three distinct patterns with increasing temperature over the range of 20 to 40 °C (70 to 100 °F):

• Corrosion rate increases for iron

• Corrosion rate decreases for zinc

• Corrosion remains constant for copper

Fig 40 The effect of temperature on the corrosion rates of iron, zinc, and copper Source: Ref 10

The temperature of interest may not be the average daily temperature It may be more important to know the dew-point temperature or the test panel surface temperature From an atmospheric corrosion standpoint, dry, hot conditions are preferable to cooler, moist conditions

Sunlight influences the degree of wetness and affects the performance of coatings and plastics Sunlight may also stimulate photosensitive corrosion reactions on such metals as copper and iron In addition, it may stimulate biological reactions, such as the development of fungi Ultraviolet (UV) light and photo-oxidation can cause embrittlement and surface cracks in polymers This can be avoided by the addition of UV stabilizers (for example, carbon black)

Wind. The direction and velocity of the wind affect the rate of accumulation of particles on metal surfaces Also, wind disperses the airborne contaminants and pollutants Figure 36 illustrates the very severe corrosion that can occur on an ocean beach facing the prevailing wind The effect caused by the chloride ions being carried inland is illustrated in Fig

37, which shows an increased corrosion rate at 1 km (0.6 mile) inland Stronger prevailing winds can carry the airborne contaminants even further inland A marine site may be made even more aggressive by the prevailing winds bringing industrial pollutants, particularly SO2, to the marine site

Time. For many materials, there is a decrease in the corrosion rate as time increases This decrease is associated with the formation of protective corrosion layers Figure 41 provides an example of this for a low-carbon steel at eight sites in South Africa There is an initial sharp increase in the atmospheric-corrosion rate, followed by a slowing down of the corrosion rate as corrosion products form on the alloy surface This is particularly true for the sites C through G For site

B, the corrosion rate is sufficiently high to prevent the formation of a protective layer; therefore, a very high corrosion rate can be maintained The effect of corrosion on tensile strength is shown in Fig 42 for a low-carbon steel and aluminum The rate of loss in ultimate tensile strength is initially large, but as time continues, the rate of loss decreases

Fig 41 Change in corrosion rate as a function of time for eight South African sites Source: Ref 10

Fig 42 Loss in tensile strength as a function of time for (a) 1.6 mm ( -in.) low-carbon steel and (b) aluminum alloys of the same thickness at five test sites Data in (b) are averages for aluminum alloys 1100,

3003, and 3004 Source: Ref 10

Starting Date. There can be a variation in the corrosion rate that depends on when the tests were started Figure 43 compares the measured weight losses for iron and zinc in tests started at two different dates Over a 60-day test, the variation in corrosion rate for zinc is much larger than that for iron Similarly, for iron specimens at the Kure Beach, 25-m (80-ft) lot, there are variations of hundreds of microns per year in corrosion rates, as measured on samples exposed vertically for 1 and 2 years each This is shown in Fig 44 for iron calibration specimens tested from 1949 to 1979

Fig 43 Effect of different starting dates on the corrosion rate of iron (a) and zinc (b) Source: Ref 10

Fig 44 Corrosion of iron calibration specimens tested for 1 year (a) and 2 years (b) at the 24.4-m (80-ft) lot at

Kure Beach, NC Source: Ref 12

Site Variability. Large variations in atmospheric corrosion rate occur within a particular type of region An example would be the various corrosion behaviors of steel and zinc in different tropical environments, especially in a tropical seacoast for 1 year Figure 45 shows the average penetration for steel in a 1-year test at various tropical sites For zinc under similar conditions, the average penetration varied from 3 to 15 m (0.11 to 0.6 mil) Similarly, Fig 42 illustrates the loss of tensile strength with increasing time in different temperate marine environments As Fig 42 shows, there is a wide variation in the loss of tensile strength between the four seacoast locations

Fig 45 Variation in corrosion rate after 1-year exposure of steel at four different tropical sites Data are

averaged from various investigations; see Ref 10 for details Source: Ref 10

Temperate and tropical marine sites, along with inland sites, are compared in Fig 46 for copper and zinc Overall, the long-term rates are similar at both marine and inland sites

Fig 46 Comparison of corrosion rates for zinc (a and b) and copper (c and d) at tropical and temperature

exposure sites Numbers on curves are stabilized corrosion rates in microns per year Source: Ref 10

However, a similar comparison for carbon steels and low-alloy steels (Fig 47) illustrates that for these materials the tropical environment has a higher overall corrosion rate Figure 47(a) compares the stabilized corrosion rate of carbon steel at Cristobal, Panama (20 m/yr, or 0.8 mil/yr), to that at Kure Beach 250 m (800 ft) from the ocean (16 m/yr, or 0.63 mil/yr) Low-alloy steels exhibit a similar increased rate of corrosion, as shown in Fig 47(b) and a similar pattern is exhibited for carbon steels compared at inland sites (Fig 47c)

Fig 47 Comparison of corrosion rates of steels at temperate and tropical exposure sites Numbers on curve are

stabilized corrosion rates in microns per year (a) Carbon steel, marine exposure (b) Low-alloy steel, marine exposure (c) Carbon steel, inland exposure Source: Ref 10

Alloy Content. The selection of a particular alloy composition can make a significant difference in the corrosion rate of

a material For steels, a comparison can be made for carbon steels, low-alloy steels, and steels with 5% alloying elements (Fig 48) Figure 48 also compares marine versus inland exposure, and in each case, the long-term corrosion rate is greater for the marine environment Figure 48 also shows the more rapid corrosion that takes place in the first 1 to 3 years and the leveling off associated with long-term atmospheric corrosion Very similar results have been reported for a study done in South Africa at eight sites that were classified as rural to severe marine

Fig 48 Comparison of marine (a) and inland (b) corrosion rates for carbon steel, low-alloy steels, and 5% alloy

steels at the Naval Research Laboratory test sites in Panama Numbers on curves are stabilized corrosion rates

in microns per year Source: Ref 10

The results of 15.5-year studies of low-alloy steels in 13 groups conducted at the Kure Beach, NC, 250-m (800-ft) lot are shown in Fig 49, in which the mass loss per unit area is plotted as a function of the total alloy content Alloy additions of about 2 wt% result in the mass loss per area being reduced from 40 mg/dm2 to less than 12 mg/dm2

Fig 49 Corrosion data for 25 low-alloy steels tested over a 15.5-year period at the Kure Beach, NC, 250-m

(800-ft) lot Source: Ref 11

The significance of chromium as an alloying element is shown in Fig 50 for atmospheric-corrosion conditions classified

as moderate and severe marine Above 12 or 12.5 wt% Cr, the atmospheric corrosion becomes negligible; lower chromium levels result in a rapid increase in the corrosion rate This effect is also illustrated in Table 11 For AISI 300-series stainless steels tested at the Kure Beach, NC, 250-m (800-ft) lot over a 15-year period, the atmospheric-corrosion rates were equal to or less than 0.03 m/yr (0.001 mil/yr) (see Table 11)

Table 11 Average corrosion rate and pit depth for ten austenitic stainless steels

Results of a 15-yr test at the Kure Beach, NC, 250-m (800-ft) lot

Fig 50 Effect of chromium additions on the atmospheric corrosion of steels Source: Ref 10

An excellent summary of marine atmospheric-corrosion data is given in Table 12 A wide variety of metals and alloys are listed In addition, information on general corrosion, pitting, and loss of tensile strength is given for materials exposed at a seacoast site at Cristobal, Panama

Table 12 Corrosion data for noncoupled metal panels exposed at the US Naval Research Laboratory tropical seacoast site at Cristobal, Panama

pits (d), m (mils)

Loss in tensile strength (e), % Metal or alloy Surface(a)

1 year 2 years 4 years 8 years 16 years

Final corrosion rate(c), m

(mils) 8 years 16 years

Deepest pit, m

Type 430

(0.02) (0.04) (0.04) (0.04) (0.08) (0.01) (4.9) (4.9) (4.9) 0.3 <0.3 <0.3 0.3 0.5 0.3 <1125 <125 <125 <1 <1

Monel 400

(0.04) (0.04) (0.07) (0.1) (0.22) (0.01) (4.9) (4.9) (4.9) 0.2 0.5 0.8 1.5 5.0 <0.3 <125 <125 <125 <1 <1

Nickel (99%)

(0.008) (0.02) (0.03) (0.05) (0.2) (0.01) (4.9) (4.9) (4.9) 1.5 3.4 6.3 11 20 1.3 <125 <125 <125 <1 <1

(a) All specimens were degreased before exposure; any treatment prior to degreasing is listed

(b) Average penetration over a 4.23-dm2 (65.6-in.2) exposed area; calculations based on weight loss and density

(c) Rate after time-corrosion relation had stabilized; slope of the linear portion of the curve, usually after two to eight years

(d) Averages obtained by measuring the five deepest measurable (>125 m, or 5 mils) penetrations on each surface of duplicate panels (e)

Percent loss in ultimate tensile strength for 1.59-mm ( -in.) thick metal

Exposure Time. One of the difficulties with marine atmospheric corrosion testing is the length of time required for the tests For steels, a reasonable estimate of long-term corrosion performance can be made from short-term data This is not always the case; any short-term result must be used very cautiously, and it is always best to have long-term test data available

Atmospheric Corrosion Test Sites

There are a large number of atmospheric corrosion sites throughout the world; Table 13 lists some of them Where available, the 2-year corrosion rates for low-carbon steel and zinc are given Some of the sites have a marine corrosion index and/or an atmospheric corrosion index number after them The higher the index number, the more aggressive the environment

Table 13 Some marine atmospheric-corrosion test sites around the world

Corrosion rates of steel and zinc are also listed for some sites

Corrosion rate from 2-year test

Distance from

sea

Corrosivity

Point Reyes, CA Marine 0.400 0.25 11 0.183 0.50 19.71 0.0015 0.060

Brazos River, TX Industrial

Havre de Grace, MD Marine

Key West, FL Subtropical

La Jolla, CA Marine

Miami, FL Marine 4 2.5 5.9 0.04

Ormond Beach, FL Marine

Point Judith, RI Marine

Portsmouth, VA Marine

Sandy Hook, NJ Marine

Battelle, Sequin, WA Marine 0.030 0.018 6.9 0.07

Hickham AFB, HI Marine 0.150 0.09 8.7 1.4

Panama

Fort Amidor Marine 0.014 0.57 0.0011 0.045

Miraflores Marine 0.043 1.69 0.0026 0.104

Limon Bay Marine 0.062 2.45 0.0026 0.104

Galeta Point Marine 0.69 27.14 0.015 0.607

Canada

Esquimalt, Vancouver Island, BC Rural marine 0.013 0.53 0.0005 0.019

Cape Beale, NC Marine 0.025 0.015 12.4 0.20

Chebucto Head, NS Marine 0.100 0.06 13.0 1.2

Estevan Point, BC Marine 0.400 0.25 8.4 0.02

Daniels Harbor, NF Marine 0.150 0.09 17.5 0.11

Sable Island, NS Marine 13.9 0.99

St Vincents, NF Marine 0.150 0.09 14.7 0.18

Deadmans Bay, NF Marine 0.030 0.018 11.9 0.12

Durban, Salisbury Island Marine 0.010 0.006 64.0 5.7 0.056 2.20 0.015 0.607

Dyeban Bluff Severe marine 0.26 10.22 0.0032 0.126

Cape Town docks Mild marine 0.047 1.84 0.0032 0.126

Walvis Bay military camp Severe marine 0.11 4.33 0.063 2.483

Barranquilla Marine 0.010 0.006 12.6 3.3

Cartagena Marine 0.010 0.006 16.3 3.1

Galera Zamba Marine 0.190 0.12 48.0 8.8

Santa Marta Marine 0.060 0.037 1.9 0.06

(a) MCI, Marine Corrosivity Index; determined by the weight loss of an aluminum wire-mild steel bolt couple

ACI, Atmospheric Corrosivity Index; determined by the weight loss of an aluminum open helical coil specimen or an aluminum wire-plastic bolt specimen

Metallic Coatings

Jean A Montemarano and Barbara A Shaw, David Taylor Naval Ship R & D Center

Effective protection from the marine environment can be provided by metallic coatings, which include thermal spray, galvanizing, and, for certain applications, electroplating In general, metallic coatings are two to three times more expensive than their traditional organic counterparts; therefore, their use is usually justified by longer service life and reduced maintenance Metallic coatings function by providing a barrier, similar to organic coatings, for the bare metal from the marine environment and by corroding preferentially with respect to the substrate (normally, steel) when the coating is scratched or nicked Aluminum and zinc are more active in seawater than steel and are the metals most widely used as protective coatings Aluminum and zinc can be thermal sprayed Zinc, aluminum, and zinc-aluminum alloy hot dip coatings are also used for the corrosion protection of steel in marine environments Only the hot-dip zinc process, better known as galvanizing, is commercially available for the coating of fabricated articles Lastly, the electroplating process is commonly used for the zinc or cadmium plating of fasteners for marine applications

Metal Spray

Thermal spray has been used by industry and in European countries for corrosion protection since the 1940s Bridges, structural steel work, boat holds and tanks, sluice gates and canal lock gates, and offshore drilling rigs are some of the items that have been metallized (Ref 13) Thermal spray for corrosion protection is normally applied by either the wire flame (combustion) or wire arc process In either case, the metal wire is fed into a gun and melted either by a flame (normally oxyacetylene) or an electric arc The atomized particles are propelled by means of compressed air onto the surface, where they cool, forming layers of splat-quenched particles (Ref 14) Both methods are portable and can be easily automated Figures 2 and 4 in the article "Thermal Spray Coatings" in this Volume illustrate the wire flame and wire arc processes

Surface preparation, as with all coatings, is an essential part of the coating process Care must be taken to ensure that the surface is properly prepared and cleaned Grit-blasting to white metal is used to achieve the necessary surface roughness Typical grit media include aluminum oxide and chilled iron The coating is largely mechanically bonded to the substrate

in the flame or arc spray process used for corrosion control (Ref 15) If a component cannot be cleaned such that all rust and oil are removed, the thermal spray coating will not remain attached for long

For example, field tests were conducted in Norway on steel piles coated with aluminum thermal spray followed by a wash primer, a coal tar vinyl paint, and then a topcoat After 1 year or less, in spite of the organic coatings, blisters appeared in the coatings on all the piles in the splash zone The failure analysis indicated that the major contributing factor was inadequate adhesion between the steel and aluminum thermal spray coating due to poor surface preparation (Ref 16) On the other hand, carefully prepared thermal sprayed steel specimens gave 19 years of complete base metal corrosion protection in seawater and marine atmosphere tests performed by the American Welding Society (AWS) (Ref 17)

The metal coating is normally applied to a thickness of 75 to 180 m (3 to 7 mils) to provide adequate corrosion protection The thickness of the coating is selected to limit interconnected porosity (too thin a coating) and to minimize

thermal expansion mismatch (too thick a coating) with the substrate, which could result in bond-line separation (Ref 18) For marine applications, thermal sprayed aluminum coatings 180 to 250 m (7 to 10 mils) thick are used in order to limit through porosity (Ref 19)

Zinc is widely used by industry and in European countries for thermal spray for corrosion control protection In addition,

a duplex coating of aluminum followed by zinc has also been used in Europe, although aluminum thermal spray coatings are being stressed for marine applications in the United States (Ref 20) The zinc thermal spray coating has a high electrochemical activity and therefore high corrosion rates; this results in depletion of the coating, although it affords excellent cathodic protection to steel On the other hand, the aluminum coating forms a passive film of aluminum oxide, resulting in very low corrosion rates (Ref 21) In an attempt to combine the best features of both materials, zinc-aluminum coatings (85Zn-15Al) were examined for their marine corrosion protection This combination of metals can be obtained either in a prealloyed wire or by spraying the two wires simultaneously in the dual wire arc process Both coatings provided adequate cathodic protection to the substrate, but suffered severe coating degradation after just 6 months of exposure (Ref 14)

Sealing. Standard practice is to seal the thermal spray coating with low-viscosity sealers because the metal coating is inherently porous (Fig 51) The sealer is normally sprayed or brushed on, and it penetrates and fills the pores Sealers should also be used in acidic or alkali environments (Ref 18) Vinyls and thinned epoxies are typical sealers For high-temperature applications, a silicone alkyd sealer is used (Ref 22) Topcoats are normally used for cosmetic reasons The roughness of the thermal spray coatings offers an excellent surface for paint adhesion Rusting of the substrate underneath the paint film, which is the common mode of paint failure, is not the same mode of failure for thermal spray coatings (Ref 18) The topcoated thermal spray system provides excellent long-term maintenance-free corrosion protection (10 to 15 years)

Fig 51 Schematics of metal sprayed coating on steel (a) As-sprayed coating (b) Sealed coating

For ship applications, thermal spray coatings offer corrosion protection for topside weather equipment, machinery spaces, and interior wet spaces (Ref 18) Specifically, these categories include auxiliary exhaust stacks; diesel headers; steam valves, piping, and traps; boiler skirts; stanchions, pipe hangers; rigging fittings; lighting fixtures; ladders; hatches and scuttles; boat davit machinery components; bilges; and machinery foundations Figure 52 shows two topside applications For marine atmospheric service, the use of thermal spray aluminum coatings is an outstanding method of corrosion control However, use of thermal spray coatings for immersion service lacks extensive field experience Underwater hulls

of small boats have been thermal sprayed, followed by primer and antifouling topcoats More recently, thermal spray coatings, sealed with a silicone sealer, were applied to the flare boom of a North Sea oil rig platform (Ref 23) In addition, tension-leg elements, the risers, and the flare tower of a North Sea tension-leg rig platform were coated using the same materials (Ref 23)

Fig 52 Examples of aluminum flame sprayed topside weather equipment (a) Stanchion (b) Swivel arm

assembly

Comparative corrosion tests support the long-term performance of thermal spray in marine environments The longest test (19 years), which was conducted by AWS, indicates that 75 to 150 m (3 to 6 mils) of aluminum thermal spray coating, whether sealed or unsealed, provided long-term protection in total seawater immersion, splash and spray zone, and marine atmosphere tests Although no significant base metal attack occurred, some blistering and rust staining of the aluminum coating on unsealed panels were noted (Ref 17) In this study, zinc thermal spray coatings gave equivalent performance in marine atmosphere; but for total seawater immersion and splash and spray zone tests, a minimum thickness of 300 m (12 mils) was required to prevent base metal attack for unsealed panels, and a minimum of 230 m (9 mils) was required for sealed panels However, for the latter two marine environments, the unsealed zinc had completely converted to corrosion products (Ref 17)

Sealed aluminum thermal spray coatings again performed well in a 7-year study conducted by the National Bureau of Standards to ascertain what coating provided the best corrosion protection for steel piles Sand abrasion, seawater immersion, splash and spray, and marine atmosphere test zones of the coated piles were evaluated for coating performance Hot-dip zinc, sealed zinc thermal spray coatings, and unsealed aluminum thermal spray coatings performed equivalently and had at least two to three times the corrosion rate of sealed aluminum thermal spray coating (Ref 24)

A study of coated 3-m (10-ft) long panels designed to simulate coated piles also demonstrated the effectiveness of thermal spray coatings After 20 years of field exposure, unpainted aluminum thermal spray coatings and painted zinc thermal spray coatings showed excellent corrosion protection (Ref 25) After 5 years, pipe columns, pumps, and oil flow lines were internally and externally protected against the marine atmosphere in offshore oil facilities by 200 m (8 mils) of aluminum thermal spray, followed by a wash primer and two coats of aluminum sealer (Ref 26) Tensile steel links of a suspension bridge, which were coated with zinc flame spray, primed with red lead, and topcoated, showed no rusting after

44 years of marine exposure (Ref 27) After 1 years of marine atmosphere and splash and spray exposure in Norway,

flamed-sprayed zinc and arc-sprayed and flamed-sprayed aluminum steel panels showed good corrosion performance (Ref 28)

Laboratory tests, such as salt spray (fog), as well as electrochemical techniques, demonstrate the excellent corrosion performance of thermal spray coatings For example, after 1600 h of exposure in tests, aluminum flame-sprayed panels showed no rust, although zinc flame-sprayed panels showed rust after 500 h of exposure (Ref 21) Other investigations indicated that zinc thermal spray panels rusted after 100 h of exposure (Ref 29) The results from the salt spray test are difficult to correlate with actual service performance; therefore, it is not a good accelerated test method for prediction (Ref 30, 31) Electrochemical methods have also been used to evaluate the corrosion behavior of thermal spray coatings (Ref 14, 31, 32, 33, 34) For example, in one study, corrosion potential monitoring tests and potentiodynamic polarization measurements were conducted The results from the latter test indicated that the aluminum flame spray coatings provide corrosion protection by passivation of the coating but that zinc and 85Zn-15Al alloy flame spray coatings operate solely through a galvanic mechanism (Ref 14) Another study, based on potential versus time curves, indicated that aluminum became electrochemically active after spraying (Ref 33) Duplicate studies by other researchers did not find this activation phenomenon (Ref 35) Other applications and materials for thermal spray coatings are discussed in the article "Thermal Spray Coatings" in this Volume

Hot Dip Coatings

Galvanizing has been extensively used for protection against the marine environment The advantages associated with applying zinc by flame spray versus galvanizing makes the former method attractive for certain applications (Ref 36) Hot dip galvanizing produces a fully dense coating that is metallurgically bonded to the substrate In galvanizing, the size of part, heat distortion, ease of application, and the thickness and uniformity of coating are factors that must be considered The thickness of galvanized coatings can vary from 75 to 200 m (3 to 8 mils) and should be selected depending on the environment to be experienced and the desired lifetime A 43- m (1.7-mil) coating thickness is projected to give approximately 10 to 15 years of protection to steel in temperate to tropical marine atmospheric environments, but in service, this life may not be realized Thick galvanizing and thermal spray were the only protection methods recommended by the British Standards Institution for providing long-term corrosion protection in a polluted marine atmosphere (Ref 37)

The American Society for Testing and Materials (ASTM) exposed galvanized sheet specimens in two marine environments Sandy Hook, NJ, and Key West, FL in 1926 and reported that panels with a coating weight of 760 g/m2(2.5 oz/ft2) of zinc first showed rust after 13.1 and 19.8 years of exposure, respectively (Ref 38) An extensive study on the atmospheric corrosion of galvanized steel at the 244-m (800-ft) lot at Kure Beach, NC, resulted in predicted weight losses after 10 years of 103 g/m2 and 55 g/m2 for skyward and groundward marine exposures, respectively (Ref 39) Most investigators agree that the life of a zinc coating is roughly proportional to its thickness in any particular environment and

is independent of the method of application Galvanizing is used for corrosion protection on cables of suspension bridges

in Norway (Ref 16) One study indicated that galvanized steel panels were in good condition ater 1 years (Ref 28) dip aluminized coated steel panels, which also were exposed, showed rusting after this time period

Hot-Hot dip aluminum coatings, or aluminized coatings, are also used for the corrosion protection of steel in marine environments Hot dip aluminizing of fabricated articles is no longer carried out in the United States or in Europe on a commercial basis (Ref 40) The coatings in use today are produced by a continuous strip process An extensive comparative study was conducted on the atmospheric corrosion behavior of aluminized and galvanized steels (Ref 41, 42) Table 14 shows predicted 10-year weight losses of both of these coatings based on exposures conducted in the 250-m (800-ft) lot at Kure Beach, NC A further comparison of the atmospheric-corrosion behavior of aluminized and galvanized panels was conducted by ASTM After 20 years of marine atmospheric exposure (250-m, or 800-ft, lot, Kure Beach, NC), many of the galvanized steel panels were showing rust, but consistently good results were reported for the aluminized coating, which showed only minor pinholes of rust (Ref 43) Since 1972,a commercially produced aluminum-zinc (55Al-1.5Si-43.5Zn) hot dip coating has also been available for the corrosion protection of steel One study reported that after 11 years of severe marine exposure (25-m, or 80-ft, lot, Kure Beach, NC) the 55Al-Zn coated panels were in good condition, with some corrosion products starting to creep inward on the faces of the panels from the cut edges (Ref 44) The advantages of hot dip aluminum coatings are discussed in Ref 45 The corrosion behavior of aluminum coatings obtained from aluminizing baths of various compositions was studied in laboratory tests Aluminized coatings containing manganese were suggested as possible candidates for corrosion protection for coastal structures and deep sea oil rigs More information on hot dip galvanized and aluminized coatings is available in the article "Hot Dip Coatings" in this Volume

Table 14 Predicted 10-year corrosion rates for galvanized and aluminized steel panels

Tested 250 m (800 ft) from the ocean at Kure Beach, NC

Predicted weight loss, g/m2

Coating

Skyward exposure

Groundward exposure

Type 1 aluminized (Al-Si) 17.8 20.1

Type 2 aluminized (pure aluminum) 11.6 17.9

Source: Ref 43

Electroplating

Electroplated zinc or cadmium is the standard coating used to provide corrosion protection to steel fasteners in the marine environment The cadmium coating is used because of its hardness, close dimensional tolerance, and barrier to hydrogen permeation into or out of steels (Ref 46) The disadvantages of cadmium plating are its short life (for example, 4 months)

in the marine atmospheric environment and concerns about occupational health due to the toxicity in the plating process Zinc plating also has a short service life Alternatives for this application include ion vapor deposited aluminum and paints containing zinc or aluminum pigment in a ceramic binder These coatings, including zinc with a potassium silicate binder and aluminum with a phosphate-chromate binder, exhibit excellent corrosion protection for fasteners (minimum 1 year marine protection) (Ref 47) They are normally applied by conventional hand spraying

Methods for electroplating aluminum are still in development, although plating using an organic aprotic solvent is a promising process (Ref 48) In laboratory polarization and galvanic tests, ion-deposited aluminum coatings performed well, indicating their potential for use on aircraft fasteners (Ref 49) Zinc and aluminum coatings for aircraft fastener applications showed variable results in corrosion tests (Ref 50) Aluminum and zinc pigmented performed better than electroplated zinc, ion vapor deposited aluminum, and electroplated cadmium on steel fasteners in laboratory seawater immersion tests (Ref 46) However, hydrogen permeability through the coating, as well as the corrosion performance of the coating, must be considered for a given fastener application (Ref 49)

Organic Coatings

J.S Smart III, Amoco Production Company; R Heidersbach, California Polytechnic State University

Organic coatings are the principal means of corrosion control for the hulls and topsides of ships and for the splash zones

on permanent offshore structures Most stationary offshore oil industry platforms are not painted below the waterline, and most marine pipelines are factory coated with special proprietary coatings (see the articles "Corrosion in Petroleum Production Operations" and "Corrosion of Pipelines" in this Volume)

Figure 53 shows the marine environments that are destructive to shipboard coatings Similar environments are found on offshore oil production platforms (Fig 54), lighthouses, docks, and other marine structures

Fig 53 Environments that are destructive to shipboard coatings (a) Antennas and superstructures (b) Deck

areas (c) Underwater hull

Fig 54 Zones of severity of environment for a typical offshore drilling structure

Before the 1960s, most marine coatings were fairly simple and could be applied by laborers such as seamen or maintenance personnel Although the advent of high-performance marine coatings in the 1960s changed this, the performance of marine coatings has improved to such an extent that topside coating lives of 20 years have been experienced on some offshore oil production platforms

Surface Preparation

Proper surface preparation is the most important consideration in determining the performance of organic coating systems Surface cleanliness and proper surface profile are both important Surface preparation frequently accounts for two-thirds of total painting costs for offshore structures

The Steel Structures Painting Council (SSPC), the National Association of Corrosion Engineers (NACE), and standards groups in Sweden, Germany, the U.K., and Japan have all issued standards for surface preparation These are listed in Table 15 Wet abrasive blast cleaning and waterblasting are not yet included in the standards, but are now being extensively used Wet blasting is useful for dust control and for avoiding electrical sparking in Class I (explosive) areas Generally, a small amount of nitrite inhibitor is added to the water to prevent re-rusting before priming

Table 15 Recommended surface preparation methods for various metallic substrates

Substrate Recommended surface

Solvent cleaning (degreasing) of new surfaces, followed by application of one coat of a wash primer

Aluminum Anodizing or chromate conversion coating whenever possible Otherwise, clean as for steel using only stainless steel

Gritblasting is usually used for surface preparation for marine coatings The severe corrosion exposure conditions in offshore and coastal locations require the best possible surface preparation

Inorganic zinc primers, which are frequently used in marine applications, require white metal gritblasting to remove all surface contamination because inorganic zinc has both a chemical bond and a mechanical bond to the surface Epoxy primers can be applied over commercial grade surfaces for land-based exposures, but require near-white metal surfaces to maintain performance offshore

Table 16 lists the characteristics of several types of grit Grit that is used offshore is not recoverable; this limits the economical choices to either boiler slag or copper slag Other types are too expensive unless they can be recycled Silica sand is generally not used because of the possibility of silicosis, its friable nature, and its rounded shape, which is not conducive to high productivity

Table 16 Properties of abrasives

Mineral 5-7 Rounded 2000 125 Variable 5 Medium Good

Flint 6.7-7 Angular 1280 80 Light gray 90+ Medium Good

By-product abrasives

Manufactured abrasives

Silicon carbide 9 Angular 1680 105 Black nil Low Good

Aluminum oxide 8 Blocky 1920 120 Brown nil Low Good

Glass beads 5.5 Spherical 1600 100 Clear 67 Low Good

Metallic abrasives

Steel shot (a) 40-50(b) Round Excellent

Steel grit (made by crushing steel shot) (a) 40-60(b) Angular Excellent Source: Ref 51

(a) Steel shot produces a peened surface, while steel grit produces an angular, etched type of surface

texture

Topside Coating Systems

Organic coatings are usually composed of three components: binders (resins), pigments, and solvents Not all paints, however, have all three components For example, solventless paints have been developed in response to environmental restrictions on the use of volatile solvents Solventless paints can be applied at thicknesses to 13 mm ( in.); such thick films would not be possible in a paint containing volatile solvents, because the thickness of the film would prevent solvent evaporation

Paints can be classified by the type of binder or resin into the following categories

• Air-drying oils (for example, linseed oil, alkyds)

• Lacquers (vinyls, chlorinated rubbers)

• Chemically cured coatings (epoxies, phenolics, and urethanes)

• Inorganic coatings (silicates)

The article "Organic Coatings and Linings" in this Volume contains detailed information on the formulation of all of these types of organic coatings

Inorganic zinc-rich primers are based on various silicate binders There are several types of self-curing and postcuring primers The binder serves as a strong, adherent matrix for the zinc metal The zinc dust must be present in sufficient amounts to provide metal-to-metal contact between both the zinc particles and the steel surface The zinc dust provides protection to the steel substrate in the same manner as in galvanizing If a break develops in the coating, the zinc acts as a sacrificial anode and corrodes preferentially; this provides protection of the iron for long periods Laboratory tests and field experience indicate that inorganic zinc-rich primers can at least double the life of a coating system and can often increase it tenfold To be effective, however, inorganic zinc-rich primers must be applied to a clean surface

Organic zinc-rich primers are alternatives to the inorganic zinc-rich coatings when conditions are not appropriate for inorganic zinc-rich coatings Organic zinc-rich primers can be formulated with epoxy, urethane, vinyl, and chlorinated rubber binders The most common binder used for marine applications is polyamide epoxy Zinc-rich epoxies provide a lower degree of conductivity and cathodic protection than inorganic zinc but impart several other desirable characteristics:

• Organic zinc-rich primers frequently may be applied over old paint, which makes them a good choice for maintenance painting

• The good adhesion of the epoxy binder makes surface preparation requirements less stringent than those for inorganic zinc-rich coatings Near-white metal surfaces are adequate for offshore applications, and commercial grade gritblasting can be used in less severe environments

• The epoxy binder provides some protection to the zinc, and this allows moderate exposure of the primer

to the marine environment without corrosion of the zinc and formation of zinc corrosion products Zinc corrosion products can cause intercoat adhesion problems and paint blistering

Topcoats

Topcoats for steel serve mainly to protect the primer and to add color and appearance To serve this function, they must be:

• Barrier coatings impervious to moisture, salt, chemicals, solvents, and ion passage

• Strong and resistant to mechanical damage

• Of adequate color and gloss retention

Some of the most common topcoats in use are discussed below, and detailed information on each of these types is available in the article "Organic Coatings and Linings" in this Volume

Alkyds are the most common and versatile coatings in existence, but they are seldom used in severe marine applications, because of their poor performance over steel This poor performance is due to the oil base of the alkyd As corrosion proceeds on steel, hydroxyl ions (OH-) are generated at cathode sites Hydroxyl ions saponify the oils in the coating, and this results in coating failure

Alkyds are the product of the reaction of a polybasic acid, a polyhydric alcohol, and a monobasic acid or oil The number

of possible combinations is large; therefore, a wide range of performance is available Alkyds are used in marine service

in relatively mild applications, such as interior coatings for cabins, quarters, engine rooms, kitchens, heads, and some superstructure applications

Vinyls also have a broad range of desirable properties Most vinyl resins are the product of the polymerization of polyvinyl chloride (PVC) and polyvinyl acetate (PVA) Vinyls are solvent-base coatings that form a tight homogenous film over the substrate They are easy to apply by brush, roller, and spray Intercoat adhesion is excellent because of the solvent-base nature of the coating Vinyls do not oxidize or age, and they are inert to acids, alkalies, water, cement, and alcohols They do soften slightly when covered with some crude oils Vinyl coatings dry quickly and can be recoated in a short time (often in minutes), depending on the solvent used Vinyl coatings are also flexible and can accommodate the motion of the steel beneath them, such as when a ship or platform is launched

Vinyls were extensively used on ships and offshore platforms for many years, and they are still in use in many areas However, they have given way to epoxies in most marine applications because vinyl coatings are relatively thin and are not very strong Film thickness is typically only 50 m (2 mils) per coat, and the coatings cannot withstand mechanical abuse In addition, vinyls are not very effective for covering rough, previously corroded surfaces

Chlorinated rubber coatings are based on natural rubber that has been reacted with chlorine to give a hard quality resin that is soluble in various solvents Chlorinated rubbers have been used for many years as industrial-type paints because of their low moisture permeability, strength, resistance to UV degradation, and ease of application Chlorinated rubbers have found application on ships and containers, railroads, and as traffic paint for road stripes For many years, chlorinated rubber coatings were used to paint ships because of their ease of application and repairability, tolerance of poor surface preparation, fast drying characteristics, and relatively good wear and abrasion resistance They

high-are still used to a great extent on ships and high-are the standard paint system for containers Modern fleet owners, however, have phased out chlorinated rubbers in favor of higher-quality coating systems, such as epoxy, for reasons to be discussed below

Epoxies. The combination of excellent adhesion (some can be applied underwater), good impact and abrasion resistance, high film builds (up to 6.4 mm, or in., on a wet, vertical surface), relatively low cost, and excellent chemical and solvent resistance has made epoxy coatings the workhorses of modern marine coatings These properties result in service lives of 7 to 12 years on ships, offshore platforms, and coastal applications when epoxy topcoats are applied over inhibited epoxy or inorganic zinc-rich primers Because epoxies are chemically cured, a wide range of properties can be achieved by varying the molecular weight of the resin, the type of curing agent, and the type of pigments or fillers used

Immersion Coatings

Immersion coatings for marine service have far greater requirements than other organic coatings They must resist moisture absorption, moisture transfer, and electroendosmosis (electrochemically induced diffusion of moisture through the coating) They also must be strong and have good adhesion

Most ship hulls and many marine structures use cathodic protection to supplement the protection afforded by organic coatings (see the section "Cathodic Protection" in this article) This is desirable because it is virtually impossible to apply and maintain a defect-free organic coating system on a large structure

Barrier Properties. To be effective in seawater immersion service, an organic coating must have a low moisture vapor transfer rate as well as low moisture absorption Moisture absorption is the molecular moisture absorbed into and held within the molecular structure of the coating This property is not important to the effectiveness of the coating unless the moisture absorption lowers the dielectric characteristics of the coating and increases the passage of electrical current Moisture vapor transfer, on the other hand, is important, particularly when the coating is exposed to an external current (as in cathodic protection) Generally, the lower the moisture vapor transfer rate of a coating, the more effective the coating

Where electroendosmosis may be encountered, adhesion is also very important Most organic coatings are negatively charged, and under cathodic protection, the cathode has an excess of electrons, which makes it negatively charged This being the case, coatings with a high moisture vapor transfer rate or questionable adhesion would be more subject to damage and blistering by cathodic potentials

Mechanical Properties. Coatings used on marine structures must be strong Most damage to marine coating systems

is mechanical, not a breakdown of the coating from exposure to seawater Immersion coatings must have good impact and abrasion resistance and must be able to flex well enough to maintain contact with the steel substrate when it is bent Rubbing by mooring ropes, chains, and crane wire ropes, as well as impact from cargo handling, work parties, and berthing operations, are major causes of damage

Types of Immersion Coatings. Many of the common paint formulations can be used for immersion service, but the most common coatings in use are coal tar epoxies and straight epoxies

Coal tar epoxies were introduced in 1955 and are the most common coatings in use on fixed marine structures (Ref 52) These thermosetting materials are available with a variety of setting temperatures and chemical curing systems Coal tar epoxies require near-white surface preparation and are very adherent and abrasion resistant They tend to be brittle and should not be used on flexible structures Straight epoxies have been commercially available longer than coal tar epoxies (Ref 53) Epoxies are usually applied in thinner coats and are more expensive than coal tar epoxies Epoxies have become the material of choice for immersion service because of their superior performance (Ref 51, 53) They have replaced chlorinated rubbers for most ship hull applications, and they are available in a variety of polyamide- or amine-based formulations (Ref 52) Detailed information on these and other coating materials is available in the article "Organic Coatings and Linings" in this Volume

Antifouling Topcoats. Most shipboard applications require antifouling topcoats The formulations for these coatings are changing because of environmental legislation In some parts of the world, copper-containing antifouling coatings are still popular, but in North America, these coatings have been replaced by organo-tin compounds

Cathodic Protection Criteria

A number of criteria are used to determine whether or not a structure is cathodically protected These criteria, which are covered in NACE RP-01-76 (Ref 54), include potential measurements, visual inspection, and test coupons

Potential Measurements. Reference 46 specifies a negative (cathodic) voltage of at least 0.80 V between the platform and a silver-silver chloride reference electrode contacting the water Normally, voltage is measured with the protective current applied The 0.80 V standard includes the voltage drop across the steel/water interface, but does not include the voltage drop in the water

Application of the protective current should produce a minimum negative (cathodic) voltage shift of 300 mV The voltage shift is measured between the platform surface and a reference electrode contacting the water; it includes the voltage drop across the steel/water interface but not the voltage drop in the water

Visual inspection should indicate no progression of corrosion beyond limits acceptable for platform life (Ref 54)

Corrosion test coupons must indicate a corrosion type and rate that is within acceptable limits for the intended platform life (Ref 54)

A number of other criteria are also possible, but in practice, -0.80 V versus Ag/AgCl is the most commonly used Other reference electrodes can be used for marine applications They are listed in Table 17

Table 17 Reference electrodes used for cathodic protection systems on offshore structures

Type of electrode Protection potential of steel, V

Ag/AgCl -0.80 (or more negative)

Cu/CuSo 4 -0.85 (or more negative)

Zinc +0.25 (or less positive)