Volume 13 - Corrosion Part 13 pdf

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 CO 2 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 m. 3000× Two chemical species, oxygen and hydrogen, that are often implicated (or even rate-controlling) in corrosion are also important in the metabolism of the bacteria. A given bacterial slime film can be either a source or a sink for either oxygen or hydrogen. Moreover, these films are rarely continuous. Usually, they provide only spotty coverage of the metal surface. Thus, they are capable of inducing oxygen (or other chemical) concentration cells. Bacterial action on decaying organic matter in the slime film can also result in the production of ammonia and sulfides. Ammonia causes stress- corrosion cracking of copper alloys, and sulfides have been implicated in accelerated localized and/or uniform corrosion 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 H 2 S. 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 Atmosphere Location mm/yr mils/yr Marine Point Reyes, CA 0.5 19.71 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 Industrial East Chicago 0.084 3.32 Marine Bayonne, NJ 0.077 3.05 Urban Pittsburgh, PA 0.03 1.20 Suburban (semi-industrial) Middletown, OH 0.029 1.13 Rural State College, PA 0.023 0.90 Marine Esquimalt, BC, Canada 0.013 0.53 Desert Phoenix, AZ 0.0046 0.18 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 r elative 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 SO 2 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/m 2 /d results in a corrosion rate of less than 0.1 g/dm 2 /mo, while a salinity of 1000 mg/m 2 /d results in a corrosion rate of almost 10 g/dm 2 /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 SO 2 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 SO 2 levels is shown in Fig. 35, in which data are plotted from three Norwegian test sites. The data show that as SO 2 concentrations are increased the corrosion rate, measured as weight loss, increases. For example, at an SO 2 concentration of 25 g/m 3 , the corrosion rate is approximately 55 g/m 2 /mo, while for an SO 2 concentration of 100 g/m 3 the corresponding corrosion rate is approximately 170 g/m 2 /mo. A summary of Scandinavian data for carbon steel and zinc showed the following relationships between corrosion rate and SO 2 concentration: steel = 5.28 [SO 2 ] + 176.6 (Eq 7) and zinc = 0.22 [SO 2 ] + 6 (Eq 8) where is the atmospheric corrosion rate in g/m 2 /yr, and [SO 2 ] represents the concentration of SO 2 in g/m 3 . Similar types of relationships have been shown for other alloy systems and locations. [...]... 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 AISI type 301 302 304 321 347 316 317 308 309 310 Average corrosion rate mils m . 33 shows the effect of moving inland along the coast of Nigeria from the 4 5-, 36 5-, and 1190-m (5 0-, 40 0-, and 130 0-yd) sites at Lagos. From studies done on Barbados, the effect of distance. 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. Beach, NC. (a) 25-m (8 0- 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