Handbook Corrosion (1992) WW Part 2 ppsx

mean resistivities of less than 2000 · cm and a mean redox potential more negative than 400 mV on the normal hydrogen scale corrected to pH 7. Soils that were borderline based on these two tests tended to be aggressive if their water content was over 20%. With regard to redox potential alone, soil corrosivity varied as follows (Ref 48); Soil E h Corrosivity < 100 mV Severe 100-200 mV Moderate 200-400 mV Slight > 400 mV Noncorrosive Other attempts to assess the risk of corrosion by SRB have been sporadic. One investigation attempted to assess the severity of the SRB hazard on the inside of submarine pipelines carrying North Sea crude oil by measuring both the numbers of SRB present in the oil and the activity (or vigor of growth) of the organisms (Ref 49). The risk was assessed as extreme if both the numbers of organisms and their activity were rated high, and the risk was considered to be minimal if both were rated low. Efforts to solve the anaerobic iron and steel corrosion problem as outlined in Ref 48 include: • Repl acing the iron or steel with noncorrodible materials, such as fiberglass, PVC, polyethylene, and concrete • Creating a nonaggressive environment around the steel by backfilling with gravel or clay- free sand to encourage good drainage (that is, oxygenating t o suppress SRB), making the environment alkaline, or using biocides (in closed industrial systems) • Using cathodic protection, although potentials of -0.95 V versus Cu/CuSO 4 (or even more negative) are often required; at these potentials, the risk of hydrogen cracking or blistering should be assessed • Using various barrier coatings, some with corrosion inhibitors and/or biocides Aerobic Corrosion. Corrosion of iron and steel under oxygenated conditions generally involves the formation of acidic metabolites. The aerobic sulfur-oxidizing bacteria Thiobacillus can create an environment of up to about 10% H 2 SO 4 , thus encouraging rapid corrosion. Other organisms produce organic acids with similar results. This corrosion can be localized or general, depending on the distribution of organisms and metabolic products. If all the bacterial activity is concentrated at a break or delamination in a coating material, the corrosion is likely to be highly localized. If, on the other hand, the metabolic products are spread over the surface, the corrosion may be general, as has been reported for carbon steel tendon wires used to prestress a concrete vessel in a nuclear power plant (Ref 46). In this case, the wires were coated with a hygroscopic grease prior to installation. A study to determine the cause of corrosion concluded that the wires, shown in Fig. 27, were corroded by formic and acetic acids excreted by bacteria in breaking down the grease. Fig. 27 Carbon steel wires from a prestressing tendon of a nuclear power plant showing the damage resulting from the formation of organic acids in the tendon due to the breakdown of gre ase by the bacteria present in the tendon. Source: Ref 46 Other cases of aerobic corrosion of iron and steel begin with the creation of oxygen concentration cells by deposits of slime-forming bacteria. Such corrosion is often accelerated by the iron-oxidizing bacteria in the formation of tubercules. This topic is addressed in the discussion "Tuberculation" in this section. Biological Corrosion of Stainless Steel There are two general sets of conditions under which localized biological corrosion of austenitic stainless steel occurs (Fig. 25). These will be illustrated by two generalized case histories. Typical examples of microbiologically induced localized corrosion of stainless steel are shown in Fig. 28. Fig. 28 Localized biological corrosion of austenitic stainless steel. (a) Crevice corrosion of type 304 stainless steel flange from a cooling wate r system. Staining shows evidence of adjacent biomounds. The corrosion attack reached a depth of 6 mm ( in.). Courtesy of W.K. Link and R.E. Tat nall, E.I. Du Pont de Nemours & Co., Inc. (b) Pits on the underside of type 304 stainless steel piping used in a waste treatment tank (after sandblasting to remove biomounds). Courtesy of G. Kobrin and R.E. Tatnall, E.I. Du Pont de Nemours & Co., Inc. Hydrotest or Outage Conditions. As originally reported in Ref 50, a new production facility required type 304L and 316L austenitic stainless steels for resistance to nitric and organic acids. All of the piping and flat-bottomed storage tanks were field erected and hydrostatically tested. The hydrotest water was plant well water containing 20 ppm chlorides and was sodium softened. The pipelines were not drained after testing. The tanks were drained, but were then refilled for ballast because of a hurricane threat. Two to four months after hydrotesting, water was found dripping from butt welds in the nominally 3-mm ( -in.) wall piping. Internal inspection revealed numerous pits in and adjacent to welds under reddish-brown deposits in both piping and tanks. Upon cleaning off the deposit, the researchers found a large stained area with a pit opening. Metallographic sectioning showed a large subsurface cavity with only a small opening to the surface. Photographs of the weld corrosion deposits and the resulting pitting corrosion are shown in Fig. 26 to 35 in the article "Corrosion of Weldments" in this Volume (see the discussion "Microbiologically Induced Corrosion"). Pitted welds in a type 316L tank showed some evidence of preferential attack of the -ferrite stringers, as shown in Fig. 29. It is not yet known why such attack often concentrates at the weld line. Fig. 29 SEM micrograph showing matrix remaining after preferential corrosion of the - ferrite phase in a type 316 stainless steel. 300×. Courtesy of J.G. Stoecker, Monsanto Company Well water and deposits both showed high counts of the iron bacteria, Gallionella, and the iron/manganese bacteria, Siderocapsa. Deposits also contained thousands of parts per million of iron, manganese, and chlorides. Sulfate-reducing and sulfur-oxidizing bacteria were not present in either water or deposits. The proposed mechanism for the attack involves: • Original colonization by the iron and manganese bacteria at the weld seams to create an oxygen concentration cell • Dissolution of ferrous and manganous ions under the deposits • Attraction of chloride ions as the most abundant anion to maintain charge neutrality • Oxidation of the ferrous and manganous ions to ferric and manganic by the bacteria to form a highly corrosive acidic chloride solution in the developing pit Many failures of this type have been reported in the chemical-processing industries in new equipment after hydrotesting but prior to commissioning in service. Similar failures have been reported in older equipment in both the chemical- processing and nuclear power industries when untreated well or river water was allowed to remain stagnant in the equipment during outage periods. Occasionally, the pitting will be accompanied by what appear to be chloride stress- corrosion cracks under the deposits (Ref 45, 46). Examples of transgranular cracks in a type 304 stainless steel tank are shown in Fig. 30. Fig. 30 Cracks emanating from pits in a type 304 stainless tank that was placed in hot demineralized water service with an operating temperature that fluctuated from 75 to 90 °C (165 to 195 °F). (a) Photomicrog raph of a section through a typical biological deposit and pit in the wall of the tank. 25×. 10% oxalic acid etch. (b) Higher-magnification view of cracks. These branched transgranular cracks are typical of chloride stress- corrosion cracking of austenitic stainless steel. 250×. 10% oxalic acid etch. Source: Ref 51 Crevice or Gasket Conditions. A different set of conditions has lead to the localized corrosion of asbestos-gasketed flanged joints in a type 304 stainless steel piping system (Ref 52). Inspection of the system after about 3 years of service in river water revealed severe crevice corrosion in and near the flanged and gasketed joints. The corrosion sites were covered by voluminous tan-to-brown, slimy biodeposits, as shown in Fig. 31(a). Under the deposits were broad, open pits with bright, active surfaces (Fig. 31(b)). The surfaces under the gasket material and adjacent to the corroded areas were covered with black deposits, which emitted H 2 S gas when treated with HCl. Fig. 31(a) Single remaining biodeposit adjacent to resulting corrosion on a type 304 stainless steel flange. Numerous other similar deposits were dislodged in opening the joint. This flange was covered by a type 304 blind flange and was sealed with a bonded asbestos gasket. Source: Ref 52 Fig. 31(b) Close-up of gouging-type corrosion under deposits shown in Fig. 31(a) after cleaning to remove black corrosion products. Source: Ref 52 The biodeposits were high in iron, silt- and slime-forming bacteria, and iron bacteria, but not chloride, manganese, and sulfur compounds. Sulfate-reducing bacteria were found only in the black deposits. These bacteria had survived continuous chlorination (0.5 to 1.0 ppm residual), caustic adjustment of pH to 6.5 to 7.5, and continuous additions of a polyacrylate dispersant and a nonoxidizing biocide (quarternary amine plus tris tributyl tin oxide) (Ref 52). The suspected mechanism involves: • Colonization by slime-forming bacteria at low-velocity sites near gasketed joints • Trapping of suspended solids rich in iron by the growing biodeposit, thus creating an environment conducive to growth of the filamentous iron bacteria. • Rapid depletion of oxygen in the crevice area by a com bination of biological and electrochemical mechanisms (Ref 53), creating an environment for the SRB • Breakdown of pass ivity by a combination of oxygen depletion and SRB activity, causing localized corrosion Standard approved methods for controlling the biological corrosion of stainless alloys are currently being developed. Some general guidelines for avoiding problems in hydrotesting, however, are given in Ref 50. These guidelines are summarized as follows. First, demineralized water or high-purity steam condensate is used for the test water. The equipment should be drained and dried as soon as possible after testing. Second, if a natural freshwater must be used, it should be filtered and chlorinated, and the equipment should be blown or mopped dry within 3 to 5 days after testing. Biological Corrosion of Aluminum Pitting corrosion of integral wing aluminum fuel tanks in aircraft that use kerosene-base fuels has been a problem since the 1950s (Ref 54). The fuel becomes contaminated with water by vapor condensation during variable-temperature flight conditions. Attack occurs under microbial deposits in the water phase and at the fuel/water interface. The organisms grow either in continuous mats or sludges, as shown in Fig. 32, or in volcanolike tubercules with gas bubbling from the center, as shown schematically in Fig. 33. Fig. 32 Microbial growth in the integral fuel tanks of jet aircraft. Source: Ref 42 Fig. 33 Schematic of tubercule formed by bacteria on an aluminum alloy surface. Source: The Electrochemical Society The organisms commonly held responsible are Pseudomonas, Cladosporium, and Desulfovibrio. These are often suspected of working together in causing the attack. Cladosporium resinae is usually the principal organisms involved; it produces a variety of organic acids (pH 3 to 4 or lower) and metabolizes certain fuel constituents. These organisms may also act in concert with the slime-forming Pseudomonads to produce oxygen concentration cells under the deposit. Active SRB have sometimes been identified at the base of such deposits. Control of this type of attack has usually focused on a combination of reducing the water content of fuel tanks; coating, inspecting, and cleaning fuel tank interiors; and using biocides and fuel additives. More information can be found in Ref 44 and 54. Biological Corrosion of Copper Alloys Far less is known about the influence of micro-organisms on the corrosion of copper and copper alloys than was the case for iron and steel. The well-known toxicity of cuprous ions toward living organisms does not mean that the copper-base alloys are immune to biological effects in corrosion. It does mean, however, that only those organisms having a high tolerance for copper are likely to have a substantial effect. Thiobacillus thiooxidans, for example, can withstand copper concentrations as high as 2%. Most of the reported cases of microbial corrosion of copper alloys are caused by the production of such corrosive substances as CO 2 , H 2 S, NH 3 , and organic or inorganic acids. Copper-nickel tubes from the fan coolers in a nuclear power plant were found to have pitting corrosion under bacterial deposits (Fig. 34). Slime-forming bacteria acting in concert with iron- and manganese-oxiding bacteria were responsible for the deposits. Fig. 34 Pitting corrosion in 90Cu- 10Ni tubes from a fan cooler in a nuclear power plant. Pits are located under the small deposits associated with the deposition of iron and manganese by bacteria. Source: Ref 46 In another case, Monel heat-exchanger tubes were found to have severe pitting corrosion (Fig. 35) under discrete deposits rich in iron, copper, manganese, and silicon, with some nickel. Associated with the deposit were slime-forming bacteria, along with iron- and manganese-oxidizing bacteria. Several million SRB were found within each pit under the deposit. It was thought that the deposit-forming organisms created an environment conducive to growth of SRB, which then accelerated corrosion by the production of H 2 S. Fig. 35 Pitting corrosion in Monel tubes from a heat exchange r. Each pit was originally covered by a discrete deposit containing large numbers of SRB. Source: Ref 46 It is quite common to have bacterial slime films on the interior of copper alloy heat exchanger and condenser tubing. Usually, these films are a problem only with heat transfer as long as the organisms are living. When they die, however, organic decomposition produces sulfides, which are notoriously corrosive to copper alloys. Occasionally, NH 3 -induced stress-corrosion cracking has been directly attributed to microbial NH 3 production. Tuberculation The formation of tubercules by biological organisms acting in conjunction with electrochemical corrosion occurs in many environments and on many alloys. An example of tuberculation in a steel economizer tube in sulfuric acid service is shown in Fig. 36. This example shows that it is possible for tubercules to form without the presence of any microorganisms; the phenomenon usually takes place in biologically active aqueous systems. Fig. 36 Steel hairpin bend tube used in the economizer of a sulfuric acid waste heat boiler. The tube exhibits tuberculation associated w ith oxygen attack. The bottom photograph shows the tubercules in greater detail. Source: Ref 55 The process of tubercule formation is a complex one. A number of the reactions that can take place are illustrated for a ferrous alloy in Fig. 37. The volcanolike structure often starts with a deposit of slime-forming and iron-oxidizing bacteria at a point of low flow velocity. This creates an oxygen concentration cell, thus promoting dissolution of iron as Fe 2+ under the deposit. As the Fe 2+ ions move outward, they are oxidized to Fe 3+ ; this occurs electrochemically as they encounter higher oxygen concentrations and/or by the action of iron bacteria. The resulting corrosion product, Fe(OH) 3 , mingles with the biodeposit to form the wall of the growing tubercule. When bacteria are present, the tubercule structure is usually less brittle and less easily removed from the metal surface than when they are absent. The outside of the tubercule becomes cathodic, while the metal surface inside becomes highly anodic. Fig. 37 Schematic diagram of electrochemical and microbial processes involved in tuberculation. Not all of these processes may be active in any given situation. As the tubercule matures, some of the biomass may start to decompose, providing a source of sulfates for SRB to use in producing H 2 S in the anaerobic interior solution. In some cases, the sulfur-oxidizing bacteria may assist in the formation of the sulfates. Depending on the ions available in the water, the tubercule structure may contain some FeCo 3 and, when SRB are present, some FeS. Finally, if there is a source of chlorides and if the iron-oxidizing bacteria Gallionella are present, a highly acidic, ferric chloride solution may form inside the tubercule. Generally, not all of the above reactions will take place in any single environment. As the individual tubercules on a surface grow under the influence of any combination of reactions, they will eventually combine to form a mass that severely limits flow (or even closes it off altogether), leaving a severely pitted surface underneath. References 1. "Standard Test Method for Filiform Corrosion Resistance of Organic Coatings on Metal," D 2803, Annual Book of ASTM Standards, American Society for Testing and Materials 2. R. Preston and B. Sanyal, J. Appl. Chem., Vol 6, 1956, p 26-44 3. W. Funke, Prog. Org. Coatings, Vol 9 (No. 1), April 1981, p 29-46 4. R. Ruggeri and T. Beck, Corrosion, Vol 39 (No. 11), Nov 1983, p 452-465 5. W. Slabaugh and E. Chan, J. Paint Technol., Vol 38, 1966, p 417-420 6. W. Slabaugh, W. DeJager, S. Hoover, and L. Hutchinson, J. Paint Technol., Vol 44 (No. 56), March 1972, p 76-83 7. W. Ryan, Environment, Economics, Energy, Vol 1, Society for the Advancement of Material and Process Engineering, May 1979, p 638-648 8. P. Bijlmer, Adhesive Bonding of Aluminum Alloys, Marcel Dekker, 1985, p 21-39 9. T.S. Lee and R.O. Lewis, Mater. Perform., Vol 24 (No. 3), 1985, p 25 10. H.P. Godard, W.B. Jepson, M.R. Botwell, and R.L. Kane, Crevice Corrosion of Aluminum and Crevice Corrosion of Titanium, in Corrosion of Light Metals, John Wiley & Sons, 1967, p 45, 319 11. T.S. Lee, R.M. Kain, and J. W. Oldfield, "Factors Influencing the Crevice Corrosion Behavior of Stainless Steels," Paper 69, presented at Corrosion/83, Houston, TX, National Association of Corrosion Engineers, 1983 [...]... X (ferric sulfate-sulfuric acid test) Alloy Corrosion rate, mg/dm2/d 900-°C (1650-°F) anneal Annealing time at 620 °C (1150 °F) 10 min 5h 15,700 27 0 62 81 85 43 26 4 50 67 85 43 43 5950(a) 8030(a) 990 50 40 53 822 0(a) 12, 400(a) 890 50 37 50 78 15,600 940 138 80 74 27 0 15,500 500 1 32 80 70 22 6 50 104 21 4 25 8 98 1 02 58 50 26 -3 4h 77 26 -2 2h 37 26 -1 1h 15,600 26 -0 30 min 95 160 96 93 97 58 50 (a) 56 h in... wt% Ti, wt% Nb, wt% Ti or Nb/(C + N), % Result 18Cr-2Mo 0. 022 0.16 7.3 Fail 0. 028 0.19 6.8 Fail 0. 027 0 .23 8.5 Pass 0.057 0.37 6.5 Pass 0.079 0.47 5.9 Pass 0.067 0. 32 4.8 Fail 0.067 0.61 9.1 Pass 0.030 0.19 6.3 Pass 0. 026 0.17 6.5 Fail 0. 026 0 .22 8.5 Fail 0. 026 0 .26 10.0 Pass 0. 026 0.17 6.5 Fail 0. 025 0.33 13 .2 Pass 18Cr-2Mo 26 Cr-1Mo 26 Cr-1Mo This guideline is empirical and cannot be explained... reduce the corrosion resistance Table 4 The effect of crystal structure on the corrosion behavior of an Fe-47Cr alloy Corrosion rate, g/dm2/d Solution Ferrite phase Ratio(a) Reducing 10% HCl boiling 1461 7543 5 .2 10% H2SO4 boiling 29 39 7 422 2. 5 50% H2SO4 boiling 5088 528 0 1.04 50% H2SO4 + Fe2(SO4)3 boiling 0.0195 0.196 10 50% H2SO4 + CuSO4 boiling 0.0170 0.415 24 65% HNO3 boiling 0. 020 5 0.861 42 HNO3 +... Society (UK), Sept 1984 24 A.J Sedriks, Int Met Rev., Vol 27 (No 6), 19 72 25 T.S Lee and A.H Tuthill, Mater Perform., Vol 22 (No 1), 1983, p 48 26 Y.M Kolotyrkin, Corrosion, Vol 19, 1963, p 26 1t 27 J.L Crolet, J.M Defranoux, L Seraphin, and R Tricot, Mem Sci Rev Met., Vol 71 (No 12) , 1974, p 797 28 H Spahn, G.H Wagoner, and U Steinhoff, Paper A -2, Firminy Meeting, France, June 1973 29 M.G Fontana, The... Marine Corrosion, Causes and Prevention, John Wiley & Sons, 1975, p 117 18 R.M Kain, "Effect of Alloy Content on the Localized Corrosion Resistance of Several Nickel Base Alloys in Seawater," Paper 22 9, presented at Corrosion/ 86, Houston, TX, National Association of Corrosion Engineers, 1986 19 R.M Kain, Corrosion, Vol 40 (No 6), 1984, p 313 20 R.M Kain, Mater Perform., Vol 33 (No 2) , 1984, p 24 21 R.M... F.L LaQue, Localized Corrosion, National Association of Corrosion Engineers, 1974, p i.47 31 H.H Uhlig and R.W Revie, Corrosion and Corrosion Control, 3rd ed., John Wiley & Sons, 1984, p 13-14 32 H.H Uhlig, Corrosion Handbook, John Wiley & Sons, 1948, p 165 33 M.G Fontana, Corrosion Engineering, McGraw-Hill, 1986, p 66 34 U.R Evans, Corrosion, Vol 7 (No 23 8), 1951 35 A.J Sedriks, Corrosion of Stainless... 0.004 Pass 0.004 Fail 0.0 02 0.009 Fail 0.0 02 0.005 Pass 0.004 0.010 Partial failure 0.003 0.016 Fail 0.013 26 Cr-1Mo 0.0 02 0.010 18Cr-2Mo N 0.006 Fail For 18Cr-2Mo alloys to be immune to intergranular corrosion, it appears that the maximum level of carbon plus nitrogen is 60 to 80 ppm; for 26 Cr-1Mo steels, this level rises to around 150 ppm The notation of partial failure for the 26 Cr-1Mo steel containing... of the effect of heat treatment on the microstructure of 29 Cr-4Mo alloys, both and phases were found in material held in the 700- to 925 -°C ( 129 0- to 1695-°F) range (Ref 27 ) Longterm aging of the 29 Cr-4Mo steel did not render it susceptible to intergranular corrosion in the boiling 50% H2SO4 + Fe2(SO4)3 solution This work also included 29 Cr-4Mo-2Ni alloys, and and phases were seen to form much more quickly... and J.G Stoecker, Microbiologically Influenced Corrosion, in Process Industries Corrosion- Theory and Practice, B.J Moniz and W.I Pollock, Ed., National Association of Corrosion Engineers, 1986, p 22 7 -24 2 46 D.H Pope, "A Study of Microbiologically Influenced Corrosion in Nuclear Power Plants and a Practical Guide for Countermeasures," Final Report EPRI NP-45 82, Electric Power Research Institute, 1986 47... External Biological Corrosion, in Biologically Induced Corrosion, S.C Dexter, Ed., National Association of Corrosion Engineers, 1986, p 26 8 -27 4 50 G Kobrin, Reflections on Microbiologically Induced Corrosion of Stainless Steels, in Biologically Induced Corrosion, S.C Dexter, Ed., National Association of Corrosion Engineers, 1986, p 33 51 J.G Stoecker and D.H Pope, Study of Biological Corrosion in High . 1984 24 . A.J. Sedriks, Int. Met. Rev., Vol 27 (No. 6), 19 72 25 . T.S. Lee and A.H. Tuthill, Mater. Perform., Vol 22 (No. 1), 1983, p 48 26 . Y.M. Kolotyrkin, Corrosion, Vol 19, 1963, p 26 1t. Microbiologically Influenced Corrosion, in Process Industries Corrosion Theory and Practice, B.J. Moniz and W.I. Pollock, Ed., National Association of Corrosion Engineers, 1986, p 22 7 -24 2 46. D.H. Pope,. Result 0.0 02 0.004 Pass 0.010 0.004 Fail 18Cr-2Mo 0.0 02 0.009 Fail 0.0 02 0.005 Pass 0.004 0.010 Partial failure 0.003 0.016 Fail 26 Cr-1Mo 0.013 0.006 Fail For 18Cr-2Mo alloys to