Microbial corrosion attacks

Many of the by-products of microbial metaboHsm, including organic acids and hydrogen sulfide, are corrosive. These materials can concentrate in the biofilm, causing accelerated metal attack. Corrosion tends to be self-limiting due to the buildup of corrosion reaction products. However, microbes can absorb some of these materials in their metaboHsm, thereby removing them from the anodic or cathodic site. The removal of reaction products, termed depolari tion stimulates further corrosion. Figure 10 shows a typical result of microbial corrosion. The surface exhibits scattered areas of localized corrosion, unrelated to flow pattern. The corrosion appears to spread in a somewhat circular pattern from the site of initial colonization. 

For transportation of the artefact it is essential to maintain treatment initiated in the field. If the items cannot be moved in their storage tanks, they must be packed as mentioned for the large artefacts above, i. e. use sawdust, etc. If by the smell of the corrosion products (presence of hydrogen sulfide gas), microbial corrosion of the artefact is suspected, it would be advisable to add a biocide in order to minimise this type of attack until it can be treated in the laboratory. 

The most aggressive corrosive attacks occur in the presence of microbial communities that contain a variety of bacteria. In these communities, the bacteria act cooperatively to produce favorable conditions for the growth of each species. For example, obligate anaerobic bacteria can thrive in aerobic environments when present beneath biofilms/deposits in which aerobic bacteria consume the oxygen. In the case of underground pipelines, the severe attack has been associated with acid-producing bacteria in such bacterial communities (Fig. 4.9). 

Biofilm formation at the air-water interface. The bathtub ring often formed at the air-water interface around the sides of the basin is likely to be a biofilm due to microbial activity. This film acts like a trap and is known to concentrate caesium and other radioactive isotopes contained in the basin water. This biofilm should be removed mechanically by wet brushing, using water to hold down any airborne activity. A 35% solution of hydrogen peroxide has proven effective in suppressing microbial activity and could be used to assist in biofilm removal without corrosive attack on aluminium alloys. 

Microbial corrosion is probably not caused by a direct bacterial attack on the metallic phase. It is affected by metabolic products which interfere with diverse steps of the corrosion process (Gordon et al., 1981 Hamilton, 1985 Morton, 1987 N. N., 1990 a, b, 1991 Rose, 11981 Summers and Silver, 1978 Thierry, 1987 Weiner et al., 1988). Taking into consideration the living conditions of microorganisms, it appears... 

Microbial effects caused by certain typ>es of bacteria or microbes when their metabolism produces corrosive species in an otherwise innocuous environment, or when they produce deposits which can lead to corrosion attack. 

In Chapter 7,1 try to show that microbial corrosion can have more or less similar patterns despite different systems in which it is occurring. This chapter shows how microbial corrosion in fire water lines could be similar to that happening within the legs of a submersible off-shore platform, and how buried pipelines and steel piles of a jetty could experience almost the same scenarios of microbial attack. 

As mentioned earlier, microorganisms can attack drilling fluid additives and introduce corrosive agents to the system. Therefore, it is very important to monitor their activity and detect any source of problem as early as possible. API RP 38 is probably the most widely used testing procedure in the industry [201]. The methods that can be used to monitor the microbial activity can include the following [201,208] ... 

Attack by organisms other than SRB. Ammonia and amines are produced by microbial decomposition of organic matter under both aerobic and anaerobic conditions (ammo-nification). (Stott)5 These compounds are oxidized to nitrite by aerobic bacteria such as Nitrosomonas or Nitrobacter species. Nitrobacter is very efficient at destroying the corrosion-inhibition properties, of nitrate-based corrosion inhibitors by oxidation, unless a biocidal agent is included in the formulation. The release of ammonia at the surfaces of heat-exchanger tubes has a detrimental effect. (Stott)5... 

Aluminum alloys can also be attacked by microorganisms. For example, there have been MIC problems with aluminum fuel tanks and transfer lines. In this case, microorganisms grow in the water layer under the fuel to produce volcano-shaped tubercles, frequently evolving gas. Pitting occurs under the tubercles. MIC attack on copper alloys is usually insignificant. However, corrosion of copper condenser tubes by microbially produced ammonia has been reported. In addition, sulfuric acid has been produced by microbial activity by corrosion of underground copper pipes. 

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