Microbial corrosion

Corrosion in ships can also be caused by MIC. In this type of corrosion, microbial organisms present in the environment can accelerate corrosion. For example, SRB, which are present in stagnant water of many harbors, can build up on the hulls of ships. Other corrosion-causing bacteria, such as acid-producing and anaerobic bacteria, are also present in ballast tanks as well as in the liquid products that some tankers carry. The microbes cause a localized change in the environment, which can promote aggressive pitting and other types of corrosion. 

Carbon dioxide (COj) corrosion Hydrogen sulfide (HjS) corrosion Preferential weld corrosion Erosion and erosion-corrosion Crevice corrosion Flange face corrosion Cavitation Dead-leg corrosion Under-deposit corrosion Microbial corrosion Oxygen corrosion Galvanic corrosion External corrosion Corrosion under insulation (CUI)... 

The corrosive microbial effect on metals can be attributed to removal of electrons from the metal and formation of corrosion products principally by ... 

In 1990, NACE officially accepted the teim Microbiologically Influenced Corrosion to address this type of corrosion (see Materials Performance (MP), September 1990, p.45). This type of corrosion is also called microbiologically induced corrosion , microbial corrosion or bioconosion. In this book, all of these terminologies will be used interchangeably. 

Corrosion associated with the action of micro-organisms present in the corrosion system. The biological action of organisms which is responsible for the enliancement of corrosion can be, for instance, to produce aggressive metabolites to render the environment corrosive, or they may be able to participate directly in the electrochemical reactions. In many cases microbial corrosion is closely associated with biofouling, which is caused by the activity of organisms that produce deposits on the metal surface. 

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. 

Biofilms can promote corrosion of fouled metal surfaces in a variety of ways. This is referred to as microbiaHy influenced corrosion. Microbes act as biological catalysts promoting conventional corrosion mechanisms the simple, passive presence of the biological deposit prevents corrosion inhibitors from reaching and passivating the fouled surface microbial reactions can accelerate ongoing corrosion reactions and microbial by-products can be directly aggressive to the metal. 

Continuous chlorination of a cooling water system often seems most pmdent for microbial slime control. However, it is economically difficult to maintain a continuous free residual in some systems, especially those with process leaks. In some high demand systems it is often impossible to achieve a free residual, and a combined residual must be accepted. In addition, high chlorine feed rates, with or without high residuals, can increase system metal corrosion and tower wood decay. Supplementing with nonoxidizing antimicrobials is preferable to high chlorination rates. 

Recently, there has developed a greater recognition of the complexity of the MIC process. MIC is rarely hnked to a unique mechanism or to a single species of microorganisms. At the present state of knowledge, it is widely accepted that the growth of different microbial species within adherent biofilms facihtates the development of structured consortia that may enhance the microbial effects on corrosion. 

Microbiauy induced corrosion (MIC) probes. Devices are available to measure the amount of microbial activity in some environments. MicrobiaUy induced corrosion is known to be an actor in many corrosion-related problems in processing plants. The monitoring devices for MIC are limited in their range and, at present, are available only for a few specific environments. This is an exciting area for development of corrosion probes and monitoring systems. 

The triggering mechanism for the corrosion process was localized depassivation of the weld-metal surface. Depassivation (loss of the thin film of chromium oxides that protect stainless steels) can be caused by deposits or by microbial masses that cover the surface (see Chap. 4, Underdeposit Corrosion and Chap. 6, Biologically Influenced Corrosion ). Once depassivation occurred, the critical features in this case were the continuity, size, and orientation of the noble phase. The massive, uninterrupted network of the second phase (Figs. 15.2 and 15.21), coupled... 

A relatively high degree of corrosion arises from microbial reduction of sulfates in anaerobic soils [20]. Here an anodic partial reaction is stimulated and the formation of electrically conductive iron sulfide deposits also favors the cathodic partial reaction. 

One of the reasons why it is important to remove suspended solids in water is that the particles can act as a source of food and housing for bacteria. Not only does this make microbiological control much harder but, high bacteria levels increase the fouling of distribution lines and especially heat transfer equipment that receive processed waters (for example, in one s household hot water heater). The removal of suspended contaminants enables chemical treatments to be at their primary jobs of scale and corrosion prevention and microbial control. 

Tank materials and polish are critical for microbial and corrosion control. 

There are several methods of monitoring microbial-influenced corrosion. Some methods are as follows ... 

Methods to prevent or reduce problems associated with microbial corrosion will be discussed later. Some of them are ... 

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] ... 

Because systems are normally not designed for use with this type of fluid, certain aspects should be reviewed with the equipment and fluid suppliers before a decision to use such fluids can be taken. These are compatibility with filters, seals, gaskets, hoses, paints and any non-ferrous metals used in the equipment. Condensation corrosion effect on ferrous metals, fluid-mixing equipment needed, control of microbial infection together with overall maintaining and control of fluid dilution and the disposal of waste fluid must also be considered. Provided such attention is paid to these designs and operating features, the cost reductions have proved very beneficial to the overall plant cost effectiveness. 

Soil resistivity The role of soil in the electrical circuitry of corrosion is now apparent. Thus the conductivity of the soil represents an important parameter. Soil resistivity has probably been more widely used than any other test procedure. Opinions of experts vary somewhat as to the actual values in terms of ohm centimetres which relate to metal-loss rates. The extended study of the US Bureau of Standards presents a mass of data with soil-resistivity values given. A weakness of the resistivity procedure is that it neither indicates variations in aeration and pH of the soil, nor microbial activity in terms of coating deterioration or corrosion under anaerobic conditions. Furthermore, as shown by Costanzo rainfall fluctuations markedly affect readings. Despite its short comings, however, this procedure represents a valuable survey method. Scott points out the value of multiple data and the statistical nature of the resistivity readings as related to corrosion rates (see also Chapter 10). 

Methods of protecting materials against microbial corrosion include ... 

In recent years it has become apparent that widespread microbial infections of materials in the manufacturing industries can lead to corrosion for the reason briefly outlined above. Examples include the instant rusting of machined parts, corrosion of machine tools, aircraft fuel tanks, hydraulic systems, strip steel etc. 

The advent of Biotechnology now mainly directed at medical diagnostics and more recently to the food industry is likely to yield more rapid and simple tests for measuring microbial mass, enzymes etc. and these, e.g. a clip slide measuring ATP, adapted to corrosion diagnosis ... 

To conclude it must be stressed that recent work has directed attention to the interplay between different microbial species in most of the corrosion effects described. Microbial corrosion is therefore one special instance of the rapidly developing field of Microbial Ecology. ... 

Hill, E. C. in Microbial Aspects of Corrosion, Ed. Miller, J.D.A., Medical and Technical Publishing, Aylesbury (1971)... 

---