Biological Corrosion Failures

Fig. 1 The pH and oxidation reduction potential for growth of anaerobic bacteria able to reduce nitrate or sulfate (dots in plots) and for soils dominated by the microbial metabolism (boxes). Aerobic bacteria grow over a wide range of pH at Eh 300 mV (normal hydrogen electrode). Source: Ref 19 MICROORGANISMS can directly or indirectly affect the integrity of many materials used in industrial systems. Most metals, including iron, copper, nickel, aluminum, and their alloys, are more or less susceptible to damage (Ref 1– 3). Only titanium and its alloys appear to be generally resistant (Ref 4). This review focuses initially on the mechanisms of microbially induced or influenced corrosion (MIC) of metallic materials as an introduction to the recognition, management, and prevention of microbiological corrosion failures in piping, tanks, heat exchangers, cooling towers, and so on. Numerous reviews of MIC have appeared over the last decade (Ref 2, 5–12). Two recent publications (Ref 13, 14) present broader discussions of MIC, including a useful introduction to microbial problems seen with nonmetallic materials such as polymers, composites, concrete, glass, wood, and stone. Viable microorganisms can be found over a surprisingly wide range of temperature, pressure, salinity, and pH (Ref 1). In the 1950s, pioneering work by Zobell isolated sulfate-reducing bacteria (SRB) that grew at 104 C (219 F) and pressures of 1000 bar from oil-bearing geological formations deep underground (Ref 15). Microbial communities exist in environments as diverse as subzero snowfields to deep ocean thermal vents. Halophiles evolved to live at extreme salinities turn pink the evaporation pans used to win salt from seawater. Sulfur-oxidizing bacteria create very acidic conditions (pH 1) by producing sulfuric acid as an end product of their metabolism, while other microorganisms survive the opposite end of the pH scale. Given these examples, it should not be surprising that microorganisms have been implicated in the accelerated corrosion and cracking of a correspondingly wide range of industrial systems. For example, the involvement of thermophilic SRB in the severe intergranular pitting of 304L stainless steel condenser tubes in a geothermal electrical power plant operating at 100 C ( 210 F) has been reported (Ref 16). In another example, microbiological activity and chloride concentrated under scale deposits were blamed for the wormhole pitting of carbon steel piping used to transport a slurry of magnesium hydroxide and alumina at pH 10.5 (Ref 17). Whatever the environmental conditions, microorganisms need water, a source of energy to drive their metabolism, and nutrients to provide essential building materials (carbon, nitrogen, phosphorus, trace metals, etc.) for cell renewal and growth. An understanding of these factors can sometimes help in failure investigations. Energy may be derived from sunlight through photosynthesis or from chemical reactions. The importance of photosynthetic metabolism is limited in the context of this article to above-ground facilities or submerged structures that receive sunlight. For closed systems and buried facilities, microbial metabolism is based on energy derived from oxidation reduction (redox) reactions. Under aerobic conditions, reduction of oxygen to water complements the metabolic oxidation of organic nutrients to carbon dioxide. Under anaerobic conditions, electron acceptors other than oxygen can be used. Figure 1 illustrates the range of pH and redox potential where anaerobic forms of microbial metabolism tend to be found (Ref 18). Whatever the metabolism, electrochemical reactions catalyzed by enzymes provide energy for cell growth. Many of these reactions are not important under abiotic conditions, because they are kinetically slow in the absence of organisms. By promoting these reactions, microbes produce metabolites and conditions not found under abiotic conditions. In some cases, electrons released by the oxidation of metals are used directly in microbial metabolism. In other cases, it is the chemicals and conditions created by microbial activity that promote MIC. Secondary effects can also be important. These include such things as the biodegradation of lubricants and protective coatings designed to prevent wear or corrosion in an operating system, or the alteration of flow regimes and heat-transfer coefficients due to the biological fouling of metal surfaces. Given the potential impact of MIC on a wide range of industrial operations, it is not surprising that microbiological effects are of significant concern in failure analysis and prevention. Microbially induced corrosion problems afflict water-handling operations and manufacturing processes in oil and gas production, pipelining, refining, petrochemical synthesis, power production, fermentation, waste water treatment, drinking water supply, pulp and paper making, and other industrial sectors. Microbially induced corrosion is also a concern whenever metals are exposed directly to the environment in applications including marine or buried piping, storage tanks, ships, nuclear waste containers, pilings, marine platforms, and so on.