Microbial Induced Corrosion

Abstract

Microbial Induced Corrosion

Microbiologically Influenced Corrosion (MIC) can occur in a range of materials used in the petroleum industry. MIC is not a single, easily identifiable corrosion mechanism: the presence of activity of microorganisms simply enhances the rate of electrochemical corrosion reactions which occur in an a-biotic environment. The most well known ‘problem’ group of organisms are the sulphate-reducing bacteria (SRB) which generate toxic and corrosive hydrogen sulphide (H2S) during their growth, but other organisms are also involved in MIC.

Corrosion prevention measures in the petroleum industry are generally designed to minimise electrochemical (a-biotic) corrosion rates, for example by the complete removal of oxygen from seawater used in secondary oil recovery by water injection. Film-forming corrosion inhibitor chemicals are also frequently applied, again in an attempt to prevent or limit electrochemical corrosion. Biological fouling of industrial systems processing water is, however, almost inevitable and this increases the risk of corrosion. Any use of microbiological processes in the petroleum industry (for example MIOR techniques) must take account of the possibility of microbiologically influenced corrosion of materials, especially if the process is designed to stimulate the growth of microorganisms.

A biofilm developing on a metal surface can dramatically change the electrochemistry at the surface. Figure 1 shows how the, respiration of an active biofilm less than 100μm thick is able to create totally anaerobic micro environments at the steel surface, even if the bulk fluid is oxygen-rich. In a biologically active environment, antimicrobial chemicals (“biocides” or “bactericides”) are used to minimise the development both of biofilms and the effects of MIC.

Almost all structural materials can be damaged by the effects of the presence or activity of one microbial group or another. For example, copper-containing materials have anti fouling properties because copper ions are toxic to higher organisms. This, however, is not the case for bacteria, which are more resistant to high concentrations of copper. Bronzes and copper alloys can be severely affected by microbial growth. Stainless steels are also susceptible to damage in anaerobic environments and even Monel and more exotic metals may be damaged by pitting attack initiated beneath localised microbial colonies. These more expensive materials are usually used in thin sheets or for highly critical components. Although overall corrosion rates are generally lower for these materials, the time to failure may be equally rapid simply because there is material to perforate or because dimensional tolerances are more exacting. Corrosion damage may be caused either directly and indirectly by a number of microorganisms.