The phenomenon known as 'Microbiologically Influenced Corrosion' (MIC), is currently, in different fields of industry, a very complex process still in need of a lot of further research, especially with regard to metallic systems in contact with stagnant water or in a marine environment, in which embrittlement effects of hydrogen can occur. The microorganisms can form complex biofilms with local chemical conditions which can change with time. In all cases, the chemistry under the biofilm has chemical characteristics very different from the bulk solution, and corrosion progresses as a result of the concentration of different species by the bacteria, and the low pH value generated at the base of the pits. To assess the complex causes of the MIC of stainless steels that arise in industry, it is necessary to consider the stability and nature of the passive film, formed on the metal surface, and relate these to different factors including the oxide film thickness, chemical composition, the chromium depletion in the alloy beneath the oxide, as well as the nature of the biological colonisation. This paper summarises and reviews the principal factors controlling the susceptibility of stainless steels to MIC, taking into account various microstructural changes, metallurgical variables and surface effects with the object of improving the resistance of these versatile alloys to MIC.
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