Corrosion on metals is electrochemical, you need an electrolyte and a conduction path.
When you talk about Microbiologically Influenced Corrosion you have to know which aspect of it you are talking about.
1. In some rare cases the microbes are actually metabolizing the metal, this is highly uncommon.
2. The microbes often act as a deposit and lead to crevice corrosion. Since most engineering alloys rely on an oxide surface film for corrosion resistance. Coating the surface does not allow fresh oxygen from the environment to reach the surface for refreshing of the film. The layer further serves to trap impurities from the environment such as Cl creating a lower oxygen, lower pH, higher impurity crevice.
3. There are many situations where the microbes either produce a waste product that attacks the metal or when they die the decomposition products are highly corrosive. These effects are of course amplified because you are under a surface deposit.
Coatings come in two varieties, those that try to supress the microbial growth and the ones that just try to isolate and protect the surface. One problem is that with any coating if there are any voids or holidays you will see greatly accelerated corrosion at those locations.
All of these are why people use simulated environments for testing. For example with stainless steels we use a ferric chloride solution that mimics the chemistry that forms in a crevice during actual crevice or MIC corrosion. This works on the assumption that you can't prevent corrosion from starting but allows you to evaluate resistance to propagation by testing at different temperatures.
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P.E. Metallurgy, consulting work welcomed
