Corrosion, or rust (in common language) is always a consequence of electron transfer. Whereby elementary iron is converted into iron 2+ and/or iron 3+, bound to other molecules such as oxygen or sulphur. For some years now, the scientific world has been defining two basic mechanisms that lead to conversions from elementary iron. The first and best known form is:
CMIC – Chemical Microbial Influenced Corrosion.
The basic principle is that the micro-organisms are producing an acid; this acid has corrosive properties and ensures that elemental iron is subsequently converted into other forms. It is therefore an indirect form. It is therefore not the organism itself that is harmful, but the metabolic product it produces.
The most well-known form is that of SRBs. Sulphate-reducing bacteria or Archaea that produce hydrogen sulphide from elemental sulphur or other sulphur compounds. Less well known, but from a corrosion point of view at least as relevant, is the ability of sulpher oxidizing organisms to produce thiosulfate. This salt has been described to result into intergranular stress corrosion cracking (IGSCC) in sensitized stainless steels. But besides these two examples, organisms are capable of producing many more corrosive and less corrosive acids.
The next mechanism is
EMIC – Electrical Microbial Influenced Corrosion.
EMIC differs from CMIC in that it directly influences electron transfer at the surface of a metal. Whereas CMIC depends on chemical processes between the micro-organisms and the metal, with EMIC the micro-organisms take control. The overarching principle of EMIC is thus that electrons are extracted from the metal by direct intervention of a micro-organism. It is actually a number of tricks to extract energy from the metal. You are now entering an exciting area , because this is an area where a lot of research is still taking place.
Within our masterclass on Microbial influenced Corrosion, these mechanisms will be explained in more detail.