Sulphate reducing bacteria (SRB) are a group of different species of bacteria that are well known for their possible influence in corrosion processes. They are bacteria or Archaea that can reduce sulphate (SO42-) and use it as an source for harvesting energy, since it is their terminal electron acceptor.
We as humans, but also mammals in general use oxygen as an electron acceptor and use carbons (food) as an electron donor for this but SRB originated from a time when there was no oxygen in earth’s atmosphere. As a consequence you can find SRB’s in places without oxygen, for example, deep underground, pipelines with hydrocarbons or in marine sediments. Sulphate reduction is not only performed by bacteria but also by some archaea. The reduction process can be separated into two parts, the reduction of sulphate to sulphite and sulphite to sulphide.
What is the Corrosion mechanism of SRB’s?
SRB’s and CMIC
We are up to a stage where scientists have proved two different mechanisms of SRB’s causing corrosion. The first is Chemical corrosion caused by the production of hydrogen sulphide (H2S) and electronic corrosion with direct electron transfer (DET). Chemical corrosion is linked with hydrogen usage and sulphide production. SRB consume hydrogen for their metabolic processes. Most species of the Desulfovibrio genus are able to grow in a medium with hydrogen as the energy source.
Hydrogen sulphide is known for its smell (rotten eggs), its toxicity and its corrosive potential against iron. The corrosion caused by hydrogen sulphide is often centred in pits and creates a black crust of iron sulphide (FeS). Not all metals have the same susceptibility for corrosion due to H2S . Aluminium is resistive due to the hydrogen sulphide forming corrosion inhibiting complexes. And it’s interesting to note that, in comparison with Iron, Copper is even more susceptible to corrosion by hydrogen sulphide due to it being a stronger oxidizing agent.
A more in dept explanation of the process is that during metabolic processes hydrogen is oxidized to hydrogen ions and bacteria gain energy through this process. The role of hydrogen is more important than just an energy carier. Hydrogen is used to control the redox level of cytochromes. You could almost say that the SRB can control the throttle. As a consequence, some bacteria also experience a hydrogen burst in their initial phase of growth which could last a few hours. After this burst, almost no hydrogen is excreted by the bacteria instead hydrogen sulphide is excreted. These are relatively new insights from recent literature, but interesting to reflect to practical corrosion cases.
CMIC as an isolated corrosion mechanismsalone of SRB is something to worry about but usually results into corrosion processes of a few tenths of mm per year.