A base metal, such as iron (Fe) goes into aqueous solution as positively charged cation, Fe. As the metal is oxidized under anaerobic conditions by the protons of water, H ions are reduced to form molecular H. This can be written in the following ways under acidic and neutral conditions respectively:
:Fe + 2 H  →  Fe + H
:Fe + 2 HO  →  Fe(OH) + H
Usually, a thin film of molecular hydrogen forms on the metal. Sulfate-reducing bacteria oxidize the molecular hydrogen to produce hydrogen sulfide ions (HS) and water:
:4 H + SO  →  HS + 3 HO + OH
The iron ions partly precipitate to form iron (II) sulfide. Another reaction occurs between iron and water producing iron hydroxide.
:Fe + HS  →  FeS + H
:3 Fe + 6 HO  →  3 Fe(OH) + 6 H
The net equation comes to:
:4 Fe + SO + H + 3 HO  →  FeS + 3 Fe(OH) + OH
This form of corrosion by sulfate-reducing bacteria can, in this way, be far more harmful than anaerobic corrosion.