These heat tint layers promote the initiation and growth of corrosion pits even in seemingly
harmless environments [3,4].
Natural waters contain always a diversity of microbes and thus these seemingly harmless
environments might turn out to be aggressive. Microbes can change the environment's
corrosiveness either chemically or physically. This phenomenon is called microbially induced
corrosion (MIC). It is generally accepted that the ennoblement (increase of the open circuit
potential) of stainless steel surfaces detected all over the world in ocean waters, brackish waters,
rivers and even tap water is caused by microbial colonisation. This ennoblement can exceed the
pit initiation potential. However, the mechanism of ennoblement is not clear and many different
hypotheses and theories have been suggested during the years. One of these is the role of
manganese oxidising bacteria, which can oxidise Mn2+ to the strong oxidant Mn4+ causing the
potential of the metal to rise. Sulphate reducing bacteria (SRB) have often been detected together
with iron and manganese oxidising bacteria on failure areas of stainless steels. SRBs can lower
the redox potential within the deposits on the surface by producing sulphides (S2-). Another way
for SRBs to enhance the corrosion of stainless steel is to form thiosulphate (S2O3
2-). SRBs
consume sulphate (SO4
2-) by reducing it to sulphide, which can be oxidised to thiosulphate in the
presence of air. In simulated paper machine water, it has been demonstrated that thiosulphate
produced by the metabolic activity of SRBs caused severe pitting corrosion to stainless steel[2].
In this paper, failure analyses of two case histories of fire protection systems suspected to be
caused by MIC are presented.