The microbially influenced corrosion (M.I.C.) is linked to bacterial presen
ce. This type of corrosion can affect any industrial area in contact with m
icroorganisms. Bacteria are able to form films on the majority of elements
immerged in aquatic environment. Those biofilms can induce localized corros
ion on metallic surfaces, the biofilm formation is the obligatory first ste
p of the biocorrosion The study of biofilms composition formed on courant s
teels immerged in a natural fresh water has been made with electron microsc
opic techniques coupled with different X-ray analysis. The different result
s permit us to propose schemas of biofilms formed on a carbon steel (38CrS2
) and three stainless steels (304, 304 L, 316L). The relations between iden
tified products near bacteria and metabolic activity of the microorganisms
have been established in order to evidenced the potential corrosive action
of living organisms in the biofilm.