Four austenitic and two duplex stainless steels were studied in three different paper mills. Thin aerobic biofilms were observed on the specimens. No SRB (sulphate reducing bacteria) could be detected. The alloy, AISI 304L, suffered from pitting corrosion caused by thiosulphate which was a decomposition product of sodium dithionate. Occasionally very high total chlorine concentrations were noticed in the biofilm but the chlorine was predominantly combined as organic chlorine. A high amount of chlorine can lead to an increased risk of pitting corrosion of stainless steels.

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