Effect of biofilm on ennoblement and localized corrosion of stainless steel in fresh dam-water

AISI 304 stainless steel specimens were exposed in-situ to fresh dam-water for nearly 2 years. Open circuit potential (OCP) of the specimens becomes remarkably ennobled after exposure for about 40 days. The ennobled OCP of coupons is greatly affected by both immersion depth and sunlight. It is found that ammonium, nitrate, nitrite and chloride ions are enriched in the biofilm. It is suggested that the reduction of nitrate may be partly responsible for the OCP ennoblement. Localized corrosions at welds and crevices of specimens are attributed to both the OCP ennoblement of specimens and the chloride ion enrichment in the biofilm.