Contributions of Anions, Biofilms and Alloying Components to the Ennoblement of Stainless Steel in Natural Waters

摘要

Biofilms from fresh water and sea water usually ennoble the open circuit potential of 316 stainless steel by about 400 mV and 260 mV respectively. In 3% sodium chloride a shift in open circuit potential (OCP) to the noble direction by about 180 mV is also observed. In triply distilled water with oxygen the ennoblement range is smaller when compared to other systems. The nature and stability of the film formed under such conditions have been examined by photoelectrochemical techniques. A high value of photopotential was observed in presence of a biofilm compared to that in presence of other anions like chloride and oxygen. This indirectly indicates that negatively charged bacteria enhance the oxide film thickness which favours ennoblement. Also, the percentage amount of various major elements like chromium, molybdenum, nickel and iron are correlated with the ennoblement range. The comparison of ennoblement for various materials indicates that 316 stainless steel has optimum concentration of various elements for higher ennoblement. This also explains why inherently corrosion resistant materials have a smaller ennoblement range. These results give additional evidence for the adsorbed inhibitor theory proposed by CECRI.