Understanding the Mechanism of Ennoblement of Stainless Steel: A multidisciplinary Approach

摘要

Ennoblement is a phenomenon exhibited by stainless steel (SS) exposed to natural waters. It is characterized by an approximately 400 mV increase in corrosion potential. This increase in corrosion potential can aggravate pitting corrosion. Biofllms growing on SS surfaces are hypothesized to cause this phenomenon. We expose SS to natural waters. After Ennoblement of the samples occurred, microbial deposits and inorganic surface chemistry were studied using XPS and other surface analytical tools. The major accomplishments of this project were (1) to demonstrate that stainless steel Ennoblement in natural fresh water arises from manganic oxide mineralization, (2) to induce Ennoblement under laboratory conditions using pure cultures of the manganese oxidizing genus Leptothnx discophora,0 and (3) to deteimine that microbially deposited MnO2 can be electrochemically reduced to Mn2+ while Fe is oxidized; manganese oxyhydroxide, MnOOH, is an intermediate product. These findings open the door to assessing and controlling the corrosive impact of Ennoblement by providing a rational explanation for the phenomenon and by allowing the process to be manipulated under controlled conditions. It is the first step in developing new corrosion resistance strategies for stainless steel and other passive metals and alloys.