The influence of bacteria on the passive film stability of 304 stainless steel

The stability of stainless steel 304 in the presence of aerobic and anaerob ic bacteria was investigated in a continuous flow system using electrochemi cal impedance spectroscopy and scanning electron microscopy (SEM) examinati ons. The results show that the open circuit potential (OCP) of stainless st eel was ennobled (shifted in the electropositive direction) by about +150 m V in the presence of the aerobic bacteria. The observed change of OCP in th e electropositive direction can be explained by an increase in the rate of the cathodic reaction. Also the presence of an aerobic biofilm led to a dec rease in the polarization resistance of stainless steel is not only due to the growth of micropits, as shown from SEM micrographs, but also due to thi nning of the passive film. Ln the presence of only Pseudomonas fragi, the e lectrochemical impedance response showed a capacitive behavior with RP on t he order of 500 k Omega. The addition of anaerobic sulfate reducing bacteri a (SRB) to the test medium decreased RP to 12 k Omega due to an increase in localized corrosion, as indicated by SEM examination. (C) 1999 Elsevier Sc ience Ltd. All rights reserved.