EFFECT OF BIOFILMS ON CREVICE CORROSION OF STAINLESS-STEELS IN COASTAL SEAWATER

The effect of biofilms an crevice corrosion of stainless steels (SS) UNS S31603 (type 316L SS), 531725 (type 317LM SS), N08904 (type 904L SS), and N08367 (6XN) in coastal seawater was investigated using the remote crevice assembly technique. One set of naturally initiated tests and one set of preinitiated tests were performed. For UNS N08367, anodes in natural initiation tests did not corrode, while preinitiated corrosion did not propagate in natural or control seawater. Biofilms did not significantly affect initiation times for UNS S31603 and 531725, while for the corroded samples of UNS N08904, biofilms significantly decreased crevice corrosion initiation times. Biofilms greatly increased the propagation rate for UNS S31603, 531725, and N08904, as measured by maximum and average depths of attack, weight loss, and current density. Theoretical weight losses (W-T) calculated using Faraday's law and the measured current densities were in good agreement with the measured weight losses (W-m). For anodes in preinitiated tests, current densities calculated from cathodic polarization curves also were in good agreement with the measured current densities. The increased propagation rate of crevice corrosion was caused by an increase in the cathodic reaction rate, which was due to the action of biofilms. Effective control conditions were achieved in the long-term tests by a combination of heat treating the water at 80 degrees C before exposure and periodically exchanging the cathode panels after 1 h of immersion in fresh water at 60 degrees C.