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

The effect of biofilms an crevice corrosion of stainless steels (SS) U NS S31603 (type 316L SS), 531725 (type 317LM SS), N08904 (type 904L SS ), and N08367 (6XN) in coastal seawater was investigated using the rem ote crevice assembly technique. One set of naturally initiated tests a nd one set of preinitiated tests were performed. For UNS N08367, anode s in natural initiation tests did not corrode, while preinitiated corr osion did not propagate in natural or control seawater. Biofilms did n ot significantly affect initiation times for UNS S31603 and 531725, wh ile for the corroded samples of UNS N08904, biofilms significantly dec reased crevice corrosion initiation times. Biofilms greatly increased the propagation rate for UNS S31603, 531725, and N08904, as measured b y maximum and average depths of attack, weight loss, and current densi ty. Theoretical weight losses (W-T) calculated using Faraday's law and the measured current densities were in good agreement with the measur ed weight losses (W-m). For anodes in preinitiated tests, current dens ities calculated from cathodic polarization curves also were in good a greement with the measured current densities. The increased propagatio n rate of crevice corrosion was caused by an increase in the cathodic reaction rate, which was due to the action of biofilms. Effective cont rol conditions were achieved in the long-term tests by a combination o f heat treating the water at 80 degrees C before exposure and periodic ally exchanging the cathode panels after 1 h of immersion in fresh wat er at 60 degrees C.