THE PASSIVE BEHAVIOR OF 304 STAINLESS-STEEL IN DIMETHOXYETHANE SOLUTIONS

304 stainless steel, an alloy that is more corrosion resistant in aqueous solutions than carbon steel, was examined to assess the processes involved in forming a passive film in an aprotic, nonaqueous solvent such as dimethoxyethane (DME). Electrochemical tests show that the passive ranges of 304 stainless steel in ''dry'' DME solutions containing LiAsF6, LiClO4 and TBAP were similar. Breakdown of the passive state occurred above 1700-1800 mV (sce). The high anodic potentials were due to the expected superior nature of the air-formed film on 304 stainless steel. Removing the surface oxide him on 304 stainless steel resulted in rapid repassivation in the DME/0.5 M LiAsF6 solutions at potentials below breakdown. Post-test examinations did not clearly define the passivating mechanism in DME/0.5 M LiAsF6 solutions but, in DME/saturated LiAsF6, evidence indicated repassivation around 1800-2000 mV (sce) by salt him formation. Small water additions to DME/0.5 LiAsF6 solutions caused slight surface activation of the stainless steel, but no premature breakdown of the passive film. Small acid additions or removal of water below 50-100 ppm triggered surface pitting of the 304 stainless steel above the breakdown potential in DME/0.5 M LiAsF6 solutions. A critical amount of water appears to be required to passivate the stainless steel surface and neutralize acids produced during electrolyte oxidation.