CREVICE REPASSIVATION POTENTIAL OF ALLOY 22 IN HIGH-NITRATE DUST DELIQUESCENCE TYPE ENVIRONMENTS

摘要

The nitrate ion (NO_3~-) is an inhibitor for crevice corrosion of Alloy 22 (N06022) in chloride (Cl~-) aqueous solutions. Naturally formed electrolytes may contain both chloride and nitrate ions. The higher the ratio R = [NO_3~-]/[Cl~-] in the solution the stronger the inhibition of crevice corrosion. Atmospheric desert dust contains both chloride and nitrate salts, generally based on sodium (Na~+ and potassium (K~+). Some of these salts may deliquescence at relatively low humidity at temperatures on the order of 150°C and higher. The resulting deliquescent brines are highly concentrated and especially rich in nitrate. Electrochemical tests have been performed to explore the anodic behavior of Alloy 22 in high chloride high nitrate electrolytes at temperatures as high as 150°C at ambient atmospheres. Naturally formed brines at temperatures higher than 120°C do not induce crevice corrosion in Alloy 22 because they contain high levels of nitrate. The inhibitive effect of nitrate on crevice corrosion is still active for temperatures higher than 100°C.