Relationship between Pitting and Intergranular Corrosion of Nitrogen-bearing Austenitic Stainless Steels

摘要

Nitrogen-bearing types 304 and 316 stainless steels thermally aged at 823, 873 and 923 K for various durations were investigated to understand the relation between the sensitized microstructure (intergranular corrosion) and pitting corrosion resistance. The sensitized microstructure was assessed as per ASTM A262 practice A (electrolytic etch test) and electrochemical potentiokinetic reactivation (EPR) tests; and pitting corrosion resistance was evaluated in an acidic chloride medium by potentiodynamic anodic polarization method. The results indicated that the pitting resistance decreased as the degree of sensitization increasee, and this was attributed to the formation of a heterogeneous microstructure consisting of Cr-rich M_(23)C_6 carbides, Cr-depleted regions and the carbide/matrix interfaces resulting from the sensitizing treatment. Pitting attack was found at triple points, grain boundaries and precipitate/matrix interfaces. Time-temperature- sensitization-pitting diagrams were developed for both the alloys based on the results, interrelating the sensitized microstructure and the pitting attack.