Effect of Nitrogen Addition on the Intergranular Corrosion of 304L Stainless Steel in Nitric Acid Medium

Critical components in reprocessing plants like continuous dissolver and centrifugal extractor demand austenitic stainless steels (SS) with good corrosion resistance coupled with high wear resistance in nitric acid medium. High nitrogen containing, impurity controlled, and vanadium-added 304LN1 (0.132% N), 304LN2 (0.193% N), and 304LN3 (0.406% N) SS with improved hardness were developed and corrosion studies were performed as per ASTM A262 Practice-A & C test and double loop electrochemical potentiokinetic reactivation test. The 304LN stainless steels were evaluated in forged, hot rolled, and hot rolled + heat treated conditions (948 K: 1, 10, 25, 50, 100, 260 h; 1,223 K: 1 h; 1,323 K: 1 h; and 1,473 K: 1 h). The study revealed good corrosion resistance for 304LN1 and 304LN2 SS as per ASTM A262 Practice-C test in sensitized condition (948 K/1 h). The degree of sensitization (DOS) and corrosion rates of high nitrogen 304L SS increased during sensitization heat treatment (948 K) done up to a certain aging duration, and thereafter desensitization was observed with decrease in corrosion rate and % DOS. Among the high nitrogen 304L SS, 304LN3 with 0.4% nitrogen content showed accelerated inter-granular corrosion associated with grain droppings. Heat treatment of these high nitrogen 304L SS at solution annealing temperature gave encouraging results. This study increases the confidence in considering high nitrogen alloyed 304L SS with improved hardness for reprocessing applications by achieving adequate corrosion resistance with controlled alloying elements and impurities, and proper heat treatment/microstructure.