Role of nitrogen on the corrosion behavior of austenitic stainless steels

The role of nitrogen in the mechanisms of localized corrosion resistance and repassivation were electrochemically investigated using a nitrogen-bearing austenitic stainless (SUS316L) steel in 0.1 and 0.5 M Na2SO4 solutions and a 3.5% NaCl solution. Almost 100% of the nitrogen Compounds dissolved into the bulk solution after crevice corrosion were transformed into NH3. That is, the mole amount of ammonia in the solution was approximately equivalent to the mole amount of nitrogen dissolved in the steel. This suggests that NH4+ consuming H+ in the pit controlled the local decrease of pH and promoted the repassivation. NO3-N and NO2-N were not detected by chemical analyses in the high potential and thermodynamically stable zone as NO3- in the potential-pH diagram. The repassivation in nitrogen-bearing SUS316L steel in a 0.1 M Na2SO4 solution was studied using the scratching electrode technique, which measured the partially destroyed passivation films on the steel. This technique showed that nitrogen dissolved in the steel has a strong repassivation capacity. (C) 2002 Elsevier Science Ltd. All rights reserved.