Influence of Impurities on Intergranular Corrosion of Extra High Purity Austenitic Stainless Steels

Abstract

An intergranular corrosion is observed in austenitic stainless steels exposed to high temperature, concentrated nitric acid (HNO3) solution with highly oxidizing ions. It is an important degradation mechanism of austenitic stainless steels for use in a nuclear fuel reprocessing plant. The intergranular corrosion is caused by the segregation of impurities to grain boundaries and the resultant formation of active sites. Extra High Purity (EHP™) austenitic stainless steel was developed with conducting the new multiple refined melting in order to suppress the total harmful impurities less than 100ppm. The intergranular corrosion behavior of EHP alloys with various impurities was examined in boiling HNO3 solution with highly oxidizing ions to find a correlation between the intergranular corrosion and the impurities of EHP alloys. A good correlation was confirmed between the degree of intergranular corrosion and the corrosion rate. The relationships between the corrosion rate and the impurities content of EHP alloys was determined using a multiple regression analysis. The influence on corrosion rate became small in order of B, P, Si, C, S and Mn. It was important to control B in intergranular corrosion behavior of EHP alloys.