Intergranular corrosion for extra high purity austenitic stainless steel in boilng nitric acid with Cr(VI)

Ioka, Ikuo  ; Kato, Chiaki   ; Kiuchi, Kiyoshi; Nakayama, Jumpei

Austenitic stainless steels suffer intergranular attack in boiling HNO with oxidants. The intergranular corrosion is mainly caused by impurities at the grain. An extra high purity austenitic stainless steel (EHP alloys) was developed with conducting the new technique in order to suppress harmful impurities less than 100 ppm. The corrosion behavior of type 310 EHP alloy in HNO with highly oxidants was investigated. The straining, aging and recrystallizing (SAR) treated type 310 EHP alloy showed superior corrosion resistance for intergranular attack than solution annealed (ST) type 310 EHP alloy with same impurity level. Boron segregation at the grain boundary was detected in only ST specimen using a FTE method. It is believed that the segregated boron along the grain boundaries in type 310 EHP alloy was one of main factor of intergranular corrosion. The SAR treatment was effective to restrain the intergranular corrosion for type 310 EHP alloy with B up to 7 ppm.