The effects of C, P, S, and N on the transpassive corrosion of high purity(HP) Fe-18Cr-14Ni alloys were studied in 1 3 kmolm 3 \[0.36 kmol/ft 3\] nitric acid(HNO3) and 13 kmolm 3 HNQ containing ceric(Ce 4+) ions. Alloy specimens of 99.99 % purity were fabricated by acold-crucible method in argon(Ar) gas atmosphere to prevent contamination and used to isolate the effect of each target element. The results showed that C and P were the responsible elements for transpassive intergranular corrosion in 13 kmolm 3 HNO3. The cntical concentrations, preventing intergranular corrosion, were less than 500 ppm for solution annealed HP18Cr-14Ni alloys. The addition of only 50 ppm of each impurity caused deep intergranular corrosion in HP1 8Cr-14Ni alloys corroded in 1 3 kmolm 3 HNO3 with Ce 4. ions. Thus the responsible elements and the critical concentrations for intergranular corrosion were different in both the test solutions. Aging treatment at 973 K for lh degraded the corrosion resistance of all the impurity doped HP18Cr-14Ni alloys. Especially, 500 ppm C showed a detrimental effect on the corrosion resistance of the alloy in 13 kmolm 3 HNO3 through sensitization.
Keywords: austenitic stainless steel, carbon, impurity, intergranular corrosion, nitric acid, nitrogen, phosphorus, segregation, sulfur, transpassive corrosion