Corrosion Behavior of High-Purity Fe-Cr-Ni Alloys in the Transpassive Condition

The corrosion behavior of high-purity (99.99%) Fe-Cr-Ni alloys was investigated in 13 kmol/m³ [0.36 kmol/ft³] nitric acid (HNO³ ) with and without ceric (Ce⁴⁺) ions to clarify the effect of purity on the transpassive corrosion of stainless steel. Almost no intergranular corrosion was observed in the high-purity alloys, although the corrosion rate of the matrix region was almost the same as that of a commercial stainless steel with the same Cr and Ni contents. Because of the improved intergranular corrosion resistance of the high-purity alloys, the beneficial effect of the purification became significant in the corrosion condition where the grain separation became pronounced in the commercially pure alloys. A purity of 99.99% was good enough to use the solution-annealed Fe-Cr-Ni alloys as the base materials to examine the effect of impurities and/or alloying elements on transpassive corrosion. However, further purification will be necessary to use in the aged condition since intergranular corrosion was observed in aged high-purity alloys.