Synergistic Effect of Impurities on the Corrosion Behavior of Fe-18%Cr-14%Ni Alloys in Concentrated Nitric Acid

Synergistic effect of Si, Mn, C, P, and S on the transpassive corrosion of high-purity (HP) Fe-18%Cr-14%Ni (HP18%Cr-14%Ni) alloys was studied in boiling 13 kmol/m3 nitric acid (HNO3). The specimens were fabricated by a cold crucible method in a high-vacuum chamber to reduce contamination. The addition of Si < 1% and Mn < 2% had no effect on the corrosion rate of HP18%Cr-14%Ni alloys, and the addition of Si < 1% also had no effect on the corrosion rate of HP18%Cr-14%Ni-1%Mn alloys, although 1% Si induced intergranular corrosion in both alloys. Thus, HP18%Cr-14%Ni-1%Mn-0.5%Si alloys were selected to evaluate the effect of minor impurities such as C, P, and S. The addition of 100 ppm P, or co-addition of 100 ppm C, 100 ppm P, and 100 ppm S, induced inter-granular corrosion of the same degree in solution-annealed HP18%Cr-14%Ni-1%Mn-0.5%Si alloys. This result suggested a synergistic effect of Si and P to induce intergranular corrosion, since a single addition of 0.5% Si or 100 ppm P did not induce intergranular corrosion in HP18%Cr-14%Ni alloys.