Effect of Silicon and Manganese Additions on the Transpassive Corrosion of High-Purity Fe-18% Cr-1 4% Ni Alloys

Effects of Si and Mn on the transpassive corrosion of high-purity Fe-18% Cr-14% Ni alloys were studied in 13 kmol/m³ (0.36 kmol/ft³) nitric acid (HNO₃) and 13 kmol/m³ HNO₃ containing Ce⁴⁺ ions. Alloy specimens of 99.99% purity were fabricated by a cold-crucible method in an Ar gas atmosphere to prevent contamination and were used to isolate the effect of each target element. Results showed that Si and Mn were not the main cause of intergranular corrosion of the alloy in 13 kmol/m³ HNO₃. In 13 kmol/m³ HNO₃ containing Ce⁴⁺ ions, Si accelerated (at concentrations ≤ 1%) and mitigated (at concentrations ≥ 1.5%) intergranular corrosion of the alloy. The 5% Si alloy showed very low corrosion potentials within the passive region in the test solution, and the absence of intergranular corrosion in the alloy could be attributed to the low corrosion potential induced by the Fe- and Ni-rich surface film. Mn slightly mitigated intergranular corrosion of Si-doped, high-purity alloys in 13 kmol/m³ HNO₃ containing Ce⁴⁺ ions, although Mn-doped, high-purity alloys showed intergranular corrosion in the solution.