Electrolyte Based Modeling of Corrosion Processes in Sulfuric Acid Mixtures, Part 1: Nonoxidizing Conditions

The corrosion behavior of stainless steels and Ni-based alloys in nonoxidizing sulfuric acid mixtures at concentrations below approximately 30 mol/kg H2O is modeled. The redox potential in sulfuric acid across a broad concentration range, from 0 to 80 mol% (0 to 95.6 wt%), is determined by the proton reduction reaction. Thus, in the absence of other oxidizing species, sulfuric acid behaves as a nonoxidizing (reducing) acid. The calculated corrosion rates, using an electrochemical model up to about 30 mol/kg H2O (about 75 wt%), are in agreement with experimental values. The predicted polarization curves of anodic and cathodic processes show that the alloys in these environments are in active dissolution regime, consistent with experimental data. The model predictions of corrosion rates in H2SO4+HCl, H2SO4+HF, and H2SO4+HCl+HF mixtures are in agreement with weight-loss corrosion data. The corrosion rate of alloys in the nonoxidizing sulfuric acid mixtures correlated to an equivalent alloy composition given by (Ni0.7-Cr0.1+Mo+0.5 W). The effect of alloying elements under these conditions may be related to their beneficial effect on active dissolution and proton reduction reaction rates.