Anodic Polarization Behavior of Fe-Si Alloys in Sulfuric Acid Solutions

Anodic polarization behavior of annealed, high purity Fe and five binary Fe-Si alloys in hydrogen saturated, 1, 5, and 10N sulfuric acid solutions was investigated at 22 ± 1 C (72 F) using a potentiostatic technique. All specimens exhibited active, passive, and transpassive behavior in all acid environments. Additions of 12 arid 15 wt % Si eliminated the critical current density plateau, significantly decreased the value of critical current density, and increased the width of the passive region. Corrosion potentials, E_corr, for pure Fe and the Fe-Si alloys were linear functions of pH over the pH range −1.07 to +1.09. Slope dependency of the corrosion potential with pH (dE_corr/dpH) varied from −0.046 to −0.068, independent of composition. Critical potentials, E_cr, for the 88Fe-12Si and 85Fe-15Si alloys shifted linearly in the noble potential direction with decreasing pH; slope dependency (dE_cr/dpH) for these alloys was −0.053 and −0.066, respectively. As the Si content was increased from 0 to 9 wt %, the passive current density, i_p, increased. Maximum i_p occurred with the addition of 6 wt % Si. Addition of 12 wt % Si markedly decreased the value of i_p. All specimens exhibited transpassive Tafel behavior. Visible oxygen evolution was not observed for the 88Fe-12Si and 85Fe-15Si alloys.