Polarization Behavior of Iron-Cobalt Alloys in Sulfuric Acid Solutions

Anodic polarization behavior of iron, cobalt and seven iron-cobalt alloys in H2-saturated 1, 5 and 10 N sulfuric acid solutions was investigated at 22 C using a potentiostatic technique. Cobalt and the 95 w/o cobalt alloy did not passivate in any acid studied. In 10 N acid, specimens containing 60 w/o or more cobalt failed to exhibit a normal active-to-passive transition. Specimens containing 75 and 80 w/o cobalt were passivated only marginally in 5 N acid. Specimens which did not have a normal passive region exhibited “secondary” passivation at potentials just prior to visible oxygen evolution. Tafel slopes for anodic dissolution and oxygen evolution varied from 0.040 to 0.065 and 0.065 to 0.095 volt, respectively and appeared to be composition and pH independent. Corrosion potentials were linear functions of pH over the pH range +1.87 to −1.08; dEcorr/dpH varied from −0.035 to −0.045, independent of composition. Ecorr decreased linearly with composition from a value of −0.360 volt vs SCE for pure cobalt to −0.505 volt for pure iron. Critical current density increased slightly with increasing cobalt content and obeyed the expression: logicr = logicro + kpH. The passive current density increased with both increasing cobalt content and increasing acid concentration. Values of io, H2, icorr and βc” for iron and cobalt in H2-saturated, 1, 5 and 10 N acid were determined from cathodic polarization curves. Hydrogen overvoltage at 1 ma/cm2 for iron and cobalt appeared to increase linearly with pH; dη/dpH ≅ −0.045. Effect of time on the cathodic polarization parameters for iron in H2-saturated 1 N acid was determined.