Effect of Polarization Rate in Potentiokinetic Anodic Polarization of iron in 1N H2SO4

Abstract

Abstract Potentiokinetic data are reported for single crystal and polycrystalline specimens of high purity (56 ppm total impurities) and sulfurized (150 ppm sulfur) iron in 1N H2SO4. Eight polarization rates, ranging from 20 to 1100 mV/min, were investigated. The shape and position of potentiokinetic anodic polarization curves were found to vary appreciably and reproducibly with polarization rate. The observed logarithmic variation of three characteristic passive and transpassive current densities (ix) with polarization rate (η̇) can be expressed by a relation of the form where k and r are constants. The exponent r is given the name “polarization-rate-sensitivity.” At faster polarization rates, current densities tended to be higher and secondary passivity more pronounced. Secondary passivity merged with ordinary passivity at a material-dependent “critical” polarization rate, at which passive current densities were nearly independent of potential. Passive current densities increased with increasing potential at faster polarization rates, corresponding to Langmuir adsorption conditions, and decreased with increasing potential at slower polarization rates, corresponding to Temkin adsorption conditions. The presence and absence of sulfur and of grain boundaries had a negligible quantitative effect on the appearance of the potentiokinetic anodic polarization curve of iron in dilute sulfuric acid solution. However, a small effect of these chemical and physical variables could be expressed by different relations between currents and polarization rate.