Investigation of Anodic Behavior of Nickel in H2SO4?Solutions Using Galvanostatic Polarization Technique. II. Initiation and Inhibition of Pitting Corrosion by Some Inorganic Passivators

摘要

The anodic polarization of nickel in dilute sulfuric acid media was investigated in the absence and presence of ClO4 - ions, at a constant current density. The anodic polarized curve is characterized by a sudden jump in potential owing to the decay of H-over voltage, followed by two oxidation arrests, passive and O2 evolution regions. The presence of ClO4 - ions distorts the polarized curve owing to the damage in the passive film with the initiation of localized pitting corrosion, at pitting potential, Epit. Epit displaces into active direction with rising amounts of the ClO4 - ions. The presence of various concentrations of Na2HPO4, Na2MoO4 and Na2CrO4 shift the Epit into more positive values, confirming the suppression of pitting corrosion. The anions of these salts displace the Epit into the more noble direction due to a competition with the ClO4 - ions. The inhibition effect towards the pitting corrosion of Ni follows the sequence Na2HPO4 > Na2MoO4 > Na2CrO4. Thermodynamics activation energies required for the destruction of the passive film, Ea, are estimated and explained in the absence and presence of the different anions.