Pit Formation in the Cathodic Polarization of Passive Iron IV. Repair Mechanism by Molybdate, Chromate and Tungstate

The effect of molybdate, chromate and tungstate on the pitting resistance of iron has been investigated, and the repair mechanism is considered. Molybdate enhanced the pitting resistance with increasing the concentration, which was attributed to the polymolybdate’s ability to adsorb or precipitate as a salt. Chromate inhibited the pitting formation effectively at higher concentration, but pitting was expedited at lower concentration. The inhibition of pitting by chromate was related to the deposition of a protective layer involving chromium hydroxide or oxide at a lower concentration; however, chromate stimulated the anodic reaction, which created a situation in favor of pitting formation. Tungstate had no positive effect on the pitting resistance; pitting was accelerated at a higher concentration (≥0.2 M), which was rationalized by assuming that the polytungstate neither adsorbed on passive iron nor precipitated as a salt with iron ion.