INHIBITION OF THE CREVICE CORROSION OF IRON IN CHLORIDE SOLUTIONS BY CHROMATE

The crevice corrosion of iron in neutral solutions is promoted by chloride ions and inhibited by chromate. For any given concentration of chloride in the range investigated (10-4-0.6 M/1), the crevice corrosion of iron can be inhibited by using a sufficient amount of chromate, with the concentration of chromate required decreasing with the concentration of chloride. A linear relationship is observed between the logarithm of the chloride ion activity and the logarithm of the minimum activity of chromate ion required for protection against crevice corrosion. This relationship is explained on the basis of competitive adsorption between aggressive (CI-) and inhibitive (CrO4=) ions within the crevice, with each ion adsorbing according to a Temkin isotherm. The steady-state electrode potential of corroding iron within crevices is -620 to -660 mV vs. Ag/AgCl, independent of bulk solution composition and in reasonable agreement with the thermodynamic value calculated for an iron crevice or pit. © 1979, The Electrochemical Society, Inc. All rights reserved.