The role of chloride ions in pitting of carbon steel studied by the statistical analysis of electrochemical noise

The role of chloride ions in metastable pitting of A516-70 carbon steel was studied based on the statistical analysis of recorded current and potential noise. It is shown that the amplitude and the occurrence frequency of current and potential transients, indicating the initiation of metastable pits, increase with the concentration of chloride ions. The time dependence of pit initiation rate (lambda) is well described by equations of lambda(t) = lambda(0)exp(at), 0 less than or equal to t < tau and lambda(t)= lambda(tau)exp(-bt), t greater than or equal to tau. The interaction between chloride ions and passive film has the reaction order of 3. The average peak potential depends upon the chloride concentration by a logarithm law. An analysis of the probability distribution of pit repassivation time vs. chloride concentration shows that the main role of chloride ions in pitting is to increase the chance of the breakdown of passive film, rather than to inhibit the surface repassivation. (C) 1999 Published by Elsevier Science B.V. All rights reserved.