Evaluation of Theories for the Initial Stages of Pitting Corrosion

Abstract

<p>It is assumed in this study that the protection imparted by passive layers is dependent upon the balance between the breakdown of the protective film and the healing of the produced flaws. Therefore, the occurrence of a pit must coincide with a change in the local conditions in a flaw which did not heal.</p> <p>Using this hypothesis, five proposed theories for pitting corrosion are compared using data published in the literature. The processes assumed to occur during pit nucleation are simulated. The solution compositions and potentials from reported experimental determinations of pitting potentials are used as initial conditions. The potential and concentrations at the base of the flaw are computed by using mass transport equations.</p> <p>The values obtained are then used to compare the five models. For each theory, an expression depending on potential and concentrations at the metal surface was identified, so that where a theory is correct, this expression is smaller (or larger) than a parameter. This parameter, should be constant, and independent of the particular set of experimental conditions (e.g. pH, chloride concentration in the bulk solution), as all the calculations are made for conditions corresponding to the pitting potential, i.e. conditions borderlining between pitting and passivity.</p> <p>The particular case of iron in borate buffers is considered here. The computed results indicate that the most probable mechanism controlling pitting is the adsorption of the chloride ions at the metal surface. The repassivation by precipitation of ferrous hydroxide is always thermodynamically favoured, but apparently slow.</p>