Estimation of corrosion cavity growth rate for predicting system service life

Abstract

It has been shown that under real, practical conditions, values of the critical pit depth of the system, x cr, and typical service life, t s, impose significant restrictions on the corrosion current densities (averaged and initial) and thus on the potential and concentration drops that might be observed in the corrosion cavity. Thus, it is shown that, if x cr does not exceed the order of 1–10 mm, and if the order of t s is not less than 1 year, initial corrosion current densities in real, open pits cannot exceed values of 10 −4–10 −3 A/cm 2, with the understanding that the polarization curve (corrosion current density vs. potential and the surface concentration of species) does not change as the pit propagates. Simple analytical expressions for predicting potential and concentration drops in open corrosion cavities, and for predicting cavity propagation rate in systems containing uni- and bi-valent anions in stagnant electrolytes under well-mixed, external conditions have been obtained. These expressions can be used to extrapolate the results of short-term corrosion experiment to future times. In particular, it is shown that, if localized corrosion takes place in an electrolyte having a conductivity of the order of that of seawater, it is possible to neglect the potential drop in an open corrosion cavity and hence to consider the rate of pit propagation as being constant under constant environmental conditions. The same conclusion holds for the pitting of 316 L steel, practically for any composition of the external electrolyte.