The kinetics of chloride-induced filiform corrosion on aluminum alloy AA2024-T3

A scanning Kelvin probe (SKP) is used to study the kinetics and mechanism of filiform corrosion (FFC) as it occurs on polyvinyl butyral-coated AA2024-T3 aluminum alloy. In order to control the quantity of electrolyte entering the filament-head population, FFC is initiated by injecting a measured quantity of aqueous HCl directly into a penetrative coating defect. Subsequently, time-dependent free corrosion potential (E-corr) maps are obtained by repeated in situ scanning of a fixed sample area at 20 degreesC and 93% relative humidity. The number of moles of HCl (N-HCl) introduced during initiation is varied systematically and the resulting E-corr distributions analyzed to provide detailed information on filament dimensions and FFC propagation rates. Equations are derived relating the volume of individual filament-head electrolyte droplets to filament-head radius. Equations are also derived relating the total (population) filament-head electrolyte volume to N-HCl. Using these relationships a series of kinetic equations are derived predicting propagation rates for individual filaments and filament populations under circumstances where rates of coating delamination are surface controlled or controlled by underfilm ion migration. Comparison with experimental kinetics shows that for the system studied the rate of coating delamination is surface controlled, i.e., proportional to the interfacial area existing between the filament-head electrolyte droplet and the metal substrate. (C) 2003 The Electrochemical Society.