Chromate Inhibition of Filiform Corrosion on Organic Coated AA2024-T3 Studied Using the Scanning Kelvin Probe

Abstract

The influence of in-coating strontium chromate pigment dispersions on the kinetics and mechanism of HCl-induced filiform corrosion (FFC) affecting polyvinyl butyral coated AA2024 T3 aluminum alloy is studied using a scanning Kelvin probe. FFC initiation and propagation are detected by imaging time-dependent changes in local free corrosion potential distribution patterns. Area rates of coating delamination fall from to zero as the volume fraction is increased from zero to 0.06. Increasing over the same range produces minimal change in filament-head values but depresses filament tail and intact coating values by up to 0.3 V. It is proposed that filament tail depression results from increased polarization of cathodic reduction reactions at sites of high pH, where insoluble is the predominant reduction product. It is further proposed that intact coating depression results from reversible, nonfaradaic adsorption of at the oxide-covered aluminum surface. Both effects are implicated in FFC inhibition, as is pH buffering by which acts in conjunction with adsorption to forestall anodic attack at the coating-substrate interface. © 2004 The Electrochemical Society. All rights reserved.