Characterization of pitting corrosion in bare and sol-gel coated aluminum 2024-T3 alloy

The prevention of pitting corrosion in aerospace aluminum alloys by the app lication of protective sol-gel coatings requires a thorough understanding o f pit formation kinetics and morphology developments in such surface coatin g systems. This study reports results of chemical and electrochemical metho ds of pitting corrosion tests for bare and sol-gel coated Al 2074-T3 alloy. Specific attention is focused on the characterization of pitting in sample s coated with vinyl-silicate and epoxy-silicate sol-gel coatings. Specimens were exposed to a variety of chemically aggressive environments, based on 3-5% NaCl solutions with addition of HCl and H2SO4, including a standard CA SS solution. The exposure of bare samples to these environments produced ex tensive surface corrosion, but pits were nor observed for sol-gel coated sa mples. Anodic polarization tests with potentials above that required for pi tting in bare samples were used to initiate pitting corrosion in sol-gel co ated samples. A corrosion current monitoring test provided a method of cont rolling the pit formation process, which provides well-defined pits in term s of spatial density and geometry. A two-stage kinetic phase in pit develop ment was observed and correlated with pit morphological developments in sol -gel coatings. An initial low current stage was associated with pit penetra tion through the coating to the surface and the secondary high current stag e was associated with an active growth stage which grew in sub-coating surf ace interface regions. Results of this research provide a basis for designi ng and improving corrosion protection systems based on the application of s ol-gel coatings. (C) 2001 Elsevier Science B.V. All rights reserved.