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

Abstract

The prevention of pitting corrosion in aerospace aluminum alloys by the application of protective sol–gel coatings requires a thorough understanding of pit formation kinetics and morphology developments in such surface coating systems. This study reports results of chemical and electrochemical methods of pitting corrosion tests for bare and sol–gel coated Al 2024-T3 alloy. Specific attention is focused on the characterization of pitting in samples 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 H 2SO 4, including a standard CASS solution. The exposure of bare samples to these environments produced extensive surface corrosion, but pits were not observed for sol–gel coated samples. Anodic polarization tests with potentials above that required for pitting in bare samples were used to initiate pitting corrosion in sol–gel coated samples. A corrosion current monitoring test provided a method of controlling the pit formation process, which provides well-defined pits in terms of spatial density and geometry. A two-stage kinetic phase in pit development was observed and correlated with pit morphological developments in sol–gel coatings. An initial low current stage was associated with pit penetration through the coating to the surface and the secondary high current stage was associated with an active growth stage which grew in sub-coating surface interface regions. Results of this research provide a basis for designing and improving corrosion protection systems based on the application of sol–gel coatings.