Characterization of Localized Surface States of Al 7075-T6 during Deposition of Cerium-Based Conversion Coatings

Abstract

The combination of chloride ions and H2 O2 in solutions used to deposit cerium-based conversion coatings led to localized dissolution of aluminum alloy 7075-T6 substrates. Potentiodynamic scans indicated that exposure of the alloy to a solution containing 0.3 M chloride ions and 1 M H2 O2 led to active dissolution. This process resulted in selective etching of the aluminum alloy substrate that produced a nonuniform surface and voids that penetrated a few micrometers into the alloy. When H2 O2 was replaced by an alternative oxidizing agent, NaClO4, cerium-based conversion coatings were deposited without substrate dissolution. Chloride ions and H2 O2 selectively etch aluminum alloy 7075-T6 due to electrochemical reactions that take place. The reactions readily dissolved the native oxide that was present, allowing for the aluminum substrate along with embedded intermetallic particles to be exposed to the electrolyte, which propagated localized dissolution. © 2010 The Electrochemical Society.