William R. Pinc

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

The combination of chloride ions and H 2 O 2 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 H 2 O 2 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 H 2 O 2 was replaced by an alternative oxidizing agent, NaClO 4 , cerium-based conversion coatings were deposited without substrate dissolution. Chloride ions and H 2 O 2 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.

Electrochemical and Structural Changes in Cerium Based Conversion Coatings during Exposure to Salt Spray

Cerium-based conversion coatings on aluminum alloy 2024-T3 substrates were characterized after various exposure times in salt spray corrosion testing. Coatings posttreated in phosphate solutions exhibited no visible corrosion after seven days. Impedances doubled for these coatings after 12 hours of testing and a ~150 nm thick alumina layer developed after 24 hours of exposure, leading to the conclusion that posttreated coatings facilitated the formation of a protective alumina layer. As-deposited coatings exhibited significant corrosion after 24 hours of exposure and had impedances that were an order of magnitude lower than post-treated coatings. Subsurface voids present underneath coatings corroded into nodules of alumina on the surface of asdeposited coatings. In contrast, the voids corroded into small pits with little corrosion product present in post-treated coatings, which was attributed to a protective oxide present around the voids.