The Formation, Structure, Electrochemical Properties and Stability of Trivalent Chrome Process (TCP) Coatings on AA2024

We report on the formation, structure, electrochemical properties and stability of trichrome process coatings (TCP) on AA2024-T3. The coating is 50-100 nm thick and forms over most of the alloy surface. It consists of hydrated zirconia (ZrO2 center dot 2H(2)O) and its formation under open circuit conditions is driven by an increase in the interfacial pH caused by (i) dissolution of the oxide layer and (ii) oxygen reduction mainly at the Cu-rich intermetallics. The coating appears biphasic with a hydrated zirconia overlayer and a fluoroaluminate interfacial layer (KxAlF3+x). Cr(III) oxide is coprecipitated with the hydrated zirconia with Cr-rich regions in and around pits. Some anodic and cathodic protection is provided by physically blocking Al-rich sites (oxidation) and Cu-rich IMCs (reduction). This is evidenced by a 10 x greater polarization resistance, R-p, for the TCP-coated alloys, and suppressed anodic and cathodic currents (air saturated 0.5 M Na2SO4, room temperature) in potentiodynamic polarization scans. In the short-term (4-h immersion), the coating is stable, as evidenced by unchanging passivation performance. While no Cr(VI) species were detected in the coating immediately after formation, Raman spectroscopy revealed consistent evidence for transient formation of Cr(VI) species in the coating after immersion in different air-saturated electrolyte solutions from 1-14 days. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3607980] All rights reserved.