Communication-Role of Trivalent Chromium on the Anti-Corrosion Properties of a TCP Conversion Coating on Aluminum Alloy 2024-T3

Electrochemical methods were used to study the role of Cr(III) on the anti-corrosion behavior of a trivalent chromium process conversion coating on AA2024-T3. The same conversion coating was investigated with and without Cr(III) added to the bath. Polarization curves (naturally-aerated 0.5 M Na2SO4 + 0.1% NaCl) revealed similar anodic current suppression by both coatings. Cathodic currents were suppressed more by the coating with Cr(III). Rotating disk voltammetric data revealed that cathodic currents for oxygen reduction were invariant with the rotation rate (rpm)(1/2) for the alloy coated with the Cr(III) conversion coating. The trivalent chromium process coating appears to inhibit oxygen reduction by providing a diffusional barrier and by blocking sites (Cr(OH)(3)) for O-2 chemisorption on cathodically-active intermetallics. (C) 2018 The Electrochemical Society.