The Role of Chromium (III) in the Corrosion Inhibition of AA2024-T3 By Trivalent Chromium Process Coatings

High strength aluminum alloys (e.g., AA2024-T3) are widely used as aircraft structural components. The inclusion of two major alloying elements, Cu and Fe, make the metal more susceptible to localized damage in the form of pitting and intergranular corrosion. The susceptibility of the alloy to localized corrosion is due to the fact that galvanic cells form between the Cu- and Fe-rich intermetallic phases, sites that support relatively rapid oxygen reduction reaction kinetics, and the aluminum, which undergoes dissolution. The intermetallic phases are more noble than the surrounding aluminum matrix leading to the localized pitting or dissolution of the aluminum near the intermetallics. Inhibiting this localized corrosion is essential for achieving long performance lifetime of these structural components.

Conversion pretreatment coatings are one component of a multilayer coating system that is typically used to protect aerospace aluminum alloys from corrosion. The multi-layer coating system consists of a conversion coating, a primer and an organic topcoat. The focus of current research in our group is on the application of alternate, more environmentally-friendly conversion coatings that serve as suitable replacements for the currently-used chromate conversion coatings (CCC). The corrosion inhibition provided by the multilayer coating system depends on the chemical composition, physical structure and thickness and uniformity of the conversion coating. The need for some level of corrosion inhibition by the conversion coating arises when there is coating breakthrough, in other words a scratch through the coating system that exposes some bare metal. Conversion coating thicknesses are  generally in the 50-200 nm range and they provide corrosion inhibition, at least in part, by providing a barrier to oxygen and water/electrolyte accessibility to the metal surface.

The trivalent chromium process (TCP) conversion coating is an environmentally-friendly potential replacement for the currently used chromate conversion coating. TCP is regarded as one of the leading drop-in replacements for CCC. Prior work has shown that the TCP coating inhibits corrosion on aluminum alloys, in part, by serving as a barrier layer. A key question with regard to the mechanism of corrosion inhibition is the role of Cr(III) or the transiently formed Cr(VI) that has been shown to form in the coating. We conducted studies using a  trivalent chromium  conversion coating from Henkel Corp., named Alodine® T5900™ , and a formulation of the same coating devoid of Cr(III). The formation, structure and corrosion inhibition of the coatings was investigated on   AA2024-T3. The coating was formed by immersion on a  polished, degreased and deoxidized specimen. These conversion coatings are about 100 nm thick when formed by immersion and consist of a biphasic structure with a hydrated zirconia (ZrO₂·nH₂O) top layer and a fluoroaluminate (e.g., K_xAlF_3+x) interfacial layer.  Electrochemical methods were used to assess the degree of corrosion inhibition. Despite having the same physical structure, the conversion coating containing the Cr(III) exhibited a polarization resistance 10x greater than that for the coating devoid of it. Polarization curves revealed that the TCP coating provided cathodic inhibition. Rotating disk voltammetry was to assess the effect of the coating on the oxygen reduction reaction (ORR)  kinetics. The results demonstrated that the TCP coating, as formed, inhibits the ORR. The reduced rate of oxygen reduction is attributed to (i) the formation of a poorly conducting layer on the Cu-rich intermetallics that blocks sites for O₂ chemisorption and serves as a barrier to electron transfer and (ii) the formation of diffisusion barrier layer for O₂ to the reaction sites.