Abstract
Trivalent chromium process conversion coatings are being investigated as alternates for commonly used chromate conversion coatings on aerospace aluminum allloys. In our studies, the TCP coating is formed by immersion on a polished, degreased and deoxidized specimen. The ca. 100 nm-thick biphasic coating consists of a hydrated ZrO2 layer that is about 50 nm thick with an Al-O-F interfacial layer that is also approximately 50 nm thick. Cr(OH)3 is co-deposited within the hydrated ZrO2 layer at less than 5 at. %. There is a need to better understand the mechanisms by which these coatings inhibit corrosion on these alloys and how well the coatings perform during different accelerated degradation tests. In this presentation, we will report on the formation, chemical composition and electrochemical properties of one commercial TCP coating (Bonderite T5900) on AA2024-T3. The corrosion inhibition provided by the coating depends on the coating bath formulation as well as the physicochemical properties (roughness and elemental composition) of the aluminum alloy after degreasing and deoxidation. The coating provides both anodic and cathodic protection by serving as a barrier layer. In terms of the cathodic inhibition, the kinetics of the oxygen reduction reaction are reduced because the coating functions as a diffusional barrier and coating species chemisorb on Cu intermetallic sites to block O2 chemisorption. In this presentation, we will report on the electrochemical characterization of coated and uncoated AA2024-T3 before and after exposure to a 14-day SO2 atmospheric corrosion test (ASTM G87). SEM and EDXS, optical and stylus surface profilometry and weight loss measurements were used to assess the coating peformance. The results show that the TCP coating is stable on the alloy surface during exposure to the SO2/H2O environment and significantly reduces the corrosion damage and metal loss as compared to the uncoated controls.
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