| High strength aluminum alloys are widely used in aviation and aerospace industries because of their desirable strength/weight ratio that results from the alloy addition (e.g., Cu, Fe etc). However, this alloy addition causes pitting corrosion of the aluminum surrounding those intermetallic inclusions. One efficient way to inhibit the corrosion is through the protective coating system that contains the topcoat, primer, and conversion coating. The conversion coating is in direct contact with the alloy surface and is expected to provide both good corrosion protection and adhesion. The chromate conversion coating (CCC) has been widely used in aviation/aerospace industry and provides excellent active corrosion protection and adhesion to aluminum alloys. Unfortunately, the Cr(VI) is toxic and chromate is a carcinogen. Therefore, a trivalent chromium process (TCP) coating was developed as a drop-in replacement of CCC.;This dissertation focuses on a fundamental understanding of the formation mechanism, chemical structure, and basic electrochemical properties of the TCP coating on three high strength aluminum alloys: AA2024-T3, AA6061-T6, and AA7075-T6. The formation of the TCP coating is driven by an increase in the interfacial pH. The coating is about 50-100 nm thick and has a biphasic structure consisting of a ZrO2/Cr(OH)3 top layer and an AlF63-/Al(OH)3 interfacial layer. The coating contains hydrated channels and or defects.;The TCP coating provides good corrosion protection to all three aluminum alloys evidenced by e.g., increased Rp, decreased icorr, and higher Epit for the coated alloys than uncoated samples. However, the protection by TCP is increased for AA2024-T3 < AA7075-T6 < AA6061-T6. The protection mechanisms include (i) reducing oxygen reduction kinetics, (ii) partially blocking the mass transfer of dissolved O2 and ions to the metal, and (iii) insulating the electron transfer through the non-conductive oxide layer. The TCP coating (Cr(III)/Zr-based) provides better corrosion protection compared to other non-Cr alternatives of CCC, e.g., Ti/Zr- and Zn/Zr-based conversion coatings,;Another significant finding of this work is that the TCP coating provides some active corrosion protection by transiently forming Cr(VI) species (e.g., CrO42-). The formation involves two steps: (i) O 2 diffuses through the defects to the Cu-rich intermetallic sites where it is reduced to H2O2; (ii) H2O2 oxidizes nearby Cr(III) to Cr(VI).;The insights gained from this fundamental study provide information to improve the corrosion protection provided by TCP. For example, the coating contains defects through which dissolved O2 and ions can still diffuse to the underlying metal. Therefore, its corrosion protection can be improved by (i) compressing the coating structure and (ii) reducing the alloy sites where defects form. The former is achieved by optimizing the curing temperature and time and the corrosion resistance is increased by ~4x; the latter is through optimization of the deoxidation/desmutting time and solution and the corrosion protection property is improved by ~5x. |
