The aerospace industry uses a variety of metals and alloys, primarily aluminum alloys, for the different structural components in aircraft including the fuselage, landing gear, tail fins, and the many other parts. As these components are metal based, the control and mitigation of corrosion in service is of paramount importance. The military and civilian aviation sectors spend considerable sums of money annually on corrosion prevention and maintenance. Components made with aluminum alloys are generally placed in service with a multilayer coating system to prevent corrosion. This coating system consist of a conversion coating, primer, and topcoat. The coating system can inhibit corrosion in multiple ways, but the general mechanism involves barrier layer protection that reduces contact of the environment with the underlying metal. Legacy conversion coatings and primers have chromate (Cr(VI)) as a component. While chromate is an excellent corrosion inhibitor, it is toxic and constitute a significant environmental hazard. There is a current technological need to (i) replace chromate conversion coatings and primers with nonchromateor zero-chrome coating systems and (ii) understand how to properly pretreat the aluminum alloys surfaces for application of such surface finishes. The trivalent chromium process (TCP) coating is the leading replacement non-chromate conversion coating and praseodymium and new aluminum-based coatings are replacement primers being investigated. There is also a scientific need to better understand how to properly pretreat aluminum alloys in order to properly form conversion coatings and primers that effectively prevent environmental degradation and corrosion. These surface pretreatments typically include abrasion and polishing, wet chemical cleaning, and deoxidation or desmutting.In this dissertation project, fundamental research was conducted to better understand how surface pretreatments of aluminum alloys impact the formation of TCP conversion coatings and the mechanisms by which TCP conversion coatings inhibit electrochemical corrosion in laboratory measurements and during accelerated degradation testing. Research was also conducted to learn how effectively TCP coatings can seal porous anodic oxide coatings on aluminum alloys thereby improving the barrier properties and electrochemical corrosion resistance. The specific surface pretreatments investigated included laser cleaning and hyperpassivation of aluminum alloyAA2024-T3, in comparison with conventional wet chemical processing. Additionally, studies were performed to learn the mechanisms and effectiveness of TCP sealants for anodic coatings formed on this aluminum alloy during sulfuric acid (SA) and sulfuric acid/boric acid (SABA)anodization.
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