Formation and stability of lithium protective layer for corrosion protection of AA2024-T3 and AA2198-T8 aluminium alloys

Abstract

As a possible replacement for chromate-based coatings in the aerospace industry, lithium leaching coatings have been widely studied in recent years. The formation mechanism of protective layer on AA2024-T3 aluminum alloys has been investigated intensively. In order to explore more possibilities for its application in aerospace industry, AA2198-T8 aluminum alloys is also be considered in research for its unique chemical composition which contains lithium. In this thesis, three kinds of different polymer coatings which are non-inhibiting coating, coating containing Li2CO3 and proprietary industrial Li2CO3-loaded coating XP420 are used. These three polymer coatings were applied on the AA2024-T3 and AA2198-T8 aluminum alloys samples. A defect was artificially made on the these samples. Then the samples were divided into two groups, one group is used directly for the experiment to investigate the formation of protective layer, one week neutral salt spray test were applied to another group in order to form the protective layer in advance and later used for the experiment. The second group is mainly focus on investigating the stability of protective layer. What is more, low volume of electrolyte was used during experiment considering the real environment of coating’s application. The research results showed that: (1) Under low volume electrolyte of 5ml aqueous 0.1M NaCl solution, the protective layer is successfully formed; (2) Li2CO3-loaded coating shows better corrosion inhibition performance compared to the non-inhibiting coating; (3) AA2198-T8 aluminum alloy containing lithium in its element composition further contributes to the formation of protective layer; (4) In general, proprietary industrial Li2CO3-loaded coating XP420 are more stable when exposed to the corrosion condition