Review of the state of art of Li-based inhibitors and coating technology for the corrosion protection of aluminium alloys

The quest for novel alternatives to hexavalent-chromium-based corrosion inhibitors is of utmost significance and urgency. Strict international health and safety regulations, due to growing concerns regarding the impact of hexavalent chromium on human health and the environment, have pushed the commercial introduction of many alternative inhibitor types, but the implementation of alternative active protective primers for structural parts in the aerospace industry is still pending. This endeavour has proven to be remarkably challenging, as the potential replacement coating types must meet numerous functional requirements encompassing cost-effectiveness and exceptional corrosion protection for intrinsically corrosion susceptible aerospace aluminium alloys. In recent years, considerable attention has been drawn to lithium salts as environmentally friendly corrosion inhibitors forming the basis for a novel active protective coating technology. The involvement of lithium ions has been shown to play a pivotal role in the conversion process of aluminium alloy surfaces by stabilizing the reaction products, thereby facilitating the gradual development of a protective layer with a multi-layered configuration, which exhibits considerable variability in morphology, depending on local chemical and electrochemical conditions. The versatility of the lithium-based corrosion protection extends to their application as corrosion inhibiting pigments in organic coatings or as a pre-treatment, directly forming conversion layers, thereby enhancing their practical implementation. However, previous chromate replacement reviews only introduced the promising outcomes provided by the lithium technology, omitting key details of its development and formation mechanism. This paper critically reviews and summarizes the studies conducted to date on lithium-based inhibitor technologies for the corrosion protection of aluminium alloys as well as topics to be investigated in the future.