Hydrothermally prepared layered double hydroxide coatings for corrosion protection of Mg alloys – A critical review

Layered double hydroxide (LDH) coatings emerge as a sound strategy to mitigate corrosion of magnesium (Mg) alloys owing to their unique anion-exchange capability to entrapping aggressive anions, such as chloride. The most common approach for fabricating protective LDH coatings upon the surface of Mg alloys is hydrothermal treatment, which is ascribed to their simple manipulation, rich diversity in chemistry and structure of resultants, and high coating adhesion via chemical bonding. This article reviews the roles of key processing variables of hydrothermal manufacturing of LDH coatings to provide insights for design and optimising LDH coatings with satisfactory corrosion protection to Mg alloys. The selected key variables include chemistry and microstructure of Mg alloy substrate, components of LDH solution, hydrothermal operational conditions (mainly involving solution pH, reaction temperature and reaction time duration), and anionic types used in post anion-exchange treatment. The contributions of those variables to the growth behavior of LDH coatings are discussed. The relationships between LDH coating structure and its corrosion mitigation are also established. Finally, the strength and limitations of existing work are critically articulated and future research directions are proposed accordingly.