Layered double hydroxide (LDH) coatings on magnesium (Mg) alloys shine brightly in the field of corrosion protection because of their special ion-exchange function. State-of-the-art steam coating as a type of LDH film preparation technique has emerged in recent years because only pure water is required as the steam source and its environmentally friendly LDH coating fits the current need for green development. Moreover, this coating can effectively inhibit the corrosion of the Mg alloy substrate due to the chemical bonding between the coating and the Mg alloy substrate. This review systematically explains cutting-edge advancements in the growth mechanism and corrosion behavior of LDH steam coatings, and analyzes the advantages and limitations of the steam-coating method. The influencing factors including pressure, CO₂/CO₃²⁻, aluminum content of the substrate alloy, solution type, and acid-pickling pretreatment, as well as the post-treatment of steamcoating defects, are comprehensively elucidated, providing new insights into the development of the in situ steam-coating technique. Finally, existing issues and future prospects are discussed to further accelerate the widespread application of Mg alloys.

In this study, the effects of intermetallic compounds (Mg₁₇Al₁₂ and Al₈Mn₅) on the Mg-Al layered double hydroxide (LDH) formation mechanism and corrosion behavior of an in-situ LDH/Mg(OH)₂ steam coatings on AZ80 Mg alloy were investigated. Citric acid (CA) was used to activate the alloy surface during the pretreatment process. The alloy was first pretreated with CA and then subjected to a hydrothermal process using ultrapure water to produce Mg-Al-LDH/Mg(OH)₂ steam coating. The effect of different time of acid pretreatment on the activation of the intermetallic compounds was investigated. The microstructure and elemental composition of the obtained coatings were analyzed using FE-SEM, EDS, XRD and FT-IR. The corrosion resistance of the coated samples was evaluated using different techniques, i.e., potentiodynamic polarization (PDP), electrochemical impedance spectrum (EIS) and hydrogen evolution test. The results indicated that the CA pretreatment significantly influenced the activity of the alloy surface by exposing the intermetallic compounds. The surface area fraction of Mg₁₇Al₁₂ and Al₈Mn₅ phases on the surface of the alloy was significantly higher after the CA pretreatment, and thus promoted the growth of the subsequent Mg-Al-LDH coatings. The CA pretreatment for 30 s resulted in a denser and thicker LDH coating. Increase in the CA pretreatment time significantly led to the improvement in corrosion resistance of the coated AZ80 alloy. The corrosion current density of the coated alloy was lower by three orders of magnitude as compared to the uncoated alloy.