Protection of magnesium alloys: From physical barrier coating to smart self-healing coating

With promising mechanical properties and biodegradability, magnesium (Mg) and its alloys are considered as potential candidates in both industrial and biomedical fields. However, rapid corrosion is a major obstacle that severely prevents their widespread applications. Surface modification is a cost-effective way to prevent the corrosive medium from contacting with Mg substrate. Generally, fabricating a dense and stable coating as a physical barrier can effectively inhibit the corrosion of Mg substrates. However, in some complex service environments, the coatings are inevitable to be damaged during use. In this case, it is essential to equip coatings with smart self-healing abilities. This article reviews recent advances in conventional physical barrier coatings and smart self-healing coatings on Mg alloys for industrial and biomedical applications. For physical barrier coatings, metal-based coatings, metal hydroxides and hydrotalcite coatings, metal oxide coatings, silane sol-gel coatings, polymer coatings, Ca–P coatings, and others like graphene-based coatings are mainly reviewed. For smart self-healing coatings, chemical conversion coatings, encapsulation coatings, and layer-by-layer assembled coatings are reviewed.