Recent progress in self-repairing coatings for corrosion protection on magnesium alloys and perspective of porous solids as novel carrier and barrier

Featuring low density and high specific strength, magnesium (Mg) alloys have attracted wide interests in the fields of portable devices and automotive industry. However, the active chemical and electrochemical properties make them susceptible to corrosion in humid, seawater, soil, and chemical medium. Various strategies have revealed certain merits of protecting Mg alloys. Therein, engineering self-repairing coatings is considered as an effective strategy, because they can enable the timely repair for damaged areas, which brings about long-term protection for Mg alloys. In this review, self-repairing coatings on Mg alloys are summarized from two aspects, namely shape restoring coatings and function restoring coatings. Shape restoring coatings benefit for swelling, shrinking, or reassociating reversible chemical bonds to return to the original state and morphology when coatings broken; function self-repairing coatings depend on the release of inhibitors to generate new passive layers on the damaged areas. With the advancement of coating research and to fulfill the demanding requirements of applications, it is an inevitable trend to develop coatings that can integrate multiple functions (such as stimulus response, self-repairing, corrosion warning, and so on). As a novel carrier and barrier, porous solids, especially covalent organic frameworks (COFs), have been respected as the future development of self-repairing coatings on Mg alloys, due to their unique, diverse structures and adjustable functions.