“Smart” micro/nano container-based self-healing coatings on magnesium alloys: A review

Coating technologies are a commonly used way to protect metals against corrosion. However, with more and more severe service environments of materials, many protective coating systems often are not environmentally friendly or toxic as in the case of chromates. Based on the world's abundant ideal magnesium (Mg) and its alloy, the smart self-healing anticorrosive coating can autonomously restore the damaged part of the coating according to the environmental changes, strengthen the corrosion protection ability, and prolong its service life. This paper reviews the research progress of smart self-healing coatings on Mg alloys. These coatings mostly contain suitable corrosion inhibitors encapsulated into micro/nano containers. Moreover, the different self-healing mechanisms and functionalities of micro/nano containers are discussed. The micro/nano containers range from inorganic nanocontainers such as mesoporous nanoparticles (silica (SiO2), titanium dioxide (TiO2), etc.), over inorganic clays (halloysite, hydrotalcite-like, zeolite), to organic nanocontainers such as polymer microcapsules, nanofibers, chitosan (CS) and cyclodextrin (CD), as well as, carbon materials such as graphene and carbon nanotubes and hybrids such as metal organic frameworks. The functioning of micro/nano containers can be divided in two principal groups: autonomous (based on defect filling and corrosion inhibition) and non-autonomous (based on dynamic bonds and shape memory polymers). Moreover, multi functionalities and composite applications of various micro/nano containers are summarized. At present, significant progress has been made in the preparation methods and technologies of micro/nano containers. Achieving long-term self-healing properties of coatings sensing of coating failure and early warning after self-healing function failure can be expected as the main development direction of self-healing corrosion protection coatings in the future.