Mechanistic analysis of improving the corrosion performance of MAO coatings on Ti-6Al-4V alloys by annealing

Titanium and its alloys find extensive application within the realms of dentistry and orthopedics due to their remarkable chemical stability. Nevertheless, they are still susceptible to corrosion in harsh environments. Micro arc oxidation (MAO) emerges as a potent methodology for enhancing the corrosion resistance of titanium and its alloys, achieved through the generation of a ceramic stratum atop the material's surface. The corrosion resistance of MAO coatings in clinical applications is closely related to their microstructure. This manuscript systematically explores the effect of annealing temperature on the phase composition and microstructure of MAO coatings. Furthermore, the evolution of the surface properties and the crystal structure of coatings is meticulously discussed and illustrated. The results show that the corrosion resistance of coatings exhibits an ascending trajectory followed by a subsequent decline, coinciding with the augmentation of the annealing temperature. This behavior can be attributed to the structural evolution of coatings caused by the gradual increase of the annealing temperature. At the optimum temperature (400 degrees C), the annealing treatment leads to the crystallization of the coating as well as to a decrease in the surface porosity, resulting in the formation of a compact and continuous multilayer microstructure. The decrease in porosity can be attributed to the diffusion-reaction of oxygen from the air with titanium from the discharge channel (titanium oxide), resulting in the formation of TiO2 particles that effectively fill the pores. The decrease in corrosion resistance of the coatings after annealing at 800 degrees C is due to the phase transition of TiO2 which leads to the expansion of cracks inherent in the oxide layer to form A-H cracks. Therefore, this study shows that the microstructure of MAO coatings can be adjusted by post annealing treatment (400 degrees C) to obtain a desirable oxide layer with appropriate porosity, crystallinity and chemical composition, thus improving its corrosion resistance.