Effect of annealing on microstructure evolution and corrosion resistance of an amorphous Cr-Al-C coating

Highlights: • Amorphous Cr-Al-C coating firstly transformed into solid solution, and then into Cr₂AlC MAX phase during annealing. • All the coatings exhibited passivation behaviors, and amorphous coating possessed the lowest passive current density. • All the passive films exhibited p-type semiconductor characteristics. • The passive film on the amorphous coating was most protective for containing a relatively high content of Cr₂O₃. The microstructure evolution of an amorphous Cr-Al-C coating prepared by magnetron sputtering was investigated. With increasing annealing temperature, three kinds of coatings, including amorphous coating, partially crystallized coating ((Cr,Al)₂C_x solid solution), and fully crystallized coating (Cr₂AlC MAX phase), were obtained. The electrochemical measurements in 3.5 wt. % NaCl solutions indicated that all of the coatings exhibited passivation behaviors, and amorphous coating possessed the lowest passive current density. All the passive films exhibited p-type semiconductor characteristics, and the passive film on amorphous coating provided the best protection due to the presence of a relatively high content of Cr₂O₃.