Effect of Al content on the corrosion resistance of TixAly coatings deposited on a titanium alloy

This study investigates the structural features, surface mechanical properties, and corrosion performance of Ti x Al y coatings deposited on Ti6Al4V alloy, focusing on the influence of Al content. Coatings were fabricated by electrospark deposition (ESD) method using electrodes containing 25-100 at.% Al. Microstructure, phase constitution and mechanical response were characterized by SEM, XRD, EDS, and Vickers microhardness testing. Corrosion behavior was evaluated by potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution. The results indicate that electrode composition significantly influenced phase evolution, coating thickness, morphology, and electrochemical response. The coatings exhibited a three-layered microstructure comprising a uniform region, a transition zone, and an interfacial layer. With increasing Al content, the dominant phase of the coating changed from Ti 3 Al to TiAl and then to TiAl 3 . Simultaneously, the chemically uniform region thickened and the transition zone narrowed, while the coating/substrate interface remained gradual. All coatings increased surface hardness to ∼700 HV (about twice that of the substrate), and the 50%Al coating achieved the highest cross-sectional microhardness (∼1200 HV). The 25%Al sample exhibited the most uniform and least rough surface, whereas 75%Al showed the highest roughness, significant heterogeneity, and surface cracking. Electrochemical results indicated that 50%Al provided the highest charge-transfer resistance and the lowest corrosion current density, representing the best corrosion performance. In contrast, Al-rich coatings displayed low-frequency inductive behavior and higher susceptibility to localized corrosion. Overall, an intermediate Al content, particularly 50%Al, offers the most favorable combination of structural stability, microhardness enhancement, and corrosion resistance in chloride medium.