Effect of Multilayer and Gradient Structures on the Corrosion Resistance of (Zr,Hf,Al)N Coatings

Abstract

Severe micro-galvanic corrosion in harsh marine environments greatly limits the corrosion resistance of high-strength aluminum alloys. To address this issue, this study employs a filtered cathodic vacuum arc deposition (FCVAD) system to deposit three Al-(Zr,Hf,Al)N composite coatings with different structures on the surface of 7050 aluminum alloy: a constant-thickness multilayer (S1), a nanolayer thickness gradient (S2), and a compositional gradient (S3). Their microstructural evolution and initial electrochemical corrosion failure modes in a 3.5 wt.% NaCl solution are systematically investigated. The results indicate that, due to the lack of nanolayer interfaces and the compositional-gradient strategy forcing a lower coating thickness, the electrolyte in the S3 coating rapidly penetrates along through-thickness columnar grain boundaries and pinhole defects, thereby resulting in severe corrosion failure. In contrast, the S2 coating, designed by dynamically increasing the substrate rotation speed, exhibits superior protective performance, with its corrosion current density as low as 3.58 × 10−7 A·cm−2, and obtains the optimal low-frequency impedance(|Z|0.01 Hz). The increased interfacial density in the upper region of the S2 coating effectively interrupts the continuous growth of columnar grains and hinders the vertical diffusion of corrosive media, thereby transforming the failure mode from deep discrete pitting to shallow flake-like spallation. This study compares the short-term electrochemical failure modes of the three structured coatings mentioned above, providing a certain reference basis for the microstructural design of coatings in Cl-containing environments.