HVAF-Sprayed Bi-Layer Coatings: Effect of Microstructure and Chemical Composition of the Intermediate Layer on Corrosion Behavior

Abstract

Abstract In a variety of engineering applications, components are exposed to corrosive/erosive environment. Protective coatings are essential to improve the functional performances and/or extend the lifetime of the components. Thermal spraying as a cost-effective coating deposition technique offers high flexibility in coatings’ chemistry/morphology/microstructure design. However, the pores formed during spraying inherently restrict the use of coatings for corrosion protection. In view of the above gap to have a high quality coating, bi-layer coatings have been developed to boost the corrosion performance of the coatings. In a bi-layer coating, an intermediate layer is deposited on the substrate before spraying the coating. The electrochemical behavior of each layer is critical to ensure a good corrosion protection. The corrosion behavior of the layers strongly depends on coating composition and microstructure, which are affected by feedstock material and spraying process. In the present work, Cr3C2-NiCr top layer with different intermediate layers (i. e., Fe-, and Ni-based) were sprayed by high-velocity air fuel (HVAF) process. Microstructure analysis, as well as electrochemical tests, e.g., open-circuit potential (OCP) and polarization were performed. The results showed a direct link between the OCP of each layer in a bi-layer coating and corrosion mechanisms. It was found that the higher corrosion resistance of Ni-based intermediate layers than Fe-based coatings was due to higher OCP of the coating in the galvanic couple with top layers. Splat boundaries and interconnected pores reduced the corrosion resistance of the intermediate layers, however a sufficient reservoir of protective scale-forming elements (such as Cr or Al) improved the corrosion behavior.