| Abstract: |
In a variety of engineering applications, components are exposed to corrosive 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 inherent pores formed during spraying limit 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 improve the corrosion resistance 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 and intermediate layers (Fe-, Co- and Ni-based) were sprayed by high-velocity air fuel (HVAF) process. Microstructure analysis, as well as various electrochemical tests, e.g., open-circuit potential (OCP), polarization and electrochemical impedance spectroscopy (EIS) were performed. The results showed a direct link between the corrosion potential (Ecorr) of the intermediate layer and the corrosion mechanisms. It was found that the higher corrosion resistance of Ni-based intermediate layers than Fe- and Co-based coatings was due to higher Ecorr of the coating in the galvanic couple with top layers. Inter-lamellar boundaries and interconnected pores reduced the corrosion resistance of intermediate layers, however a sufficient reservoir of protective scale-forming elements (such as Cr or Al) improved the corrosion behavior.
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