Microstructural and electrochemical characterization of Ni-based bi-layer coatings produced by the HVAF process

• Published in: Surface & coatings technology Vol. 304; pp. 606 - 619
• Main Authors: Sadeghimeresht, E., Markocsan, N., Nylén, P.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 25.10.2016
• Subjects: Corrosion; EIS; HVAF; Manufacturing and materials engineering; Polarization; Production Technology; Produktions- och materialteknik; Produktionsteknik; Thermal spray; HVAF; Polarization; Corrosion; Thermal spray; EIS
• ISSN: 0257-8972; 1879-3347; 1879-3347
• DOI: 10.1016/j.surfcoat.2016.07.080

Bi-layer coating systems comprising a Cr3C2-NiCr coating with different underlying coatings of Ni, NiCr, NiCoCrAlY, and CoNiCrAlY were deposited on 304L stainless steel substrates using the high-velocity air fuel (HVAF) process. The corrosion behavior of the coating systems was studied using electrochemical and immersion tests in 3.5wt% NaCl at 25°C. The higher open-circuit potential (OCP) value of the NiCoCrAlY coating (−120mV/SCE) compared to the value of the Cr3C2-NiCr coating (−230mV/SCE) revealed that the underlying NiCoCrAlY coating was sacrificially protected by Cr3C2-NiCr coating. The polarization resistance (Rp) of the bi-layer coatings with the different underlying coatings of Ni, NiCr, NiCoCrAlY and CoNiCrAlY was approximately 77, 189, 487, and 74kΩ·cm2 respectively, while the value was 101kΩ·cm2 for the single-layer Cr3C2-NiCr coating and 30kΩ·cm2 for the 304L substrate, which confirmed a higher corrosion protection of the NiCoCrAlY coating. The electrochemical impedance spectroscopy (EIS) results showed that the corrosion resistance of the 304L substrate was significantly improved by adding an intermediate layer of NiCoCrAlY to the Cr3C2-NiCr coating. Results of the immersion tests confirmed that the underlying coatings in the different bi-layer coating systems acted as protective barriers. Moreover, the NiCoCrAlY coating showed the best corrosion protection among the investigated underlying coatings. •Corrosion initiated from the topcoat due to microgalvanic cells between NiCr and Cr3C2.•Corrosion propagated through the coating defects, e.g., pores, and interlamellar boundaries.•Bi-layer coating systems provide promising results in corrosion protection of the substrate.•NiCoCrAlY under coat that has a high OCP ensures maximum protection of the substrate.•Cr3C2-NiCr/NiCoCrAlY was the best system to protect the substrate.