The effect of NiCr intermediate layer on corrosion behavior of Cr₂O₃ ceramic coated materials

The NiCr plasma-sprayed deposition, as an intermediate layer, is normally used to improve the adhesion between the metal substrate and the ceramic coating. Defects, such as pores and micro-cracks, generally exist in the ceramic layer, and lead to corrosion beneath the coating. The present work used plasma-sprayed specimens, such as the Cr₂O₃ ceramic coatings with and without a NiCr intermediate layer and the single NiCr coatings, etc. which were deposited on a SUS316L stainless steel substrate. The results of corrosion tests and electrochemical experiments indicated that the corrosion resistance of the 80NiCr (80Ni-20Cr) coating was weaker than that of the SUS316 substrate, while that of 50NiCr (50Ni-50Cr) was better than that of the substrate. The existence of 80NiCr plasma-sprayed intermediate layer decreased the corrosion resistance of the Cr₂O₃ ceramic coated specimen, while the 50NiCr increased it. The microstructure analysis demonstrated that both the 80NiCr and 50NiCr plasma-sprayed coatings consisted of the matrix and a second-phase structure. Energy dispersive spectroscopy (EDS), wave dispersive spectroscopy (WDS) and X-ray diffractometry (XRD) analysis showed that the components of the matrix and the second-phase in 50NiCr were different from those in 80NiCr. In addition, this paper preliminarily discusses the corrosion mechanism of 80NiCr and 50NiCr intermediate layers. The different chromium content in the matrix of the 80NiCr and 50NiCr coatings might have caused the difference in the corrosion resistance between them. It can be predicted that the decrease of porosity by means of plasma spraying or sealing treatment of the ceramic coating, as well as by regulating the components of the NiCr intermediate layer, would improve the corrosion resistance of Cr₂O₃ ceramic coated materials. (C) 2000 Elsevier Science S.A. All rights reserved.