Influence of plasma spraying parameters on microstructure and corrosion resistance of Cr₃C₂-25NiCr cermet carbide coating

In this work, Cr₃C₂-25NiCr coatings were deposited on 410 stainless steel substrate by using the atmospheric plasma spray technique, at varying spaying parameters. The porosity and microhardness, adhesion strength and corrosion behaviour of coatings were examined in relation to these spraying parameters.

The microstructure of prepared coatings was examined by using scanning electron microscopy. The coating compositional analysis was performed by using X-ray diffraction (XRD) technique. The corrosion resistance of coated steel was investigated by potentiodynamic polarization. Results indicate that optimal factors for minimalizing the porosity were as follows: 10 g/min feed rate, 600 A plasma current and 100 mm spraying distance. The spraying factors influencing corrosion resistance of coating were also evaluated.

Under this optimal condition, the porosity of coating reached its minimal value of 3.1 per cent. The microhardness and adhesion of coatings also reached their maximum values of 64.8 Rockwell Hardness scale C and 60 MPa, respectively. XRD results indicated the transformation of Cr₃C₂ originating from Cr₃C₂-25NiCr source powder into Cr₇C₃ and Cr₂₃C₆ crystalline phases, due to the high temperature during spraying process. The undetectable Cr₃C₂ peaks indicating that this phase was remained in coating at very low concentrations. The potentiodynamic polarization and salt spray tests confirmed the highest corrosion resistance for the coating prepared by optimal spraying parameters.

The application of Cr₃C₂-NiCr cermet carbit coating for protection of steel from corrosion-erosion is very promising.