Structural changes in high-velocity oxy-fuel thermally sprayed WC–Co coatings for improved corrosion resistance

• Published in: Surface & coatings technology Vol. 188; pp. 1 - 6
• Main Authors: Godoy, C., Lima, M.M., Castro, M.M.R., Avelar-Batista, J.C.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.11.2004
• Subjects: Corrosion; Duplex coating; Thermal spray; Wear; Duplex coating; Corrosion; Wear; Thermal spray
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2004.07.123

In this work, an attempt to improve the corrosion resistance of WC–Co coatings was made by either modifying the coating composition with the addition of a NiCr alloy or by performing a post-melt treatment, in order to disrupt the intrinsic lamellar microstructure of the coating. A duplex system comprising a WC–Co top layer and a NiCrAl interlayer was also produced. Five coating systems deposited onto an AISI 1020 steel substrate were investigated: a WC–Co, a duplex WC–Co/NiCrAl system, NiCrAl, and as-sprayed and post-melted 50% (WC–Co)+50% (NiCr). Immersion tests were performed in 1N HCl, and potentiodynamic tests were carried out in 1N H 2SO 4. The highest mass loss was recorded for the NiCrAl/steel system in immersion tests. Chemical analyses of the solution indicated that the introduction of a NiCrAl interlayer between the steel and WC–Co coating provided very effective protection of the substrate against corrosion in HCl solution. Iron was present in all solutions analyzed, but its content was significantly lower in the duplex coating solution. The post-melt treatment decreased the dissolution of alloy components, confirming that a non-lamellar structure improves the corrosion resistance. Potentiodynamic curves in H 2SO 4 showed that both non-duplex and duplex WC–Co systems exhibited a similar corrosion behavior. However, chemical analyses of the solutions after potentiodynamic tests indicated a higher dissolution of iron for the non-duplex WC–Co system in comparison to the duplex one. The post-melt treatment of the 50% (WC–Co)+50% (NiCr) coating and the addition of a corrosion-resistant NiCrAl interlayer to the WC–Co/steel system were both effective in improving the corrosion performance of thermally sprayed WC–Co coatings in a 1N H 2SO 4 medium.