ESEM evaluation of Inconel-625 thermal spray coating (HVOF) onto stainless steel and carbon steel post brine exposure after tensile tests

High-velocity-oxy-fuel (HVOF) coating process is widely used in Oil and Gas Industry due to improved corrosion, wear and abrasion resistances of the coating. Coatings applied by the HVOF process exhibit density, hardness and bond strength characteristics that can significantly outperform operating components. However, surface preparation and spraying conditions could play a crucial role in the produced coating adherence and integrity to the base material; consequently, proper preparation of the substrate surface and proper selection of coating parameters yields sound and durable coatings. The present study investigates effect of surface preparation on the coating–substrate adherence. Inconel-625 was HVOF sprayed onto both stainless steel and carbon steel substrates. Coated specimens were exposed to brine solution for 2 and 3 weeks periods. Tensile tests were performed prior and post brine exposure. The microstructure at coat–substrate interface after the tensile tests was examined using environmental electron microscopy (ESEM). The results indicated that the improper surface preparation was the cause for the early tensile failure of coating. Moreover, oxidized carbon steel surfaces were detected lowering the bonding strength of the coating.