One of the principal aims was to determine if HVOF sprayed coatings of stainless steel and nickel alloys have similar corrosion properties in seawater compared to conventional wrought materials. The work reported here measured the level of corrosion performance that can be expected from coatings of corrosion-resistant alloys deposited onto a steel substrate using commercially available HVOF spraying systems. Two alloy types were considered, a stainless steel with a composition similar to 316L (UNS S31603) and a nickel base alloy with composition similar to alloy 625 (UNS N06625). The cyclic potentiodynamic polarisation method was used to examine the corrosion behaviour of these coatings and the same alloys in wrought form. A HVOF sprayed coating of nickel alloy 625 was found to be more corrosion resistant in seawater than a coating of 316L stainless steel. However, the nickel alloy coating did not match the corrosion resistance of the same nickel alloy in wrought form, but may have the ability to offer corrosion resistance (in seawater) approaching that of wrought stainless steel. The lower level of corrosion resistance of the nickel alloy coating compared to the same material in wrought form is believed to be due to microstructural differences and in particular related to preferential attack along the inter-particle (splat) boundaries of the coating.
Key Words: Coatings, thermal spraying, HVOF, nickel alloys, stainless steel, corrosion protection, seawater corrosion