Salt fog corrosion behavior of high-velocity oxygen-fuel thermal spray coatings compared to electrodeposited hard chromium

The corrosion behavior of several coating/substrate combinations was determined using the ASTM B117 Salt Fog Test. The coatings were electrodeposited hard chromium (EHC) and two high-velocity oxygen-fuel (HVOF) thermal-sprayed coatings, tungsten-carbide/cobalt (WC/Co) and Tribaloy 400 (T400). The substrates were 4340 steel, 7075 aluminum alloy, and PH13-8 stainless steel. On the 7075 Al alloy, a sulfamate nickel layer was deposited prior to the deposition of hard chromium. The results indicated that on the 4340 steel none of the coatings provided significant protection, with equivalent performance between the EHC and WC/Co coatings and slightly poorer performance for the T400. On the 7075 Al alloy, the EHC with sulfamate nickel exhibited excellent performance as no pits or blisters were noted on the faces or edges of the samples. The WC/Co showed no pitting or blistering on the faces but had a significant amount of pitting along the edges. The EHC and WC/Co coatings performed well on the 13-8 stainless steel as no pits or blisters were noted on the faces or edges. The T400 coatings had rust stains on the faces and edges but defects could not be seen with the unaided eye or at a 7× magnification.