Erosion of hard surface coatings for use in offshore gate valves

This paper describes the work carried out in assessing the performance of hard chromium, electroless nickel composite and two HVOF tungsten carbide coatings as candidate hard surface coatings for use in offshore gate valves. The coatings were erosion tested in a slurry jet facility and their performance compared with a D-gun 86WC–10Co–4Cr coating that is currently widely used in service. Sand erosion tests were conducted at 28 m s−1 and with 135 and 235 μm sand, respectively. The eroded surfaces were examined so that the erosion mechanisms of each coating could be determined. The results indicate that the Diatec applied HVOF 86WC–10Co–4Cr coating is the most promising alternative, representing an improvement in erosion resistance of more than 50% over the D-gun applied coating of identical nominal composition. For some applications of low-erosivity, hard chromium may be a more cost-effective option. The variation in the erosion performance of the thermally sprayed coatings is linked to directional fracture toughness and crack propagation paths which are influenced by the presence of pores, inhomogeneous carbide distributions, Co6W6C and substrate grit blast remnants.