Erosion–corrosion behaviour of high-velocity oxy-fuel Ni–Cr–Mo–Si–B coatings under high-velocity seawater jet impingement

In this paper, erosion–corrosion characteristics of self-fluxing Ni–Cr–Mo–Si–B coatings deposited onto carbon steel (BS970 EN8) and stainless steel (UNS S31603) substrates using a commercial high-velocity oxy-fuel (HVOF) thermal spray process are reported. The coatings were studied in as-sprayed, vacuum-sealed (by polymer impregnation) and vacuum furnace fused conditions. In addition, comparisons of the HVOF coatings were made with an uncoated wrought stainless steel (UNS S31603). The erosion–corrosion characteristics were assessed under the influence of a high-velocity single-phase artificial seawater jet (without added solids), impinging perpendicularly onto the coating surface at 72 ms−1 flow velocity. Anodic polarisation tests were performed at both 18 and 50 °C to assess the in situ corrosion performance. Using static seawater results as a reference, the effects of the impinging flow on both the electrochemical response and the damage mechanisms are discussed in this paper. The application of cathodic protection (CP) reduced the extent of deterioration. The study showed the effect of sealing by polymer impregnation did not significantly alter the erosion–corrosion behaviour of the sprayed coating. However, there was an improvement in erosion–corrosion durability afforded by the post-fusion process. The mechanisms by which the improved performance of fused coatings is achieved are discussed.