The influence of carbide dissolution on the erosion–corrosion properties of cast tungsten carbide/Ni-based PTAW overlays

WC/Ni-based plasma transfer arc welded (PTAW) overlays are frequently used in the oil sands industry for applications requiring extremely high wear resistance. These overlays usually consist of a dense distribution of either tungsten monocarbide (WC) or cast tungsten carbides (WC/W2C) in a NiBSi or NiCrBSi matrix. A previous erosion–corrosion (E–C) study of cast tungsten carbide/NiBSi-based PTAW overlays not only highlighted the expected preferential E–C attack of the non Cr-bearing matrix but also the significantly greater effect that carbide dissolution of the cast tungsten carbides had on the overall E–C performance of the overlay. It is recognised that cast tungsten carbide particles are more susceptible to dissolution during deposition than WC. As a result, a new process that modifies the outer periphery of cast tungsten carbides by producing a tungsten monocarbide shell has been developed. Overlays consisting of these carbides in a NiBSi binder were subsequently produced with the aim of reducing the extent of dissolution during deposition. The aims of this study were (i) to assess the effectiveness of the tungsten monocarbide outer shell of cast tungsten carbides to resist dissolution during the PTAW deposition process; and (ii) to highlight any improvements in the E–C properties using a novel slurry pot erosion corrosion (SPEC) tester, which incorporates a three-electrode cell to enable the separate components of erosion, corrosion and synergy, to be established. Results were compared with a commercially available NiBSi PTAW overlay containing unmodified cast tungsten carbides.