The Effects of Tribological Contact Conditions and Solution pH on the Wear-Corrosion Performance of D-Gun Sprayed WC-10Co-4Cr Coating

WC-based thermal-spray type coatings are extensively used in the oil and gas industry and are exposed to corrosive environments during operation often leading to premature failure of the coating due to the synergistic effects of wear and corrosion. It is regularly observed that thermal spray coatings used on the helical mud motor rotors have a very different wear-corrosion performance as compared to thermal spray coatings used for other applications, such as radial bearings, despite very similar operating environments. This is due to factors such as pH, time of exposure and contact conditions. As such, the influence of these individual parameters on the wear-corrosion performance of thermal spray coatings needs to be fully understood to develop and tailor surface engineering solutions for specific applications within the bottom hole assembly (BHA). To achieve this, the present paper attempts to decouple the effects of contact conditions and corrosion by examining the wear-corrosion performance of Detonation gun (D-gun) sprayed WC-10Co-4Cr coating using a modified micro-abrasion tester capable of in situ electrochemical measurements and a modified ASTM G65 rubber-wheel abrasion tester under neutral and alkaline test conditions using the same type (SiC) and size (4.5 microns) of abrasive particles. To mimic prolonged exposure to alkaline drilling fluids, select coating specimens were also ‘pre-exposed’ to NaOH solutions with pH 11 before the wear-tests. Scanning electron microscopy and in situ electrochemical noise data has been used to compare the wear-corrosion mechanisms of the coating in the different tests. In addition to parameters such as severity of contact, it is possible to relate the overall wear rates to individual parameters of contact conditions and pH.  Additionally, effects of ‘static’ corrosion (pre-exposure), such as depth of attack and partial passivation of the coating were examined using X-ray photoelectron spectroscopy (XPS) and focussed ion beam (FIB) sectioning of the samples.