The present work is a Schlumberger funded PhD project entitled ‘Abrasion-corrosion of downhole drilltool components’. The objective of this project was to replicate the wear-corrosion mechanisms oftungsten carbide (WC)-based hardmetals and coatings occurring in downhole environments (pH 9-11)under controlled laboratory conditions, to identify and establish a better understanding thesemechanisms and the factors influencing them so as to minimise the material wastage during service.The presence of hard and soft phases within WC-based hardmetals and coatings results in complex wearmechanisms. In addition, the presence of a corrosive environment downhole further complicates thecontact conditions and can lead to accelerated surface degradation and even catastrophic failures. AScanning Electron Microscope (SEM) investigation of worn drill-tool components revealed thepresence of micro-scale (by abrasives similar size to the carbide grains i.e. less than 5 ?m) and macroscaleabrasion (by abrasives orders of magnitude larger in size compared to the carbide grains). Thewear-corrosion testing of candidate materials was investigated using a micro-macro dual approachcomprising of micro-scale abrasion testing (University of Southampton) and the modified ASTM G65tester (National Physical Laboratories, Teddington). To mimic exposure to alkaline drilling fluids forlong durations, selected samples were exposed to pH 11 NaOH solution / drilling fluid for 168 h prior towear testing. Screening of candidate materials on the basis of their wear-corrosion performance usingmicro-abrasion tester was performed and WC-10Co-4Cr coating along with sintered WC-5.7Co-0.3Crwere selected for in-depth analysis and the micro-macro dual test programme.The WC-10Co-4Cr coating exposed to pH 11 and pH 7 distilled water (for comparison), revealed thepresence of an intense localised corrosion in the form of ‘corrosion trenches’ due to the preferentialdissolution of decarburised metallic tungsten (W), which occurred around the periphery of the carbidegrains. These ‘corrosion trenches’ were found to be one-carbide deep and resulted in the carbide beingheld loose in the corrosion trenches. Alternatively, for the sintered WC-5.7Co-0.3Cr, exposure to pH 11did not show any evidence of localised corrosion. However, exposure to pH 7 distilled water resulted inthe preferential dissolution of the binder phase.For the first time, a modified micro-abrasion tester capable of in situ electrochemical measurements wasdeveloped to monitor the corrosion kinetics during micro-scale wear-corrosion. Interestingly, the lowestwear occurred under pH 11 conditions. It was proposed that the presence of Co(OH)2 based passivefilms, also detected by XPS analysis, appears to influence the rate of binder-phase removal by alteringthe stiffness of the abrasive-surface contact and lowering the friction between abrasives and the surfaceand in turn lowers the overall wear rates. This was also corroborated by the observed wear mechanismof preferential removal of the binder-phase leading to the undermining of carbides. Conversely, for thesintered WC-5.7Co-0.3Cr, despite the lack of surface passivation under similar test conditions, the wearrates were found to be independent of pH.The influence of abrasive size on the wear-corrosion performance of sprayed WC-10Co-4Cr coatingwas investigated using the modified ASTM G65 test. It was revealed that in addition to the size ofabrasives, the wear rates are dependent on the overall wear mechanisms. In general, severe damage tothe coating was caused by delamination due to the propagation of sub-surface cracks resulting in thedoubling of wear rates. The sub-surface cracking of the coating increases with an increase in theabrasive size. Alternatively, for the sintered WC-5.7Co-0.3Cr, an increase in the extent of cracking inthe carbide grains increased with the abrasive size. An order of magnitude increase in wear resultedfrom the extensive carbide cracking and the subsequent removal of the carbide grains. The dualapproach successfully replicated the wear in downhole conditions by examining the influence of contactconditions and abrasive size on the wear-corrosion of WC-based sintered hardmetals and sprayedcoatings to inform a better design / selection of surfaces subjected to downhole environments.
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