Abrasion-corrosion of downhole drill tool components

Abstract

The present work is a Schlumberger funded PhD project entitled ‘Abrasion-corrosion of downhole drill tool components’. The objective of this project was to replicate the wear-corrosion mechanisms of tungsten carbide (WC)-based hardmetals and coatings occurring in downhole environments (pH 9-11) under controlled laboratory conditions, to identify and establish a better understanding these mechanisms 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 wear mechanisms. In addition, the presence of a corrosive environment downhole further complicates the contact conditions and can lead to accelerated surface degradation and even catastrophic failures. A Scanning Electron Microscope (SEM) investigation of worn drill-tool components revealed the presence of micro-scale (by abrasives similar size to the carbide grains i.e. less than 5 ?m) and macroscale abrasion (by abrasives orders of magnitude larger in size compared to the carbide grains). The wear-corrosion testing of candidate materials was investigated using a micro-macro dual approach comprising of micro-scale abrasion testing (University of Southampton) and the modified ASTM G65 tester (National Physical Laboratories, Teddington). To mimic exposure to alkaline drilling fluids for long durations, selected samples were exposed to pH 11 NaOH solution / drilling fluid for 168 h prior to wear testing. Screening of candidate materials on the basis of their wear-corrosion performance using micro-abrasion tester was performed and WC-10Co-4Cr coating along with sintered WC-5.7Co-0.3Cr were 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 the presence of an intense localised corrosion in the form of ‘corrosion trenches’ due to the preferential dissolution of decarburised metallic tungsten (W), which occurred around the periphery of the carbide grains. These ‘corrosion trenches’ were found to be one-carbide deep and resulted in the carbide being held loose in the corrosion trenches. Alternatively, for the sintered WC-5.7Co-0.3Cr, exposure to pH 11 did not show any evidence of localised corrosion. However, exposure to pH 7 distilled water resulted in the preferential dissolution of the binder phase. For the first time, a modified micro-abrasion tester capable of in situ electrochemical measurements was developed to monitor the corrosion kinetics during micro-scale wear-corrosion. Interestingly, the lowest wear occurred under pH 11 conditions. It was proposed that the presence of Co(OH)2 based passive films, also detected by XPS analysis, appears to influence the rate of binder-phase removal by altering the stiffness of the abrasive-surface contact and lowering the friction between abrasives and the surface and in turn lowers the overall wear rates. This was also corroborated by the observed wear mechanism of preferential removal of the binder-phase leading to the undermining of carbides. Conversely, for the sintered WC-5.7Co-0.3Cr, despite the lack of surface passivation under similar test conditions, the wear rates were found to be independent of pH. The influence of abrasive size on the wear-corrosion performance of sprayed WC-10Co-4Cr coating was investigated using the modified ASTM G65 test. It was revealed that in addition to the size of abrasives, the wear rates are dependent on the overall wear mechanisms. In general, severe damage to the coating was caused by delamination due to the propagation of sub-surface cracks resulting in the doubling of wear rates. The sub-surface cracking of the coating increases with an increase in the abrasive size. Alternatively, for the sintered WC-5.7Co-0.3Cr, an increase in the extent of cracking in the carbide grains increased with the abrasive size. An order of magnitude increase in wear resulted from the extensive carbide cracking and the subsequent removal of the carbide grains. The dual approach successfully replicated the wear in downhole conditions by examining the influence of contact conditions and abrasive size on the wear-corrosion of WC-based sintered hardmetals and sprayed coatings to inform a better design / selection of surfaces subjected to downhole environments.

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ORCID for Robert Wood: orcid.org/0000-0003-0681-9239

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