Exposure effects of alkaline drilling fluid on the microscale abrasion-corrosion of WC-based hardmetals

The microscale abrasion-corrosion performance has been examined for two sintered hardmetals, WC-6Co and WC-11Ni, and two WC-10Co-4Cr sprayed coatings after exposure to a water-based alkaline drilling fluid for 168 h. A series of tests were also performed on freshly polished sintered hardmetals and coatings using both NaOH (pH 11) and neutral SiC aqueous slurries. Contrary to expectations, micro-abrasion under alkaline conditions generally resulted in lower wear rates producing a negative 8-18% abrasion-corrosion synergy, with the exception of the sintered WC-11Ni. Typically, wear scars for the sintered hardmetals were 7-8 carbides deep and the wear mechanism was binder depletion and undermining of the carbides. While the corrosion performance of the sintered hardmetals was only slightly affected when exposed to the alkaline drilling fluid, there was a marked decrease in the anodic behaviour for the WC-10Co-4Cr coatings. SEM analysis revealed that exposure to the drilling fluid resulted in a localised corrosion zone at the binder-carbide interface approximately 100 nm wide. This selective corrosion will alter the surface composition in addition to influencing mechanical properties such as the surface roughness, hardness, surface compliance and synergistic effects. The synergy results for both sintered and sprayed coatings are discussed in terms of the wear mechanisms at the microscale, selective phase removal and the effects of localised passivation.