Scanning photoelectron microscopy discloses the role of Cr and Mo in the selective corrosion of hardmetal grades with Co-Ni binders

Abstract

Advanced applications of hardmetal (HM) in chemically aggressive ambients call for the improvement of their corrosion resistance, combined with optimal abrasion and toughness properties. This goal is chiefly achieved with binder alloying. In particular, Cr and Mo additions to Co-Ni matrices have proved effective in many cases, but the physico-chemical mechanisms underlying this functional improvement are poorly understood. This situation, on the one hand does not enable the fine-tuning of the composition, and, on the other hand, does not allow to reliably extrapolate the performance of these grades to new ambients. The aim of this work is to propose a molecular-level approach to the understanding of the electrochemical corrosion of HM, based on space-resolved compositional and chemical-state analysis of the surface of HM subjected to controlled electrochemical corrosion. This type of analysis is enabled by synchrotron-based scanning photoelectron microscopy (SPEM) and photoelectron microspectroscopy (μ-XPS). Specifically, we studied a WC-based grade with 15 w% binder with 4.67 Co/Ni ratio, alloyed with 0.78 w% Cr and 0.94 w% Mo, corroded potentiostatically in neutral sulphate aqueous solution in the pseudopassive and transpassive ranges. SPEM and μ-XPS allowed to locate and distinguish the chemical state of the binder and carbide elements in the respective phases and to distinguish the surface stoichiometry brought about by corrosion in different conditions. In particular, we found that pseudopassive conditions lead to the formation of surface films, enriched in W(VI) and Cr(III). Transpassive conditions cause notable surface enrichment in Cr(III) as well as to the formation of mixed Cr-W oxides in the WC regions. Mo is leached from the binder at all potentials investigated, while Mo(0) is present in the WC region under pseudopassive conditions, while transpassivity leads to its oxidative removal. Chloride in the electrolyte favours dissolution of oxidized metals, in particular causing the leaching of Cr(III), while Mo(III) is found at the surface of WC grains.