TEM and ToF-SIMS studies on the corrosion behavior of vanadium and chromium containing WC–Co hard metals in alkaline solutions

The corrosion behavior of hard metals with VC and Cr3C2 grain growth inhibitors was investigated in alkaline solutions by electrochemical methods. The two inhibitors have opposite effects on the corrosion behavior: Cr3C2 significantly improves the corrosion behavior, whereas VC-containing alloys show a poor resistance. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and analytical transmission electron microscopy (TEM) analyses of the distributions of Cr and V in the composite material, as well as in the surface layers formed during corrosion were employed to clarify the influence of these elements on the corrosion behavior. The measurements showed that VC is precipitated mostly along the WC/binder interface after the liquid-phase sintering process, while Cr3C2 is almost homogeneously dissolved in the binder. As VC is chemically instable in alkaline solutions, it completely dissolves out of the binder. In accordance with this observation no V was found in the corrosion product layer on the surface. As WC is more noble than Co, galvanic coupling between the two phases reinforces the Co dissolution, while the WC-phase is protected cathodically. Contrary to VC, chromium is stable in alkaline environments and forms a passivating Cr2O3 layer. Enrichment of Cr in the corrosion product layers was detected by TEM and ToF-SIMS. Due to surface passivation by Cr2O3, galvanic coupling effects between Co and WC play a much less important role in the corrosion process of the composite material.

Research Highlights
► The addition of VC and Cr3C2 inhibitors causes very good mechanical properties by regulating a small WC grain-size
► In case of VC no corrosion protection is given due to galvanic coupling of WC and Co → binder removal in alkaline solutions
► Cr3C2 containing hard metals show a much higher corrosion resistance by forming a Cr2O3 passive layer in alkaline solutions → galvanic coupling plays a minor role