The Role of Galvanic Corrosion in Erosion-Corrosion of Structured Composite Overlays

Abstract

The erosion-corrosion behaviour of two structured composite overlays has been investigated using a slurry pot erosion-corrosion tester in a slurry of silica sand in 3.5% NaCl solution. These overlays comprise hard tiles that are embedded in a NiBSi-sCTC (spherical cast tungsten carbide) metal matrix composite (MMC). The hard tiles are cemented tungsten carbide (WC) and thermally stable polycrystalline (TSP) diamond, respectively. Both overlays showed high synergy contribution to the total erosion-corrosion loss, with synergy loss accounting for 99.3% and 65.4% of the total loss for the overlay with cemented WC tiles and TSP diamond tiles, respectively. The synergy was dominated by corrosion-enhanced erosion which accounted for 98.7% and 92.5% of synergy for the WC tiled and the TSP diamond tiled overlays, respectively. The high corrosion-enhanced erosion and synergy for these materials can mainly be attributed to the formation of galvanic coupling between the hard tile and the embedding coating (NiBSi-sCTC). In the overlay with cemented WC tiles, the Co binder in the cemented WC tile was preferentially corroded at very high rate, diminishing its support to the WC grains and leading to rapid removal (pull-out) of the WC grains by impacting erodent particles. For the overlay with TSP diamond tiles, the diamond tile acted as cathode which led to severe corrosion-enhanced erosion in the embedding coating. The results have been discussed based on mixed potential theory. This study clearly demonstrates that macro-scale galvanic coupling among different parts of the machine structure can have important impact on the wear-corrosion performance of wear components.

Keywords: Slurry erosion, erosion-corrosion, Tribo-corrosion, galvanic corrosion effect, structured composite overlay, tungsten carbide overlay