Erosion-corrosion degradation mechanisms of Fe-Cr-C and WC-Fe-Cr-C PTA overlays in concentrated slurries

In this investigation the microstructure and erosion-corrosion behaviour of a Fe-Cr-C overlay (FeCrC-matrix) produced by plasma transferred arc welding (PTA) and its metal matrix composite (FeCrC-MMC) were assessed. The FeCrC-MMC was obtained by the addition of 65 wt.% of tungsten carbide (WC). The erosion-corrosion tests (ECTs) were carried out using a submerged impinging jet (SIJ); after the ECTs the surface of the overlays was analysed to identify the damage mechanisms. Two different temperatures (20 and 65 degrees C) and sand concentrations (10 and 50 g/l) were used in a solution of 1000 ppm of Cl- and a pH value of 8.5; the conditions were chosen to be representative of the recycling water in the tailings line in the oilsands industry. The FeCrC-matrix showed a dendritic structure and a high concentration of carbides in the interdendritic zone. The addition of the WC reinforcing phase promoted the formation of W-rich intermetallic phases, increased the microhardness values of the matrix phase of the FeCrC-MMC overlay and dramatically improved its erosion-corrosion performance as expected. For the FeCrC-matrix overlay the main erosion-corrosion degradation mechanisms were severe plastic deformation and the formation and removal of material flakes due to consecutive impacts. At 65 degrees C the dendritic zone was severely corroded in the area of low impact frequency. The FeCrC-MMC showed greater attack of the matrix phase compared to the WC grains; at high sand concentration the WC grains were severely fractured and flattened. The anodic polarisation analysis showed active corrosion behaviour of the FeCrC-MMC at both temperatures and sand concentrations; however the temperature dramatically increased the corrosion process of the surface studied under erosion-corrosion conditions. The paper assesses the degradation mechanisms of both FeCrC-matrix and FeCrC-MMC with the aim of understanding what aspects of MMCs must be adapted for optimum erosion-corrosion resistance. (C) 2009 Elsevier B.V. All rights reserved.