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

Abstract

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 °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 °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.