Electrochemical and Microstructural Response of Metal Matrix Composites Under Different Erosion-Corrosion Conditions

The electrochemical and microstructural response of metal matrix composites (MMCs) under different erosion-corrosion conditions was assessed in this paper. The MMCs were plasma transferred arc overlays produced by reinforcing two hardfacing alloys (iron-based and nickel-based) with tungsten carbide (WC) particles. The microstructures were examined using scanning electron microscopy (SEM) and stereological techniques. The electrochemical response under erosion-corrosion was assessed by in-situ potentiostatic and potentiodynamic techniques as a function of the sand content (10 and 50 g/l), flow stream velocity (5, 10 and 14 m/s) and slurry temperature (20 and 65°C). Finally, after the erosion-corrosion tests, the MMCs microstructural components were analyzed using optical microscopy (OM) and SEM. The microstructural analysis showed that, WC grains were partially dissolved in the molten matrix, promoted the formation of secondary phases embedded in the matrix phase. The in-situ electrochemical results showed that there was a correlation between the MMCs microstructural characteristics and their electrochemical response under erosion-corrosion. In the in-situ potentiostatic tests, the monitored current density of the nickel-based MMC was significantly lower at all sand content. Furthermore, under certain erosion-corrosion conditions, it showed a pseudopassive behavior in the potentiodynamic tests,. On the other hand, the iron-based MMC showed a fully active response at all erosion-corrosion conditions. The erosion-corrosion tests in conjunction with SEM and OM showed that at the lower stream velocity: WC grains and secondary phases suffered little structural damage, small impact scars were produced in the matrix phase and the degradation process was mainly governed by the MMC corrosion resistance. Finally, under more severe erosion-corrosion conditions, the structural integrity of every microstructural component was significantly affected and the corrosion processes undermined the WC grains and secondary phases support from the matrix phase.