Erosion-corrosion of WC-Co-Cr cermet coatings
The erosion-corrosion characteristics of a WC-Co-Cr coating were assessed under an impinging jet of 3.5% NaCl solution at a velocity of 12 ms-1 both free from solids and containing solids up to 2500 mg/l at a range of impingement angles. The total weight loss increased with increasing solids in the liquid stream and maximum material loss occurred in the area directly under the jet where high impact angles occurred. The material loss tended to reduce as the impingement angle reduced. The mechanism and contribution to the overall material loss in erosions-corrosion processes were studied. Surface profiling revealed the erosions mechanism to be that of solid particles causing craters on the surface of the coating, not associated with any specific microstructural aspect of the coating. The total contribution to overall material loss from corrosion processes was substantial (up to 30%). While the impinging jet increased the corrosion rate, in the absence and presence of solids, these corrosive effects could be inhibited, particularly in the presence of solids, by the application of cathodic protection which reduced overall material loss by up to 50%. The synergistic component of material loss can be referred to as an indirect corrosion effect, and in this work, the synergistic effect was significant (up to 40%). However, more complex direct corrosion effects were found in the form of galvanic currents between the area directly under the impinging jet and the outer area which significantly increase the corrosion rate on the impinged area. This study, which focuses on the interactions between corrosion and erosion, has improved the understanding of erosion-corrosion processes on complex composite materials.