Corrosion and synergy in a WCCoCr HVOF thermal spray coating-understanding their role in erosion-corrosion degradation

The material degradation of a high velocity oxy-fuel (HVOF) WCCoCr thermal spray coating is addressed in this paper focusing on the effect of corrosion and the degradation caused by the interaction between corrosion and erosion (synergy) on the overall material loss in erosion-corrosion environments. The mechanical and electrochemical components, which contribute to degradation are isolated and evaluated using an experimental protocol comprising electrochemical techniques and scanning electron microscopy (SEM). Tests under erosion-corrosion conditions were performed in 3.5% NaCl solution with silica sand concentrations of 200, 500 and 1000 mg/l, at two temperatures (20 and 50 °C) and using an impingement velocity of 17 m/s. Austenitic (UNS S31603) and super duplex (UNS S32760) stainless steels were used as a reference and the performance of these three materials is presented. The results show that corrosion and corrosion-erosion interactions play an important role in the overall material degradation of the HVOF coating and that the dominance of each process is critically dependent on the environment. Some discussion of potential means of improving the coating durability is presented through consideration of the dominant processes in erosion-corrosion.