Mechanisms and Kinetics of WC-Co-Cr High Velocity Oxy-Fuel Thermal Spray Coating Degradation in Corrosive Environments

Aspects of microstructure on the synergy and overall material loss of thermal spray coatings in erosion–corrosion environments

“…Co 3 W 3 C) [11]. The phases identified by the broad peaks in the 2θ range from 35 • to 45 • represent the dissolution of Co to form Co 3 W 3 C and also formation of W 2 C and W [47][48][49].…”

“…4a and b and it is observed that there is a clear difference in the spectra. The values of 2θ for WC, Co, Co 3 W 3 C, W 2 C and W were obtained through the powder diffraction standards from 1999 JCPDS-International Centre for Diffraction Data [48]. It is clear that there is no evidence of any amorphous phases in the HVOF coating since the XRD peaks are essentially sharp and well-defined.…”

A comparative study on the microstructure and surface property evaluation of coatings produced from nanostructured and conventional WC–Co powders HVOF-sprayed on Al7075

“…Current stabilization during potentiodynamic polarization of WC-Co (in a bulk form and as a coating) has been observed in several works and it has been ascribed to pseudopassivity: In aqueous H 2 SO 4 , pseudopassivity at low anodic potentials has been related to the oxidation of W dissolved in the binder [52,[90][91][92], whereas at high anodic potentials it has been associated with the oxidation of W and C in the carbide phase [52,[90][91][92]. In aqueous NaCl, pseudopassivity has been credited to surface depositions associated with Co dissolution [93] and oxidation of WC to WO 3 (as an intermediate reaction before dissolution to WO 4 2− ) [13]. Although the high current values suggest pseudopassivity, the positive hysteresis loop indicates efficient activation of surface protection mechanisms during this stage.…”

“…The above works are reviewed in more detail in a previous effort by the authors [12]. A general classification of the corrosion behaviour of WC-based coatings by Souza and Neville [13], involves two key processes: (a) corrosion of matrix and consequent release of WC particles by loss of the supporting matrix; and (b) corrosion of both carbide phase and matrix, the degradation of WC also constituting an electrochemical process.…”

Wear and corrosion behaviour of HVOF WC–CoCr/CVD DLC hybrid coating systems deposited onto aluminium substrate

“…11C) followed by a subsequent decrease. The initial increase reflects a passivation phenomenon, possibly caused by partial plugging of the pores by corrosion products, coming either from the aluminium substrate or from the WC-CoCr interlayer: in WC-Co-based cermet coatings, selective corrosion of the metal matrix and re-precipitation of corrosion products, with partial occlusion of the surface, were indeed reported to occur [62,63]. As the electrolyte continues to infiltrate the porosities of the coating system, however, active corrosion of the metal matrix in the cermet and/or of the aluminium substrate is increasingly promoted, so that the corrosion resistance of the system decreases.…”