Study of corrosion and tribocorrosion of Fe₃Al-based duplex PVD/HVOF coatings against alumina in NaCl solution

In this study, we assess the corrosion and tribocorrosion of the duplex (thin-on-thick) coating systems formed by thin CrN or DLC layers sputter deposited on top of thick Fe₃Al layers thermal sprayed using high velocity oxy-fuel (HVOF) technique on 304-type stainless steel (SS304) substrates. The corrosion behavior of the duplex coatings was evaluated using a three-electrode cell, and the tribocorrosion assessment was carried out using a reciprocating ball-on-flat tribometer in which the tribological contact was immersed in the electrolyte. The microstructure and surface morphology of the coatings were analyzed using X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. Results show that the corrosion resistance of the Fe₃Al coatings increased by approximately 1 and 3 orders of magnitude with the addition of CrN and DLC top layers, respectively. The electrochemical impedance spectroscopy analysis reveals that the poor corrosion behavior of the Fe₃Al coating can be attributed to the presence of defects (porosity, cracks and inter-splat boundaries) in the HVOF layer that favor the infiltration of electrolyte and the formation of corrosion products. The tribocorrosion experiments confirm that addition of DLC and CrN layers is particularly beneficial when the duplex system is subjected to wear tests during cathodic polarization. However, in the anodic regime, defects formed at the surface by the wear process are detrimental since the electrolyte infiltrates the top layer and causes the dissolution of the Fe₃Al interlayer. We demonstrate that under this situation, material losses are due to synergistic effects of wear and corrosion.