High temperature corrosion behavior of Ni and Co base HVOF coatings exposed to NaCl-KCl salt mixture

The high-temperature corrosion behavior of NiMoCrW and CoNiCrAlY base coatings produced by high velocity oxy-fuel technique (HVOF) has been analyzed in dry air atmosphere and in presence of NaCl-KCl salt mixture at 650 degrees C for 360 h. Their behavior was compared with that of NiCr coating used as reference. Their corrosion resistance in presence of NaCl-KCl was measured by dimensional metrology analysis and the corroded samples were characterized by XRD and SEM techniques. The coatings exhibited excellent oxidation resistance in dry air atmosphere due to the formation of Cr2O3 and Cr-rich spinets, as well as the formation of Al2O3 in CoNiCrAlY coating. Their corrosion resistance in presence of NaCl-KCl was significantly lower due to chlorine-induced active oxidation and due to the formation of chromates, identified by XRD as K2CrO4. However, both coatings presented higher corrosion resistance in presence of the alkali salts compared to that of NiCr coating. The presence of Mo in the NiMoCrW coating improved the high-temperature corrosion resistance in presence of the alkali salts and the formation of a stable Al2O3 oxide scale on CoNiCrAlY coating reduced the penetration of chlorides and the degradation of the protective oxide scale by chromates formation.