An investigation on hot corrosion behavior of spherical graphite cast iron coated by HVOF method

Industrial applications have a significant impact on heat and corrosion resistance of materials at high operating temperatures. Some coatings are applied to protect materials from corrosive environment. Since automotive engines contain vanadium and sulphate in dirty fuels, these salts increase the rate of corrosive wear at high temperatures. In this study, the resistance of the coated materials to these undesirable salts at high temperatures in the laboratory environment and the possible macro and microstructural changes that may occur in the field were investigated. Samples prepared from spherical graphite cast iron material were coated with Amdry 9624 (NiCrAlY) and Amdry 9951(CoNiCrAl) metallic powders by High Volume Oxy-Fuel (HVOF) method from Thermal Spraying methods. The adhesion properties of the coating layer were analyzed by Rockwell-C Adhesion method. In addition, Vanadium Pentaoxide (V2O5) and Sodium Sulphate (Na2SO4) salt mixture was applied to the coating surfaces and then Hot Corrosion test was carried out in a well type oven at 500 ° C, 600 ° C and 700 ° C for 6 hours. Macro and microstructural changes on the surface of the samples subjected to the Temperature Corrosion test were characterized by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) methods. It was determined that the adhesion strength of both coatings obtained from two different metallic powders as a result of the Rockwell-C adhesion test was in the acceptable (HF1) category according to VDI 3198 principles. Two different metallic powders, Amdry 9624 and Amdry 9951, were coated by HVOF method and samples tested at 500°C for hot corrosion were found to have no corrosion products with corrosive salts and interaction with the atmosphere. At temperatures of 600°C and 700°C, Amdry 9624 coatings were found to be Ni3V2O8, NaCrV2O7 and AlV2O4, Amdry 9951 coatings CO1.5Ni1.5 (VO4) 2 and AlV2O4 vanadate compounds as corrosion products. It has been determined that the corrosion products in the form of vanadate compounds on the surfaces of the samples subjected to the hot corrosion test have thin, long and colonsal structures in the SEM images. It has been found that both coatings protect the substrate material from corrosive external influences and no diffusion mechanism from the coating layer to the substrate material occurs. It has been observed that there is no obvious reduction in coating thicknesses after the hot corrosion test.