A mechanistic exposure experiment was performed on the commercially available and welded Ni-Cr-Mo-Fe alloy samples used in the piping materials of the coal gasification pilot plant. Thermodynamic Ellingham-Pourbaix stability diagrams were constructed to provide insight into the mechanism of the observed corrosion behavior. The thermodynamic inference on the corrosion mechanism was supplemented with the morphological, compositional and microstructural analyses of the exposed samples using scanning electron microscopy, X-ray diffraction and energy-dispersive X-ray spectroscopy analyses. X-ray diffraction result revealed stable corrosion products of NiO, MoNi4 and Cr4.6MoNi2.1 after accumulated total exposure duration of 139 h to the corrosive atmosphere. Scanning electron microscopy and energy-dispersive X-ray spectroscopy positively identified formation of rather continuous and adherent pre-oxidation corrosion products although extensively peeled-off oxides were finally observed as corrosion scales on the post-exposure alloy samples, which were attributed to the chlorination/oxidation into thin (spalled) oxides.

In this study we analyzed the rolling contact fatigue behavior of two types of coatings made by thermal coating, by the method of atmospheric plasma spraying (APS) from two commercially available powders: Ni5Al5Mo and Al2O3 – 13 TiO2. The contact fatigue behavior was studied on an installation specially designed. The specimens were tested for 54 hours (at 1380 rpm), at a load of 944 N. For both types of coatings, the appearance of a wear path was observed, much more obvious in the case of the Ni matrix layer, also confirmed by profilometry. The mechanism of the wear phenomenon was predominantly of plastic deformation type (the material was pushed towards the edges of the wear path) in the case of NiAlMo coating. In the case of ceramic coating, the wear path width was very small (300-450 μm), with very few changes at the surface level of the coating, which recommends this type of material for applications that require wear resistance to rolling.