A mechanistic exposure experiment was performed on welded samples of the commercially available Haynes ® Hastelloy ® C–4 ® Ni-Cr-Mo-Fe alloy (65 wt.-% Ni, 16 wt.-% Cr, 16 wt.-% Mo, 3 wt.-% Fe, 2 wt.-% Co, 1 wt.-% Mn, 0.7 wt.-% Ti, 0.5 wt.-% Cu, 0.08 wt.-% Si and 0.01 wt.-% C) at coal gasification pilot plant facilities affiliated with the Institute for Advanced Engineering in Yongin-si, South Korea. The alloy samples were preoxidized at 400 °C under a stagnant air atmosphere for 24 h prior to exposure to the corrosive environment (60 % CO, 28.4 % H 2 , 2.5 % CO 2 , 0.8 % CH 4 , 600 ppm H 2 S and 110 ppm carbonyl sulfide under 2.005 MPa pressure and 170 °C). Thermodynamic Ellingham-Pourbaix stability diagrams were constructed to provide insight into the mechanism of the observed corrosion behavior prevailing in the piping materials between the particulate removal unit and water scrubber of the coal gasification pilot plant. The thermodynamic inference of the corrosion mechanism was supplemented with morphological, compositional and microstructural analyses of the exposed samples using scanning electron microscopy, X-ray diffraction and energy dispersive X-ray spectroscopy analyses performed on the external and cross-sectional surfaces of the recovered corrosion test samples to comprehensively examine the corrosion scale. The X-ray diffraction results revealed stable corrosion products of NiO, MoNi 4 and Cr 4.6 MoNi 2.1 after a total accumulated exposure duration of 139 h to the corrosive atmosphere. Scanning electron microscopy and energy dispersive X-ray spectroscopy positively identified the formation of rather continuous and adherent preoxidation corrosion products in the alloy samples, although extensively peeled off oxides were eventually observed as corrosion scale on the post-exposure alloy samples, which were attributed to the combined effects of the evaporation of the hydrated Fe, Al and Cr chlorides and their subsequent transformation into thin (spalled) oxides.