Corrosion behavior of a precipitation hardened Ni-Cr-Co-Mo alloy under partial slagging coal gasification conditions

Abstract

A mechanistic exposure experiment was performed on the commercially available precipitation hardened VDM (R) alloy C-263 (Nicrofer (R) 5120 CoTi) Ni-Cr-Co-Mo alloy samples (21 wt.-% Cr, 21 wt.-% Co, 6.1 wt.-% Mo, 2.4 wt.-% Ti, 0.7 wt.-% Fe, 0.6 wt.-% Al, 0.6 wt.-% Mn, 0.4 wt.-% Si, 0.2 wt.-% Cu, 0.08 wt.-% C and bal. Ni) at coal gasification pilot plant facilities affiliated with the Institute for Advanced Engineering in Yongin, South Korea. 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 on the corrosion mechanism was supplemented with the morphological, compositional and microstructural analyses of the exposed samples using scanning electron microscopy 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. Corrosion products with conspicuous spallation were observed after 139 h exposure to the corrosive environment (60 vol.-% CO, 28.4 vol.-% H-2, 2.5 vol.-% CO2, 0.8 vol.-% CH4, 600 ppm H2S, and 110 ppm carbonyl sulfide under 2.005 MPa and 170 degrees C). Scanning electron microscopy and energy-dispersive X-ray spectroscopy positively identified formation of rather extensively peeled-off oxides as corrosion scales on the post-exposure alloy samples, which were attributed to the combined effects of evaporation of hydrated Fe, Al, and Cr chlorides and their subsequent transformation into thin (spalled) oxides. This article contains technical contents on the effects of Cr, Mo, Fe, and Al on the observed corrosion behavior which supplement and cast engineering insight into the previously published MP 2-2018 article on corrosion behavior of Ni-Cr-Mo-Fe alloy in a similar environment.