A mechanistic exposure of welded Ni-Mo-Cr-Fe alloys in coal gasification syngas plants for morphological and microstructural examination and corrosion mechanism study

Abstract

A mechanistic exposure experiment was performed on the commercially available and welded Haynes® HYBRID-BC1® Ni-Mo- Cr-Fe alloy samples (62wt% Ni, 22wt% Mo, 15wt% Cr, 2wt% Fe, 0.5wt% Al, 0.25wt% Mn, 0.08wt% Si, and 0.01wt% C) at coal gasification pilot plant facilities affiliated with the Institute for Advanced Engineering. The alloy samples were pre-oxidized at 400°C under stagnant air atmosphere for 24 h prior to exposure to the corrosive environment (60% CO2, 28.4% H2, 2.5% CO2, 0.8% CH4, 600ppm H2S, and 110ppm carbonyl sulfide under 2.005 MPa 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 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 performed on the external and cross-sectional surfaces of the recovered corrosion test samples to comprehensively examine the corrosion scale. X-ray diffraction result revealed stable corrosion products of NiO and MoNi4 after accumulated total exposure duration of 139 h to the corrosive atmosphere. Scanning electron microscopy and energy-dispersive X-rayspectroscopy positively identified formation of rather continuous and adherent pre-oxidation corrosion products in the alloy samples although extensively peeled-off oxides were finally observed 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. © 2017 Nova Science Publishers, Inc. All rights reserved.