ABSTRACT

Metal dusting can be a severe failure mechanism of metallic materials exposed to industrial process environments containing high carbon activity. There have been many studies on the metal dusting behaviors of alloys in 1 atm environments, but some recent laboratory-scale studies have shown that typically-resistant alloys are apt to undergo more severe metal dusting attack under high-pressure conditions. The current study analyzes the metal-dusting behavior of a Ni-based Ni29Cr2Al alloy weldment tested under 19 bar total pressure at 620 °C. Of particular interest is the mechanism by which a higher pressure favors the initiation of dusting attack. It will be shown that regions of the heat-affected zone (HAZ) and weld surfaces were much more prone to dusting attack than the as-processed wrought alloy surface. However, regions of the weldment which formed fine grains at the surface could exhibit excellent resistance to metal dusting. The effects of such microstructural factors on oxidation resistance were directly related to conferring dusting resistance.

INTRODUCTION

Metals and alloys are susceptible to carburization and metal dusting in carbon-bearing processes associated with refinery and petrochemical industries. In ammonia, methanol, and hydrogen plants, the process environments often contain varying levels of hydrogen, carbon monoxide, carbon dioxide and steam, creating reducing environments with high carbon activity (ac>1) when operated in the intermediate temperature range of 450-900 °C. Under such conditions, degradation by metal dusting is a major issue that could result in catastrophic failure.1-6 It is widely accepted that metal dusting involves carbon diffusion into the metal part to create carbon oversaturation, leading to carbide formation in Fe-based alloys and direct graphite formation in high Ni alloys. For Fe-based alloys or other carbide-forming alloys, carbides subsequently decompose to form graphite and metallic particles.7-10 Metal dusting exhibits maximum corrosion attack in a small temperature range of about 600-700 °C, which is often attributed to the high carbon activity at these temperatures.11 When temperature increases, carbon activity decreases, and thus causes less driving force for metal dusting attack. At lower temperatures, the carbon deposition kinetics too slow to be of significance.4 Grabke et al.7 considered the rate at temperatures below 540 °C to be controlled by cementite decomposition for iron and low alloy steels. Though not clearly resolved from a mechanistic standpoint, it is ultimately the combination of thermodynamics and kinetics that contrive to favor maximum metal dusting attack in the intermediate temperature range.

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