Carbon formation phenomena and the initial stage of metal dusting corrosion –an experimental investigation

Abstract

Metal dusting corrosion is high temperature degradation of metals and alloys into dust-like fine particles. It is often encountered in petrochemical industry, where metals and alloys extensively exposed to carbon containing gases at high temperature and pressure conditions. It is a costly issue in the industry; millions of dollars have been invested annually in the fields of monitoring, controlling and prevention of metal dusting corrosion to avoid potential dangers in the environments that are considered by explosive and/or poisonous gases under high pressure and temperature conditions.

Metal dusting initiates as a result of unwanted carbon formation on the inner surface of chemical engineering installations. At high temperature, solid carbon diffuses into the metal/alloy matrix to form a carburized layer on the surface of the material. Under certain conditions, this carburized region may become unstable and decompose into carbon and metal/alloy particles (coke), as well as other corrosion products. Such metal particles may further catalyze the carbon deposition and the process is hence accelerated.

The initial stages of metal dusting, i.e. the first stages of carbon formation that initiate the process that we find are not well described, are analogues to carbon formation issue on the catalysts used in the same syngas technology. Improved understanding of the initial phase of metal dusting and the role of the alloy surface was obtained through a comprehensive experimental investigation, together with surface and bulk characterization. Pre-oxidized Ni-base alloy samples were subjected to different syngas atmospheres. Samples after carburization gas treatment were first analysed by visual inspection, optical imaging and light optical microscopy. The surface morphology and composition of the fresh, pre-oxidized and CO exposed alloy samples were studied by means of scanning electron microscopy (SEM) in conjunction with energy-dispersive X-ray spectroscopy (EDS) and Auger electron spectroscopy under ion-sputtering. The resulting carbonaceous products were characterized by using transmission electron microscopy (TEM) and EDS. The bulk composition of the fresh alloy was determined by electron probe micro-analysis via wave length dispersive X-ray spectroscopy.

As shown by Auger depth profile analysis, the increasing oxidation temperature was found to result in a surface oxide layer with better resistance to carbon formation under 10%CO-Ar mixture at 1 bar, irrespective of the oxygen concentration of the pretreatment gas. The carbon formation appears to be associated with inclusion of Ni and Fe species in the surface oxide layer, which subsequently reduce to promote the kinetics of the process. SEM imaging confirms that the extent of the carbon formation during CO exposure is also depending on the initial surface microstructure and composition.

Initial stages of metal dusting corrosion and carbon formation under syngas at industrially relevant conditions (at 20 bar) were also studied. Results show that a little or no carbon formed at low temperature, and the alloy matrix remained reasonably intact, with some smaller changes occurring within the Cr-rich surface oxide. The higher carbon formation occurring under the exposure at medium and high temperatures demonstrates that the reactions leading to carbon formation are kinetically limited. Co-current oxidation and Cr-enrichment of the near-surface layer was also observed under the mixture having low CO partial pressure, and pitting of the samples was revealed after removal of most of the carbonaceous products on the surface. The carbon formation is even higher under the high CO partial pressure at medium or high temperature conditions. Severe metal spalling of the alloy was also observed under these conditions, with complete disintegration of the Cr-rich surface oxide layer and carburization of the alloy matrix extending far into the bulk. Presence of reducible Ni and Fe phases within the Cr-rich oxide formed during the pre-oxidation were associated with the carbon formation and the TEM/EDS evidence confirms that Ni−Fe alloy particles have acted as catalysts for the formation of filamentous carbon