Analysis of an aluminide coating on austenitic steel 800HT exposed to metal dusting conditions: Lessons from an industrial hydrogen production plant

Metal dusting takes place in carbon-supersaturated gaseous atmospheres at temperatures comprised between 400 and 800 degrees C and is most commonly encountered in steam reforming processes such as hydrogen production from methane. It can lead to a severe loss of metal, resulting in high-cost maintenance and serious safety issues. Aluminide coatings are known to protect Fe and Ni based alloys under these conditions and are currently being used in the petrochemical industry. However, if not properly applied, they can lead to early failures. Analysis of an aluminide coating deposited by pack cementation on austenitic steel 800HT tubes belonging to an industrial catalytic reactor for hydrogen production by reforming was carried out. The coated tubes placed on a large multi-tubular heat exchanger reforming reactor were exposed to fairly severe conditions for more than six years. Significant spallation was observed in some parts, while a few pits could be observed on the surface of some tubes by simple visual inspection. Metallographic analyses of the pits were carried out and the results showed evidence of metal dusting. The reasons for the coating failures were investigated and coating spallation resulted from an unfavorable combination of material/coating and operation conditions that induced interdiffusion between the substrate and the coating leading to Kirkendall porosity formation and finally coating spallation. Regarding metal dusting, the analyses suggested three possible causes of coating failure: 1) an excessive thickness of the coating with brittle (Fe, Ni)(2)Al-5 which developed cracks that propagated into low Al content coating zones, the coating substrate interdiffusion zone (IDZ) or the substrate itself, 2) partial coating delamination due to short cracks located close to each other, which exposed low Al content coating zones and/or 3) an excessive thickness of the coating with inter-granular corrosion due to the presence of chloride resulting in delamination, again exposing low Al content coating areas.