A study of the factors contributing to the metal dusting of Fe-Cr-Ni alloys in highly carburising atmospheres

Metal dusting is a recurring problem within the petrochemical industry that has serious consequences for plant efficiency and reliability, as well as for operational safety. The underlying factors governing its occurrence are not fully understood and therefore studies under closely monitored conditi ons in atmospheres that simulate those encountered in service are essential . For this preliminary study, candidate reformer and pyrolysis tube materials have been selected and exposed at 560 degrees C for periods of up to 200 h ours in a highly reducing atmosphere. The intention has been to focus on so me of the main variables that are thought to possibly contribute to metal d usting and evaluate their contribution through a series of controlled exper iments. In addition to alloy composition, the principal parameters that hav e been addressed are surface condition and system pressure. In the series of experiments described in this paper, machined and ground a s well as polished specimens have been exposed to an H-2-24.4%CO-2.4%H2O ga s mixture at pressures of nominally 1.5, 2 and 5 bar (absolute). As a conse quence of the character of this gas (i.e. carbon activity, a(c), >>1), exte nsive coke deposition occurs on the metallic specimens. Thus, a thermal cyc le has been introduced into the test procedure, generally at 25 hour interv als, so that the samples can be inspected and coke removed. Cross-sectional optical and scanning electron microscope examinations of th e morphology of the coke deposit and the nature and extent of attack have p rovided a deeper understanding of the importance of alloy composition and t he contributions made by surface treatment and system pressure.