The subject of this study is related to the performance of austenitic steels coils and tubes. in a range of temperatures between 425 to 870°C for the transport of reducing gases, in an installation involving the direct reduction of iron-ore by reforming natural gas. Evidence is presented that metal dusting is not the only unique high temperature corrosion mechanism that caused catastrophic failures of austenitic 304 (UNS S30400) coils and FIK-40 (UNS J94204) tubes. sensitization as well as stress corrosion cracking occurred in 304 stainless steel coils, and metal dusting occurred in tubes of HK- 40, a high resistance alloy. The role of a continuous injection of H2S to the process is suggested to avoid the high temperature metal dusting corrosion mechanism found in these kind of installations.
Direct Reduction Process
The direct reduction process involves the continuous reduction of high-grade iron ore by reforming natural gas, The gas first flows through desulphurizing drums and then through preheater coils in the reformer stacks. Steam is then mixed with the preheated natural gas, and the mixture passes through hot, catalyst-filled tubes within reformer fu maces. The reforming reaction takes place at about 870°C according to the equation:
The gases emerge with a composition (vol. %) averaging about (in vol. %) 60.08% H2, 12.42% CO2, 20.97% CO, 5.30% unconverted methane, 1.19% N2 and 0.04% H2O. The reformed gases are partially cooled by passing them through a preheated boiler, to generate steam. The gases then pass into gas-preheating furnaces where they are heated up to 9 15 °C before they flow through the reduction retorts. The preheated primary reducing gases enter the ore-filled retorts, which are in the primary cycle of ore reduction.
It has been pointed out 1 that, when iron-, nickel-, and cobalt-based alloys are in contact with reducing atmospheres, such as those mentioned above, they can suffer significant attack by carburizing components of the environments, leading to catastrophic carburization such as metal dusting, especial] y in atmospheres of high carbon activities, and at intermediate temperatures between 450 and 800°C. Metal dusting is a catastrophic carburization process in which alloys containing Fe, Ni and Co are vulnerable. The result is the disintegration of the alloy into a dust, consisting of particles of carbon, carbides, metal and oxides, which are blown off in erosive gas streams leaving behind pits or general abrasion. This phenomenon of metal dusting, has keen simulated for low and high alloy steel s for carbon activities aC1 1 in flowing CO-H2-H2O, and its proposed mechanism has been confirmed.
In reformer gas environments, carbon formation at relatively low temperatures (<900 °C) can occur by two reactions, the disproportionation of CO (Boudouard) and the reduction of CO:
2CO = CO2 + C (Boudouard reaction)
The thermodynamics of the two reactions are similar for reformer gases and it is usual lo concentrate on the Boudouard reaction (because of the gas composition founded in the direct reducticion process), which is catalysed strongly by Fe, Ni and Co.