Mathematical Model for Metal Dusting on Low Alloy Steels

Metal dusting is a kind of catastrophic corrosion phenomenon that can be observed in a wide range of petrochemical industry, it occurs in high-temperature equipments and tubes that exposed to hydrocarbon atmosphere and involves the disintegration of metals or alloys into graphitic carbon, fine metal or carbide particles. It is generally considered that the process of metal dusting can be divided into three stages: carburization, precipitation of carbides and decomposition of carbides, the precipitated carbides may include one or several kinds of formation such as Fe₃C, Fe₅C₂ and Fe₂C. The experiment was carried out on the dusting of low alloy steel Cr5Mo in CO-H₂ gas mixture at 600 °C, the experimental results proved that the process of metal dusting for this kind of low alloy steels could be seen as uniform corrosion. By X-ray diffraction analysis, the formations of corrosion samples from exterior surface to interior matrix were: brittle layer of Fe₅C₂, intermediate layer of Fe₃C plus Fe, inner metallic matrix. To describe the process of metal dusting in Cr5Mo, the mathematical model for the three stages of metal dusting was offered respectively: 1^st stage, based on correlative theories on metal carburization, the distribution of carbon concentration in initial stage could be simulated by finite difference method; 2^nd stage, based on 3-D diffusion theories of metal solid solution, Fick’s diffusion equation and jump mass balance equation was applied at interface. Reaction of metal dusting was simplified to be the proceeding of carbon diffusing into metallic matrix through Fe₅C₂ and Fe₃C+Fe layers, then the growing rate equation of corrosion layer was represented. 3^rd stage, based on the reaction rate equation, the carbides products were considered to be decomposed thoroughly to calculate the rate of disintegration reaction. Using the model with 3 stages mentioned above, the depth of corrosion samples for simplified 1-D plate of Cr5Mo steel was calculated. Compared with the actual measured corrosion depth of Cr5Mo samples subject to carburizing atmosphere after different exposure time, the calculated result showed a good agreement for the experimental result, so this model could be applied for the description of metal dusting in low alloy steels, and the method for calculating corrosion rate might offer some theoretical supports for further research such as life prediction of chemical process equipment subject to attack of metal dusting.