Forms of corrosion High temperature / Chemical Metal dusting

摘要

Metal dusting is a catastrophic car-burization process to which alloys containing iron, nickel and cobalt are vulnerable. It results in the disintegration of the alloy. Metal dusting has been observed in coal gasification, iron ore reduction and synthesis gas production processes. The phenomenon occurs at temperatures between 400 deg C and 800 deg C and was first described in the late fifties. In ammonia plants metal dusting can occur in the process equipment between the secondary reformer outlet and the inlet of the high temperature shift converter. Metal dusting results in the disintegration of the alloy into dust consisting of particles of carbon, metal and oxides. The disintegration can show up as overall corrosion but sometimes also a pitting like attack occurs with large pits with diameters up to several centimeters. Sometimes the morphology equals an orange skin appearance. Anyway the disintegration of the materials of construction impairs the mechanical integrity of the equipment. Part of the dust is transported by the (high velocity) process gas to the top layer of the high temperature shift catalyst. The nickel in the dust at the top of the catalyst bed will have the effect of increasing the amount of by-product hydrocarbon formation like ethane and propane. Beside the impact on mechanical integrity metal dusting will decrease ammonia production.