Wastage of austenitic heater tubes used in a melamine production plant: plant experience and mechanistic insight

摘要

Melamine is mostly made from urea with alumina catalyst at around 400°C. The nitride layer is formed on the surface of heater tubes due to a presence of ammonia that is a by-product of melamine. In a reactor of melamine production plant, a nitride layer formed on 310S stainless steel (25Cr-20Ni) heater tubes showed a severe spalling susceptibility resulting in a high thinning rate. After replacement to 304 stainless steel (18Cr-8Ni, generally not as corrosion resistant as 310S) heater tubes, spalling was rather suppressed. The purpose of this study is to elucidate a spalling mechanism of 310S heater tubes and a spalling suppression mechanism of the 304 heater tubes.