アンモニア合成塔での現用鋼の水素侵食および窒化による腐食脆化について

抄録

The corrosion embrittlement of commercial carbon steels, low alloy steels and stainless steels in a New Fauser ammonia converter by hydrogen and nitriding attack has been investigated. The degree of embrittlement was examined by an optical microscope, carbon and nitrogen analyses, tension, Charpy impact, bend and hardness tests. New parameters, i.e., P: Cr/52+Mo/96 and S: Cr/52+Ti/48+Al/27 were introduced to evaluate the degree of embrittlement. The results were as follows: (1) At the inlet side (temperature≅430°C) in the converter, embrittlement was mainly caused by hydrogen attack. Steels having P value less than 0.02, were severely embrittled. The embrittlement became minimum at P value of 0.05, and then increased slowly with P value by nitriding attack. (2) At the outlet side (temperature≅465°C) of first stage in the converter, nitriding attack became predominant, and the degree depended on the thickness of nitriding layer which could not be estimated simply by the contents of nitride-forming elements, e.g., Cr, Ti. Al. The effect of other elements, e.g., Cu, Ni, must be taken into account. (3) The embrittlement by hydrogen or nitriding attack, therefore, depended on the content of alloying elements in steel. (4) Toughness determined by impact test and ductility by tension test were good parameters of the embrittlement, but hardness was not. Approximate degrees of embrittlement were also estimated by bend test.