Stress corrosion cracking of pyrotherm reformer tube for steam-reforming hydrogen production

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A section of Pyrotherm G 25/35 Nb reformer tube was rupture-failed in a steam-reforming hydrogen plant and analysed to identify the causes of failure. Examination of the internal surface of the pipe indicated signs of heterogeneous corrosion attack in localized areas near to the primary crack site. Some of these areas were associated with fissures, although they did not penetrate through the pipe wall. Measurement of the pipe wall thickness revealed that fair amounts of the material had been consumed by corrosion. Cross-sectional examination of the dissected pipe in areas showing signs of corrosion attack and fissures revealed the presence of radial macrocracks, originating from the internal surface, and numerous microcracks in the pipe interior. Most microcracks were formed along the grain boundaries of the spin-cast microstructure. Further examination of the macrocracked surfaces revealed the presence of a granular microstructure, indicative of a brittle failure mode. Based on the characteristics exhibited by the macrocracking, the rupture failure of the reformer tube is attributed to stress corrosion cracking (SCC). The SCC is believed to be produced through synergistic reactions amongst sulfur-containing derivatives in the natural gas (feedstock), hydrogen and superheated steam in the processed gases under a mechanically-stressed environment. The presence of the mechanical stress is attributed to the bending of the pipe caused by improper suspension design.

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