Catastrophic carburisation in petrochemical industry

Abstract

This paper reports catastrophic carburisation (metal dusting corrosion) of radiant tube of a Pacol reactor which has undergone service life of 67,000 hours in a petrochemical plant. The 6" dia. tube conforms to Sch. 40 of ASTM A335-P11 (1.25Cr-0.5Mo steel). The failure of tubes occurred due to non¬uniform thinning of tube wall leading to fracture. The failed tube was investigated by x-ray diffraction phase analysis, scanning electron microscopy, energy dispersive spectroscopy and optical microscopy for characterisation of reaction products. The inner wall of tube surface contained wustite, magnetite, Cr2C, M23C6, rounded iron metal particles and irregular shaped carbon soot. The presence of wustite in scale deposit revealed that the tube temperature was above or around 600°C, while the absence of hematite, which grow in presence of water vapour and thermodynamically unstable to high reducing gas atmosphere, was consistent with the atmospheric condition in the tube. The structural identification of Cr2C and M23C6 phases and morphological features near the base metal of inside surface revealed highly carburised inner tube metal surface. The wall thinning in thicker region of the tube is in the form of rounded pits, whereas in thinner region of tube, metal loss in the form of uniform localised thinning was encountered. The carbon soot deposits were mostly on the thicker wall region. All these observations indicate that the metal wastage was due to carburising condition.