Abstract: Two heat exchange tubes in an exchanger tube bundle of a hydrocracking unit for reaction products and low pressure oil broke and came out. Various physical and chemical analyses show that the presence of Cl ion enrichment in the tube resulted in pitting corrosion on the inner wall. Under the effects of shell side fluid impact and heat exchanger structure, the tube bundle is easy to vibrate and needs to bear alternating loads. Under the action of cyclic stress caused by pitting corrosion and alternating load, the corrosion fatigue cracks on the inner wall of the heat exchange tube continued to expand, and eventually overload breakage occurred.

Key words: hydrocracking unit, pitting corrosion, corrosion fatigue, fracture analysis