Microbially induced corrosion (MIC) has been identified as the main cause of the rupture of a key heat exchanger shell at a local refinery. The failure occurred at the 6 o'clock position, close to the girth weld, halfway along the length of the carbon steel shell. An ultrasonic survey revealed up to 90% wall thinning in the ruptured zone. Despite extensive crystallization at the external cooling seawater inlet of the cooling system tubing, there was no evidence of internal leakage of seawater into the shell compartment. The total dissolved solids in the condensate dropout sample were found to be 262 mg/L. The investigation also confirmed that the girth weld had been satisfactorily conducted, including post heat treatment, and that the failure occurred outside the heat affected zone (HAZ). Quantitative polymerase chain reaction (qPCR) analysis of the dry surface film from a crack in the ruptured area confirmed the suitability of the operating environment for prokaryotic growth and the presence of both groups of highly corrosive acid producing bacteria (APB) and denitrifying bacteria (DNB) in moderate numbers (1.92 x 10(3) APB/g and 1.81 x 10(5) DNB/g respectively). The colonization of different groups of corrosion-causing bacteria of a sour system resulted in the localized generation of highly aggressive metabolic by-products causing substantial loss of wall thickness of the heat exchanger shell. The localized wall thinning by over 90% of its original thickness led to the loss of pressure retention capacity of the shell and the rupture of the wall. (C) 2016 Elsevier Ltd. All rights reserved.