In-service failure of mechanically galvanised low alloy steel bolts of three 25,000 m3 bolted water tanks

Abstract

Several single-lap bolted joints fastening the steel plates of three 25,000 m3 water tanks showed premature failure. Most fractured bolts were in the lower sector of the tanks.The tempered martensitic steel bolts were mechanically galvanised, and the poor quality of the galvanisation allowed the corroding agents (Cl− and SO4−2) to percolate through the Zn and Sn coating particles, finally reaching the steel surface of the bolts. Additionally, multiple 6 μm-deep intergranular pre-cracks were observed at the threads’ roots of the new bolts. During service, these pre-cracks accumulated the corrosive agents, creating numerous 10 μm-deep corrosion pits, nucleating the bolts’ environmentally assisted cracks. Microfractographic examination of the bolts featured stable growth of brittle cracks (intergranular and quasi-cleavage cracking) followed by mixed-mode cracking and a small region of ductile overload fracture. This failure sequence confirms that the fracture mechanism of the low alloy steel bolts is due to environmentally assisted cracking. Finite elements analysis investigated the effect of the bolt diameter, bolted joint design and pre-load force on the tensile stress distribution. Improvements in the bolts’ quality control and bolted joint design were discussed to prevent new failures.