Cracking Reasons of Zinc Coated High-Strength Bolt for Seawater Pump in Nuclear Power Plant

A double-headed zinc coated high-strength bolt in a seawater pump of nuclear power plant cracked. The failure reasons of the bolt were analyzed through testing of material properties (chemical composition, microstructure and mechanical properties) and morphology observation of fracture and cracks. The results show that the failure of the high-strength bolt was the result of hydrogen induced cracking (HIC). When the high-strength bolt with damaged zinc coating was in contact with seawater, a galvanic couple formed between the surface zinc coating and the bolt substrate, and the substrate worked as cathode on which hydrogen was precipitated. Some atomic hydrogen diffused into the bolt substrate, resulting in hydrogen damage. HIC happened on the bolt under the continuous action of preloading force.