Influence of baking time on hydrogen embrittlement in alkaline nickel-zinc electroplated steel fasteners

The hydrogen embrittlement (HE) phenomena pose a considerable risk, especially in the application of high-strength steel fasteners. If HE in fasteners is not eliminated, it is a phenomenon that has catastrophic consequences such as crack growth in matter, fracture initiation, and finally instant failure. Hydrogen gas produced during side reactions in the electroplating process of steel fasteners is known to be riskier compared to atmospheric hydrogen exposure. In this study, Ni-Zn electroplated 12.9 bolts have been baked in 200°C for 4, 8 and 12 h to observe its effect on the prevention of HE. The elongation values of the samples have been examined with a slow strain rate test (SSRT), and the best recovery condition of the ductility of the materials was determined. The effects of different baking times on the microstructure are observed by an optical microscope. The morphological structure of the coating is investigated by SEM. An electromagnetic acoustic spectroscopy technique was performed to know the effect of different bake times executed at constant temperature and the effect upon dislocation mobility. The results show that eight hours of baking is sufficient for HE elimination, and therefore twelve hours would be unnecessary in terms of energy saving. Furthermore, the non-destructive testing technique known as acoustic resonance may be used to detect HE.