Zn-Ni alloy plating from sulfuric acid baths has excellent hydrogen embrittlement resistance for high strength steels. In this study, we investigated the factors that contribute to the hydrogen embrittlement resistance of Zn-Ni alloy plating. In Zn-Ni alloy plating, microcracks are formed in the coating. Hydrogen embrittlement was accelerated by mechanical sealing of these microcracks, and the microcracks that form in the coating were a pathway for hydrogen release. Therefore, these microcracks are found to play an extremely important role in suppressing hydrogen embrittlement. It is suggested that a gap in the plating film that can release hydrogen-induced vacancies is important to suppress hydrogen embrittlement due to plating.

In this study, 6061-T6 aluminum alloys were plated with electroless Ni-P with different phosphorus content and three-point bending and rotary bending fatigue tests. The effects of hydrogen due to plating on mechanical properties were investigated. It was shown that the ductility decreased immediately after low-phosphorus type and high-phosphorus type Ni-P plating by three-point bending test because a hydrogen was introduced into the alloys. The fatigue strength of the low-phosphorus type Ni-P plated specimen was higher than that of the untreated specimen, while that of the high-phosphorus type plated specimen was much lower. It is clear that the fatigue strength differs greatly depending on the phosphorus content in the plating film. 6061-T6 aluminum alloy have been reported to exhibit no hydrogen embrittlement in low strain rate tensile tests under wet condition. However, it was found that hydrogen embrittlement occurred when 6061-T6 aluminum alloy was plated with the high-phosphorus type electroless Ni-P and fatigue-tested on rotary bending machine.

Effects of various surface treatments on hydrogen embrittlement of 6061-T6 aluminum alloy were investigated. Rotary bending fatigue tests showed that electroless Ni-P plating on 6061-T6 reduced the fatigue strength considerably because of hydrogen absorbed into the substrate by plating. These findings suggest the possibility of evaluating hydrogen embrittlement of 6061-T6 by conducting rotary bending fatigue tests after electroless Ni-P plating.

Hydrogen embrittlement affected by three types of plating (low-P and high-P types electroless Ni-P plating and electrolytic Zn-platin) was investigated by means of slow-strain-rate three-point bending test on three aluminum alloys (2017-T3, 6061-T6 and 7075-T651). Hydrogen generated by the Ni-P and zinc plating was absorbed by the aluminum alloy substrates, and the trap sites in the aluminum alloy substrates for the absorbed hydrogen differed between Ni-P and zinc plating. Hydrogen embrittlement was able to be evaluated by the three-point bending tests on plated aluminum alloys. Zinc plating did not cause embrittlement for all the alloys, but Ni-P plating induced embrittlement only for the 6061-T6 aluminum alloy. The result of embrittlement of the Ni-P plated 6061-T6 aluminum alloy corresponds to the highest amount of hydrogen desorbed below 240°C than the other alloys.