An investigation of hydrogen embrittlement of high strength steel due to zinc/nickel alloy electroplating

The work described in this thesis examines the embrittling effect of both cadmium and zinc-nickel electroplating on quenched, tempered and plated and plated and baked tensile specimens of AISI4340 steel. Measurements were taken of ultimate tensile strength, reduction in area to obtain an embrittlement index and Vickers hardness. In this way a direct comparison of the extent of embrittlement for the two alternative processes was made, while at the same time, keeping a quality control check on successive batches of specimens. An examination of the microstructure and fracture surfaces was made. The effect of baking to remove hydrogen dissolved in the electrodeposit and de-embrittle the steel was considered. Being porous to hydrogen, some types of cadmium electrodeposits allow hydrogen gas to exit the substrate during baking. Nickel, however is capable of forming a barrier to hydrogen diffusion and it was found that this could impede the ingress of hydrogen without preventing the degassing of the specimens. Permeation experiments were conducted, using a cell based on a Devanathan and Stachurski permeation cell, to compare and contrast the amount of hydrogen that entered a steel substrate during zinc-nickel alloy electroplating under various conditions. The amounts of hydrogen that permeated the diffusion membrane were then examined in the light of the results of the tensile tests. It was found that a close correlation existed between the amount of hydrogen entering the substrate and the extent of embrittlement. This correlation allowed apparently anomalous embrittlement observations to be explained in terms of the pH of the plating solution. The results suggested a modification to the currently accepted hydroxide suppression mechanism for zinc-nickel alloy deposition was possible. By way of testing predictions made under the modified deposition mechanism, Auger and XPS analyses were used to search for a nickel rich layer predicted to exist adjacent to steel substrate.