Permeation measurements to study hydrogen uptake by steel electroplated with zinc–cobalt alloys

Hydrogen permeation measurements were performed to investigate hydrogen uptake by a steel substrate when electroplated with zinc–cobalt alloys. The influence of plating bath composition and pH were studied and the effect of absorbed hydrogen on embrittlement of high strength steel was measured in slow strain rate tests. It was shown that the majority of the hydrogen generated during electroplating was either evolved from the surface or trapped in the coating. Only a very small proportion, equivalent to 0.1–0.6% of the total charged passed, was absorbed by the steel and was responsible for causing embrittlement. The average amount of hydrogen required to cause hydrogen embrittlement failure of quenched and tempered AISI 4340 steel was 1.5 × 10−7 g H atoms cm−2. There was a linear relationship between the rates of hydrogen uptake from the different coatings and the extent of hydrogen embrittlement produced in the high strength steel. In certain conditions, a cobalt-rich layer formed at the steel/coating interface and this layer was shown to be beneficial in restricting hydrogen uptake and embrittlement.