Abstract
Electrodeposition of zinc, cadmium, nickel or zinc-nickel on steel is accompanied by hydrogen evolution. The risk for hydrogen embrittlement of high strength steels can be avoided by using appropriate plating conditions. During the development of a non-embrittling ZnNi plating process the amount of diffusible hydrogen effusing from the steel and the coating during thermal treatment (24 h/190°C) was quantified. The risk for hydrogen embrittlement of high strength steel at the different steps of the coating process was determined with constant load tests on electroplated notched round tensile specimens. It was found that metal coatings may serve as a remarkable hydrogen reservoir. Thus, zinc and nickel coatings store more hydrogen during the plating process than the high strength base material. However, electroplated cadmium coatings perform differently than other plated metals. Although bright-cadmium plating leads to hydrogen embrittlement of the tested high strength steel, less diffusible hydrogen was determined than in case of non-embrittled specimens with other metal coatings.
Introduction
Work is ongoing to replace cadmium coatings, still used today in military and aerospace applications (e.g. landing gears of airplanes), by environmentally more friendly zinc-nickel coatings. However, the central problem is to avoid hydrogen embrittlement by atomic hydrogen co-produced in the electroplating process. For cadmium platings this problem has been solved since many decades by using the LHE-cadmium process1 (LHE = Low Hydrogen Embrittlement). For ZnNi coatings previous own investigation led to the development of a LHEZnNi plating process2 which prevents hydrogen embrittlement of the plated high strength steel and is ready for the industrialization phase.
During the development of the LHE-ZnNi plating process much work focussed on the question: where is the co-produced hydrogen going and how much? When measuring the amount of hydrogen that effuses during the standard thermal treatment (24 h, 190°C) from notched round tensile specimens, it was observed that more hydrogen effuses from coated specimens than from specimens whose coatings had been mechanically removed after electroplating. This led to the assumption that a considerable amount of hydrogen is trapped in the metal coating.
