Hydrogen evolution and permeation into steel during zinc electroplating; effect of organic additives

The Devanathan and Stachurski diffusion membrane method was used to study t he evolution of hydrogen and its permeation into a steel sheet during catho dic charging from a chloride electrolyte or during zinc electroplating. The influence of four different organic compounds, which are the components of various formulations derived to improve zinc electrocoatings, were also te sted. At a high-charging current density, the permeation transients obtaine d in a chloride electrolyte without zinc ions exhibit a maximum attributed to hydrogen trapping in the subsurface layer on the entry side. The concent ration of adsorbed hydrogen on the steel surface depends not only on the ca thodic current density and the composition of the solution, but also on the influence of the organic additives on the recombination of hydrogen atoms. During zinc electrodeposition, the coating covers the substrate in a few s econds and acts as a barrier for hydrogen absorption. The permeation rate d epends on the cathodic current density but also on the concentration of ZnC l2 in correlation with the porosity of the coating. It is shown that steel substrate hydrogenation (beneath the zinc coating) is strongly reduced in t he presence of a combined additive, composed of four compounds in appropria te amounts as well as in the presence of PEG(6000) in the plating bath. Thi s effect, which is correlated to the modification of the hydrogen evolution process, can be used to hinder the severe drawbacks caused by hydrogen pen etration into the steel substrate.