Hydrogen re-embrittlement of high strength steel with different zinc-containing topcoats

Low alloyed high strength steels are increasingly used in many industry sectors to reduce weight and, hence, to save fuel, reduce emissions, and to minimize resource utilization. Those alloys show, however, a tendency for hydrogen induced cracking (HAC), where the hydrogen is formed either during processing or due to service corrosion processes. The aim of this study was to evaluate the impact of various zinc-containing topcoats on hydrogen re-embrittlement. In order to cause hydrogen induced cracking, hydrogen does not only need to be formed, but also needs to penetrate and migrate into the material. The hydrogen entry is influenced by various parameters, e.g. temperature, electrostatic potential, humidity, pH-value, oxygen concentration, chloride and promoter concentration, and time. In this study, the influence of different environmental conditions on the hydrogen induced corrosion was investigated by means of electrochemical permeation measurements. The electrochemical permeation measurements were performed under different concentrations of the local oxygen content (5 % NaCl; pH = 3; flushing with different gases). For these chosen environmental conditions, the results show that the local oxygen content has a significant influence on the resulting permeation current. To validate the results of the electrochemical permeation measurements and to examine the susceptibility against hydrogen re-embrittlement of those zinc containing topcoats, incremental step-load-tests were performed under the same environmental conditions. It is shown that the investigated topcoats (electroplated zinc, electroplated zinc-nickel, galvanized zinc, zinc-flake (GEOMET 321A)) impose a different susceptibility on hydrogen re-embrittlement. Under the tested environmental conditions specimens with electroplated Zn sustain a lower fracture load compared to specimens in heat treated black condition. In contrast, an electroplated zinc-nickel topcoat leads to a higher fracture load.