Electrochemical Hydrogen Permeation Tests Under Potentiostatic Hydrogen Charging Conditions Conventionally Used for Hydrogen Embrittlement Study

Abstract

Electrochemical hydrogen charging is a commonly used method for hydrogen embrittlement study of high strength steels as well as immersion in ammonium thiocyanate solutions. Hydrogen concentration in specimens in wide range, from low hydrogen concentration simulating that caused by atmospheric corrosion to relatively high hydrogen concentration to emphasize the effect of hydrogen on hydrogen embrittlement fracture, can be obtained either by changing hydrogen charging solutions with/without catalyst poisons and by controlling current density and potential applied to specimens. However, in many cases, hydrogen concentration is controlled in a trial-and-error manner. To understand the behavior of hydrogen entry in various hydrogen charging conditions and to obtain guidelines for efficient electrochemical hydrogen charging, electrochemical hydrogen permeation tests were performed in galvanostatic hydrogen charging conditions and the effects of current density and solution were reported in our previous study. In the present study, aiming at the same purpose, potentiostatic hydrogen charging was carried out using varied conditions and the hydrogen entry was monitored by means of electrochemical hydrogen permeation test and the results were compared with that obtained by using galvanostatic hydrogen charging. In the same way of the previous study, 0.5 mm-thick pure Fe sheet specimens set in a Devanathan-Stachurski cell were used in this study. The hydrogen input side of the specimen was potentiostatically polarized in 3 wt% NaCl solutions containing varied concentration of ammonium thiocyanate and a 0.1 M NaOH solution, open to air at room temperature. The Ni-plated hydrogen output side was polarized at 20 mV vs. Hg/HgO in a 0.1 M NaOH solution and the electrochemical hydrogen permeation current density was monitored in various potentiostatic hydrogen charging conditions. Hydrogen permeation current density was increased almost linearly with decrease in hydrogen charging potential. Exceptionally, at relatively high potential region the hydrogen permeation current density was lower than the extrapolation of the line probably because oxygen reduction decreases the efficiency of hydrogen entry. The efficiency of hydrogen entry expressed by the ratio of hydrogen permeation current density to hydrogen charging current density at the applied potential was decreased with decrease in potential accompanied with increase in cathodic current density. The increase in ammonium thiocyanate concentration led to enhancement of hydrogen entry drastically. These results were almost the same as that obtained for galvanostatic hydrogen charging in the previous study, and the relationship between hydrogen charging potential and hydrogen permeation current density of the potentiostatic hydrogen charging was similar to the relationship between potential of hydrogen input side resulted by galvanostatic hydrogen charging and hydrogen permeation current density. The relationship obtained in the present study can be used as guidelines for potentiostatic hydrogen charging. Though it is assumed that potentiostatic and galvanostatic hydrogen charging are similar to each other in principle, stability of hydrogen permeation current density showed some differences. In the presentation, stability of hydrogen permeation current density under the potentiostatic and galvanostatic hydrogen charging will be also discussed.