Change in States of Hydrogen Present in Cold-Rolled Pure Iron Charged by Hydrogen Gas and Electrolysis

Abstract

  States of hydrogen present in cold-rolled pure iron specimens charged by hydrogen gas and cathodic electrolysis have been compared using thermal desorption analysis to obtain the fundamental properties for evaluating the hydrogen embrittlement susceptibility of hydrogen gas pipeline steels. Hydrogen was charged into the cold-rolled pure iron specimens in gaseous hydrogen at pressures of 4, 7, and 10 MPa at temperatures of 30, 60, 90, 120℃, and in an aqueous solution of NaOH with pH of 13, to which 5 g•L⁻¹ of NH₄SCN was added, at current densities in the range of 0 to 50 A•m⁻² and at temperatures of 30, 60 and 90℃. For charging at 30℃ under various hydrorgen pressures, hydrogen peak temperatures are approximately 60℃ and the peak profiles are identical. In contrast, when the charging temperature is increased to 90℃ under various hydrorgen pressures, hydrogen desorption newly occurred above approximately 100℃. The hydrogen peak at 60℃ corresponds to hydrogen trapped at dislocations and vacancies, whereas the hydrogen desorption above approximately 100℃ corresponds to hydrogen trapped at vacancy clusters formed during hydrogen charging at higher temperatures. In addition, equilibrium hydrogen content in solid solution and at trapping sites decreases with increasing charging temperature, since enthalpy of hydrogen solution is −27.3 kJ•mol⁻¹. These results indicate that higher-temperature charging causes the states of hydrogen present in cold-rolled pure iron to change due to the formation of vacancy clusters and reduces the equilibrium hydrogen content including that in solid solution and at trapping sites.