Development of Complex Hydrogen Penetration Ratio As a New Transfer Function for Analysis on Hydrogen Penetration Mechanisms into Metals

Abstract

Degradation of steels related to the hydrogen embrittlement is a serious problem from the standpoint of infrastructure preservation. The hydrogen embrittlement is related to the hydrogen penetration reaction (HPR) into steel. Some electrochemical methods [1-4] were employed to investigate the HPR mechanisms. One of the major electrochemical methods is electrochemical impedance spectroscopy (EIS). The EIS was applied to the investigation of HPR mechanisms because the time constants observed in impedance spectrum can be discriminated [5]. Another one is Devanatnan-Stachurski permeation technique [6, 7]. In this technique, two independent electrochemical cells are arranged on two surfaces of metal sheet and anodic and cathodic polarizations can be performed with each cell using the three-electrode system. The amount of hydrogen penetration rate can be evaluated by measuring the oxidation current of hydrogen atoms diffused in the metal sheet at the anode side. In this study, we proposed a new transfer function based on the electrochemical reactions and mass transfer during HPR by combining EIS and Devanatnan-Stachurski permeation technique. The complex hydrogen penetration ratio (CHPR), ι, as a new transfer function is defined from ratio of ΔI per and ΔI ent. ι = ΔI per / ΔI ent (1)In this equation, the ΔI per is the current response related to the hydrogen penetrated through metal sheet at anode (detection) side, and the ΔI ent is the current response of cathode (entry) side. The equation (1) can be expressed by the following equation as the theoretical equations for ΔI per and ΔI ent were derived based on Fick's 2nd law,. ι = 1 / K ab exp (-h (jω / D)1 / 2) (2)In the equation (2), K ab is the hydrogen entry ratio against the total cathodic reaction rate, h is the thickness of metal sheet, j is imaginary number unit, and D is the hydrogen diffusion coefficient.The simulations of ι were conducted with the equation (2) assuming arbitrary values of D. The simulation results indicated that the phase shift of ι became large with the decrease of D. The new method to determine the value of D from the phase shift of CHPR was developed.