Influence of cementite morphology on the hydrogen permeation parameters of low-carbon steel

Abstract

In this investigation the effect of cementite morphology and distribution on the hydrogen permeation parameters of a low-carbon steel [chemical composition (in wt%): C 0.07, Mn 0.28, Si ~< 0.01, S 0.017, P 0.009 and A10.079] was studied utilizing an improved electrochemical technique. A two-compartment electrochemical permeation cell (based on the traditional apparatus described by Bees and Zfichner [1]) with the necessary dedicated hardware and software was developed to permit automated, high-precision isothermal permeation tests with real-time computer monitoring and data analysis via a programmable multichannel control unit. The results presented here were obtained using the above experimental system, selecting the double-potentiostatic option as the mode of operation, and a 2 s data-sampling rate. The specimens used for this investigation were of the order of 0.6 mm thick, having been prepared from 1 mm-thick sheet material by the usual metallographic polishing technique. Both compartments of the electrochemical cell were filled with deaerated 0.1 M NaOH, continuous nitrogen bubbling being maintained throughout the test. Hydrogen was generated cathodically at the sample surface in one of the compartments, utilizing a constant potential of 1 .35V versus saturated calomel electrode (SCE) and detected by anodic polarization at the corrosion potential in the adjacent compartment after permeating through the thin-sheet metallic specimen which separated the two. The progress of the hydrogen permeation process was followed by monitoring the evolution, over time, of the anodic current flow, measured between the specimen and the counterelectrode of the detection compartment. Thermostatic control of both cells was maintained throughout the test, the temperature (300 K) being monitored and controlled with an accuracy of +0.1 °C via a system of silicon-transistor sensors. Relatively small variations in temperature may significantly effect the precision and scatter of the data obtained. An analysis of the resultant data in terms of the unidirectional solid-state diffusion of hydrogen in the thin metallic sheet, as described by Fick's laws, permits the determination of the three interrelated parameters which quantify the permeation behaviour characteristics of the material: the permeability [P(t)], the apparent diffusivity (Dapp) and the apparent solubility (Sapp) [1, 2]. The term solubility is used here to signify the total hydrogen content,