An adsorption study of hydrogen on iron and its relation to hydrogen embrittlement

Abstract

The chemical reactions of hydrogen gas on iron surfaces have been determined by simultaneously measuring the volume of gas adsorbed and the corresponding magnetization change. The combination of these experimental results as well as the kinetics of the reactions is used to explain the temperature dependence observed in crack growth studies performed in gaseous hydrogen. The reaction is shown to be a two step process involving the formation of an adsorbed molecular precursor prior to the formation of the embrittling hydrogen ion. The adsorption isotherm and magnetizationadsorption isotherm for H₂ on Fe at 77 K were determined to be Langmuirian. This, plus a first order adsorption rate are given as evidence for the existence of a chemisorbed molecular H ⁺₂ precursor at this temperature. The mechanical test data of other investigators for slow crack growth in gaseous H₂, which show a nonmonotonic change of crack growth rate with temperature, become explainable based on a measured decrease in the adsorption of H₂ at temperature above 300 K and the two step adsorption process. At temperatures below 300 K the formation of an H^- ion from the adsorbed precursor H ⁺₂ ion is the rate controlling process in gaseous hydrogen embrittlement. At temperatures above 300 K, the decreasing net adsorption rate of H ⁺₂ becomes the limiting process.