Coadsorption of CO and H₂ on an Iron Surface and Its Implication on the Hydrogen Embrittlement of Iron

Abstract

We investigated the competitive coadsorption of carbon monoxide and hydrogen gas on an iron surface with a 110 facet using density functional theory. Our study discusses the hydrogen dissociation reaction on a fresh iron surface and a surface with varying carbon monoxide coverage. Additionally, we investigated the carbon monoxide surface adsorption as a function of the carbon monoxide surface coverage. Our results show different trends for the carbon monoxide adsorption and hydrogen dissociation on surfaces with low and high CO coverage. Those opposite trends were related to the charge of the surface iron atoms and the available surface electron density which is necessary to facilitate the carbon monoxide adsorption and catalyze the hydrogen dissociation reaction. The subsurface diffusion of predissociated surface hydrogen atoms has been included in the model. It was found that the atomistic hydrogen diffusion into the material is also related to the carbon monoxide surface coverage. Our theoretical results confirmed that a small amount of carbon monoxide as an impurity in the hydrogen gas can mitigate the effect of hydrogen embrittlement by significantly reducing the rate of hydrogen dissociation on the iron surface and thus reduce the hydrogen uptake into the bulk of the material. To verify the theoretical results, we carried out a fracture toughness test of pure iron in a high-purity H 2 , CO and H 2 mixture, and N 2 gases. This material suffered from hydrogen embrittlement, in other words, reduction in the fracture toughness due to hydrogen. We could derive the complex dependence on the hydrogen embrittlement manifestation as a function of the H 2 /CO gas mixture ratio and gas exposure time.

Effect of Gas Pressure on Hydrogen Environment Embrittlement of Carbon Steel A106 in Carbon Monoxide Mixed Hydrogen Gas

“…The CO mitigation effect on the HEE in a fracture toughness test was reduced with a decrease in the loading rate. [17] The hydrogen gas pressure dependence on the mitigation effect of CO on the susceptibility to HEE observed in this study can also be ascribed to the same mechanism. This consideration is the case where a new fresh Fe surface is continuously created by crack extension during the fracture toughness test after the CO coverage reaches the maximum value.…”

“…The coadsorption of H 2 and CO on the Fe surface has recently been investigated in detail using first principle calculations. [17] The results showed that CO surface coverage is crucial for understanding the mitigation effect of CO impurities in H 2 gas on the susceptibility to HEE. The maximum CO coverage on the Fe surface was estimated to be limited to approximately 75 pct.…”

“…Furthermore, the loading rate also has been confirmed to affect the mitigation effect of CO on HEE susceptibility. [17] A single crosshead speed that can be expected to be sufficiently slow to saturate the effect of hydrogen on the HEE susceptibility was chosen in this study. [9] In some fracture toughness tests conducted in the present study, the crack opening displacement, COD, exceeded the capacity of the clip gauge measurement before the J-Da curve crossed the 0.2 mm offset line.…”

“…The CO surface coverage would mitigate the molecular to atomistic hydrogen dissociation; however, the CO surface coverage cannot completely prevent the molecular to atomistic hydrogen dissociation since the activation barrier of this process would be up to 0.87 eV. [17] Thus, some of the fracture toughness test results that showed complete suppression of the HEE in the presence of CO are not ascribed to complete inhibition of hydrogen uptake but were jointly achieved with suppression of hydrogen diffusion. Therefore, while the barrier would significantly reduce the H 2 dissociation rate under normal conditions, at longer time periods or as a result of external stimuli such as applied pressure, the surface concentration of atomistic hydrogen would gradually increase, which will cause HEE.…”

“…Based on first principle simulations, [17] in the current study, a classical MD approach was employed to elucidate the pressure dependence of the CO and H 2 interaction with the Fe surface. While classical mechanics would allow us to explore the time dependence of the surface coverage, the molecular hydrogen interaction with the Fe surface should be considered to be a dissociative adsorption that requires considering the H-H bond cleavage chemistry.…”