Adsorption Behavior Of Gas Molecules On Iron Surface

Iron and iron-based alloys are one of the most widely used materials in industrial production.However,the long-term service of the steel under atmospheric conditions causes a huge loss of iron-based materials due to corrosion?including hydrogen corrosion and electrochemical corrosion?.Corrosion is the result of the interaction between various atmospheric molecules and steel materials.Therefore,it is very important to study the adsorption behavior of atmospheric small molecules on the surface of iron and stainless steel at room temperature.Density functional theory and molecular dynamics methods are used in this paper,the specific research is displayed as follows:Density functional theory?DFT?was used to investigate the effect of adsorption coverage?1/16ML and 1/4ML?and adsorption sites of hydrogen on the adsorption of H₂O,CO₂,N₂,CO and He on?-Fe?111?.The calculated results show that the austenite Fe?111?surface is highly active and susceptible to corrosion.In addition to the inert gas,chemical adsorption occurs with other gas molecules.The adsorption order of the different molecules on all constructed iron substrates is O₂>CO>N₂>CO₂>H₂O>He.Except He,pre-adsorbed H₂ enhances the adsorption of other small molecules on Fe?111?.At the same time,the increase of surface H₂ concentration enhances the diffusion of other gases to the interior of iron.After the H₂ diffusion into the interior of iron,they will be decomposed into atomic state.The stable adsorption sites for H atoms in the first and the second sub-layer of the iron substrate are Fcc site and Hcp site respectively,hydrogen atoms are easily existed at the surface and near the surface.The adsorption coverage and adsorption site of hydrogen atoms have a great influence on the adsorption strength of small molecules on the iron surface.The more"deep"sites of the hydrogen atoms in the substrate,the weaker the adsorption strength of small molecules.When the hydrogen adsorption coverage is enhanced from 1/16 to 1/4ML,the adsorption of small molecule is weakened.In general,the presence of surface hydrogen molecules contributes to the adsorption of small-molecule on austenitic Fe?111?surface,while the hydrogen atoms weaken the binding of other gas molecules to the surface of Fe?111?surface except He.These results provide an important basis for the regulation of the gas reactions on the Fe?111?surface and the further study on the adsorption of small molecules on the stainless steel surface.The effects of time and temperature on the adsorption of gas small molecules on stainless steel surface are calculated by molecular dynamics method in this section.The results show that at 5 ps,the adsorption of O₂,H₂O,CO₂ and CO on the surface of stainless steel occurs.When the time reaches 1ns,only CO is adsorbed.When the temperature rose to 300K,CH₄,H₂O and CO adsorbed on the surface;500K,O₂ also adsorbed on steel surface;at 800K,except H₂O,other gases decomposed.The above results provide an important theoretical basis for studying the corrosion mechanism of austenitic stainless steel.