Effects Of Hydrogen On Corrosion And Anodic Behavior Of Iron In Neutral And Alkaline Solutions

Hydrogen can enter into iron and its alloys through many sources such as manufacturing,fabrication and service processes.There have been reports on the effect of hydrogen in iron and ferritic steels on corrosion and electrode reactions in aqueous solutions.The effects of hydrogen on corrosion are dependent on the types of metalenvironment systems,the involved element processes and their rate-determining steps.The effects of hydrogen on corrosion processes can arise from several sources such as the change of the surface condition,the direct oxidation of hydrogen atoms,the change of active dissolution kinetics of the metals,the change of formation and dissolution kinetics of the film on the metal,and the properties and stability of the film.It is important to identify and quantify the fundamental aspects of the effects of hydrogen on the element electrochemical processes.In the present work,the effects of hydrogen in iron on open circuit states on anodic dissolution,passivation,transpassivation,pitting corrosion and corrosion products are investigated by electrochemical measurements,solution analysis,surface film analysis and surface morphology analysis.Typical test systems with non-charged iron,pre-cleaned iron and hydrogen charged iron in a Devanathan dual cell are used to distinguish the primary hydrogen effect and the resultant active surface effect.1)Anodic dissolution and passivation of iron in alkaline solutionsCharged-hydrogen shift the open circuit potential in the negative direction and causes the transition from passivity to active dissolution for iron in bicarbonate solutions of pH 8.4 and mixed bicarbonate/carbonate solution of pH 9.17.The shift of open circuit state and the appearance of active dissolution range for hydrogen-charged iron are primarily caused by the surface cleaning effect.Hydrogen in iron hinders the transition from active dissolution to passivation.Hydrogen causes the active dissolution range and current density peaks in anodic polarization curves,and decreases the electrochemical impedance for iron in dilute bicarbonate solutions.Charged-hydrogen in iron increases the anodic reaction rates but did not cause the transition of the open circuit states for iron in a carbonate solution of pH 11.46 and in sodium hydroxide solution of pH 13.Hydrogen increases the anodic current and decreases the electrochemical impedance for iron in solutions with high pH values and high concentrations of bicarbonate.ICP analyses of tested solutions confirm the enhancing effect of hydrogen on anodic dissolution.The sequence of hydrogen-charging and immersion affects the transition of the open circuit states as the results of hydrogen effect.2)Anodic dissolution,passivation,transpassivation and pitting of iron in bicarbonate solutions with chloride ionsThe shift of open circuit state and the appearance of active dissolution range for hydrogen-charged iron in bicarbonate solutions with low chloride concentrations are primarily caused by the surface cleaning effect.Both pre-cleaning and hydrogencharging accelerates the anodic dissolution and pitting corrosion at high potentials.The anodic current density of hydrogen-charged iron is slightly higher than that of the precleaned iron at potential lower than the first anodic current peak.At potentials higher than the potential for the first peak current,the effect of hydrogen-charging becomes more significant until the transpassivation potential.At high anodic potentials,pitting of hydrogen-charged iron is more severe than that of non-charged iron,which is less severe than that of pre-cleaned iron in bicarbonate solutions with low chloride concentrations at 0.9 V(SCE)and 1.2 V(SCE).3)Corrosion products on iron in pure water at various temperaturesIncreasing temperature from 25 oC to 70 oC significantly accelerates the corrosion of non-charged iron in pure water.Charged-hydrogen changed the types,morphologies and electrochemical properties of the surface films formed on iron.The yellow rust formed on the non-charged iron after immersion 21 h at 40 oC,while the surface of hydrogen-charged iron after immersion in the same condition was gray.The film formed on the non-charged iron after immersion 21 h at 70 oC is dark,while the surface of hydrogen-charged iron after immersion in the same condition is covered with loosely distributed yellow rust particles.The stability and semiconductor properties of the surface films formed on non-charged and hydrogen-charged iron were characterized by EIS and M-S plot measurements in boric acid-borate buffer solutions,showing significant effect of hydrogen.The effects of charged hydrogen on the properties of the oxide films were dependent on the test temperatures.Magnetite tended to be formed on hydrogencharged specimens due to the excess of ferrous ions.Electrochemical impedance and semi-conduct properties of prior-immersed iron specimens are related to the test temperature.4)Effects of hydrogen on corrosion products of iron in chloride and sulfate solutionsCharged-hydrogen affects the properties of the surface films formed on iron after 21 h of immersion in 10-4 to 0.5 mol/L NaCl solutions at 25 °C.After the immersion in 10-4 mol/L NaCl solution,the non-charged iron is partly covered by the surface film,while the hydrogen-charged iron is fully covered by the surface film.After the immersion in 0.5 mol/L NaCl solution,the surface of the non-charged iron is partly dark and partly covered with yellow rusts,while the surface of the hydrogen-charged iron has a larger area covered with yellow rusts than that of the non-charged iron.OCP values for hydrogen-charged iron in pure water,10-3 and 10-4 mol/L NaCl solutions are significantly lower than those for non-charged iron.In 0.5 mol/L NaCl solution,the OCP difference between non-charged iron and hydrogen-charged iron gradually becomes smaller with increasing immersion time.Raman spectra showed the presence of magnetite in the film on hydrogen-charged iron after immersion in 10-4 mol/L NaCl solution,which is supported by XRD results.Yellow rust and the XRD peak for hematite in the films on non-charged and hydrogen-charged iron specimens indicated that the mechanism for the effect of hydrogen on the corrosion products of iron in dilute NaCl solutions could be different from that in concentrated NaCl solutions.In solutions with a high chloride concentration,hydrogen in iron might be able to promote the formation of high valence iron oxide,while in many other cases it is found that the formation of magnetite is promoted by hydrogen in iron.Hydrogen in iron shifts the open circuit potential in the negative directions.Hydrogen could change the types of oxide film on iron.The effects of hydrogen on corrosion of iron are more significant in pure water and dilute sodium solutions than those in concentrated sulfate solutions.