Effects of an applied magnetic field on the dissolution and passivation of iron in sulphuric acid

The effects of an applied magnetic field (MF) on the electrochemical state, anodic dissolution and passivation of iron in sulphuric acid solution were studied by potentiodynamic scanning polarisation measurements, potentiostatic polarisation measurements and scanning electron microscopy observation. The magnetic field reduced the fractional surface film coverage on the electrode by enhancing the film dissolution process. This made the electrode prone to active dissolution. With increasing applied potentials the magnetic field accelerated the anodic dissolution at relatively low potentials, changed the oscillation or passivation to permanent active dissolution at intermediate potentials, and maintained the passive state at high potentials. Potentials for the onset of passivation moved in the noble direction when the magnetic field was imposed. An electrode kinetics formulation for the effects of the magnetic field on the dissolution and passivation is proposed. In the presence of a magnetic field and at specific anodic potentials, scalloping occurred due to accelerated localized dissolution. The scalloping areas were on both sides of the electrode and oriented parallel to the direction of the earth’s gravitation field. The ratios of the scalloping area caused by a 0.4 T magnetic field on the whole electrode surface were 0.69 (at 200 mV), 0.66 (at 350 mV) and 0.75 (at 400 mV), respectively. In contrast, uniform electrode surfaces were observed at these anodic potentials in the absence of the magnetic field. Uneven dissolution of iron in the presence of a magnetic field was related to the relative configuration between the magnetic field direction and the electrode surface and also to the special concentration gradient of reactive species at the electrode circumferential area.     

磁场对Cu/NaCl体系表观Tafel区阳极溶解的作用

采用线性电位扫描极化曲线测试 ,以及恒电位极化下磁场扰动法研究了磁场对铜在氯化钠溶液中阳极溶解的作用。在阳极表观 Tafel区 ,无磁场时恒电位极化然后外加磁场使阳极电流密度增大 ;有磁场时恒电位极化然后撤去磁场使阳极电流密度减小。磁场通过加速电极溶液界面的传质过程而加速阳极溶解。由于 Cu/ Na Cl与Fe/ H2 SO4体系阳极 Tafel区速率控制步骤不同 ,会导致不同的磁场效应作用     

Effect of high gradient magnetic fields on the anodic behaviour and localized corrosion of iron in sulphuric acid solutions

The influence of high gradient magnetic fields on the anodic dissolution of iron in sulphuric acid solutions and the localization of the corrosion attack is investigated by means of potentiodynamic and potentiostatic polarization experiments and subsequent surface profile analysis. A localization of the material loss is observed in every potential region of the anodic Fe dissolution except from the passive region. The impact of the magnetic field on the anodic current density and the localization of the corrosion attack are explained by the action of the Lorentz force and the magnetic field gradient force.     

Quantitative characterization by micro-electrochemical measurements of the synergism of hydrogen, stress and dissolution on near-neutral pH stress corrosion cracking of pipelines

Occurrence of stress corrosion cracking of pipelines under a near-neutral pH condition depends on the synergism of stress, hydrogen and anodic dissolution at the crack tip of the steel. In this work, micro-electrochemical techniques, including localized electrochemical impedance spectroscopy and scanning vibrating electrode technique, were used to characterize quantitatively the synergistic effects of hydrogen and stress on local dissolution at crack-tip of a X70 pipeline steel in a near-neutral pH solution. Results demonstrate that, upon hydrogen-charging, the anodic dissolution of the steel is enhanced. The resistance of the deposited corrosion product layer depends on the charging current density. There is a non-uniform dissolution rate on the cracked steel specimen, with a highest dissolution current density measured at crack-tip. For a smooth steel specimen, the synergistic effect factor of hydrogen and stress is equal to 5.4, and the total effect of hydrogen and stress on anodic dissolution of the steel is 7.7. In the presence of a crack, the hydrogen effect factor, stress effect factor and the synergistic effect factor are approximately 4.3, 1.3 and 4.0, respectively. The total effect factor is up to 22.4, which is very close to the 20 times of difference of crack growth rate in pipelines in the presence and absence of the hydrogen involvement recorded in the field.     

重铬酸根与磁场对铁在硫酸溶液中阳极极化行为的影响

采用动电位极化曲线法研究了外加磁场及不同浓度重铬酸根对Fe在0．5mol/L H2SO4溶液中阳极极化行为的影响,结果表明：浓度≤0．02mol/L的重铬酸根不改变阳极极化曲线的构型;重铬酸根浓度≥0.05mol/L时,阳极极化曲线上出现传质控制特征的斜率突变转折段,并且随重铬酸根浓度增大,斜率转折段在更负的电位、更小的电流密度下出现,外加磁场后,不同阳极电位区间内会表现出电流变小、不变及增大的现象,这与相应电位下阴、阳极反应贡献的相对大小以及各自的控制步骤类型有关,考虑阳极过程中可能发生的活性溶解、表面膜生成及表面膜溶解反应,以及极反应的影响,主要考虑磁场对阴、阳极反应中传质过程的作用,得出了外加磁场作用下的极化曲线模型,该模型较好地说明了磁场对不同电位区电流密度产生不同影响的实验事定。     

磁场对铁腐蚀过程中阴极析氢和阳极溶解的影响

用动电位扫描法研究了０．５Ｔ磁场下纯铁在三种典型介质中的阴极极化曲线和两种介质中的阳极极化曲线．发现磁场会引起一个新的过电位一磁致过电位ｍａｇ,使铁上阴极析氢和铁的阳极溶解有不同程度的加速,尤以在酸性介质中为甚．认为这是由于磁场使铁－水溶液界面抗磁性离子的水合作用减少和使Ｆｅ２＋有吸附于铁电极表面的倾向,导致双电层结构的改变和腐蚀体系的活性增加．     

磁场作用下铁／硫酸体系阳极过程全息显微研究

利用电化学和一息显微摄影技术研究弱磁场（Ｂ≤２００ｍＴ）作用下, ０．５ｍｏｌ·ｄｍ＾－３Ｈ２ＳＯ４溶液中铁的阳极溶解、恒电位电流振荡以及钝化铁再活化 等过程,并从电极／溶液界面现场显微动态观察的角度探讨了磁场对上述过程的影响。磁场的存 在主要通过增加液相传质速度影响电化学过程。外加磁场对铁的活化溶解没有明显的影响,但加快了 预钝化区铁的溶解,从而显著改变铁的电化学振荡行为;钝化铁再活化过程主要取决于电极电位,与外加磁场没有关系;研究表明全息显微术是磁电化学研究中一种十分有效的方法.     

In situ monitoring the effects of a magnetic field on the open-circuit corrosion states of iron in acidic and neutral solutions

The effects of a 0.4 T horizontal magnetic field (HMF) on the open-circuit corrosion states of iron in static aqueous solutions are studied by in situ monitoring the responses of two electrochemical parameters to the applied magnetic field, i.e. the open-circuit potential (OCP) and the current under potentiostatic polarization. The applied magnetic field makes the OCP shift in the noble direction. Withdrawing the magnetic field causes a negative shift of the OCP in acidic solutions, but it does not cause any significant change of OCP in neutral solutions. Imposing a magnetic field induces a cathodic current for iron that was previously potentiostatically polarized at the OCP without magnetic field. Withdrawing the magnetic field induces an anodic current for iron that was previously potentiostatically polarized at the OCP with the magnetic field. The magnetic field effect is more significant in the acid solutions than in the salt solutions. The magnetic field effects on the oxygen reduction and on the activation-controlled iron dissolution reaction are found to be insignificant. The magnetic field effect on the hydrogen reduction reaction on iron in acidic solutions is demonstrated. Results show the possibility that a magnetic field would affect the hydrogen evolution by enhancing the electron-transfer process that has been categorized in the classical electrochemistry kinetics to be the rate-determining process. The memory effect of the magnetic field on the electrochemical reaction is identified and discussed.     

Electrochemical corrosion of X65 pipe steel in oil/water emulsion

The electrochemical corrosion behavior of X65 pipeline steel in the simulated oil/water emulsion was investigated under controlled hydrodynamic and electrochemical conditions by rotating disk electrode technique. Results demonstrated that mass-transfer of oxygen plays a significant role in the cathodic process of steel in both oil-free and oil-containing solutions. Electrode rotation accelerates the oxygen diffusion and thus the cathodic reduction. The higher limiting diffusive current density measured in oil-containing solution is due to the elevated solubility of oxygen in oil/water emulsion. The anodic current density decreases with the increase of electrode rotating speed, which is attributed to the accelerated oxygen diffusion and reduction, enhancing the steel oxidation. Addition of oil decreases the anodic dissolution of steel due to the formation of a layer of oily phase on steel surface, increasing the reaction activation energy. The steel electrode becomes more active at the elevated temperature, indicating that the enhanced formation of oxide scale is not sufficiently enough to offset the effect resulting from the enhanced anodic dissolution reaction kinetics. The corrosion reaction mechanism is changed upon oil addition, and the interfacial reaction is activation-controlled, rather than mass-transfer controlled. When sand particles are added in oil/water emulsion, there is a significant increase of corrosion of the steel. The presence of sands in the flowing slurry would impact and damage the oxide film and oily film formed on the steel surface, exposing the bare steel to the corrosive solution.     

Probing into the effects of a magnetic field on the electrode processes of iron in sulphuric acid solutions with dichromate based on the fundamental electrochemistry kinetics

The effects of an applied magnetic field on the electrode processes of iron in sulphuric acid solutions with dichromate have been investigated by electrochemical measurements. Open circuit potentials, cathodic and anodic polarisation curves, values of polarisation resistance were measured in the presence or absence of a 0.4 T horizontal magnetic field (HMF). A potentiostatic polarisation plus magnetic field perturbation technique was used to study the effect of the magnetic field on open circuit corrosion. Cathodic reaction rates at open circuit potentials for iron in sulphuric acid solutions containing dichromate ions are controlled by both the electron-transfer process and the diffusion process. A magnetic field made the open circuit potential move in the positive direction, and changes of the open circuit potentials due to the magnetic field increase with increasing dichromate concentration. When iron was potentiostatically polarised at open circuit potentials in the absence of a magnetic field, a cathodic current was observed after a magnetic field was imposed. Such cathodic currents induced by the magnetic field increases with increasing dichromate concentration. The positive shifts of open circuit potential, the decrease of polarisation resistance, and the occurrence of cathodic currents induced by the magnetic field are caused by the accelerating effect of magnetic field on the cathodic diffusion process. Measured current densities showed lower, equal, or higher values in the presence of the magnetic field than those in the absence of a magnetic field at certain anodic potentials. This effect of the magnetic field is related to the contribution of the cathodic and anodic reactions to the measured current and the types of rate-determining steps for each reaction at certain potentials. The applied magnetic field significantly decreased the polarisation resistance. The experimental results in this paper are formulated based on the fundamental electrochemistry kinetics after introducing a coefficient term of the magnetic field effect on the mass transfer process of reactive ions.     

Effects of a magnetic field on the anodic dissolution, passivation and transpassivation behaviour of iron in weakly alkaline solutions with or without halides

The effects of a 0.4 T horizontal magnetic field on the anodic dissolution, passivation and transpassivation behaviour of iron in bicarbonate solutions of various concentrations and in dilute bicarbonate solutions with or without halides are investigated by electrochemical polarisation measurements. The applied magnetic field does not affect the activation-controlled anodic current, the steady passive current and the transpassive current, but significantly affects the activation-passivation transition processes for iron in bicarbonate solutions without halides. The effects of magnetic field are strongly dependent on passivation mechanisms that result in different types of surface films and corresponding rate determining steps of film dissolution. There is a synergistic effect between the applied magnetic field and halides, chlorides or bromides, on attacking the passivation of iron in dilute bicarbonate solutions. The effects of the magnetic field are analysed based on the previously proposed electrochemical kinetics equations. The magnetic field affects the anodic polarisation behaviour through its enhancing effects on mass transport processes at the precipitation-dissolution type surface film/solution interfaces. The magnetic field shows little or no effects on continuous and steady passivation films where the oxidation rate is controlled by mass transport processes within surface films. Magnetoelectrochemistry measurements are suggested as a prospective method for researches on corrosion or passivation mechanisms.     

磁场作用下铁在盐酸和氯化钠溶液中自腐蚀状态的变化

采用自腐蚀电位测量及恒电位极化法研究了磁场对铁在酸性与中性氯化的溶液中自腐蚀状态的作用。外加磁场使自腐蚀电位正移，撤去磁场使自腐蚀电位负移，没有磁场时在自腐蚀电位下极化然后外加磁场会导致磁致阴极电流；有磁场时自腐蚀电位下极化然后撤去磁场会导致磁致阳极电流。酸性溶液体系的磁致阴极电流值明显大于中性溶液体系，表现出磁场主要通过作用于析氢反应而影响自腐蚀状态。     

Micro-electrochemical characterization of corrosion of welded X70 pipeline steel in near-neutral pH solution

The local corrosion behavior of welded X70 pipeline steel in near-neutral pH solution was studied by micro-electrochemical measurements, including scanning vibrating electrode and local electrochemical impedance spectroscopy. The microstructure of the welded steel was observed by optical microscopy and scanning electron microscopy. It is demonstrated that the microstructure of weld metal consists of acicular ferrite and grain boundary ferrite, while that of heat-affected zone is a mixture of acicular ferrite, bainitic ferrite and a few martensite/austenite microconstituents. The microstructure of base steel is typically ferrite and pearlite. Electrochemical corrosion mechanism of welded X70 steel does not experience change upon hydrogen-charging, or stressing, or both. Hydrogen-charging is capable of enhancing the local anodic dissolution of the steel. The resistance of corrosion product layer decreases with hydrogen-charging, and heat-affected zone has the largest dissolution current upon hydrogen-charging. The increase of applied stress enhanced the anodic dissolution of welded X70 steel, especially the heat-affected zone, in near-neutral pH solution. Maximum current is observed in heat-affected zone, and increases with the increase of applied stresses. The total synergistic effect of hydrogen-charging (10 mA/cm²) and applied stress (550 MPa) on anodic dissolution of welded X70 steel in near-neutral pH solution is determined to be within the range of 5.7 and 6.5, with a maximum value encountering in heat-affected zone.     

Magnetic field effects on anodic polarisation behaviour of iron in neutral aqueous solutions

Abstract

The presence of a magnetic induction field with a flux density of 0·4 T caused the anodic active-passive transition potential of iron in neutral aqueous solutions of sodium sulphate to move in the noble direction and resulted in a reduction of the passive potential range. Anodic current densities at the active-passive transition potential and in the passive potential range increased in the presence of the magnetic field. There was a synergistic effect between the magnetic field and the presence of chloride in solution on the breakdown of iron passivity.     

Corrosion and corrosion inhibition of Cu-20%Fe alloy in sodium chloride solution

The electrochemical behavior of copper (Cu), iron (Fe) and Cu-20%Fe alloy was investigated in 1.0 M sodium chloride solution of pH 2. The effect of thiourea (TU) addition on the corrosion rate of the Cu-20%Fe electrode was also studied. Open-circuit potential measurements (OCP), polarization and electrochemical impedance spectroscopy (EIS) were used. The results showed that the corrosion rates of the three electrodes follow the sequence: Cu < Cu-20%Fe < Fe. Potentiostatic polarization of the Cu-20%Fe electrode in the range −0.70 V to −0.45 V (SCE), showed that iron dissolves selectively from the Cu-20%Fe electrode surface and the rate of the selective dissolution reaction depends on the applied potential. At anodic potential of −0.45 V, thiourea molecules adsorb at the alloy surface according to the Langmuir adsorption isotherm. Increasing thiourea concentration (up to 5 mM), decreases the selective dissolution reaction and the inhibition efficiency η reach 91%. At [TU] > 5 mM, the dissolution rate of the Cu-20%Fe electrode increases due to formation of soluble thiourea complexes. At cathodic (−0.6 V), the inhibition efficiency of thiourea decreases markedly owing to a decrease of the rate of the selective dissolution reaction and/or desorption of thiourea molecules. The results indicated that thiourea acts mainly as inhibitor of the selective dissolution reaction of the Cu-20%Fe electrode in chloride solution.     

Effects of alternating current on corrosion of a coated pipeline steel in a chloride-containing carbonate/bicarbonate solution

In this work, the alternating current (AC)-induced corrosion of a coated pipeline steel was studied in a chloride-containing, concentrated carbonate/bicarbonate solution, which simulated the trapped high pH electrolyte under coating, by potentiodynamic polarization measurements, immersion tests and surface characterization technique. It was found that an application of AC resulted in a negative shift of corrosion potential of the steel, caused an oscillation of anodic current density, and degraded the steel passivity developed in the solution. With the increase of AC current density, the corrosion rate of the steel increased. At a low AC current density, a uniform corrosion occurred, while at a high AC current density, pitting corrosion occurred extensively on the steel electrode surface. At individual applied AC, there was a higher electrochemical dissolution activity of the coated steel electrode containing a 1 mm defect than that of the electrode containing a 10 mm defect.     

Determination of electrochemical parameters and corrosion rate for carbon steel in un-buffered sodium chloride solutions using a superposition model

Corrosion of carbon steel in un-buffered NaCl solutions was studied applying linear potential sweep technique to a rotating disk electrode. Current-potential curves were obtained from linear potential sweep at a rate of 1 mV s⁻¹ in solution with concentrations in the range 0.02-1 M NaCl and rotation rates in the range 170-370 rad s⁻¹, at 22 °C. Potential sweeps, which were conducted in the potential range −700 to −100 mV/SHE, were started from the cathodic limit in order to approach the measurement of corrosion under rust-free conditions. Polarization curves were analyzed with a superimposition model developed ad hoc and implemented in a computer program, which enabled determining the corrosion rate and kinetics parameters of the underlying anodic and cathodic sub-processes. The anodic sub-process, dissolution of iron, was well described in terms of a pure charge transfer controlled reaction, while the cathodic sub-process, oxygen reduction on iron, was well described in terms of mixed mass transfer and charge transfer control. Increase of electrode rotation rate increases the limiting current of oxygen reduction, which results in an enhanced corrosion rate of carbon steel. Increase of NaCl concentration has a dual effect: the limiting current of oxygen reduction decreases as a result of the influence of NaCl concentration on solution viscosity and the anodic dissolution of iron increases due to the influence of NaCl on pitting formation. However, this last mechanism predominates and a net increase in carbon steel corrosion rate is observed in this case.     

Fast chemical vapor deposition of microcrystalline silicon by applying magnetic field to hollow electrode enhanced radio frequency glow plasma

Fast chemical vapor deposition of microcrystalline silicon by applying magnetic field to hollow electrode enhanced radio frequency (rf) glow plasma has been investigated. We have already developed a plasma generation technique called hollow electrode enhanced rf glow plasma transportation (HEEPT). In this study, we equipped a HEEPT system with a hollow cylinder shaped permanent magnet around an orifice prepared at the center of the counter electrode. The plasma was characterized by plasma emission spectroscopy. Silicon thin films were deposited on a glass substrate. It was found that increasing the magnetic flux density resulted in increasing plasma emission intensity, film deposition rate, and crystallinity. The maximum deposition rate of 6.9 nm/s was achieved with high crystallinity and photo-sensitivity at a plasma excitation frequency of 13.56 MHz, a substrate temperature of 300 °C and a magnetic flux density of 75 mT. Our results indicate that the magnetic field is effective in promoting fast chemical vapor deposition of microcrystalline silicon thin films with photo-sensitivity using the HEEPT technique. We consider that the effectiveness is due to a decrease of electron temperature caused by drift motion of electrons in the magnetic field inside the orifice.     

Micro-electrochemical characterization of corrosion of pre-cracked X70 pipeline steel in a concentrated carbonate/bicarbonate solution

The electrochemical corrosion behavior of a stressed, pre-cracked X70 pipeline steel was studied in a bicarbonate/carbonate solution by electrochemical and micro-electrochemical measurements, numerical calculation and surface analysis technique. The effects of stress and potential on passivity, corrosion and electrochemical behavior of the steel at crack-tip were mechanistically determined. It was found that the passive film formed at crack-tip was less stable than that formed in the region ahead of the crack. Moreover, the crack-tip is more susceptible to pitting corrosion than other region of the specimen. The applied stress enhances the anodic dissolution of the steel. In particular, the stress concentration at crack-tip further increases the local anodic dissolution rate. The enhancement of the anodic dissolution of the steel at crack-tip is also resulted from the formation of a galvanic couple, i.e., the crack-tip as an anode and the surrounding region as a cathode.     

Quantum chemistry study on the effect of Cl ion on anodic dissolution of iron in H2S-containing sulfuric acid solutions

The effect of Cl⁻ ion on the anodic dissolution of iron in H₂SO₄ solutions containing low H₂S level has been studied by electrochemical polarization curve measurements. The total energy and binding energy of the competitive adsorption for Cl⁻ and HS⁻ ions have been calculated with CNDO/2 method, as well as the net charge distribution of iron atoms at an anodic potential. The results showed that certain concentration of Cl⁻ ion inhibit the anodic reaction of iron accelerated by HS⁻. However, when Cl⁻ ion reached saturated adsorption, it began to promote the anodic reaction of iron due to the increased negative charge of iron atoms.