Effect of hydrogen sulfide on the iron dissolution rate in a sodium sulfate solution under natural aeration conditions

The influence of hydrogen sulfide (10–100 mg/1) on the Armco iron anodic dissolution in an aerated 0.17 M Na₂SO₄ solution is investigated. During a potentiostatic anodic polarization, the hydrogen sulfide introduction makes the current increase stepwise. The magnitude of the increase depends on the duration of preliminary anodic polarization, electrode potential, and hydrogen sulfide concentration. The anodic metal dissolution activation by hydrogen sulfide is explained by chemical conversion of the oxide-hydroxide passivating film into iron sulfide that is generated at the metal surface in the form of a porous film and does not hinder the electrode dissolution. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth.     

硫酸盐还原菌的含硫代谢产物在加速碳钢腐蚀中的作用

应用电化学阻抗谱(EIS)研究了硫酸盐还原菌(SRB)对碳钢腐蚀过程的促进作用,应用稳态极化法研究了SRB代谢硫化物对碳钢阳极活性溶解的增强作用。研究结果表明,代谢硫化物通过对铁的吸附增强了碳钢阳极活性溶解,因为依据吸附态E-pH图可知,在很宽的电位范围和pH值范围内,硫在铁上都能以稳定的吸附态存在。吸附态硫提升了碳钢表面铁原子能量,使铁阳极溶解所需活化能降低,从而增强其阳极溶出过程。     

Preparation of precursors of complex titanium and iron oxides using a combined electrode

Anodic dissolution of a combined iron–titanium anode in a halide-containing electrolyte was studied by galvanostatic, potentiostatic, and gravimetric methods. The rate of the anodic dissolution of the iron and titanium constituents of the combined anode can be controlled by varying the concentrations of the electrolyte components, the anodic current density, and the ratio of the metal surface areas. The phase and elemental composition of the product, its surface morphology, and particle shape were examined by X-ray diffraction and electron microscopy.     

The study of electrochemical palladium behavior using the quartz crystal microbalance

The electrochemical quartz crystal microbalance (EQCMB) method has been used to evaluate the processes which occur in/on the palladium electrode in basic solutions. Hydrogen electrosorption in palladium is accompanied by an additional frequency shift that can be attributed to the stresses generated inside the Pd metal. A non-linear dependence between the mass change and the charge consumed during hydrogen oxidation in the Pd electrode has been found for hydrogen absorbed in the α- and β-phases. This effect precludes the objective estimation of the amount of hydrogen absorbed inside the Pd electrode. The EQCMB method has been used, however, for studying the surface electrode processes on the Pd electrode, i.e. specific anion adsorption, surface oxidation and dissolution. Also, the structure of the palladium oxide formed on the Pd surface during electrochemical oxidation is discussed in this paper and the effect of the anodic limiting potential on the oxide structure is reported. Received: 10 August 1999 / Accepted: 24 September 1999     

Role of oxidizers in the active dissolution and passivation of metals

It was established that addition of hydrogen peroxide to a solution of sulphuric acid leads to acceleration of the active dissolution of chromium and to the shift of its passivation potential in negative direction. Similar data were obtained for Fe-28% Cr alloy in H₂SO₄ solution containing H₂O₂ and for iron in sulphuric acid with K₂Cr₂O₇ additions. The conclusion was made that depending on the potential the molecules of oxygen-containing oxidizers can undergo different transmissions on the surface of the metal, including transformations connected with the direct participation of the oxidizer in the anodic dissolution of the metal or in the building of inhibiting or passivating layers on the metallic surface.     

Kinetics of Active Dissolution of Hydrophilic Metals Allowing for Anodic Activation

The anodic activation of dissolution of the iron metals is due to the accelerated formation of excess sources of steps during an activating anodic pulse. This is shown on the basis of a model of spatial separation of dissolution and passivation, with allowance made for the formation of pairs of kinks on stationary steps. The formation of pairs of kinks is also the reason for the emergence of a nonpolarizability portion in steady-state current–voltage curves pertaining to anodic dissolution of iron in acid chloride solutions.     

H~2S对硫酸溶液中铁腐蚀作用的CNDO/2研究

动电位扫描得出工业纯铁在含H~2S的H~2SO~4溶液中腐蚀的电化学行为。应用CNDO/2法确定了阴、阳极电位下H~2S,HS^-在金属Fe晶面上的稳定吸附取向与最佳吸附间距,通过对吸附物总能量、结合能、净电荷分布的计算,得出H~2S体系阴、阳极反应被加速的微观机制。阳极电位下,大量HS^-为S原子的平行吸附方式,HS^-负电荷大部分向Fe转移,促进铁的溶解。阴极电位下,少量H~2S则为H原子的平行吸附方式,Fe表面负电荷向H~2S分子中与Fe作用的H转移,促进析氢。在CNDO/2法计算的基础上,圆满地解释了实验结果。     

Rare earth hydrogen storage alloy used in borohydride fuel cells

Fuel cell using the borohydride as the fuel has attracted much attentions because of high energy density and working potential. In this work, LaNi_4.5Al_0.5 hydrogen storage alloy used as the anodic material to replace noble metals has been investigated. Experimental results showed that H₂ evolution was unavoidable during discharge process because of the hydrolysis of , but the utilization of the fuel increased with the increasing current densities. At high discharge current, the alloy electrode showed the lowest hydrogen generation rate and higher utilization of the fuel because, the generated hydrogen was absorbed and oxidized to produce electric energy similar to the behavior of hydrogen storage alloy in nickel–metal hydride batteries. The reaction mechanism of borohydride on the surface of electrode made of hydrogen storage alloy also has been discussed. Hydrogen storage alloy would be a promising material as the anodic catalyst in borohydride fuel cell.     

硫酸根离子对铁在弱碱性溶液中阳极溶解和钝化行为的影响

采用浸泡实验, 电化学测试和表面分析技术研究了硫酸根离子浓度对铁在稀碳酸氢钠溶液中开路状态和阳极极化行为的影响. 在无硫酸根离子及含有少量硫酸根离子的碳酸氢钠溶液中, 铁的开路电位约为(-0.225±0.005) V, 并呈现钝化状态, 其电化学阻抗很大, 腐蚀速率较低. 在含有较高浓度硫酸根离子的碳酸氢钠溶液中, 铁的开路电位为(-0.790±0.010) V并呈现活性溶解状态, 其电化学阻抗较小, 腐蚀速率较高, 同时阳极极化曲线上能观察到活化-钝化转变现象. 由于铁在含有较高浓度硫酸根离子的碳酸氢钠溶液中处于活化状态, 阳极极化曲线上存在数个电流峰. 足够高的硫酸根离子浓度会导致铁表面预先形成或转变而成的氧化膜失效. 相比于自然曝氧状态, 在除氧条件下较低的硫酸根离子浓度即可引起铁在碳酸氢钠溶液中由钝态向活性溶解态的转变.     

Anodic Dissolution of Iron–Nickel Alloy under Non-Steady-State Conditions

Anodic dissolution of iron–nickel electrolytic alloy in the acidic chloride solution is studied by the non-steady-state electrochemical methods. It is shown that, in the initial period, selective dissolution of the alloy with prevailing iron ionization takes place. Then, the anodic dissolution rate is limited by the nonsteady-state bulk diffusion of the components in the solid phase. The effective diffusion coefficients of iron in the alloy are calculated.     

Corrosion behavior of dimensionally stable anodes in chlorine electrolyses

The rate of anodic dissolution of ruthenium from the active coating of dimensionally stable anodes is studied as a function of time by a radiotrace technique in chloride and perchlorate solutions. In chloride solutions, the rate decreases by approximately four orders of magnitude and for a long (hundreds of hours) time. In the perchlorate solution, the dissolution rate changes only by hundredfold, for 25 min. These data confirmed the earlier conclusion that adsorbed chlorine inhibits anodic dissolution of the noble metal. The pH influence on the anode’s steady-state corrosion rate is studied. The corrosion rate sharply increases of pH increasing from 2 to 4 then slowly decreases to the values characteristic of the process rates in solutions of pH 2.     

Inhibitive assessment of 1-(7-methyl-5-morpholin-4-yl-thiazolo[4,5-d]pyrimidin-2-yl)-hydrazine as a corrosion inhibitor for mild steel in sulfuric acid solution

In this study, the inhibitive effect of synthesized 1-(7-methyl-5-morpholin-4-yl-thiazolo[4,5-d]pyrimidin-2-yl)-hydrazine (MMTPH) as a new corrosion inhibitor for mild steel in 0.5 M sulfuric acid medium is investigated employing potentiodynamic polarization, electrochemical impedance spectroscopy and linear polarization resistance techniques. The results show MMTPH reduces anodic dissolution, retards the hydrogen evolution reaction and its adsorption follows Langmuir’s adsorption isotherm. Any increase in temperature will in turn increase corrosion current densities; however, the presence of MMTPH hinders the rate. In solutions with inhibitor concentration of 200 ppm, temperature elevations as great as 30° (25–55 °C) result in a drop of about 45 % in inhibition efficiency (99–55 %). Thermodynamic adsorption parameters show that the MMTPH is absorbed by a spontaneous exothermic process and the adsorption mechanism is physical. Quantum chemical method shows that the MMTPH molecules can be directly adsorbed at the steel surface on the basis of donor–acceptor interactions between π-electrons of pyrimidine, N atoms of hydrazine and vacant d-orbitals of iron atoms.     

Anodic dissolution of iron microparticles in chloride-containing acetate buffer electrolytes studied by elimination voltammetry

Elimination voltammetry with linear scan (EVLS) was used for the study of the anodic dissolution process of iron microparticles immobilized on paraffin-impregnated graphite electrode. This process was investigated in acetate buffer solutions in the range of pH 4 to 6 and the effect of chloride ions was evaluated. It was found that the anodic dissolution process of iron powder proceeded in two peaks with different pH and KCl-dependent characterizations. Both the more negative process at about ?300 mV and the positive process at about 1,200 mV are influenced by chloride and hydroxyl ions. Chloride ions incorporate into the intermediate species and form more soluble species in the passivation layer, leading to its degradation. Detailed information about the processes was provided by the EVLS functions conserving diffusion current components. The mechanism of anodic dissolution of iron powder in acetate solutions representing passive ions and chloride ions appearing in the role of corrosion activators was discussed.     

Spatially-resolved imaging of concentration distributions on corroding magnesium-based materials exposed to aqueous environments by SECM

The spatial resolution of Mg^2 + release from magnesium and its alloys during exposure to aqueous environments has been imaged using a new, solid contact, micropipette-based magnesium-ion selective electrode employed as potentiometric tip in SECM. The detection of metal dissolution is a crucial factor to detect the local microelectrodes established on the surface of the metal, and distinguish the processes related to anodic and cathodic half-cell reactions. Concentration distribution images have been obtained for the magnesium-based alloy AZ63 when galvanically coupled to pure iron during exposure to 1 mM NaCl solution.     

Linear-sweep voltammetric studies on iron in aqueous sulphate electrolytes between 20 and 200°C

Potentiostatic polarisation studies have been made on iron in 10% Na₂SO₄ at sweep rates of 0.3–30 mV s^?1 and at temperatures of 20–200°C using a platinum-platinum oxide reference electrode. The sweeps showed characteristic features of anodic dissolution and passivation. Oxide removal was evident on the cathodic sweeps. As noted by previous workers two anodic dissolution peaks were observed with clean electrode surfaces, but only one occurred when the surface was covered with oxide. The number of dissolution peaks was also found to depend on sweep rate. In contrast to other work which suggested that there was no passive region above 95°C, passivation was shown to occur even at 200°C.     

Calculation of the Limiting Current of Metal Anodic Dissolution under Natural Convection Conditions

The mass-transfer processes during anodic dissolution of a vertically-positioned plane metal electrode in a solution of an acid or a salt of another metal (ions of all types are single-charged) are analyzed theoretically for conditions of natural convection. An equation for the average limiting current of metal anodic dissolution in the cases where the limiting current is caused by the saturation of a near-anode layer by the product of metal dissolution is derived using an approximate analytical method.     

Pathways of diffusion mixed with subsequent reactions with examples of hydrogen extraction from hydride-forming electrode and oxygen reduction at gas diffusion electrode

This paper covers the role of the rate-determining step (RDS) in anodic hydrogen extraction from hydride-forming electrode. In general, hydrogen extraction from the electrode proceeds through the following steps: (1) hydrogen diffusion within the electrode, (2) hydrogen transfer from absorbed state to adsorbed state, (3) electrochemical oxidation of hydrogen to hydrogen ion involving charge transfer, and (4) hydrogen ion conduction through the electrolyte. In most theoretical and experimental investigations, it has been assumed that the RDS of anodic hydrogen extraction is hydrogen diffusion through the electrode. In real situation, however, the overall rate of hydrogen extraction is simultaneously determined by the rates of two or more reaction steps including hydrogen diffusion. The present work provides the overview of anodic hydrogen extraction in case that diffusion is coupled with interfacial charge transfer, interfacial hydrogen transfer, and hydrogen ion conduction through the electrolyte as well as the purely diffusion-controlled hydrogen extraction. In addition, the mixed controlled diffusion model was also exemplified with oxygen reduction at gas diffusion electrode of fuel cell system.

     

Effect of chloride ions on the kinetics of nitrobenzene reduction by powdered iron

The effect of different chloride ions concentrations on the reactivity of iron particles was studied using chronopotentiometry. It was found that the increase of Cl⁻ ions concentration accelerated anodic dissolution of iron, thus enhancing its surface reactivity. This fact was confirmed also by the rate of nitrobenzene reduction by iron particles. The reactivity of powder particles of various sizes, hence of different specific surface, was investigated in chloride-free and chloride-containing acetate buffer electrolytes. Experimental results indicated that the rate of nitrobenzene reduction in the presence of chloride ions in the reaction system is faster in comparison with the rate in chloride-free medium for each studied grain fraction of particulate material.     

磁场作用下铁电极的不均匀阳极溶解

采用电化学方法以及扫描电镜形貌观察研究了磁场对铁在硫酸溶液中的阳极溶解的影响 .恒电位极化测试结果表明 ,随外加阳极电位的增加 ,磁场的存在将加速阳极溶解 ,使振荡态或钝态变为活性溶解态 ,维持钝态 .于特定电位下与重力方向平行的电极表面两侧将因局部溶解加速而出现凹陷 .由于电极周边浓度梯度场的特殊性以及磁场的作用方向导致了铁的不均匀阳极溶解     

Peculiarities of anodic behavior of gold electrode in thiosulfate electrolytes

Using the methods of quartz microgravimetry and voltammetry, the anodic behavior of gold electrode in thiosulfate electrolytes is studied in the pH range of 7 to 11. It is found that, in the potential range from 0.15 to 1.0 V (NHE), the anodic current is associated predominantly with the oxidation of thiosulfate ions, and the gold dissolution rate in this electrolyte is negligibly low (< 0.02 mA/cm²). It is shown that the study of anodic processes in the neutral thiosulfate electrolytes requires stabilization of solution acidity, because the near-anode layer can be acidified to the pH values, which are sufficient for the formation of elemental sulfur. It is found that the use of Britten-Robinson buffer solution with pH 7 as the supporting electrolyte changes significantly the polarization curve of thiosulfate ion oxidation, but does not raise the gold dissolution rate. An increase in the solution pH to 11 and an exposure of electrode at various potentials (−0.5 and 0.15 V) prior to the onset of potential scanning also do not accelerate considerably the gold dissolution in the thiosulfate electrolyte. A comparison between the regularities of gold anodic behavior in the thiosulfate solutions and earlier studied gold dissolution in the cyanide and thiocarbamide electrolytes showed that they are similar. It is supposed that the specific features of anodic processes in these cases are of a similar nature: the metal dissolution proceeds with the formation of two-ligand complexes with linear structure, which is typical for all aforementioned ligands.