Model for corrosion of metals covered with thin electrolyte layers: Pseudo-steady state diffusion of oxygen

A one-dimensional mathematical model is presented for the free corrosion of a bare metal surface (devoid of any oxide film) under a thin electrolyte layer using mixed potential theory where anodic metal dissolution is controlled by oxygen diffusion through the electrolyte layer and by the oxygen reduction at the metal surface. A pseudo-steady state is considered wherein the oxygen diffusion is at steady state while the metal and hydroxyl ions keep accumulating in the thin electrolyte layer due to a decoupling arising from the assumed Tafel laws for corrosion kinetics. Under free corrosion the oxygen diffusion is shown to depend on a non-linear boundary condition with a non-integer power on oxygen concentration at the metal surface which makes the model non-trivial. Analytical and numerical results for the oxygen concentration at the metal surface, corrosion potential, and corrosion current density are reported which depend on several kinetic, thermodynamic and transport parameters in the system. The model is applied to iron and zinc systems with input data taken from the literature. The experimental utility of the model for gathering thin-film corrosion parameters from a study of the corrosion current and potential as a function of the thickness of the electrolyte layer is discussed. Precipitation and passivity, though not the main object of study in this work, are briefly discussed.     

Zn-Fe合金电镀工艺的研究

探讨了氯化物体系Zn Fe合金电镀液中各组分及其他工艺参数对镀层中铁含量的影响。在最佳Zn Fe合金电镀工艺条件下 ,得到了含铁量为 0 41 %的Zn Fe合金镀层 ,经银白色钝化后的该种合金镀层的抗蚀能力约为同厚度纯锌镀层的 2～ 3倍。研究同时表明 ,Zn Fe合金镀液具有良好的分散能力、覆盖能力和整平能力 ,电镀电流效率高 ,镀层性能优良     

Effects of introduced electrolytes on galvanic deposition of ZnO films

The effects of supporting electrolytes on galvanic deposition of ZnO films have been investigated in detail by respectively introducing K₂SO₄, KNO₃ and KCl into Zn(Ac)₂ electrolytes. It reveals that the chemical nature of introduced electrolytes plays important roles in acting on the growth of ZnO films. ZnO nanorods tend to grow in KNO₃ and KCl electrolytes, while sheet-like zinc hydroxysulfate tends to be formed in K₂SO₄ electrolyte. Besides, KNO₃ electrolyte is inclined to accelerate the passivation of Zn anode, resulting in the sharp decrease of driving force for galvanic deposition. In contrast, KCl and K₂SO₄ electrolytes facilitate zinc dissolution by anionic adsorption on the metal surface and subsequent participation in the active dissolution process, thus leading to the incorporation of anions in nanocrystals.     

Preparation of highly efficient and stable Fe,Zn,Al-pillared montmorillonite as heterogeneous catalyst for catalytic wet peroxide oxidation of Orange II

Fe,Zn,Al-pillared montmorillonite (Mt) were prepared by ion exchange reaction. Effect of the Fe/Zn molar ratio in Fe,Zn,Al-pillaring solution on the structural properties of Fe,Zn,Al-Mt was examined. Characterization studies were performed by N₂-adsorption/desorption, ICP, FTIR, DR UV–Vis spectroscopy and XRD. The interlayer space, specific surface and volume of micropores of pillared-Mt was 18.8 Å, 157 m² g^?1 and 0.114 cm³ g^?1 respectively. Besides, the catalytic performance of Fe,Zn,Al-Mt was analyzed through wet peroxide oxidation of Orange II under mild experimental condition. The results indicated that the catalytic performance of Fe,Zn,Al-Mt was affected by the Fe/Zn molar ratio as following pattern: the obtained pillared solids with lower Fe/Zn molar ratios exhibited better catalytic performance and less iron leaching during the reaction. Moreover, the introduction of zinc into Mt shortened the induction time of reaction and improved the catalytic behaviour. Throughout this work, the chemical oxygen demand removal of OII was 77.1 % after the solution treated by Fe,Zn,Al(3/7)-Mt (defined as the Fe/Zn molar ratio of 3/7 in the pillaring solution) catalyst at temperature of 60 °C and atmospheric pressure of 1 atm.     

Surface potential distribution over a zinc/steel galvanic couple corroding under thin layer of electrolyte

In this study, surface potential and surface pH changes over a zinc/steel galvanic couple corroding in artificial seawater (ASW) at 60 and 90% RH have been investigated. The results from surface potential and surface pH measurements were substantiated by the surface observation of the corroded sample during and after the corrosion test. The potential difference over the zinc and steel surface in 90% RH was very low (less than 200 mV) showing that whole steel surface was under galvanic protection. On the other hand, in 60% RH, after several days of corrosion the potential difference between the zinc coating and the steel surface was very high (more than 500 mV) and hence the galvanic protection was limited to interface region. The X-ray analysis of the sample corroded in 60% RH has shown that the zinc corrosion products were deposited on the steel surface near the interface, the same region has shown a low pH compared to than in other part of the steel surface. This led to conclude that with the progress of corrosion, the coating surface of zinc coated steel acidifies by the hydrolysis reaction of the dissolved zinc ions, and the iron surface showed the alkalinity by the oxygen reduction reaction. Moreover, the parts of the steel surface covered with zinc corrosion products had developed relatively less noble potential than other parts indicating that zinc corrosion products took a role to protect the base steel against corrosion. It was assumed that this behavior was related to a combination of the water absorbing capability of zinc corrosion products and adsorption of zinc ion on the steel surface due to low pH.     

Effect of Al on the galvanic ability of Zn-Al coating under thin layer of electrolyte

The effect of Al on the galvanic ability of Zn-Al coating has been studied under thin electrolyte layers by measuring surface potential and surface pH. The changes of surface potential and surface pH over Zn-Al/steel galvanic couple corroding in artificial sea water (ASW) were measured at 60% and 90% RH at 298 K. In the initial stage of corrosion, Zn-55Al coating has shown better galvanic protection ability than Zn-5Al coating in both 60% and 90% RH. However, Zn-5Al coating was better in long term corrosion. The better galvanic ability of Zn-55Al coating in the initial stage of corrosion was related to the observation of pH as low as low as 2 on its surface. The low pH value was due to hydrolysis of Zn²⁺ and Al³⁺ ions. The low pH value was further confirmed by observing evolution of gas due to H⁺ reduction on the Zn-55Al coating. With the progress of corrosion, the low pH region of coating layer extended towards the base steel. This helped expand the deposition of zinc corrosion products on the steel surface. The enhanced dissolution of zinc in Zn-55Al coating led to the formation of a barrier layer which limited the galvanic protection of remaining steel. This was not the case in Zn and Zn-5Al coating. The X-ray analyses of the corroded samples have shown the deposition of zinc corrosion products on the steel surface, which greatly depended on the RH value. The part of the steel surface covered with zinc corrosion products has shown relatively less noble potential than other part indicating that zinc corrosion products took a role to protect the base steel against corrosion. The results from surface potential and surface pH measurements were substantiated by the surface observation of the corroded sample during and after the corrosion test.     

Inferring the distribution of iron oxidation species on mineral surfaces during grinding of base metal sulphides

Electrochemistry plays an important role in the flotation of base metal sulphide minerals. During grinding a galvanic interaction occurs between minerals and grinding media and controls the iron contamination on mineral surfaces, which depresses mineral flotation significantly. In this study, the galvanic interaction was quantified by measuring the iron oxidation species originated from grinding media by ethylene diamine-tetra acid (EDTA) extraction in single mineral and mixed mineral systems. It was found that the extent of galvanic interaction between minerals and grinding media was intimately associated with the electrochemical reactivity of minerals. The nobler the mineral, the stronger the galvanic interaction with grinding media, and the higher the amount of iron oxidation species from grinding media. For both galena and chalcopyrite a linear relationship was observed between the amount of iron oxidation species and flotation recovery in single mineral systems. This relationship was able to predict the iron oxidation species on galena and chalcopyrite surfaces when they were mixed with pyrite separately. The distribution of iron oxidation species onto the two minerals in the mixture changed with the ratio of the mineral surface areas and was correlated with mineral flotation recovery. The major cathodic mineral in the mixture was dictated by the combination of mineral surface area and reactivity and drew iron oxidation species from the grinding media.     

EDTA滴定法测定Zn^2＋与Fe^2＋混合物中的Zn^2＋

在质量检验工作实践中,发现有的硫酸锌样品呈浅蓝色,定性实验证明,混有硫酸亚铁。原行业标准（ＨＧ３２７７－１９８６）中测定锌含量所用的屏蔽剂（ＮＨ４Ｆ和ＫＩ）不能消除Ｆｅ＾２＋的影响,致使Ｆｅ＾２＋被以Ｚｎ＾２＋的表示方式检测出来,测定的锌含量实际上是Ｚｎ＾２＋与Ｆｅ＾２＋之和,本研究提出了先用Ｈ２Ｏ氧化Ｆｅ＾２＋变成Ｆｅ＾３＋,然后再测定锌含量,结果更为准确、重现性好。     

Simplified point defect model for growth of anodic passive films on iron

We present a simplified point defect model to describe the growth of the primary passive oxide film on the surface of iron. The model postulates a reduced set of elementary interfacial reactions to describe the formation and dissolution of the oxide film. By casting the model in dimensionless form, we obtain a relatively small set of parameters that must be assigned values. Parameter values are set by matching the film thickness predicted by the model with one experimental data point. The model is then used to predict variations in film thickness with time, temperature, and applied potential, yielding reasonable agreement with experimental data. The model also gives the correct qualitative trend in the dependence of film thickness on electrolyte pH. Although the model parameters used in our comparisons are probably not unique, they suggest that physical picture embodied in the model provides a suitable starting point for modeling the growth of passive films on Fe.     

The spatial distribution of Zn during galvanic corrosion of a Zn/steel couple

The spatial distribution of Zn²⁺ during galvanic corrosion of a model Zn/steel couple in 0.01 M NaCl was investigated using a scanning zinc disk electrode. The couple had a coplanar arrangement of a steel substrate with an electroplated zinc layer at the center. During galvanic corrosion, the marked changes in the Zn²⁺ concentration were confined to a thin solution layer ca. 1.0 mm thick above the couple surface. In this thin solution layer above the zinc layer, a higher concentration region of Zn²⁺ in the range of 5-18 mM extended around the zinc layer in the solution during galvanic corrosion. Conversely, above the steel surface distant from the zinc layer, the surface concentration of Zn²⁺ was almost zero during galvanic corrosion. On this surface, the precipitation of zinc corrosion products due to the hydrolysis reaction of Zn²⁺ was observed. The distribution of the Zn²⁺ concentration supported that Zn²⁺ acted as a buffer that suppressed the increased pH due to the cathodic reaction on the steel surface near the zinc layer and almost no corrosion products formed there. The spatial distribution of Zn²⁺ is discussed in relation to the distributions of potential and pH and the surface morphology of the galvanic couple.     

Impedance measurements of a chalcogenide membrane iron(III)-selective electrode in contact with aqueous electrolytes

The electrochemical characterization of a chalcogenide-based iron(III)-selective electrode [Fe(III) ISE] [i.e., Fe_2.5(Se₆₀Ge₂₈Sb₁₂)_97.5] was achieved using impedance spectroscopy. The influence of electrolyte composition (i.e., NO₃⁻, Cl⁻, and pH) on the membrane oxidation reaction has been examined, and a mechanism for its action is proposed. Equivalent circuit analysis was undertaken to determine the interfacial charge transfer resistance and corresponding double layer capacitance as a function of electrolyte composition and immersion time. Variations were detected in the charge transfer time constant, and this was attributed to changes in the dielectric/conduction properties of the surface layer. It was found that the Fe_2.5(Se₆₀Ge₂₈Sb₁₂)_97.5 oxidation kinetics depend on the pH, and the interfacial reaction is dictated by sluggish charge transfer. By contrast, chloride was shown to accelerate the rate of membrane oxidation presumably via the formation of soluble metal-chloride complexes. Electrochemical impedance spectroscopy (EIS) aging studies of the Fe_2.5(Se₆₀Ge₂₈Sb₁₂)_97.5 membrane in chloride electrolyte under alkaline conditions showed that the charge transfer resistance decreases with exposure time. However, extended aging revealed a change in the rate of oxidation, which was attributed to a combined diffusion/passivation effect. It is proposed that the development of a modified surface layer (MSL) and passive surface layer (PSL) are partly responsible for the electrochemical stability of the chalcogenide membrane. This paper attempts to clarify and address some of the misconceptions/issues reported previously in the literature on the chalcogenide iron(III)-selective electrode.     

Effect of Fe3+ and F− on black micro-arc oxidation ceramic coating of magnesium alloy

This study focuses on a black micro-arc oxidation ceramic coating prepared on the surface of magnesium alloy by the technology of micro-arc oxidation in the electrolyte containing F^– and Fe³⁺ as well as its mechanism of F^– and Fe³⁺. It needs coatings to experience detail analyses on their thickness, roughness, corrosion resistance, thermal control property, valence states of elements, phase composition, and morphology of coatings, respectively, through coating thickness gauge, roughness tester, electrochemical workstation, AE radiometer, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive X-ray spectroscopy (EDS). Results showed that with the help of F^– in electrolyte, Fe³⁺ can be complexed and MgF₂ can be obtained in the coating, which reduces the pores on the surface of micro-arc oxidation coating. In addition, Fe³⁺ in the electrolyte contributes to the preparation of Fe₂O₃ and Fe₃O₄ in the coating, which can blacken the surface of the coating. Both F^– and Fe³⁺ benefit to improve the corrosion resistance and thermal control performance of micro-arc oxidation coating. There is higher iron oxide in the outer layer but higher fluoride in the inner layer of the coating.     

Double-layer capacitors composed of interconnected silver particles and with a high-frequency response

Porous and multi-layer network of interconnected silver particles is deposited by galvanic displacement on a technologically relevant substrate, silicon with an aluminum/copper film. The mean particle diameter is approximately 200 nm and the particle density in a single layer is 10⁹ particles per cm². Cyclic voltammetry and electrochemical impedance spectroscopy reveal that capacitance normalized to the electrode geometric area reaches a value of 1.7 ± 0.2 mF/cm², which is about two orders of magnitude higher than that observed on a smooth silver/electrolyte interface. The specific surface area of silver particles, which are assumed to be spherical, is 2.7 m²/g. The electrolyte accessible surface area is slightly larger (3.5 m²/g) due to the surface roughness of silver particles. The frequency response of the porous network of silver particles is analyzed using the transmission line model. The “knee” frequency is determined to be around 200 Hz. The described capacitor could find applications for special electronic circuits where a high-frequency response is needed.     

EIS investigation of zinc dissolution in aerated sulphate medium. Part II: zinc coatings

The kinetics of anodic dissolution and corrosion of zinc coatings deposited onto steel sheet either by electrodeposition or by hot dipping are investigated by electrochemical impedance spectroscopy in aerated sulfate medium. The results are compared with those obtained previously on pure bulk zinc and interpreted on the basis of the model derived in the first part of this paper. It is shown that zinc coatings are less sensitive to corrosion than pure bulk zinc and changes of their behavior with time are not identical. Important differences are observed between the various coatings in the respective contributions of the three parallel paths of the dissolution process. Three kinds of oxidation products were identified by Raman spectroscopy. A compact non-stoichiometric zinc oxide was formed by surface reaction on zinc. Above it, a thick and porous layer made of zinc hydroxi-sulfate and stoichiometric ZnO, was formed by precipitation from a local saturation of the solution. A strong correlation was evidenced between the oxidation products and the various paths of the reaction model. It was assumed that the impurities, initially present in the metal, may affect the interfacial reactions, the increase of the micro-roughness, and may also reinforce the protective properties of oxidation product layers. The differences between the various zinc coating behaviors result mainly from their impurities. Their crystal preferred orientations have no significant influence.     

Interfacial Fe(III)-hydroxide formation during Fe-Pt alloy deposition

Fe-Pt films with an Fe/Pt ratio close to one can be electrodeposited from an FeSO₄-H₂PtCl₆-Na₂SO₄electrolyte. At the deposition potential, the hydrogen evolution and the reduction of the Pt complex are diffusion limited, and Fe overpotential deposition has not yet set in. The sources of the Fe incorporation are iron hydroxide formation together with Fe underpotential deposition due to Fe-Pt alloy formation. Mössbauer measurements show that the iron in the iron hydroxide is predominantly Fe(III). For stoichiometry reasons, a Pt-rich Fe-Pt phase must be present in addition to the Fe(III)-hydroxide. The Fe³⁺ that takes part in the hydroxide formation is produced in the electrolyte by the oxidation of Fe²⁺ by the complexed Pt ion. This exchange reaction results in a significantly higher Fe³⁺ content in the FeSO₄-H₂PtCl₆-Na₂SO₄ electrolyte in comparison to the same electrolyte without H₂PtCl₆. Fe(III)-hydroxide formation can be depressed by adding citric acid, that acts as buffering and complexing agent. This leads to a lower iron content of the deposits. The Fe/Pt ratio close to one that is needed for hard magnetic properties can, however, only be achieved with a significant incorporation of iron hydroxide.     

Acceleration of zinc corrosion in alkaline suspensions containing iron oxides or iron hydroxides

Fast zinc dissolution is of industrial interest in recycling galvanised steel scraps. An acceleration of zinc corrosion in alkaline solutions was observed in the presence of various iron oxides or iron hydroxides. This corrosion was investigated by weight loss, measurements of hydrogen evolution and variation of current in a galvanic cell. The mechanism of this fast zinc corrosion was investigated by electrochemical means and by X-ray diffraction and scanning electron microscopy observations of zinc surface after immersion in alkaline suspensions of iron oxides or iron hydroxides. These insoluble iron compounds were involved in a reduction step leading to iron containing microparticles characterised by a low hydrogen overpotential and which acted as cathodic areas in a galvanic corrosion of zinc.     

A thermodynamic study of the effect of small-scale electrolytes on equilibrium redox potentials

Expressions are derived to calculate the equilibrium oxidation-reduction potentials for the Al⁺³/Al, Cu⁺²/Cu, and Zn⁺²/Zn systems in small-scale electrolytes. The geometrical system consists of a droplet of electrolyte resting on a flat metal plate, and the metal is considered to be immersed in a solution of its own ions. When the radius of the drop is allowed to vary, both the size of the electrolyte and the size of the active metal beneath the droplet change simultaneously. The total free energy change for the system consists of both electrochemical and surface chemical contributions. The interfacial free energy for the solid/liquid interface has been estimated from the Girifalco-Good expression or from spreading pressure considerations. When the droplet becomes sufficiently small in radius, the surface chemical contributions become significant, and the calculated redox potential changes from its normal value to more negative values as the size of the system decreases. The magnitude of this effect depends on the particular system. For 2 M Cu⁺², the calculated redox potential for a 0.8 nm radius droplet is 0.259 V more negative than for the bulk electrolyte. The effect is much smaller for aluminum and zinc. In all three systems, calculated redox potentials approach values for the bulk solution for droplet radii of about 10 nm.     

Effect of fluoride ions on the corrosion of aluminium in sulphuric acid and zinc electrolyte

The effect of fluoride ions on the corrosion of aluminium in sulphuric acid and zinc electrolyte has been investigated through thermodynamic analysis and corrosion experiments. The solution chemistry of aluminium, zinc, and iron in aqueous solution in the absence and in the presence of fluoride ions was studied with the construction of the Eh-pH diagrams for the Al–F–H₂O, Zn–F–H₂O and Fe–F–H₂O systems at 25°C. In the presence of fluoride ions, aluminium can form a series of aluminium-fluoride complexes depending on the fluoride concentration and pH whereas zinc and iron can form soluble or insoluble metal-fluoride complex species only at relatively high fluoride concentration and at higher pH values. Experimental results show that in the presence of fluoride ions, the corrosion of pure aluminium in sulphuric acid is due to uniform dissolution and the reaction rate depends on the fluoride concentration. In zinc electrolyte containing fluoride ions, zinc deposits onto the pure aluminium substrate spontaneously and the amount of deposited zinc also depends on the fluoride concentration. On the other hand, the presence of iron in the Al–Fe alloy accelerates the corrosion of aluminium in H₂SO₄ and zinc electrolyte significantly and prevents the deposition of zinc on the aluminium surface. The effect of fluoride ions on zinc adherence to the aluminium is also discussed.     

Bi and Te thin films synthesized by galvanic displacement from acidic nitric baths

Bismuth (Bi) and tellurium (Te) thin films were formed by galvanic displacement of different sacrificial iron group thin films [i.e. nickel (Ni), cobalt (Co) and iron (Fe)] where the formation was systematically investigated by monitoring the change of open circuit potential (OCP), surface morphology and microstructure. The surface morphologies and crystal structures of galvanically displaced Bi or Te thin films strongly depended on the type and thickness of the sacrificial materials. Continuous Bi thin films were successfully deposited with the sacrificial Co. However, dendrites and nanoplatelets were formed from the Ni and Fe thin films. Te thin films were synthesized with all the three sacrificial thin films. Chemical dissolution rate of the sacrificial thin films and mixed potential strongly influenced formation of Bi or Te thin films.     

Effect of nitrite anions on the rate of iron dissolution in acid electrolytes

The effect of the concentration of nitrite ions, potential, pH and solution stirring on the dissolution rate of iron was studied. Nitrite ions were shown to accelerate active dissolution. The accelerating action of the oxidant depends on the ratio of the rates of electroreduction at the electrode surface and homogenous chemical reduction in the reaction layer adjacent to the electrode. A possible mechanism of oxidant action on the iron dissolution is considered. The electroreduction process is assumed to generate at the iron surface an activated complex accelerating metal dissolution.