Anodic Behavior of Nickel-Chromium Alloys in Sodium Chloride Solutions: Pitting and Transpassivity

Anodic potentiodynamic and chronoamperometric curves on nickel-chromium alloys (2–50 wt % Cr) are obtained in NaCl solutions at various concentration, pH, and temperature. The dependences of pitting and repassivation potentials on the concentration and temperature are determined for alloy with 20 wt % Cr. The effect of the Cr content and pH on the anodic behavior of the alloys, in particular, the transition from a pitting breakdown of passivity to transpassivation, is shown. Quantitative time dependences of the current in the passive range are obtained and the steady-state values of this current are determined. The corrosion potential of the alloy with 20 wt % Cr is measured. Cathodic voltamograms for passive and chloride-ion-activated surface of the alloy are obtained.     

The effect of phase composition of Ti-20 at % Cr alloy on its anodic activation in the chloride and bromide solutions

The effect of phase composition of Ti-20 at % Cr alloy on its anodic behavior in the solutions containing aggressive Cl⁻ and Br⁻ anions and, for comparison, non-aggressive SO₄²⁻ anion is studied. Different phase compositions of the alloy were obtained by using different modes of heat treatment. The features of anodic behavior of alloys with various phase compositions are related to the electrochemical properties of titanium and chromium, their tendency to passivation and breakdown of passivity in the presence of various anions.     

The electrochemical behaviour of bronze in synthetic seawater

This paper reports a voltammetric study of bronze in synthetic seawater (SSW). The effects of buffering and deoxygenating were particularly visible in the transpassive region. The breakdown of the anodic passive film on bronze leads to a well-defined activation peak in the transpassive region typical of a nucleation and growth of pits. The breakdown potential of the passivity was shown to vary with the experimental conditions, namely, with buffering and deoxygenating. Buffering has shown to lead to more stable passive films and deoxygenating to higher oxidation currents. Scanning electron microscopy with energy dispersive spectrometer (SEM/EDS) studies of bronze samples with 1-month exposure in non-deoxygenated buffered and non-buffered SSW under open circuit potential have shown significant differences in their morphology: a uniformly cracked surface and a surface showing large and spherical precipitates of about 50 μm uniformly distributed along the surface, respectively, for bronze coupons in buffered (pH 9) and in non-buffered SSW. The EDS technique has identified Cu, O, Cl and Na on the corrosion products of bronze in non-buffered SSW, whilst in buffered media, Sn was also identified. In non-buffered media, open circuit potentials have shown to be all the time less negative than in the buffered media. After 1-month exposure the E _OCP of bronze samples in both media seem to converge to −0.131 and −0.155 V vs Ag|AgCl, respectively. This potential can be assigned to the formation of cuprite, Cu₂O and nantokite, CuCl. The analysis of the SEM images after the removal of the corrosion products has shown descuprification with higher intensity on the surface from coupons in non-buffered SSW.     

The protectivity of aluminum and its alloys with transition metals

The mechanism of the protectivity of aluminum and supersaturated aluminum alloys containing W, Mo, Ta and Cu has been investigated in chloride environments. The potential of zero charge (PZC) of the passive film was evaluated by a method based on impedance spectroscopy. The chloride ion adsorption on the passive film was measured by means of an in situ radiotracer technique. Constituents of the passive film as a function of depth were investigated by means of ex situ spectroscopic techniques including XPS, ISS and SIMS. The PZCs of the passive films of Al and Al alloys were calculated from the flatband potentials. A linear correlation between pitting potential and the PZC was found. Adsorption of the chloride ion on the Al-Ta surface starts at more anodic potentials than those of pure Al, and this shift is in agreement with the anodic shift of the PZC. A constant surface concentration of chloride ion was observed during the induction time for breakdown. A significant de crease of OH⁻ concentration in the passive film of Al and its alloys has been found after the passive film has undergone breakdown. The mole fraction of the alloying elements in the surface region of the passive film is ca. 1–8%. The adsorption of the chloride ion on the surface of the passive film is influenced by the anodic PZC shift, which varies with the alloying element. However, retardation of the chloride penetration into the passive film by blocking of the entry site by oxide ions of the alloying element controls the rate of breakdown. Received: 18 November 1996 / Accepted: 17 February 1996     

Pitting corrosion of zinc in alkaline medium by thiocyanate ions

This paper describes the use of potentiodynamic anodic polarization, cyclic voltammetry and chronoamperometry techniques in order to study the pitting corrosion susceptibility of a Zn electrode in KOH solutions containing KSCN as a pitting corrosion agent. Measurements were conducted under different experimental conditions. The results demonstrated that in the absence of KSCN, the anodic voltammetric response displays two anodic peaks prior to reaching the oxygen evolution potential. The first anodic peak A₁ is related to the electroformation of Zn(OH)₂. Peak A₁ is followed by a wide passive region which extends up to the appearance of the second anodic peak A₂. The latter is assigned to the formation of ZnO₂. Addition of SCN⁻ ions to the KOH solutions stimulates the anodic dissolution through peak A₁ and breaks down the passive layer prior to peak A₂. The breakdown potential decreases with an increase in SCN⁻ concentration and temperature, but increases with an increase in KOH concentration and potential scan rate. Successive cycling leads to a progressive increase in breakdown potential. The current/time transients show that the incubation time for passivity breakdown decreases slightly with increasing applied positive potential, SCN⁻ concentration, and temperature.     

Enrichment,incorporation and oxidation of copper during anodising of aluminium–copper alloys

Porous anodic oxides generated on copper‐containing aluminium alloys are less regular than anodic oxides generated on pure aluminium. Specifically, a porous oxide morphology comprising layers of embryo pores, generated by a cyclic process of oxide film growth and oxygen evolution, is generally observed. In this work, the relation between the oxidation behaviour of copper during anodising and the specific porous oxide film morphology was investigated by electrochemical techniques, transmission electron microscopy and Rutherford backscattering spectroscopy (RBS). It was found that the anodising potential determines the oxidation behaviour of copper, and the latter determines the porous oxide morphology. At low voltage, relatively straight pores with continuous cell walls were obtained on Al? Cu alloys, but selective oxidation of aluminium atoms resulted in the occlusion of copper‐containing metallic nanoparticles in the anodic film. At higher potentials, copper oxidation promoted oxygen evolution within the barrier layer, and generation of a less regular film morphology. RBS, performed on Al? Cu alloy specimens, revealed a high volume fraction of copper atoms in the anodic films generated at low potentials and a reduced amount of copper atoms in the anodic oxide films generated at high potentials. Copyright © 2009 John Wiley & Sons, Ltd.     

The influence of fluoride on the physicochemical properties of anodic oxide films formed on titanium surfaces

The influence of fluoride (and its concentration) on the electrochemical and semiconducting properties of anodic oxide films formed on titanium surfaces was investigated by performing electrochemical measurements (potentiodynamic/pontiostatic polarization, open circuit potential (OCP), and capacitance measurements) for a titanium/oxide film/solution interface system in fluoride-containing 1.0 M HClO(4) solution. On the basis of the Mott-Schottky analysis, and with taking into account both the surface reactions (or, say, the specifically chemical adsorption) of fluoride ions at the oxide film surface and the migration/intercalation of fluoride ions into the oxide film, the changes in the electrochemical behavior of titanium measured in this work (e.g., the blocked anodic oxygen evolution, the increased anodic steady-state current density, the positively shifted flat band potential, and the positively shifted film breakdown potential) were interpreted by the changes in the surface and the bulk physicochemical properties (e.g., the surface charges, surface state density, doping concentration, and the interfacial potential drops) of the anodic films grown on titanium. The fluoride concentrations tested in this work can be divided into three groups according to their effect on the electrochemical behavior of the oxide films: < or =0.001 M, 0.001-0.01 M, and >0.01 M. By tracing the changes of the OCP of the passivated titanium in fluoride-containing solutions, the deleterious/depassive effect of fluoride ions on the titanium oxide films was examined and evaluated with the parameter of the film breakdown time. It was also shown that the films anodically formed on titanium at higher potentials (>2.5 V) exhibited significantly higher stability against the fluoride attack than that either formed at lower potentials (<2.5 V) or formed natively in the air.     

氯离子对模拟混凝土孔隙液中钢筋腐蚀行为的影响

应用动电位扫描法研究钢筋在模拟混凝土孔隙液中的腐蚀电化学行为以及氯离子的影响作用,并根据阳极极化曲线的变化揭示钢筋表面钝化膜的击穿电位及其变化规律,得出当溶液pH值分别为12.50和12.00时,由氯离子侵蚀引起的钢筋局部腐蚀,其钝化膜击穿电位突降的浓度临界值.     

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.     

Visualising dynamic passivation and localised corrosion processes occurring on buried steel surfaces under the effect of anodic transients

A new method of visualising dynamically changing electrode processes has been demonstrated by mapping localised corrosion processes occurring on buried steel surfaces under the effect of anodic transients. Dynamically shifting external electrical interferences such as anodic transients are known to affect the efficiency of cathodic protection (CP) of underground pipelines; however unfortunately conventional techniques including electrochemical methods have difficulties in measuring such effects. In this paper we report that the wire beam electrode has necessary temporal and spatial resolutions required for measuring and visualising the dynamic effects of anodic transients on CP, passivation and localised corrosion processes occurring on buried steel surfaces. For the first time a critical anodic transient duration has been observed and explained as the incubation period for the breakdown of passivity and the initiation of localised corrosion.     

Copper oxides: kinetics of formation and semiconducting properties. Part I. Polycrystalline copper and copper-gold alloys

The anodic formation of Cu(I) and Cu(II) oxides on polycrystalline copper and copper-gold alloys (4 and 15 at% Au) in deoxygenated 0.1 M KOH was examined by voltammetry, chronoamperometry, and chronopotentiometry with a synchronous registration of photocurrent and photopotential, in situ spectroscopy of photocurrent as well as XPS and SEM measurements. The band gap of p-Cu₂O is 2.2 eV for indirect optical transitions independent of the concentration of gold in Cu-Au alloy. It grows on CuOH or n-Cu₂O underlayer. The increase of anodic potential results in a thickening of oxide film which is a mixture of Cu(I) and Cu(II) oxides. The latter is a p-type semiconductor with a low photosensitivity. The rate of oxide formation on the alloys is lower than on copper. The structure-dependent properties of the oxide phase on the alloys and copper are different. Copper is prone to corrosive oxidation even in deoxygenated alkaline solution by the traces of molecular oxygen. The corrosive growth of Cu(I) oxide film occurs according to the parabolic law. After the cathodic polarization, the surface of copper remains free of corrosive oxide no longer than 15–20 min. The preliminary anodic formation even of a thin Cu₂O film as well as the alloying of copper with gold suppresses the corrosive oxidation of copper.     

The anodic oxidation of gold + palladium alloys

The anodic oxidation of Au+Pd alloys has been studied in solutions of 1 M H₂SO₄ and 0.1 M K₂SO₄ by voltammetric methods. A linear relationship between oxide reduction maximum and bulk alloy composition, often used to determine the surface composition of homogeneous alloys, could be shown to hold only for alloys up to 60 at% gold. At higher gold content the Au oxide peak must be additionally evaluated. With continuous cycling in acid solution the anodic dissolution of Pd, especially from gold-rich places, leads to a rather heterogeneous surface. The O--chemisorption is not governed by a transfer mechanism from Pd to Au surface atoms. The alloys are able to absorb the oxygen species generated in the positive potential region; however, this ability decreases with increase of the gold content.     

Evaluation of passive film behaviour of super austenitic stainless steels at different potential regions using dynamic electrochemical impedance spectroscopy

Potentiodynamic anodic polarisation and dynamic electrochemical impedance spectroscopic (DEIS) measurements were carried out on 316L stainless steel and alloys 926 and 31 in natural seawater in order to assess the crevice corrosion resistance. DEIS measurements were performed over a wide range of potentials covering the corrosion potential, passive region, breakdown region and dissolution region. The impedance measurements in potentiodynamic conditions clearly reveal the changes that occur in the passive layer with change in potential. The impedance spectra at different potential regions were also discussed elaborately. The surface morphology of the alloy after crevice corrosion was studied using optical microscope and atomic force microscopy.     

Pitting of Al and Al–Si alloys in KSCN solutions and the effect of light

The pitting corrosion susceptibility of pure Al and three Al-Si alloys, namely (Al-6%Si), (Al-12%Si) and (Al-18%Si) has been studied in 0.04 M KSCN solution. Measurements were carried out under the effect of various experimental conditions using cyclic polarization, potentiostatic and galvanostatic techniques. In all cases, the potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of a thin film of Al₂O₃ on the anode surface. The passive region is followed by pitting corrosion, at a certain critical potential, pitting potential (E_pit), as a result of breakdown of the passive film by SCN^? anions. Cyclic polarization measurements allowed the determination of the pitting corrosion parameters, namely the pitting potential and the repassivation potential (E_rp). Alloyed Si decreased the passive current (j_pass) and shifted both E_pit and E_rp towards more positive values. Thus alloyed Si suppressed pitting attack. The effect of illumination on passivity and the initiation of pitting corrosion on Al in KSCN solutions was also studied. It is observed that illumination of Al leads to an increase in its pitting corrosion resistance-apparent from j_pass, E_pit, and E_rp measurements in aggressive KSCN solutions.     

Formation and dielectric properties of anodic oxide films on Zr–Al alloys

Zr–Al alloys containing up to 26 at.% aluminum, prepared by magnetron sputtering, have been anodized in 0.1 mol dm⁻³ ammonium pentaborate electrolyte, and the structure and dielectric properties of the resultant anodic oxide films have been examined by grazing incidence X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy, and AC impedance spectroscopy. The anodic oxide film formed on zirconium consists of monoclinic and tetragonal ZrO₂ with the former being a major phase. Two-layered anodic oxide films, comprising an outer thin amorphous layer and an inner main layer of crystalline tetragonal ZrO₂ phase, are formed on the Zr–Al alloys containing 5 to 16 at.% aluminum. Further increase in the aluminum content to 26 at.% results in the formation of amorphous oxide layer throughout the thickness. The anodic oxide films become thin with increasing aluminum content, while the relative permittivity of anodic oxide shows a maximum at the aluminum content of 11 at.%. Due to major contribution of permittivity enhancement, the maximum capacitance of the anodic oxide films is obtained on the Zr–11 at.% Al alloy, being 1.7 times than on zirconium at the formation voltage of 100 V.     

Electrochemical polarisation and galvanic couple behaviour of the primary phase of 55% Al–Zn coating investigated using band microelectrodes (BME) and band microelectrode arrays

Electrochemical polarisation experiments have shown that anodic dissolution processes on Al–40% Zn alloys are significantly enhanced in chloride compared to sulfate-based electrolytes. The aluminium content of the alloys allowed passive behaviour to be observed in sulfate electrolyte even in the presence of zinc-rich precipitates on the surface. Electrolyte pH affected cathodic processes, which was attributed to the rate of proton reduction and the passivity of the surface. Monitoring the OCP of the alloy band during polarisation of neighbouring zinc electrodes in band microelectrode (BME) arrays showed that generation of alkaline pH at the zinc electrodes affected the OCP of the alloy when the inter-electrode spacing was 10, 50, and 200 μm. Where elements of a BME array were close enough to interact via mass transport, the overall galvanic behaviour of the cell was found to be anodic or cathodic, whereas the alloy was consistently cathodic with respect to zinc in galvanic cells at larger separations. Dedicated to the 80th birthday of Keith B. Oldham.     

Composition and Structure of Anodic Oxide Films on Titanium–Aluminum Alloys by Fast Electron Reflection Diffraction, Rutherford Backscattering, and Secondary Neutral Particle Mass Spectrometry

Anodic oxide films on some Ti–Al alloys are studied using fast-electron reflection diffraction, Rutherford backscattering, and secondary neutral-particle mass spectrometry. The films are amorphous, with a small amount of crystalline phases, and comprise a mixture of TiO₂ and Al₂O₃. The Ti/Al ratio in an anodic film corresponds to that in the alloy matrix. Constants of anodic oxidation of the alloys are determined.     

Sm-La二元稀土对铅基板栅材料显微结构和电化学性能的影响

为提高铅酸蓄电池板栅材料的性能,在Pb-Ca-Sn-Al合金中添加了Sm-La二元稀土元素,研究其含量对显微结构和电化学性能的影响。利用金相和循环伏安、线性扫描、阻抗-电位、电化学阻抗谱等电化学测试方法对板栅性能进行了表征。结果表明:Sm-La稀土元素的加入促进合金晶粒细化,随其含量的增加,析出物颗粒变小并弥散分布。与添加Sm的合金相比,复合添加Sm-La的材料具有更低的析氢电位,改善了深放电时铅合金上所形成的阳极Pb(II)膜的阻抗特性,表现出良好的性能。     

Investigation of the Anodic Behavior of Al in Room Temperature Ionic Liquid Electrolytes: BMI‐BF4, PP14‐BF4 and BMI‐BF4·PC

The anodic polarization behavior of Al, Ta and Nb foil was investigated in 1‐butyl‐3‐methylimidazolium tetrafluoroborate ionic liquid (BMI‐BF₄). Compared with that of Ta and Nb foil, it showed that a better passive film was formed on Al foil surface after the anodic polarization in BMI‐BF₄, which could resist the potential up to 94.58 V vs. Ag⁺/Ag. Besides, similar anodic behavior of Al foil was observed in N‐methyl‐N‐butylpiperidinium tetrafluoroborate ionic liquid (PP14‐BF₄), which indicated that the anodic polarization behavior of Al foil was independent of the cations of RTIL. In addition, the investigation of anodic polarization behavior of Al foil was carried out in the mixture electrolytes composed of BMI‐BF₄·PC. Differently, two breakdown potential processes of Al foil were presented compared to pure BMI‐BF₄. Further research showed that the passive film on Al foil was mainly composed of AlF₃ and Al₂O₃ after the first breakdown potential process, while the fluoride film increased with continual anodic polarization, which improved the anodic stability of Al foil and resisted higher breakdown potential. The high breakdown potential properties of Al foil in BMI‐BF₄, PP14‐BF₄ and the mixture of BMI‐BF₄·PC during the anodic polarization can be favored for R&D of the high performance electrochemical devices.