Critical Impact of Chloride Containing Reference Electrodes on Electrochemical Measurements

Chloride containing reference electrodes, such as various silver‐silver chloride electrodes, are well established in electrochemical analyses. Usually less attention is paid to the influence of potential chloride contamination of electrochemical measurements. Herein, the impact of the chloride leakage rate is studied by means of experiments concerning the anodic oxidation of aluminum. Based on the results, the authors determine critical leakage rates, depending on typical electrolyte volumes and experimental conditions. The results demonstrate that chloride leakage can significantly influence electrochemical measurements, even at ultra‐low leakage rates. A guideline for the usage of chloride containing reference electrodes is deduced from the experimental results and theoretical considerations.     

Tracer studies of anodic films formed on aluminium in malonic and oxalic acids

Using a tungsten-containing layer, incorporated into sputtering-deposited aluminium, as a tracer, the growth of porous anodic films in malonic and oxalic acid electrolytes has been investigated using transmission electron microscopy, Rutherford backscattering spectroscopy and nuclear reaction analysis. Comparisons were also made with films formed in phosphoric acid electrolyte, which have been studied previously. The findings reveal a distortion of the tracer layer within the barrier region of the porous films, evident as a lagging of the tracer beneath the pores relative to that in the adjacent cell wall region. Further, the films are significantly thicker than the layer of metal consumed during anodizing and display smooth-sided pores. The anodizing behaviours are consistent with a major role for field-assisted flow of film material within the barrier layer in the development of the pores.     

Superhydrophobic and icephobic surfaces prepared by RF-sputtered polytetrafluoroethylene coatings

A superhydrophobic and icephobic surface were investigated on aluminum alloy substrate. Anodizing was used first to create a micro-nanostructured aluminum oxide underlayer on the alloy substrate. In a second step, the rough surface was coated with RF-sputtered polytetrafluoroethylene (PTFE or Teflon^®). Scanning electron microscopy images showed a “bird's nest”-like structure on the anodized surface. The RF-sputtered PTFE coating exhibited a high static contact angle of ∼165° with a very low contact angle hysteresis of ∼3°. X-ray photoelectron spectroscopy (XPS) results showed high quantities of CF₃ and CF₂ groups, which are responsible for the hydrophobic behavior of the coatings. The performance of this superhydrophobic film was studied under atmospheric icing conditions. These results showed that on superhydrophobic surfaces ice-adhesion strength was 3.5 times lower than on the polished aluminum substrate.     

Investigation of the incorporation of electrolyte anions in porous anodic Al2O3 films by employing a suitable probe catalytic reaction

A new method has been developed capable of describing the incorporation of electrolyte anions along the pore wall surface and across both the barrier layer and the pore wall oxide after the establishment of the steady state of growth of porous anodic Al₂O₃ where other methods cannot be applied to obtain reliable results. The knowledge of the nature/composition of anodic oxides as regards the incorporation of species like electrolyte anions is of specific importance for both the understanding of the electrochemical mechanism of oxide production and growth and the scientific and technological applications of porous anodic Al₂O₃ films. The method consists of the selection and use of a suitable catalytic probe reaction on porous anodic oxides at thicknesses varying from a value near zero up to the maximum limiting thickness and the treatment of the experimental reaction rate results by a properly developed mathematical formalism. This method was employed in anodic Al₂O₃ films prepared in H₂SO₄ anodizing electrolyte at a constant bath temperature and different current densities using as a probe reaction the decomposition of HCOOH on these oxides, which is almost exclusively a dehydration reaction, at relatively high reaction temperatures, 350 °C and 390 °C, where the effect of other species except SO₄ ²⁻ incorporated in the oxide on the reaction rate is eliminated. It has been shown that the fraction of the intercrystallite surfaces occupied by SO₄ ²⁻ follows a parabola-like distribution. It has a significant value at the pore base surface, depending on the current density, then it passes through a maximum along the pore wall surface and across both the barrier layer and the pore walls near the pore bases at positions depending on the current density and then becomes almost zero at the mouths of the pores of the oxide with the maximum limiting thickness and at both the Al₂O₃/Al interface and cell boundaries. The maximum value of the surface coverage is almost independent of the current density and is always near 1, showing an almost complete saturation of intercrystalline surfaces at these positions. The above distribution of surface coverage predicts a qualitatively similar distribution of the SO₄ ²⁻ bulk concentration across both the barrier layer and pore wall oxide around the pore bases. The method may be improved and developed further either for a more detailed investigation of the above films or to investigate films prepared in other pore-forming electrolytes. Received: 30 July 1998 / Accepted: 30 September 1998     

Superhydrophobic hierarchical surfaces fabricated by anodizing of oblique angle deposited Al-Nb alloy columnar films

A combined process of oblique angle magnetron sputtering and anodizing has been developed to tailor superhydrophobic surfaces with hierarchical morphology. Isolated submicron columns of single-phase Al-Nb alloys are deposited by magnetron sputtering at several oblique deposition angles on a scalloped substrate surface, with the gaps between columns increasing with an increase in the deposition angle from 70° to 110°. Then, the columnar films have been anodized in hot phosphate-glycerol electrolyte to form a nanoporous anodic oxide layer on each column. Such surfaces with submicron-/nano-porous structure have been coated with a fluoroalkyl phosphate layer to reduce the surface energy. The porous surface before coating is superhydrophilic with a contact angle for water is less than 10°, while after coating the contact angles are larger than 150°, being superhydrophobic. The beneficial effect of dual-scale porosity to enhance the water repellency is found from the comparison of the contact angles of the submicron columnar films with and without nanoporous oxide layers. The larger submicron gaps between columns are also preferable to increase the water repellency.     

喷砂-阳极氧化-氟化处理构筑铝合金超疏水表面

为研究复合法制备超疏水表面过程中主要工艺参数对表面形貌及超疏水性能的影响, 开发了一种喷砂-阳极氧化复合方法, 在铝合金表面构筑了微米-纳米二级结构, 经氟化处理后获得了超疏水特性. 结果表明, 喷砂处理在铝合金表面通过冲蚀的凹坑构筑出微米结构, 阳极氧化则在铝合金表面通过蜂窝状氧化膜构筑纳米结构. 但单纯构筑粗糙结构或单纯改变表面化学组成均不能在铝合金表面获得超疏水特性. 单纯的微米结构或纳米结构, 即使有低表面能聚合物修饰也不能获得超疏水特性. 只有微米-纳米二级结构和低表面能聚合物的协同作用, 才能有效构筑铝合金超疏水表面. 这种铝合金与水滴接触时, 形成的气阱可减小固体表面与水滴的接触面积, 降低表面与水滴间的热量交换, 从而减缓水分子的凝结, 提高铝合金的抗霜冻性. 同时, 气阱还可有效减缓海水的腐蚀, 提高铝合金的耐海水腐蚀性.     

Synthesis of anodizing composite films containing superfine Al2O3 and PTFE particles on Al alloys

Anodized composite films containing superfine Al₂O₃ and PTFE particles were prepared on 2024 Al alloy using an anodizing method. The microstructures and properties of the films were studied by scanning electron microscopy, optical microscopy and X-ray diffraction. Friction wear tests were performed to evaluate the mechanical properties of the composites. Results indicate that the composite films with reinforced Al₂O₃ and PTFE two-particles have reduced friction coefficients and relatively high microhardness. The friction coefficient can be as small as 0.15, which is much smaller than that of an oxide film prepared under the same conditions but without adding any particles (0.25), while the microhardness can reach as high as 404 HV. When rubbed at room temperature for 20 min during dry sliding friction tests, the wear loss of the film was about 16 mg, which is about the half of that of the samples without added particles. The synthesized composite films that have good anti-wear and self-lubricating properties are desirable for oil-free industrial machinery applications.     

Compositional and structural characterization of nanoporous films produced by iron anodizing in ethylene glycol solution

We report on the composition and morphology of as-grown anodic oxide films onto the iron surface in an ethylene glycol solution containing some NH₄F and H₂O by anodizing under direct current bias. Decrease in the content of NH₄F and the temperature of electrolyte allow us to form either nanochannel or nanotubular films over a larger potential window, ca. from 30 to 100 V. By this way, the films in thickness of up to10 μm have been formed. Mössbauer spectra recorded at room to cryogenic temperatures under conversion electron and transmission modes revealed the formation of lepidocrocite (γ-FeOOH) film containing some Fe(OH)₂ and/or FeF₂·4H₂O. An increase in anodizing voltage results in fabrication of more porous and less Fe(II) compounds containing films.     

Effect of electrolyte temperature on the formation of self-organized anodic niobium oxide microcones in hot phosphate-glycerol electrolyte

Nanoporous niobium oxide films with microcone-type surface morphology were formed by anodizing at 10 V in glycerol electrolyte containing 0.6 mol dm⁻³ K₂HPO₄ and 0.2 mol dm⁻³ K₃PO₄ in a temperature range of 428-453 K. The microcones appeared after prolonged anodizing, but the required time was largely reduced by increasing electrolyte temperature. The anodic oxide was initially amorphous at all temperatures, but crystalline oxide nucleated during anodizing. The anodic oxide microcones, which were crystalline, appeared on surface as a consequence of preferential chemical dissolution of initially formed amorphous oxide. The chemical dissolution of an initially formed amorphous layer was accelerated by increasing the electrolyte temperature, with negligible influence of the temperature on the morphology of microcones up to 448 K.     

硼砂-对苯二甲酸电解液中AZ91D镁合金的阳极氧化处理

研究了硼砂-对苯二甲酸环保型电解液中AZ91D镁合金的阳极氧化.考察了对苯二甲酸对镁合金阳极氧化过程及其氧化膜性能的影响,并利用扫描电镜(SEM)、X射线衍射(XRD)、能量散射谱(EDS)、动电化极化和电化学交流阻抗谱(EIS)等进行了分析表征.结果表明,对苯二甲酸的浓度对阳极氧化成膜过程、氧化膜的表面形貌、厚度、相结构和耐腐蚀性能都有重要影响.在硼酸盐电解液中加入适量的对苯二甲酸后,氧化电流密度降低,过度放电现象受到了明显的抑制,所制得的阳极氧化膜的质量也有了显著改善.氧化膜表面变得光滑、致密,膜厚度略有降低.氧化膜与镁合金基底的结合更紧密,而且其耐腐蚀性能也得到了明显增强.该研究对于镁合金阳极氧化处理工艺的环保化及阳极氧化膜质量的提高都具有积极意义.