A study of barrier film growth on aluminium in solutions of film-promoting and aggressive ions using secondary ion mass spectrometry

A Cameca Ion Analyser has been used to examine barrier-type anodic films grown on aluminium in neutral phosphate, chromate or phosphate/chromate solutions and in phosphate and chromate solutions containing relatively small amounts of chloride ion. Barrier layers grow at relatively high current efficiencies in phosphate solution, and are characterized by a rather wide phosphorus-containing layer adjacent to the oxide/solution interface. In chromate solutions, barrier layers grow at relatively low current efficiencies and contain little, if any, chromium resulting from the film growth process itself. Barrier layers grown in chromate-phosphate mixtures are generally typical of those grown in pure phosphate solutions, even at quite high chromate/phosphate concentration ratios. The presence of chloride ions in either phosphate or chromate solutions lowers the current efficiency of barrier film growth, and is associated with pitting of the aluminium substrate at the bases of flaws in the growing film, resulting in lateral undermining of the film. The results are explained in terms of a model for film development, involving growth partly by ionic migration and partly by a dissolution/precipitation process.     

AZ31镁合金方波脉冲阳极氧化新工艺研究

采用正交设计,利用盐雾试验评价方法,研究了一种新型镁合金无铬、无氟、无磷脉冲阳极氧化工艺配方.采用扫描电镜(SEM)、能谱分析仪(EDS)、X射线衍射法(XRD)、动电位极化曲线测量方法,评价了优化工艺配方条件下脉冲阳极氧化膜的微观组织结构及其耐蚀性能.结果表明,经优化配方阳极氧化处理可以显著提高镁合金的耐腐蚀性能.     

Plasma anodized ZE41 magnesium alloy sealed with hybrid epoxy-silane coating

Anodic coatings on magnesium ZE41 alloy were formed by DC plasma electrolytic oxidation (PEO) in spark regime in solution composed of NaOH, Na₂SiO₃ and KF. The positive effect of poly(ethylene oxide) addition into the anodizing electrolyte on PEO process, anodic film porosity and its protective performance was described. Anodic films were sealed with hybrid epoxy-silane formulation. The corrosion behavior of the coated ZE41 was studied through electrochemical impedance spectroscopy (EIS) in 0.6 M NaCl solution. Resulting duplex PEO/epoxy-silane coating provides good protective performance without significant signs of corrosion during 1 month of immersion test.     

The high throughput assessment of aluminium alloy corrosion using fluorometric methods. Part I - Development of a fluorometric method to quantify aluminium ion concentration

This paper examines a new method developed for the rapid assessment of corrosion inhibitors and their combinations for aluminium alloy 2024-T3 (AA2024-T3). Two fluorometric probes, lumogallion and morin, were used to rapidly assess the concentration of aluminium ions in solution through the use of a spectrophotometric plate reader. Fourteen potential inhibitor compounds and other assay constituents were screened for fluoroprobe interference: sodium chromate, sodium metavanadate, cerium chloride, lanthanum chloride, europium chloride, gadolinium chloride, neodymium chloride, yttrium chloride, barium metaborate, sodium metatungstate, potassium phosphate, sodium metasilicate, sodium phosphate, and sodium molybdate. Calibration curves were established for the two probes.     

己二酸铵对7075-T6铝合金硫酸阳极氧化的影响

研究硫酸电解液中添加己二酸铵对7075-T6铝合金阳极氧化的影响,采用场发射扫描电子显微镜(FE-SEM)对7075-T6铝合金在不添加和添加己二酸铵的硫酸电解液中制备的阳极氧化膜表面的微观形貌进行分析,采用线性阳极极化研究7075-T6铝合金在两种电解液中的极化行为。利用动电位极化技术和电化学阻抗谱(EIS)研究两种电解液中制备的阳极氧化膜的耐腐蚀性能。结果表明:硫酸电解液中添加己二酸铵可以通过降低电流密度来改善氧化膜的结构,减少氧化膜缺陷,降低微孔孔径,提高阻挡层厚度,从而降低氧化膜的自腐蚀电流密度,提高氧化膜耐腐蚀性能。     

Direct-to-metal UV-cured hybrid coating for the corrosion protection of aircraft aluminium alloy

Finding eco-efficient and environmentally viable alternatives to chromate coatings represents a fundamental milestone in the aerospace industry. Here, we show a chromate-free approach to protective hybrid coatings on aluminium alloy (AA2024-T3) departing from photoinduced sol–gel and cationic polymerizations. Beginning with a film of n-alkyltrimethoxysilane and diepoxy monomer, we rely on photogenerated superacids to induce the single step formation of two inorganic and organic barrier networks. Such system combines the unique aspects of photopolymerization including fast reactions, temporal control, solvent-free composition and temperature independence. Used without chemical conversion coating or anodizing, some films have passed 2000 h of salt spray testing.     

Surface characterization of oxides grown on the Ti–13Nb–13Zr alloy and their corrosion protection

Oxide films were formed on the biocompatible alloy Ti–13Nb–13Zr in a phosphate buffer at open-circuit potential (E_oc), potentiodynamically up to 8 V, or by micro-arc oxidation (MAO) at 300 V. Their electrochemical properties were assessed in a phosphate buffer saline solution (PBS). EIS and SEM results showed that the E_oc and potentiodynamically formed oxide films were compact and behave as a monolayer, while the MAO oxide was a bilayered film (compact inner and porous outer layers). Open-circuit potential and EIS resistance values indicated that the MAO oxide provides the best corrosion protection for the alloy in PBS.     

Influence of surface pretreatment on the anodizing film of Mg alloy and the mechanism of the ultrasound during the pretreatment

The effect of surface pretreatments on the micro-morphology, composition and the corrosion resistance of the anodic films formed on AZ91D magnesium alloys was investigated. The results showed that the aluminium content increased layer can be formed on the surface of the AZ91D magnesium alloys by the immersion pretreatment in aluminium nitrate solution with or without ultrasound. The pretreated surface with ultrasound was more uniform except for some pits. The anodic oxidation films on the pretreated magnesium alloys with ultrasound contained more aluminium and the pretreatment improved the uniformity of the anodic films. The use of ultrasound in the pretreatment process could significantly decrease the quantity and size of the micro-pores on the anodic films. The corrosion resistance of the anodic films formed on the pretreated magnesium alloys was improved. The corrosion resistance with ultrasound was better than that without ultrasound in the pretreatment process. In this paper, the effect and mechanism of the ultrasound during the immersion pretreatment process in aluminium nitrate solution was also discussed.     

Polypyrrole electrodeposited on copper from an aqueous phosphate solution: Corrosion protection properties

Highly adherent and homogenous polypyrrole films were electrodeposited at copper from a dihydrogen phosphate solution. The polypyrrole films were electrosynthesized in the overoxidized state by cycling the copper electrode from -0.4 to 1.8 V (SCE) in a pyrrole-containing phosphate solution. The growth of the polypyrrole films was facilitated by the initial oxidation of the copper electrode in the phosphate solution to generate a mixed copper-phosphate, copper oxide or hydroxide layer. This layer was sufficiently protective to inhibit further dissolution of the copper electrode and sufficiently conductive to enable the electropolymerization of pyrrole at the interface, and the generation of an adherent polypyrrole film.Potentiodynamic polarization measurements, Tafel analyses and open-circuit potential data revealed that the polypyrrole coating effectively protects the copper substrate from corrosion in a chloride solution. However, the corrosion protection properties were reduced with longer immersion times.     

Deposition of cerium oxide thin films by reactive magnetron sputtering for the development of corrosion protective coatings

As an alternative to chromate based coatings cerium oxide thin films were deposited by magnetron sputtering on Al clad 2024 alloy in order to evaluate their corrosion protection potential. Varying plasma properties by modifying O₂ content during deposition allowed to affect the chemistry and microstructure of the synthesized films. The electrochemical characterization showed that specific self-healing mechanisms are suspected for the samples deposited with the highest amount of oxygen. Those mechanisms, leading to the reinforcement of barrier properties versus immersion time in NaCl solution, are associated with the interaction between alumina and cerium oxide film in the presence of electrolyte.     

Electrochemical study of inhibitor-containing organic-inorganic hybrid coatings on AA2024

Use of organic-inorganic hybrid sol-gel thin films is recently reported among the most promising approaches for the replacement of chromate based pre-treatment methods for high strength structural aluminium parts.To be applied in the aerospace industry, pre-treatments have to meet the demanding requirements and must be compatible to state of the art paint systems. The addition of inhibitive species is desirable for the active corrosion protection and the prevention of sub-surface migration at defect sites of the coating.In the present work hybrid films were produced by the controllable sol-gel route. Corrosion inhibitors were incorporated to the coatings in order to provide active protective effect. The barrier properties and the active corrosion protection on AA2024-T3 have been studied by Electrochemical Impedance Spectroscopy and Scanning Vibrating Electrode Technique. The obtained results were proven by conventional accelerated tests such as neutral salt spray test confirming validity of the mentioned electrochemical techniques for the evaluation of protection efficiency of hybrid sol-gel films.The addition of inhibitors to sol-gel coatings in most cases leads to an improvement of the active corrosion protection but on the other hand it may negatively influences the barrier properties of the films. Therefore, encapsulation of the inhibiting compounds is beneficial in order to combine the good barrier function of the coating with the corrosion mitigating effect of the inhibitors.     

Formation and characterisation of a chromate conversion coating on AA6060 aluminium

AA6060-T6, an AlMg0.5Si0.4 model alloy and α-Al(Fe,Mn)Si phase electrodes have been subjected to chromate treatment in a commercial chromate-fluoride based solution. The coated surfaces were subsequently examined by use of field emission SEM, TEM, AES and electrochemical measurements in 0.1 M NaCl solution in order to study the effect of substrate microstructure on coating formation and properties. Non-uniform growth of the chromate conversion coating (CCC) on AA6060-T6 resulted in a porous morphology, with cracks extending down to the base metal. Poor coverage was particularly observed at the grain boundaries. The thickness of the CCC after completed treatment was about 150-200 nm, while significantly thinner films were formed on the α-Al(Fe,Mn)Si particles. AlMg0.5Si0.4 in the artificially aged (T6) condition exhibited a coating morphology similar to AA6060-T6, while CCC formation on homogenised AlMg0.5Si0.4 was characterised by growth of localised oxide particles and filaments, resulting in poor coverage. These observations indicated that precipitation of Mg₂Si particles due to heat treatment promoted nucleation of the CCC. Chromate treatment caused a significant reduction of cathodic activity on AA6060 during subsequent polarisation in chloride solution. This was attributed to rapid formation of a thin chromium oxide film on the α-Al(Fe,Mn)Si particles during the chromate treatment, resulting in significant cathodic passivation of the phase. Inhibition of the oxygen reduction reaction at cathodic intermetallic particles is suggested as an important factor contributing to the high performance of chromate pre-treatments on aluminium.     

Corrosion and electrochemical behavior of aluminium treated with high-temperature pulsed plasma in CsCl–NaCl–NaNO3 melt

Dense protective layers of aluminium corrosion products, whose composition depends on the oxidation temperature, are formed on the surface of aluminium treated with high-temperature pulsed plasma (HTPP) without visible remelting and then held in a chloride–nitrate melt in conditions of anodic polarization. Modification of aluminium treated with HTPP changes the properties of 20 μm layer under its surface and the oxide layer formed by such treatment has different morphology: it consists of smaller crystals and so has the other protective properties as compared with aluminium untreated by plasma.     

Studying the formation process of chromate conversion coatings on aluminium using continuous electrochemical noise resistance measurements

A unique electrochemical technique, namely continuous noise resistance calculation (CNRC), was used to obtain electrochemical kinetic information from the formation process of chromate conversion coatings (CCC) on aluminium electrodes. It was found that the noise resistance (R_n) of aluminium electrodes remained almost unchanged during electrodes' immersion in a chromate containing acidic solution where the CCC films were supposed to form rapidly. This result indicates that the formation of CCC was associated with continuous corrosion of the aluminium electrodes and that the CCC films formed on aluminium surface were not intact barrier films, but most likely porous layers. The CCCs became protective only after they were aged in the environment. Based on these findings, the formation and inhibition mechanisms of CCC have been discussed.     

铝合金稀土磷化与铬磷化比较

采用电化学测试、全浸腐蚀试验和结合力测试,研究了6061铝合金铬磷化处理和以稀土为添加剂的磷酸盐处理所形成的化学转化膜的工艺性能。动力学研究表明,两者的成膜过程是不相同的。极化曲线测试结果显示,在弱极化区范围内,两者抗蚀性能相近,而在强极化区,铬磷化转化膜的抗蚀性优于以稀土为添加剂的磷酸盐化学处理所形成的转化膜,全浸腐蚀试验有类似的结果。结合力测试结果则表明,以稀土为添加剂的磷酸盐化学处理所形成的转化膜与有机涂层间的结合力要优于铬磷化转化膜。     

Electrochemical behavior of surface films formed on Fe in chromate solutions

The stability of passive films formed on Fe in K₂Cr₂O₇ solutions during exposure at open-circuit potential or by potential cycling is studied in a chromate-free solution. The electrochemical behavior of chromate-passivated Fe is investigated with cyclic voltammetry combined with LASER light reflectance measurements which allow an in situ determination of the thickness of the iron oxide film. The electrochemical behavior of chromate-passivated Fe in chromate-free solutions strongly depends on passivation treatment. Passivation of iron by immersion at open-circuit in chromate solution leads to a passive film, in which both Fe and Cr species dissolve almost independently of the presence of the other one: Fe oxide by reductive dissolution and Cr oxide by oxidative dissolution in the corresponding potential regions. Passivation of iron by potential cycling in chromate solutions leads to much less loss of the otherwise soluble oxidized chromate and reduced ferrous species in subsequent electrochemical experiments (trapping in a protective film). Concerning the dissolution behavior, the film formed on iron by cycling in chromate solution behaves similarly as the passive film on Fe-17Cr alloy. However, the remnant passive film after reductive or oxidative dissolution on the Fe-Cr alloy is of truely protective nature as compared to the films formed on iron in chromate solutions, which show only a small contribution to the potential drop.     

A two-step roll coating phosphate/molybdate passivation treatment for hot-dip galvanized steel sheet

A two-step roll coating passivation treatment employing phosphate followed by molybdate solutions has been performed on hot-dip galvanized (GI) steel sheet. The phosphate coating was primarily porous, amorphous Zn phosphate. A second step coating treatment resulted in more hemispherical Zn phosphate particles and the incorporation of molybdate ions and molybdenum oxide into the existing phosphate coating, giving rise to an improved corrosion resistance. The coating reaction during the second step roll coating treatment and the corrosion protection afforded by the second step molybdate treatment are discussed, with emphases on the comparison with the coating formed via immersion.     

Anodic oxides on InAlP formed in sodium tungstate electrolyte

Amorphous anodic oxide films on InAlP have been grown at high efficiency in sodium tungstate electrolyte. The films are shown to comprise an outer layer containing indium species, an intermediate layer containing indium and aluminium species and an inner layer containing indium, aluminium and phosphorus species_. The layering correlates with the influence on cation migration rates of the energies of In³⁺-O, Al³⁺-O and P⁵⁺-O bonds, which increase in this order. The film surface becomes increasingly rough with increase of the anodizing voltage as pores develop in the film, which appear to be associated with generation of oxygen gas.     

Impedance measurements on oxide films on aluminium obtained by pulsed tensions

We have performed this study on oxide films sealed or not in boiling water. The films are first obtained on type 1050 A aluminium substrat by pulsed tensions anodizing technique, in a sulfuric acid solution. Afterwards the, Electrochemical Impedance Spectroscopy (EIS) is employed to appreciate the films behaviour in a neutral solution of 3.5% K₂SO₄, in which the interface processes interest only the ageing phenomenon of the oxide films and not their corrosion. We have also attempted a correlation between pulse parameters of anodization and the electrical parameters characterizing these films. The sealing influence on ageing has been studied as well. For all films, ageing is appreciated using impedance diagrams evolution versus time. The results show: – the existence of two capacitive loops confirming the presence of two oxide layers characteristic of oxide films obtained in a sulfuric acid medium. The first loop, at high frequencies, is related to the external porous layer and the second one, at lower freqencies, is related to the internal barrier layer. – the thickness of the barrier layer varies between 25 and 40 nm in relation with the electrical pulse parameters. – the sealing acts favorably against anodic oxide films ageing.     

The hydration of barrier oxide films on aluminium and its inhibition by chromate and phosphate ions

Compact oxide films were formed on aluminium in a neutral borate solution (pH 7.4, 20°C) by applying a constant potential of 50 V (SCE), and then immersed in distilled water, a 0.001 mol dm^?3 chromate solution (pH 7.0), or a 0.001 mol dm^?3 phosphate solution (pH 7.0). Changes in the composition profile of the oxide caused by the immersion were examined by X-ray photo-electron spectroscopy (XPS) combined with a film sectioning technique. After a 72 h immersion in distilled water, the OH^? content in the outer part of the film increased to about three times as that before immersion, and the film sustained only 30 V instead of the initial 50 V. As the profile of BO₂^? ions remained unchanged, the deterioration of the film is exclusively due to hydration of the oxide caused by the penetration of water molecules or OH^? ions into the oxide lattice. It was found that the hydration is strongly inhibited by chromate and phosphate. XPS and chemical analysis showed that CrO₄^2? and PO₄^3? ions adsorb on the oxide surface to form a mono- or bi-layer, hindering the penetration of water molecules. The inhibiting behaviour, stabilizing oxide films, is important in explaining the function of these anions as corrosion inhibitors for metals in near neutral solutions.