Chromate conversion coatings on aluminium: influences of alloying

The growth kinetics of chromate/fluoride conversion coatings are examined for 99.99% aluminium, and Al-2.3at.%Cu, Al-1.9at.%Au and Al-20at.%Au alloys. The thickening of coatings and thinning of substrates, the latter deposited by magnetron sputtering, are determined by transmission electron microscopy. The results reveal consumption of substrates during the coating process, to the maximum immersion time of 24 min. Initial relatively rapid thinning in the period to 6 min is followed by slower, approximately constant thinning at about 9.3, 6.2, 6.0 and 5.4 nm min⁻¹ for the 99.99%Al, Al-2.3at.%Cu, Al-1.9at.%Au and Al-20at.%Au alloys respectively. The ratio of the number of chromium atoms in the conversion coating, determined by Rutherford backscattering spectroscopy, to the number of oxidized substrate atoms is about 3, indicating a low efficiency of coating growth. Alloying decreases the coating thickness, as well as metal consumption. Alloying elements initially enrich beneath the coating, in alloy layers of a few nanometres thickness. Later, copper may be oxidized, and then enter the coating, whereas gold is occluded as nanoparticles. The gold nanoparticles act as markers to indicate formation of coating material at the metal/coating interface. Sudden loss of coating on the alloys, after about 6-9 min, occurs soon after entry of alloying element species into the coating. The loss may be related to the presence of nanoparticles of gold and copper near the alloy/coating interface, the latter possibly forming by reduction of copper species, that disrupt the bonding of the coating either directly though their presence or through their action as local cathodes.     

Feasibility study to probe the growth of chromate based conversion on Al by means of rotating ring disc electrode (RRDE) measurements

The use of a rotating ring disc electrode (RRDE) is introduced in this paper to study the formation of chromate-phosphate layers on rolled aluminium surfaces. A RRDE was developed containing an Al disc, which was converted with a chromate phosphate film, and a carbon ring, where the Cr⁶⁺ reduction rate was measured in situ. The principle of the method is that Cr⁶⁺ is transported from the bulk of the solution towards the disc and the ring electrode. At the disc electrode, a fraction of Cr⁶⁺ is consumed during the conversion of the Al disc electrode. This leads to a decrease in ring current, which is directly related to the amount of Cr⁶⁺ being reduced at the disc electrode. For proper operation of the RDDE system, an electrode with optimised geometrical characteristics is used. In order to be sure that the measured current on the ring is in relation with the Cr⁶⁺ reduction rate occurring at the aluminium disc, a number of requirements need to be fulfilled. After showing that the RDDE system is indeed able to monitor in situ the Cr⁶⁺ content during conversion, the amount of Cr⁶⁺ reduced at the disc electrode is calculated from the recorded ring current time curves. The method allows (i) to follow the growth of a conversion layer quantitatively in situ and (ii) to study the influence of important process parameters such as fluoride content, chromate content of the conversion bath and influence of mass transport.     

Chromate conversion coatings on aluminium-copper alloys

The study compares the formation of chromate/fluoride conversion coatings, composed mainly of amorphous hydrated chromia, on model, solid-solution, binary Al-Cu alloys, of a range of compositions, and on 2014-T6 aluminium alloy. The model alloys, produced by magnetron sputtering, reveal the importance of copper in limiting the thickness of the coatings by promoting loss of coating material. This occurs at an earlier time in the treatment with increasing copper content of the alloy. The coating loss follows closely upon the achievement of the required level of copper enrichment for incorporation of copper into the coating, with a thin alumina film beneath the hydrated chromia sustaining the enrichment process. The coating on the 2014-T6 alloy is of non-uniform thickness, with much thinner coating developing at copper-rich second phases, consistent with the results of model alloys.     

几种改进磷化膜耐蚀性对比和自修复性初探

研制了钼酸钠添加剂改进磷化膜及钼酸钠、硝酸铈、硅酸钠溶胶封闭后处理改进磷化复合膜,拟替代高毒性铬酸盐钝化膜。采用中性盐雾试验、Tafel极化和电化学阻抗测试对比分析了这4种改进磷化膜与几种传统无铬转化膜、铬酸盐钝化膜耐蚀性的差异;用SEM和EDS研究带划痕试样经盐雾腐蚀后划痕表面的组织形貌和化学成分,初步探讨了改进磷化复合膜的自修复性。结果表明:3种改进磷化复合膜的耐蚀性优于铬酸盐钝化膜,具有自修复作用,有望成为铬酸盐钝化膜的替代品;硅酸钠溶胶封闭后处理改进磷化复合膜的耐蚀性最优。     

ELECTRO-DEIONIZATION OF Cr (VI)-CONTAINING SOLUTION. PART II: CHROMIUM TRANSPORT THROUGH INORGANIC ION-EXCHANGER AND COMPOSITE CERAMIC MEMBRANE

Cr (VI) transport through a composite ceramic membrane containing an ion-exchange component, namely xerogel of hydrated zirconium dioxide, was investigated. The diffusion coefficient of Cr (VI) species through the membrane, which has been determined under open circuit conditions, is 1.80 × 10^?10 m² s^?1. The transport number of Cr (VI) species through the ceramic membrane was found to rise with increasing voltage and reached 0.17 under “over-limiting current” conditions. On the other hand, the transport of chromate ions through hydrogel of hydrated zirconium dioxide becomes more intensive with a decrease in potential drop through the system involving ion-exchanger bed and ceramic membrane due to decrease in the membrane resistance. The diffusion coefficient of Cr (VI) ions in hydrogel of the inorganic ion exchanger was estimated as 4.36 × 10^?12 m² s^?1. A possibility of Cr (VI) removal from a weakly acidic diluted solution using an electro-deionization method was shown: the degree of solution purification was found to reach 50%. The transport of species is realized through both the solution and the ion exchanger.     

Cr(III) oxidation with lead dioxide-based anodes

A procedure for preparing PbO₂-based electrodes with a titanium substrate is proposed. A platinum underlayer is first deposited on Ti by metal organic chemical vapor deposition (MOCVD), followed by the electrodeposition of the PbO₂ layer. The prepared Ti/Pt/PbO₂ anodes were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD) before being used for oxidation of Cr(III) in sulphuric acid. The current efficiency was determined for that electrodes and the results were compared with those obtained with Pb/PbO₂ and Ebonex^®/PbO₂ electrodes with different pH conditions. The Ti/Pt/PbO₂ were found to have a very good electrochemical behaviour (current efficiency: φ=0.93 for pH 2), and may be used as dimensionally stable anodes for the oxidation of Cr(III).     

The use of macroscopic modelling of intermetallic phases in aluminium alloys in the study of ferricyanide accelerated chromate conversion coatings

Intermetallic (IM) second phases of FeAl₃, Cu₂FeAl₇, and CuAl₂, were coupled to aluminium for the macroscopic study of the deposition of chromate conversion coatings. Characterisation of the coating deposition was done using X-ray photoelectron spectroscopy, Rutherford backscattering spectroscopy, Auger electron spectroscopy, scanning electron microscopy with X-ray analysis, and Fourier transform infrared spectroscopy. The coatings covering the IM phases were only one-tenth the thickness of the matrix, and contained higher levels of F, Al, and O. Cr, O, Fe, and N, indicative of a chromate conversion coating, were detected over the matrix. Over IM phases, decomposition of [Fe(CN)₆]3−/4−, and fluoride ion attack were found to be responsible for reduced rates of deposition.     

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.     

Corrosion protection performance of novel hybrid polyaniline/sol–gel coatings on an aluminium 2024 alloy in neutral, alkaline and acidic solutions

In this study the corrosion performance of a novel coating, consisting of a combination of silica sol–gel and conductive polymer, in 3.5% NaCl at different pH levels is presented. Electrochemical Impedance Spectroscopy showed the impedance of the coating remained stable for up to 24 months in neutral 3.5% NaCl solution. No corrosion or delamination was observed after 500 h in salt spray tests and Scanning Vibrating Electrode Technique tests showed the coating exhibited a “self healing” property. The surface morphology was characterised by Scanning Electron Microscopy and the mechanical properties, notably adhesion, were studied using pull off and cross cut techniques.     

Effect of Ni ion doping on the physical characteristics and chromate adsorption behavior of goethite

Pure and 0.25% Ni²⁺ ion doped goethite samples, prepared by the coprecipitation method, were characterized for pH of Point of Zero Charge (pH_pzc), Surface area, XRD, TG-DTA, FTIR and TEM. Effect of Ni²⁺ ions doping on the adsorption of chromate on goethite was investigated at pH-3, -5 and -7 in the temperature range 303-323 K. It was found that adsorption of chromate increases with decreasing pH and increasing temperature on both pure and Ni²⁺-doped goethite. Adsorption of chromate was found to increase from 61.4% to 82.92% with Ni²⁺ ion doping in goethite matrix at the lowest pH (pH-3) and highest temperature (323 K), using 1.02 mmol L⁻¹ chromate solution. Langmuir isotherm was found applicable to the experimental data. The values of (isosteric heat of adsorption) calculated for both the samples of goethite were positive, showing endothermic nature of the adsorption process.