J.H.W. de Wit

Formation of a cerium-based conversion coating on AA2024: relationship with the microstructure

The chemical conversion treatment for aluminium alloys based on the immersion in cerium chloride/hydrogen peroxide solutions is one of the possible alternatives to the chromate conversion process for the corrosion protection of aluminium alloys. The nucleation and growth of the cerium-based conversion coating on AA2024 was studied using complementary surface analysis techniques, as atomic force microscopy in the Kelvin probe mode (SKPFM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The influence of both the intermetallic particles and the copper in solid solution was considered. According to this aim, different surface preparation procedures prior to the conversion process were investigated. Acid pickling and Ce-based deoxidising, as well, were found to cause the formation of copper or copper oxide deposits, i.e. copper smut, on the AA2024 surface. While the intermetallics do not act as preferential nucleation sites, the deposition of copper on the surface is a necessary condition in order to form a thick cerium oxide film. On the other hand, the copper smut strongly decreases the adherence of the conversion layer to the aluminium substrate.

The determination of coating performance with impedance measurements. II: Water uptake of coatings

Abstract Corrosion prevention by organic coatings is not only obtained by resistance inhibition. The transport of water and corrosive species is also important. A low permeability of water gives no guarantee for an optimal anti-corrosion performance, as this is likely to cause blistering when osmotic pressures occur due to surface contamination etc. A certain permeability is necessary to prevent blistering. Electrochemical Impedance Spectroscopy (EIS) can provide useful information in this field. This article is part two in a series. The water uptake of coatings is discussed and it is shown that the water taken up in the coating affects the coating polymer and results in swelling, which is reflected by a change of the dielectrical properties. This phenomenon shows the necessity of using a novel method of calculating the water uptake from the results of impedance measurements. For the study of the water uptake of the epoxy coatings, the constant phase element (CPE) was found to be a valuable component in the equivalent circuit used for EIS. In part I [ van Westing et al., Corros. Sci.34, 1511 (1993).], the values of the CPE parameters Y0 and n were related to the properties of the network of the coating polymer, here Y0 and n are related to the amount of water taken up by the coating and the interaction (e.g. swelling) of the coating polymer.

Relation between microstructural aspects of AA2024 and its corrosion behaviour investigated using AFM scanning potential technique

The microstructure of the aluminium alloys has a large influence on the corrosion behaviour. In particular, the intermetallic particles play an important role in the corrosion resistance of aluminium alloys and they can give rise to localized corrosion, such as pitting and exfoliation, because of the formation of galvanic cells. The shape, size and chemical composition of the intermetallic particles are determined by the processing route (heat treatment and forming) carried out on the aluminium alloy. In recent work in our lab, it was found [9] that when the 2024 aluminium alloy undergoes a long quench delay time (time spent in air after the homogenisation treatment) shell-shaped particles precipitate. These are characterized by differences in chemical composition between the core and the surface. Simultaneously, the corrosion mechanism of the AA2024 changes from general to localized. Therefore, it was thought that the shell-shaped particles increase the pitting susceptibility of the 2024 aluminium alloy. In the present work, the surface potential of these particles was measured by means of atomic force microscopy (AFM) scanning potential technique. The shell-shaped particles show a large difference in potential between the core and the shell, which results in a highly localized galvanic coupling. Therefore, the formation of shell-shaped particles largely lowers the resistance to pitting attack of the AA2024.

The determination of coating performance with impedance measurements-I. Coating polymer properties

Abstract Modern coatings with new binders and pigmentation require new methods to optimize the formulation with respect to the protection of the metal substrate against corrosion. The methods normally used for accelerated weathering do not give information about the mechanism of failure of the coatings. Mechanistic information is necessary for a fast and more specific development of new coatings for various applications. Electrochemical impedance spectroscopy (EIS) can provide information. This article is part one in a series. The coating polymer properties and coatings performance are discussed. Moreover the experimental results are validated with respect to experimental errors. Models from the literature have also been included in the discussion. For the study of the curing mechanism and mechanical properties of the epoxy coatings the constant phase element (CPE) was found to be a valuable component in the equivalent circuit used for EIS. The values of the CPE parameters Y 11 and n are related to properties of the network of the coating polymer.

Electrochemical characterisation of aluminium AA7075-T6 and solution heat treated AA7075 using a micro-capillary cell

Localised corrosion of 7xxx aluminium alloys initiates at cathodic intermetallics containing Cu and Fe due to a strong galvanic coupling with the matrix. In order to study this galvanic coupling, the electrochemical behaviour of AA7075-T6 and solution heat treated AA7075 has been investigated by means of complementary techniques: micro-capillary cell, scanning Kelvin probe force microscopy (SKPFM) and scanning electron microscopy (SEM). Characterisation with the micro-capillary cell showed that the intermetallics cause a more cathodic breakdown potential in the solution heat treated AA7075 compared with the AA7075-T6. This is associated with a higher Volta potential difference between the intermetallics and the matrix in the solution heat treated AA7075, indicating a stronger galvanic coupling for this temper. From these results, it is concluded that the breakdown potential of areas containing the intermetallics is related to the Volta potential difference between the intermetallics and the matrix.

A Comparative Study of NiO ( Li ) , LiFeO2, and LiCoO2 Porous Cathodes for Molten Carbonate Fuel Cells

Porous cathodes of NiO(Li), Co-doped LiFeO[sub 2], and LiCoO[sub 2] for the molten carbonate fuel cell (MCFC) were examined in a comparative study using electrochemical impedance spectroscopy at temperature of 923, 973, and 1023 K. Using this technique the contributions of charge transfer and diffusion to the impedance could be separated. The impedance results as a function of gas composition were compared with theoretical predictions using the thin-film model leading to the conclusion that the most predominant diffusing species in porous MCFC electrodes are molecular oxygen and carbon dioxide. The reaction mechanism is probably the same for all three cathodes involving either the reduction of peroxy-carbonate or the reduction of dissociated oxygen. The remaining difference in gas dependencies can then be explained by assuming a low coverage of oxide ions on LiFeO[sub 2] while NiO(Li) and LiCoO[sub 2] have intermediate coverage by oxide ions. From the temperature dependence of the impedance an estimate may be given of the activation energies of the polarization processes.

Effect of solution heat treatment on galvanic coupling between intermetallics and matrix in AA7075-T6

Abstract Susceptibility to localised corrosion is strongly affected by heat treatments performed on Al–Zn–Cu–Mg alloys. In order to study how galvanic coupling between intermetallics and matrix is affected by solution heat treatment, AA7075-T6 and solution heat treated AA7075 have been characterised by means of scanning electron microscopy and scanning Kelvin probe force microscopy. Solution heat treatment strongly increased the Volta potential difference between the intermetallics and the surrounding matrix showing a strong increase in galvanic coupling. This is explained by Zn and Mg enrichment of the matrix caused by dissolution of strengthening particles during solution heat treatment.

Influence of surface preparation on performance of chromate conversion coatings on Alclad 2024 aluminium alloy. part II: EIS investigation

Chromate conversion layers, formed on Alclad 2024-T3 aluminium alloy after different surface preparation procedures, were investigated by means of EIS measurements performed in aggressive and middle aggressive electrolytes. The morphology of the chromate layer varies depending on the type of solution and on the time of duration due to deposition of corrosion products inside the defects and deterioration of the layer. Therefore, three slightly different equivalent circuits have been used to model the chromate film in the two electrolytes and for different times of exposure. The proposed equivalent circuits enable the study of the effect of variation in the aluminium surface microstructure, caused by different surface pretreatment, on the performance of the deposited chromate conversion coating. The removal of galvanic couples, mainly caused by precipitates, from the aluminium surface appears to be a necessary condition for the formation of a chromate layer with good corrosion resistance. Besides, the properties of the surface of the aluminium substrate largely influence the morphology and composition not only of the chromate film but also of the corrosion products formed at weak locations, which seem to play an important role in the protection provided by the chromate conversion coating.

Electrochemical impedance spectroscopy as a tool to obtain mechanistic information on the passive behaviour of aluminium

Electrochemical Impedance Spectroscopy can be used very effectively for studies regarding the passivity of metals. In general there are two different methods to interpret measured impedance data. The first method uses electrical equivalent circuits. Alternatively a transfer function can be calculated, ie a theoretical impedance expression based on an assumed physical model. The latter method is a more fundamental approach but often corroding systems including passive alloys are too complicated to arrive at useful transfer functions. In that case the impedance data are in general analysed according to the first method. An equivalent circuit is fitted to the measurement data. For some cases the transfer function can be derived and analysed, when accepting some approximations. As an example the passive behavior of aluminium will be discussed.

An18O tracer study on the growth mechanism of alumina scales on NiAl and NiAlY alloys

The effect of Y additions on the oxidation mechanism of NiAl at 1270 K has been investigated. Mass transport in the alumina scale was examined with18O tracers. Proton activation analysis (18O(p, α)15N) and SIMS were used to measure the18O distribution in the scale. On pure NiAl and low-doped (0.07% Y) NiAl mainly outward scale growth was observed. Addition of 0.5% Y induces oxide formation at both interfaces of the scale. Larger quantities of Y added to the alloy (>0.5%) do not dissolve completely in the NiAl, but form Y-rich segregates. Internal oxidation of these intergranular segregates was observed. The relation between the modification of the scale-growth mechanism and the improved adherence of the scale to the alloy due to the addition of Y is discussed.