H. Terryn

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.

Effect of bath concentration and curing time on the structure of non-functional thin organosilane layers on aluminium

The corrosion resistance of aluminium alloys can be improved by different surface treatments such as painting. A pre-treatment based on chromate is the current method used to increase the corrosion resistance and the adhesion of the organic layer. Silane films seem to be an interesting alternative system to replace the toxic chromates. In this paper, the characterisation of bis-1,2-(triethoxysilyl)ethane (BTSE) thin layers has been evaluated by coupling optical techniques like spectroscopic ellipsometry (SE) and infra-red spectroscopic ellipsometry (IRSE) along with electrochemical methods (electrochemical impedance spectroscopy (EIS)). This approach has been chosen to have a better understanding of the protection provided by these organosilane thin films. It will be demonstrated that the BTSE bath concentration modifies the thickness of the layers and that the curing of this thin film can also improve the barrier properties by forming a denser layer.

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.

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.

Determination of the thickness of thin silane films on aluminium surfaces by means of spectroscopic ellipsometry

Abstract The thickness of thin films of non-functional silane bis-1,2-(triethoxysilyl)ethane (H 5 C 2 O) 3 Si-CH 2 CH 2 -Si(OC 2 H 5 ) 3 (BTSE) deposited on aluminium surfaces is investigated using spectroscopic ellipsometry (250–1700 nm). The data processing of the ellipsometry spectra is carried out by means of simulation and regression techniques. New advances in data processing, e.g. multiple sample analysis and determination of thickness non-uniformity, are applied to characterise these thin polymer films realistically. The influence of the concentration of the BTSE solution and the curing of the film is investigated. Optical thickness estimates are corroborated by independent auger electron spectroscopy and transmission electron microscopy analysis.

Composition and thickness of non-functional organosilane films coated on aluminium studied by means of infra-red spectroscopic ellipsometry

Surface treatments of metals, leading to the deposition of thin films, are generally performed in order to create or change certain surface properties such as corrosion resistance or adhesion. For the past few years, the use of organosilanes for aluminium surface treatments are more and more considered as they provide both corrosion protection and adhesion properties. It has been previously shown that the silane bath concentration strongly influences the thickness of the silane films deposited on aluminium substrates. A linear increase of the layer thickness with the silane bath concentration has been underlined. In this paper, the chemistry of the deposited silane films has been studied as a function of the silane bath concentration. For this, Infra red spectroscopic ellipsometry (IRSE) has been used. It is a method similar to reflection absorption infra red spectroscopy (RAIRS), which is a non-destructive infra red reflection technique, allowing chemical characterisation of thin films on metals. It will be shown that differences in the positions and strength of absorption bands are visible when the silane concentration of the solution is varied. Such changes can result in a difference in the film chemistry or in the film thickness. Since IRSE offers chemical information along with the thickness information, modifications occurring in an IRSE spectrum can be understood more easily. It will be demonstrated that the use of an optical model, describing the absorption bands and thickness of the films, permits the determination of the chemistry and the thickness of the silane films.

Study of the self-assembling of n-octylphosphonic acid layers on aluminum oxide.

The deposition of n-octylphosphonic acid on aluminum oxide was studied. The substrate was pretreated in order to achieve a root-mean-square roughness of <1 nm, a hydroxyl fraction of 30%, and a thickness of approximately 170 nm. It was proven using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) that, rather than a monolayer, an organic multilayer was formed. The growth mechanism was identified as a Stranski-Krastanov one. It was also shown that the use of AFM, probing the surface topography, is essential for a reliable quantification and interpretation of data obtained with XPS.

A Generalized Electrochemical Aggregative Growth Mechanism

The early stages of nanocrystal nucleation and growth are still an active field of research and remain unrevealed. In this work, by the combination of aberration-corrected transmission electron microscopy (TEM) and electrochemical characterization of the electrodeposition of different metals, we provide a complete reformulation of the Volmer-Weber 3D island growth mechanism, which has always been accepted to explain the early stages of metal electrodeposition and thin-film growth on low-energy substrates. We have developed a Generalized Electrochemical Aggregative Growth Mechanism which mimics the atomistic processes during the early stages of thin-film growth, by incorporating nanoclusters as building blocks. We discuss the influence of new processes such as nanocluster self-limiting growth, surface diffusion, aggregation, and coalescence on the growth mechanism and morphology of the resulting nanostructures. Self-limiting growth mechanisms hinder nanocluster growth and favor coalescence driven growth. The size of the primary nanoclusters is independent of the applied potential and deposition time. The balance between nucleation, nanocluster surface diffusion, and coalescence depends on the material and the overpotential, and influences strongly the morphology of the deposits. A small extent of coalescence leads to ultraporous dendritic structures, large surface coverage, and small particle size. Contrarily, full recrystallization leads to larger hemispherical monocrystalline islands and smaller particle density. The mechanism we propose represents a scientific breakthrough from the fundamental point of view and indicates that achieving the right balance between nucleation, self-limiting growth, cluster surface diffusion, and coalescence is essential and opens new, exciting possibilities to build up enhanced supported nanostructures using nanoclusters as building blocks.

Chromate Conversion Coating on Aluminum Alloys I. Formation Mechanism

The formation of chromate conversion coatings (CCCs) on commercially pure Al (AA1050) during immersion in a chromic/ hydrofluoric acid solution has been investigated. A film formation mechanism is proposed based on the combination of open circuit potential measurements and surface analysis techniques, such as Auger electron spectroscopy and atomic force microscopy. It is suggested that the conversion of the Al surface takes place in three distinct stages: activation of the surface, initiation of the film formation, and growth of the conversion layer. In addition to the classical approach, a sol-gel model was considered for the initiation and growth of the film. The effect of the free fluoride and chromic acid concentration was studied using a thermodynamic model whereas the influence of the Al oxide film was investigated by forming an anodic barrier layer prior to the conversion process. It was shown that the rate-determining step in the CCC formation is the activation of the Al surface. Therefore, the morphology and structure of the conversion layer is determined not only by the bath composition but also by the thickness of the Al oxide film.

IRSE study on effect of thermal curing on the chemistry and thickness of organosilane films coated on aluminium

Abstract This paper focuses on the use of infra red spectroscopic ellipsometry (IRSE) to characterise thin organosilane films deposited on aluminium. IRSE combines in one measurement the possibilities of traditional FT-IR methods (molecular information) with those of visible spectroscopic ellipsometry (SE) (morphological—film thickness and optical—refractive index/extinction coefficient information). In this study, this optical method is used to get relevant informations concerning the influence of a thermal curing on various characteristics of the silane films. It will be shown that, by using an appropriated optical model for the interpretation of the data, IRSE permits to determine quantitatively the curing effect on the film thickness, non-uniformity and chemistry. It will be also demonstrated that the results obtained from the IRSE analysis correlate well those obtained from SE and electrochemical impedance spectroscopy (EIS).