Effect of curing on the corrosion protective properties of thin organosilane films on aluminium

Film properties and corrosion protection given by thin organosilane films (bis-1,2-(triethoxysilyl)ethane: BTSE) on aluminium were investigated as a function of curing. The thickness was determined using spectroscopic ellipsometry (SE). Impedance spectra, modelled by an electrical equivalent circuit, show three time constants. The variation of the elements in these three time constants as a function of the immersion time in a NaCl solution indicate the structure of the film after curing. It can be concluded that the barrier properties of the organosilane films are influenced more by the curing temperature than by the curing time.     

Silane solution stability and film morphology of water-based bis-1,2-(triethoxysilyl)ethane for thin-film deposition on aluminium

Thin-film silane coating (<1 μm) has been introduced as a chromium free multi-metal surface pre-treatment for corrosion protection, adhesion promotion and surface passivation of metals such as aluminium, steel, zinc, magnesium and others. Bis-1,2-(triethoxysilyl)ethane (BTSE) has received much attention as it is, after hydrolysis, highly reactive towards (covalent) metal/film bonding and cross-link formation for the creation of barrier properties. Much of the past work on BTSE was performed on methanol-based laboratory solutions due to the low solubility of BTSE in water. For industrial applications these solutions are not considered suitable anymore because of the high process cost as well as ecological and health issues associated with methanol and the high monomer content of such solutions. For industrial practice water-based silane solutions are being considered. In the present work water-based BTSE solution is compared to a reference methanol-based solution. The silane solution is analysed using ²⁹Si NMR spectroscopy, and the deposited silane films are morphologically characterised using infra-red spectroscopic ellipsometry and field-emission gun-scanning electron microscopy.     

Synthesis and characterisation of surface gradient thin conversion films on zinc coated steel

Surface gradient layers on hot-dip galvanised steel were synthesised in order to determine the barrier properties and corrosion resistance of thin amorphous conversion coatings as a function of layer thickness and processing time. For this purpose, a dip coating procedure was established that yields well-defined gradient layers. As a model system for conversion film formation on zinc coated steel, a zirconium based bath chemistry was used. The synthesised zirconium oxyhydroxide gradient films were characterised by localised electrochemical techniques, such as Scanning Kelvin Probe (SKP) and electrochemical impedance spectroscopy using an electrochemical capillary cell. Microscopic infrared reflection absorption spectroscopy (μ-FT-IRRAS) measurements and small-spot X-ray photo electron spectroscopy (XPS) were used as complementary surface analytical techniques. The applied analysing techniques provide a spatial resolution of 100-1600 μm. Thereby, a complete variation of thin film properties, such as thickness, barrier properties, corrosion resistance and chemical composition can be measured as function of the time of film growth on a sample with a length of a few centimetres. This approach allows a precise and accurate determination of structure-to-property relationships of thin conversion films. Moreover, it could be shown that a surface gradient film analysis significantly rationalises experimental time and increases the reliability of the experimental results.     

Bonding and corrosion protection mechanisms of γ-APS and BTSE silane films on aluminum substrates

Films of γ-aminopropyltriethoxysilane (γ-APS), 1,2-bis[triethoxysilyl] ethane (BTSE) and their mixtures adsorbed onto pure aluminum from aqueous solutions were characterized by means of ellipsometry, infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS). It was found that after hydrolysis in water the silanes were readily adsorbed onto aluminum oxide surfaces initially forming hydrogen bonds. Upon curing, such bonds are replaced by metallosiloxane bonds, Si - O - Al. The remaining silanol groups in the film condense and form Si - O - Si bonds. As the Si - O - Al bonds are known to hydrolyze, the corrosion protection is related to the hydrophobicity of the siloxane films formed on the metal substrate. BTSE films are acidic as they contain free silanol groups, therefore these are compatible with some paints but not with others. Electrochemical impedance spectroscopy (EIS) results, salt spray test results and filiform corrosion test results showed that some silane treatments, such as two-step γ-APS/BTSE and BTSE only, provided better corrosion protection on aluminum substrates as compared with a chromate treatment. Mechanisms of adhesion and corrosion protection of these silane films on aluminum substrates are proposed.     

A comparative study on the corrosion resistance of AA2024-T3 substrates pre-treated with different silane solutions: Composition of the films formed

This work reports a comparative study on the corrosion resistance of AA2024-T3 pre-treated with three different silane solutions. The silanes used for the pre-treatments of the AA2024-T3 panels were: 1,2-bis(triethoxysilyl)ethane (BTSE), bis-[triethoxysilylpropyl]tetrasulfide (BTESPT) and γ-mercaptopropyltrimethoxysilane (γ-MPS). The analytical characterisation of the silane films was performed by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The corrosion performance of the pre-treated substrates was evaluated by electrochemical impedance spectroscopy (EIS). The results show that the pre-treatments based on silanes provide good corrosion protection of unpainted AA2024-T3. Painted substrates, previously pre-treated with the silane solutions also revealed improved corrosion resistance and good adhesion properties. Fatigue tests show that the silane pre-treatments do not affect the fatigue behaviour of the AA2024-T3. The work also discusses the formation of the protective silane films.     

Corrosion of pure magnesium under thin electrolyte layers

The corrosion behavior of pure magnesium was investigated by means of cathodic polarization curve, electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) under aerated and deaerated thin electrolyte layers (TEL) with various thicknesses. Based on shot noise theory and stochastic theory, the EN results were quantitatively analyzed by using the Weibull and Gumbel distribution function, respectively. The results show that the cathodic process of pure magnesium under thin electrolyte layer was dominated by hydrogen reduction. With the decreasing of thin electrolyte layer thickness, cathodic process was retarded slightly while the anodic process was inhibited significantly, which indicated that both the cathodic and anodic process were inhibited in the presence of oxygen. The absence of oxygen decreased the corrosion resistance of pure magnesium in case of thin electrolyte layer. The corrosion was more localized under thin electrolyte layer than that in bulk solution. The results also demonstrate that there exist two kinds of effects for thin electrolyte layer on the corrosion behavior of pure magnesium: (1) the rate of pit initiation was evidently retarded compared to that in bulk solution; (2) the probability of pit growth oppositely increased. The corrosion model of pure magnesium under thin electrolyte layer was suggested in the paper.     

Electronic properties of thermally formed thin iron oxide films

The oxide layer, present between an organic coating and the substrate, guarantees adhesion of the coating and plays a determinating role in the delamination rate of the organic coating. The purpose of this study is to compare the resistive and semiconducting properties of thermal oxides formed on steel in two different atmospheres at 250 °C: an oxygen rich atmosphere, air, and an oxygen deficient atmosphere, N₂. In N₂, a magnetite layer grows while in air a duplex oxide film forms composed by an inner magnetite layer and a thin outer hematite scale. The heat treatment for different amounts of time at high temperature was used as method to sample the thickness variation and change in electronic and semiconducting properties of the thermal oxide layers. Firstly, linear voltammetric measurements were performed to have a first insight in the electrochemical behavior of the thermal oxides in a borate buffer solution. Electrochemical impedance spectroscopy in the same buffer combined with the Mott-Schottky analysis were used to determine the semiconducting properties of the thermal oxides. By spectroscopic ellipsometry (SE) and atomic force microscopy (AFM), respectively, the thickness and roughness of the oxide layers were determined supporting the physical interpretation of the voltammetric and EIS data. These measurements clearly showed that oxide layers with different constitution, oxide resistance, flatband potential and doping concentration can be grown by changing the atmosphere.     

In situ spectroscopic ellipsometric study of porous alumina film dissolution

Porous alumina films have been synthesised by anodisation of high purity aluminium in 0.4 M phosphoric acid at constant current density. The dissolution process of those coatings in 2 M phosphoric acid has been investigated by in situ spectroscopic ellipsometry (SE) combined with electrochemical measurements. A sharp drop of the open current potential is observed due to film removal. The dissolution time depends on total barrier layer thickness. Those results are confirmed by SE measurements. A three layer optical model has been used to interpret SE spectra. During the dissolution process, the porous top layer has a quasi-constant thickness, whereas porosity increases significantly. The dissolution proceeds within the pores. Only the thickness of pure alumina barrier layer (upper part) decreases during the dissolution process whereas the interface barrier layer (lower part) does not change.     

Electrochemical impedance spectroscopic investigation of the role of alkaline pre-treatment in corrosion resistance of a silane coating on magnesium alloy, ZE41

The protective performance of the coatings of bis-1,2-(triethoxysilyl) ethane (BTSE) on ZE41 magnesium alloy with different surface pre-treatments were evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 0.1 M sodium chloride solution. Electrical equivalent circuits were developed based upon hypothetical corrosion mechanisms and simulated to correspond to the experimental data. The morphology and cross section of the alloy subjected to different pre-treatments and coatings were characterized using scanning electron microscope. A specific alkaline pre-treatment of the substrate prior to the coating has been found to improve the corrosion resistance of the alloy.     

Investigation of the chromate conversion coating on Alclad 2024 aluminium alloy: effect of the pH of the chromate bath

The parameters of the chromate bath, like temperature, pH, and fluoride content, strongly affect the morphology and chemical composition of the chromate conversion coating and as a consequence have a large influence on its corrosion performance. In this paper, electrochemical impedance spectroscopy (EIS) was used in combination with other techniques to investigate the role played by the pH of the chromate bath on the properties of the chromate film formed on Alclad 2024 aluminium alloy. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and spectroscopic ellipsometry (SE) have shown the formation of a thicker and less dense chromate layer when the pH of the chromate bath is changed from 2.4 to 1.2. The analysis of the EIS spectra have highlighted that this change in pH leads to the formation of more protective and more resistant chromate corrosion products (CCP) inside the defects of the chromate film. When a thin, dense and protective layer of CCP is formed in the defects, the corrosion behaviour of the chromate conversion coating improves for two main reason: (a) further attack of the defects is avoided or delayed; (b) the change in volume caused by the formation of the CCP is limited resulting in a low level of stress in the film, which as a consequence is not detached from the aluminium substrate.     

In situ ellipsometry studies on swelling of thin polymer films: A review

The properties of a thin polymer film can be significantly affected by the presence of a penetrant. This can have potential implications for many technological applications, such as protective and functional coatings, sensors, microelectronics, surface modification and membrane separations. In situ ellipsometry is a powerful technique for the characterization of a film in contact with a penetrant. The main advantages of ellipsometry include the very high precision and accuracy of this technique, combined with the fact that it is non-intrusive. Recent advances in the speed and automation of the technique have further expanded its application.This article provides an overview of the research that has been done with in situ UV–vis ellipsometry on penetrant-exposed polymeric films, in the last 15–20 years. The focus is predominantly on films that are not attached covalently to a substrate. Polymer brushes and grafts are therefore excluded. This review addresses a variety of topics, covering instrumental aspects of in situ studies, approaches to data analysis and optical models, reported precision and repeatability, the polymer-penetrant systems that have been studied, the kind of information that has been extracted, and other in situ techniques that have been combined with ellipsometry. Various examples are presented to illustrate different practical approaches, the consequences of the optical properties of the ambient, and the various ways that have been employed to bring polymer films in contact with a penetrant, ranging from simple ex situ-like configurations (i.e., drying studies) to complex high pressure cells. The versatility of in situ ellipsometry is demonstrated by examples of the distinctive phenomena studied, such as film dilation, penetrant diffusion mechanisms, film degradation, electrochemical processes, and the broad variety of polymer-penetrant systems studied (glassy and rubbery polymers, multilayer stacks, etc.). An outlook is given on possible future trends.     

Effects of electrodeposition potential on the corrosion properties of bis-1,2-[triethoxysilyl] ethane films on aluminum alloy

Bis-1,2-[triethoxysilyl] ethane (BTSE) films were prepared on 2024-T3 alloys by using potentiostatic method for corrosion protection. This work mainly investigated the effects of electrodeposition potential on the corrosion properties of silane films. Films prepared at cathodic potentials display an improvement in corrosion inhibition properties, while those prepared at anodic potentials present the deterioration of protectiveness. In the case of cathodic deposition, when the potential shifts negatively from the open-circuit potential (OCP), corrosion protection of the obtained films initially increases and then decreases, with the optimal deposition potential at −0.8 V/SCE. As indicated in scanning electron microscopy (SEM) images, films deposited at the optimum potential present the most uniform and compact morphologies. In addition, steady-state polarization and current-time curves have been also recorded on Al alloys in BTSE solutions during the deposition, respectively.     

Comparison between wet deposition and plasma deposition of silane coatings on aluminium

Silane coatings are applied to metal surfaces for various purposes, e.g., to form a protective layer against corrosion or to act as a primer for subsequent coating. In this work bis-1,2-(triethoxysilyl) ethane (BTSE) was used as a precursor to deposit coatings on Al 99.99% substrates with three different techniques: dipcoating (water based solution), vacuum plasma and atmospheric plasma. Infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS) and field emission gun-scanning electron microscope (FE-SEM) were used to characterise the structure, composition and surface morphology of the silane coatings. The aim of this investigation is to compare the surface and bulk characteristics of the films prepared with the three different methods, in order to get information on how the BTSE molecule is modified by the deposition technique.     

Influence of laser surface hardening on the corrosion resistance of martensitic stainless steel

Martensitic high nitrogen stainless steels offer a combination of wear-, corrosion- and fatigue properties. But for some applications a higher surface hardness is required. A laser hardening with rapid heating (without smelting) and cooling (quenching) rates can improve the surface hardness with compressive residual stresses in the near surface layer. Yet, some cases of pitting corrosion in chloride media are reported.In this study, the influence of process parameters, composition of the atmosphere and the overlapping ratio, has been investigated. With complementary surface analytical methods and electrochemical techniques the relation between surface structure and composition and corrosion behavior in chloride media has been studied.It has been shown that, during the laser treatment the surface must be shielded with argon in order to avoid the formation of a porous layer of iron oxides, which is dramatically detrimental to the corrosion resistance.After the laser treatment a mixture of martensite and retained austenite is obtained, depending on the surface temperature and overlapping ratio. With a surface temperature of 1200 °C and a minimal overlapping ratio (10%), a thin surface layer of retained austenite, wherein nitrides are dissolved, improves the corrosion resistance. The hardness increases with the amount of distorted martensite and reaches a maximum at 1000 °C.     

Recent advances in some optical experimental methods

Light has been used as a non-destructive probe for the the in situ investigation of processes at electrodes. The determination of concentration profiles near electrodes by optical interferometry has been improved by considering the bending of light inthe boundary layer and its reflection from the electrode edge. Use of the Doppler shift of scattered laser light has been investigated for the measurement of convection velocities in boundary layers. The developmentof a fast and sensitive automatic ellipsometer has made it possible to follow the growth of surface layers at sizeable current densities. Consideration of the optical effect of mass-transport boundary layers on the observation of surface layers by ellipsometry has resulted in the novel use of ellipsometry for the determination of interfacial concentration at electrodes. A colorimetric analysis of interference colors, produced by thin films on electrodes, has shown that optimum angles of incidence exist for the observation of interference colors with polarized light. Interference color series have been derived theoretically which take phase change and amplitude attenuation in reflection into account.     

Characterization of porous aluminium oxide films from a.c. impedance measurements

An equivalent circuit (EC) that reproduces the a.c. impedance of porous aluminium oxide films in a highly approximate manner is proposed. The results reveal that electrochemical impedance spectroscopy (EIS) is a powerful tool for obtaining detailed information on the electrochemical properties of both the porous and barrier layer on which the corrosion resistance of aluminium depends. The impedance at a given frequency can be used for accurate calculation of the electrochemical parameter for the oxide film represented by each element of the EC. In this way, the effects of any factor on sealing and ageing of anodized aluminium oxide films can be precisely analysed. The EIS technique provides an effective, advantageous alternative to existing seal quality control tests.     

The effects of curing temperature on bilayer and monolayer hybrid films: mechanical and electrochemical properties

Thermal curing contributes to the formation and performance of hybrid films (HFs), because it enhances barrier properties by reticular densification, which originates from a less porous layer, and improves corrosion protection and mechanical properties. Increasing the number of deposited layers further enhances these properties. In this work, a HF formed by an alkoxide precursor solution of 3-(trimethoxysilylpropyl) methacrylate and tetraethoxysilane with cerium nitrate and polyethylene glycol was applied to the galvanized steel. The films were obtained by either monolayer or bilayer dip coating, and they were cured at different temperatures (60 and 90 °C) for 20 min. The results indicate that both the temperature and the number of layers interfere on the ability of HFs to act as effective barriers against corrosion. The bilayer system cured at 60 °C showed the best electrochemical impedance results; however, the monolayer HF cured at 90 °C presented higher wear resistance.     

Corrosion behavior of amorphous carbon deposit in 0.89% NaCl by electrochemical impedance spectroscopy

Amorphous carbon films were deposited on SS316L substrates using a DC magnetron sputtering system, aiming at the application of the coated SS316L for biomedical implants. The biocompatibility and chemical stability of the carbon layers have been previously demonstrated. The films were deposited on top of sputtered titanium coatings introduced as a buffer layer to enhance film-substrate adhesion. The corrosion resistance of the a-C/Ti/SS316L systems was investigated by electrochemical techniques. The electrolyte used in this work was 0.89 wt.% NaCl at pH 7.4, which simulates body fluid ionic concentrations. The coated samples displayed corrosion resistance values in the saline solution much higher than the stainless steel substrates and the role of the Ti coating thickness was analysed in order to determine the optimal system for biological applications.     

Sequentially adsorbed electrostatic multilayers of polyaniline and azo polyelectrolytes

In this study, multilayer films composed of alternate polyaniline (PANI) and azo-polyelectrolyte (PNACN, PPAPE, PNANT or PNATZ) layers were fabricated through the electrostatic layer-by-layer self-assembly scheme. In the electrostatic adsorption process, PANI was used as the polycation and the azo polyelectrolytes were used as the polyanion. The multilayer growth was monitored by UV-vis spectroscopy and optical ellipsometry. The photoresponsive and electrochromic properties of the multilayer films were studied by UV-vis spectroscopy and electrochemical measurement. Results show that the multilayer films exhibit linear increases in both the absorbance and film thickness with the increase of the deposition cycles. The thickness contributed by each individual layer depends on the pH of the PANI and azo-polyelectrolyte dipping solutions. The multilayer films can show photoinduced dichroic properties contributed by the azo-polyelectrolyte layers, but the ability to form surface-relief-grating (SRG) upon Ar⁺ laser irradiation is relatively weak. The multilayer films possess the characteristic absorption bands related to the azo chromophores and PANI. As the location of the PANI bands depends on its oxidation states, the multilayer films can show sensitive electrochromic variation. For instance, the PANI/PNACN multilayer films can undergo a transition from transparent yellow-green through deep green to opaque black when the potential changes from −0.1 to 0.8 V. It is demonstrated that using the azo polyelectrolytes with different hues, enriched spectrum of the colors can be obtained by the electrochemical transitions. The multilayer films containing both photoresponsive and electroactive components can be expected for applications in optics, photonic devices and others.     

Ion implantation induced phase transformation in carbon and boron nitride thin films

The mechanism behind energetic ion impact induced stress reduction in highly stressed tetrahedral amorphous carbon and cubic boron nitride thin films is investigated by real time in situ spectroscopic ellipsometry and ex situ electron microscopy. Highly stressed carbon and boron nitride films were grown by filtered cathodic vacuum arc and RF magnetron sputtering, respectively. The films were then implanted by 5–10 keV argon ions and the film optical properties and thickness monitored in situ by spectroscopic ellipsometry. In both cases the films were observed to expand due to a reduction in the density of the ion-modified layer. Cross-sectional transmission electron microscopy and electron energy loss spectroscopy of the carbon films showed that this reduction in density is associated with a conversion of diamond-like bonding to graphite-like bonding. In situ stress measurements performed on the boron nitride films revealed a simultaneous reduction in stress with expansion of the material.