999 publications from this institution
Lately different incorporation methods for inhibitors into organic coatings are described in literature [1,2]. Direct addition has several well-known drawbacks (incompatibility with the coating, uncontrollable inhibitor leaching, etc.). Alternatively inhibitors are accommodated in containers like Layered Double Hydroxides (LDH). In an encapsulation like this, not only the inhibitor release can be controlled and restricted depending on the pH, but also higher concentrations of insoluble substances in coatings can be achieved. In this publication a comparison of different incorporation methods for corrosion inhibitors for hot dip galvanized steel into self-healing organic coatings will be presented. Firstly the self-healing ability of an Polyesterurethaneacrylate coating containing these inhibitor recipients will be shown. Secondly their leaching out process will be studied with advanced local electrochemical methods. Scanning Vibrating Electrode Technique (SVET) allows the measurement of the electric field in solution by means of a vibrating microelectrode. As a result potential or – through calibration – ionic current density mappings can be recorded. Scanning Ion-selective Electrode Technique (SIET) is a micro-potentiometric technique which allows the local determination of individual ion concentrations. Other ions are avoided through an ion-exchange liquid in the tip of the microelectrode. With Scanning Electrochemical Microscopy (SECM) the local concentration of one selected electro-active species is monitored. By means of a redox reaction cycle between the ultra-microelectrode tip and the surface the discrimination of areas of different substrate activity is possible. All these techniques enable one to follow the leaching out process in-situ and with high spatial resolution revealing information about its mechanism.
In this paper the relation between the activating step and the formation of the zinc phosphate layer is studied on aluminium substrates pretreated in different ways. For the first time direct proof of the presence of the titane phosphate nucleation agents on the metal surface was found. It is shown that during the activation titanium phosphate particles are adsorbed flat on the aluminium surface, having typical dimensions of approximately 7–8 nm in thickness and 0.1–0.3 μm in length. The aluminium surface condition, which was varied by the pretreatment (decreasing, etching and electropolishing) has an important influence on the amount of particles adsorbed on the aluminium surface. The titanium phosphate nuclei act as nucleation agents as the first generation of zinc phosphate crystals nucleate immediately on these centres. These small nucleated crystals grow laterally in a 2D direction leading to flat walled crystals contacting the aluminium substrate. The orientation and form of these zinc phosphate crystals is dominated by the form of the nucleation agent, resulting in an epitaxial growth mechanism. The differences in the surface condition after activation are reflected in the amount of zinc phosphate crystals which are initiated.
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. © 2004 The Electrochemical Society. All rights reserved.
Corrosion of reinforcement steel forms a huge problem worldwide. A possible remedy for this problem is the application of corrosion inhibitors. The purpose of this research is to evaluate different inhibitors for both types of corrosion that can occur in reinforced concrete: pitting corrosion (by chloride attack), and uniform corrosion (by a drop in pH). Potentiodynamic polarization measurements in realistic concrete pore solutions (CPS) revealed three critical situations — representing pitting and uniform corrosion, and the combination — in which the inhibitor needs to act. According to polarization experiments and micrographs Na 2 MoO 4 is the best performing inhibitor for the three cases. Our XPS measurements are in line with the proposed inhibitor mechanism by Vukasovich (1986), which is very complex and depends on whether the surface is active or passive to start with. In conclusion Na 2 MoO 4 is a possible — and environmental-friendly — candidate to prevent corrosion of reinforcement steel.
The AC electrograining of aluminium involves the application of an alternating voltage or current between aluminium electrodes in a suitable electrolyte, to produce uniformly roughened surfaces for lithography. The resultant morphology is dependent upon the electrical, solution and substrate conditions employed. Here the influence of substrate conditions is investigated. The investigation was focused on the influence of manganese, manganese being the element which distinguishes the commercially two most commonly employed aluminium alloys for the manufacture of lithographic plates. The influence of manganese present in solid solution or in intermetallic particles was investigated for AC electrolytic graining in both hydrochloric and nitric acid. The degree of uniformity of the final morphology obtained by treatment in both hydrochloric and nitric acid increases with decreasing amount of manganese present in solid solution. For AC electrograining in hydrochloric acid the influence of manganese on the graining behaviour only becomes evident in the second stage of treatment, where hemispherical pits are created. During the etch attack, compositional or metallurgical features have only a minor influence. The influence of manganese on AC electrograining of aluminium in nitric acid is evident from the very first stage of treatment.
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The degradation of irradiated human insulin in aqueous solutions was investigated in order to protect the protein against ionizing radiation. The influence of the drug concentration, excipients and irradiation temperature were studied. Aqueous solutions at pH 2 were irradiated by gamma rays or by accelerated electrons. Two different high-performance liquid chromatography (HPLC) methods were used: reverse-phase high-performance liquid chromatography (RP-HPLC)/UV and size exclusion liquid chromatography (SEC/UV) to investigate both the fragmentation and the formation of higher molecular weight proteins. In solution without excipients irradiated at ambient temperature at 10kGy, the loss of human insulin is almost complete. Addition of radio-protecting excipients (free radicals scavengers) and cryo-irradiation allowed to decrease insulin degradation. The best radio-protector used was ascorbic acid in aqueous solution and oxidized glutathione in the frozen solutions. Only the combination of these two approaches (addition of scavenger and freezing) enables the irradiated human insulin in aqueous solution to meet the European Pharmacopoeia requirements for chemical potency (≥90%).
Atmospheric corrosion is a complex problem, essentially an electrochemical process under confined thin electrolyte layers. Predictions require better mechanistic models to understand the underlying fundamental subprocesses on a microscopic level. We developed a mechanistic multi-ion transport and reaction model (MITReM) that considers time-dependent accumulation of concentrations in thin (NaCl) confined electrolyte layers. The model predicts local concentration and electrolyte potential distributions, from which corrosion rates are derived. Simulations in the confined dynamic film show that the chloride accumulation causes a deviation from the reverse proportionality of the corrosion current with the film thickness below films of 10 μm. The oxygen distribution and local corrosion current densities demonstrate that the contribution of the edge effect decreases for thinner electrolyte layers. The influence of the electrode geometry on the current density also decreases with decreasing film thickness. These qualitative insights are essential for the development of atmospheric corrosion prediction tools and aid in the design and interpretation of electrochemical techniques for confined thin film electrolytes.
Abstract Corrosion of reinforcement steel forms a huge problem worldwide. A possible remedy for this problem is the application of corrosion inhibitors. The purpose of this research is to evaluate different inhibitors for both types of corrosion that can occur in reinforced concrete: pitting corrosion (by chloride attack), and uniform corrosion (by a drop in pH). Potentiodynamic polarization measurements in realistic concrete pore solutions (CPS) revealed three critical situations — representing pitting and uniform corrosion, and the combination — in which the inhibitor needs to act. According to polarization experiments and micrographs Na 2 MoO 4 is the best performing inhibitor for the three cases. Our XPS measurements are in line with the proposed inhibitor mechanism by Vukasovich (1986), which is very complex and depends on whether the surface is active or passive to start with. In conclusion Na 2 MoO 4 is a possible — and environmental-friendly — candidate to prevent corrosion of reinforcement steel.
The goal of this study is to determine the effect of the molecular structure of silane monomers on the corrosion protection properties of the formed silane layers. During the curing process, the silane monomers crosslink to form a molecular network, which acts as a barrier and retards the ingress of corrosive species. Non-cured and “fully” cured silane films with a comparable thickness were applied on cold rolled steel surfaces. The layer thickness and the chemical state of the films were measured with Spectroscopic Ellipsometry (SE). Electrochemical impedance spectroscopy (EIS) measurements and linear voltammetry clearly show that the number of silanol groups of the fully hydrolyzed silanes and the presence of an organo-functional group have a big influence on the barrier properties of silane layers and hence on their corrosion protection properties.
