Papers by Author: Mário G.S. Ferreira

Abstract: Pre-painted metal sheets (coil-coatings) are widely used in casings of household appliances, roofs and side walls in buildings and body parts for the automotive industry. One of the main problems of this type of material is the difficulty to weld it. In an attempt to overcome this problem weldable organic coatings are being experimented. Primers with heavy loads of zinc powder are being employed with success to spot-weld different components of an article. These primers may also present an anticorrosive effect through the galvanic protection offered by the zinc powder. However in aggressive conditions the zinc containing primers can corrode very fast and be consumed in a short period of time. Work has been undertaken in order to extend the lifetime of these primers but maintaining the welding and anticorrosive capabilities. The approach followed here was the modification of primers by corrosion inhibitors that can provide additional active protective effect. A set of organic and inorganic corrosion inhibitors was tested in order to partially passivate the primer. The inhibition of corrosion of zinc and iron was also investigated. Electrochemical testing was done with Electrochemical Impedance Spectroscopy (EIS) and the Scanning Vibrating Electrode Technique (SVET). Results showed that soluble inorganic salts of Ce(III) and La(III) and organic inhibitors like benzotriazole and mercaptobenzothiazole are good candidates to increase the service life of weldable primers.

Abstract: Aluminum alloys have found many applications in different branches of industry. In spite of the valuable properties, there is a significant drawback because of the strong corrosion susceptibility, especially in chloride-containing medium. The present work is focused on study of the 2024 aluminum alloy corrosion mechanism on early stages using Scanning Kelvin Probe Force Microscopy (SKPFM). The corrosion impact was studied measuring the Volta potential (VP) and topography of alloy matrix and S-phase intermetallics after immersion in different electrolytes and pH. It is shown that presence of the chloride anions in the electrolyte leads to increase of aluminum matrix potential for about 100 mV. This can be resulted from the adsorption of chloride ions and their incorporation into the native oxide layer changing semiconductive properties of the oxide. The zones surrounding the S-phase intermetallics are changed more significantly demonstrating higher increase of VP close to the inclusion. These regions are correlated with the increased oxygen content suggesting formation of thicker oxide layer due to local polarization. Addition of an inhibitor to electrolyte also leads to change in Volta potential that is reflected on lower corrosion impact of aggressive environment.

Abstract: Magnesium is one of the lightest metals and magnesium alloys have good strength to weight ratio making them very attractive for many particular applications [1]. The main drawback of magnesium alloys is their high corrosion susceptibility. Improving the corrosion protection by deposition of thin hybrid films can expand the areas of applications of relatively cheap magnesium alloys. This work aims at investigation of new anticorrosion coating systems for magnesium alloy AZ31B using hybrid sol-gel films. The sol-gels were prepared by copolymerization of 3- glycidoxypropyltrimethoxysilane (GPTMS), titanium alcoxides and special additives which provide corrosion protection of magnesium alloy. Different compositions of sol-gel systems show enhanced long-term corrosion protection of magnesium alloy. The sol-gel coatings exhibit excellent adhesion to the substrate and protect against the corrosion attack. Corrosion behavior of AZ31B substrates pre-treated with sol–gel derived hybrid coatings was tested by Electrochemical Impedance Spectroscopy (EIS). The morphology and the structure of sol-gel films under study were characterized with SEM/EDS techniques.

Abstract: Deposition of thin plasma polymer films as final layer on organic coatings can provide one promising solution to tailor the surface properties offering third functionality and increased mechanical properties. The present work is dedicated to the study of the barrier properties of polyurethane coil coatings modified by different plasma polymerization processes. Microwave (MW) and radio frequency (RF) plasmas were used to deposit thin films with different composition from various precursor mixtures. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) were used to reveal the evolution of the plasma treated coil coatings during weathering tests. Electrochemical impedance spectroscopy (EIS) was employed to study the barrier properties of the coatings. The results show a degradation effect of the plasma treatment on the barrier properties of the coil coatings especially in the case of the oxygen-containing plasmas. However addition of a fluorinecontaining component to the precursor mixture leads to the elimination of the negative effect of the plasma treatment on the barrier properties of the coil coatings. The fluorine-containing films exhibit higher weathering stability in comparison with the fluorine-free ones.

Abstract: Dense ceramic anodes of perovskite-type La1-x-ySrxCo1-zAlzO3-δ ( x = 0.45-0.70; y = 0- 0.05; z = 0-0.20) and K2NiF4-type La2Ni1-xMexO4+δ (Me = Co, Cu; x = 0-0.20), synthesized by the glycine-nitrate technique, were assessed for oxygen evolution in alkaline media. The lowest overpotentials are observed for (La0.3Sr0.7)0.97CoO3-δ, which exhibits a significant oxygen deficiency in combination with high conductivity associated with the A-site cation nonstoichiometry compensation mechanism via Co4+ formation. Perovskite-type cobaltite anodes are essentially stable in alkaline solutions, whilst La2NiO4-based electrodes exhibit degradation at the potentials where the oxygen evolution occurs, probably due to the electrochemical oxygen intercalation in the lattice.

Abstract: A bis-sulphur silane (BTESPT) was used to produce thin protective layers on AA2024- T3, a structural aluminium alloy widely used in the aeronautic industry, being the coatings analysed ex-situ by ellipsometry. Despite a slight degree of film anisotropy evidenced by measurements at different angles of incidence, an optical model of a single homogeneous phase could be employed to describe the silane based layer. The information obtained allowed to characterize the influence of the operational deposition parameters on the film thickness and structural organization of the silane phase (evaluated by its optical absorption). It is shown that the silane concentration of the solution and immersion time determine the final thickness of the formed layer while the curing time does not affect significantly this parameter. On the other hand, it was observed a marked decrease of the optical absorption of the coatings submitted to curing temperatures of at least 100°C, which should be ascribed to a structural reorganization phenomena induced by the removal of water molecules from the film. The data gathered by ellipsometry were successfully corroborated by independent surface profilometry analysis of the films.

Abstract: Several alkaline baths based on different complexing agents were examined for iron electroplating. The resultant films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was shown that adherent and smooth iron coatings with uniform microstructure can be obtained using alkaline Fe (II) baths containing pyrophosphate and tartrate ions as complexing agents. The average grain size can be substantially decreased by glycine additions in the pyrophosphate bath. The faradaic efficiency in these electrolytes may achieve up to 40-50%. The tartrate-containing baths are characterized with a higher throwing power and an increased buffer capacity with respect to the pyrophosphate-based electrolytes. The resultant Fe coatings are single-phase, whilst substantial broadening of the XRD peaks indicates nano-scale grain size. The alkaline baths based on EDTA complexes of iron (III) give black dull iron deposits and are characterized by rather low cathodic current efficiencies, especially at low current densities.