The authors present a review of their recent research work in an endeavour to interpret the influence of native oxide films on the corrosion behaviour of commercial AZ31 and AZ61 magnesium alloys or on the oxidation kinetics in air at 200°C. The tendency of some of these thin films to be sufficiently protective in mild or weak corrosive environments is examined. For obtaining oxide films with different protective properties, some of the specimens are tested with the surface in the as-received condition, while others are tested immediately after mechanical polishing. The technique applied to characterise thin (thickness of just a few nanometres) oxide films present on the surface of alloys has basically been XPS (X-ray photoelectron spectroscopy) in combination with ion sputtering. Oxidation resistance of the alloys is quantified by thermo gravimetric (TG) curves and their corrosion rate is evaluated by Electrochemical Impedance Spectroscopy (EIS) and hydrogen evolution measurement in chloride solutions with different aggressivity. Emphasis is placed on the possible effects of: (a) the different thickness of the native oxide films formed on the polished surfaces on the corrosion behaviour of the alloys; and (b) the different film homogeneity and uniformity on the oxidation results. Finally, an attempt will be made to learn more about the influence of the native oxide films that cover the substrate on the subsequent growth and protective behaviour of the sol–gel coatings.
Part of the book: New Trends in Alloy Development, Characterization and Application
The control of the texture in synthetic hydroxyapatite ceramics had limited their application in the field of the materials for bone implantation, even more when it is used as a filling in cements and other formulations in orthopedic surgery. The present article shows preliminary results demonstrating the effectiveness of a modification of the controlled rate thermal analysis (CRTA), developed by J. Rouquerol, used for the preparation of ceramic materials with controlled textural characteristics, during the formation of ceramic powders of synthetic hydroxyapatite at low temperatures. The thermal treatments of the hydroxyapatite were carried out in a device connected to a computer, to control temperature and pressure system, keeping the decomposition speed constant. Results, reported when preparing ceramic powders of hydroxyapatite at 300 and 850°C under controlled pressure, using synthetic hydroxyapatite with a Ca/P molar ratio equal to 1.64, were checked using IR spectroscopy and X‐ray diffraction, showed that the formed phase corresponds to that of crystalline hydroxyapatite, even at 300°C of maximum temperature. Values of specific surface (BET) between 17 and 66 m2/g, with pore size in the range of 50–300 Å in both cases are obtained by N2 absorption isotherms, when analyzing the isotherms of nitrogen absorption.
Part of the book: Modern Technologies for Creating the Thin-film Systems and Coatings
In order to elucidate the corrosion mechanism of Magnesium (Mg), assess its corrosion rate and evaluate the viability of effective corrosion protection methods, a number of different and complementary techniques are required. Aqueous corrosion is, in nature, an electrochemical process and as such electrochemical methods represent a powerful tool for the study of Mg corrosion. In this chapter the main electrochemical techniques used to study the corrosion of Mg are reviewed along with other simple non-electrochemical methods such as weight loss and hydrogen evolution measurements. The electrochemical techniques covered in this review include conventional DC and AC electrochemical techniques and the latest advances in local electrochemical methods for the evaluation and characterization of Mg corrosion. Each technique presented will be discussed, and its major advantages and drawbacks for the study of Mg corrosion will be commented. Applications range from studies of influence of the impurities in catalytic activity of high purity Mg towards hydrogen evolution, the determination of corrosion rate for Mg and Mg alloys by electrochemical methods and electrochemical study of sol-gel films as pre-treatment for Mg alloys.
Part of the book: Magnesium Alloys