Massimiliano Bestetti

Anodic oxidation and powder coating for corrosion protection of AM6oB magnesium alloys

Abstract In the present paper a protection process against corrosion for AM60B magnesium alloys is described. The coating consisted of an anodic oxide layer, obtained by microarc oxidation (MAO), covered with a powder coating layer. The MAO pretreatment improved the adhesion of the organic layer, which was responsible for the corrosion protection effect. The morphological and crystallographic structure of the anodic oxide was characterised by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Salt spray tests were carried out in order to assess the quality of the anodic oxide powder coating system.

Photoelectrochemical advanced oxidation processes on nanostructured TiO 2 catalysts: Decolorization of a textile azo-dye

This work describes a novel approach for water treatment by photoelectrocatalysis, based on nanostructured TiO2. The decolorization of aqueous solutions containing the azo dye RR243 is carried out in a tubular photocatalytic reactor working in semi batch mode under electrical polarization of the catalyst. Two different nanostructured catalysts were tested: nanotubular TiO2 obtained by conventional anodization (CA), and a novel photoactive nanoporous TiO2 obtained by plasma electrolytic oxidation (PEO). Neither UV irradiation of the TiO2 catalysts nor the electrical bias individually considered lead to a significant reduction of the dye concentration. By irradiating the catalysts with UVC light while applying an electrical bias to the same, the concentration of the dye decreased from 25 L to 2.5 mg/L using the nanotubular CA TiO2 catalyst, and to less than 1.8 mg/L using the novel nanoporous PEO TiO2 in 50 min. The main advantages of this method over current approaches for the degradation of pollutants are both the considerable processing time reduction and a suitable and easy to scale up reactor design. A further advantage is the relatively easyness in the production of the TiO2 catalysts by PEO.

Electroless and Electrochemical Deposition of Metallic Coatings on Magnesium Alloys Critical Literature Review

A critical review pertaining to surface treatments on magnesium alloys was published in 2002 (Gray & Luan, 2002). Two years later, another review paper on the same subject was published (Natarajan et al., 2004). According to the authors of both reviews, only complex and multilayer coatings are able to produce optimum results in term of corrosion and wear protection of magnesium alloys, and a great deal of research has to be done furthermore to develop technically and economically viable coating systems. One of the main challenges in the plating treatments of magnesium alloys is their susceptibility to corrosion. A way to prevent corrosion is by providing a barrier between the metal substrate and the environment. A coating has to be uniform, adherent, pore and crack free, and self-healing in the applications where damage of the coating can occur. The high reactivity of magnesium alloys causes the formation of a natural oxide when the metal is in contact with air and water. A treatment process for cleaning and removal of these oxides, that are unfavourable for coatings adhesion, is necessary before any electroplating and electroless plating treatment. The electrochemical and electroless depositions of coatings on magnesium alloys demand significant technical requirements. In fact, magnesium is subjected to corrosion in acidic aqueous environments and is only partly protected by an hydroxide layer that forms at pH higher than 9-10. Moreover, the most widely used commercial alloys, i.e. AZ91D and AM60B, contain alloying elements which form intermetallic phases (Fig. 1a). From an electrochemical point of view those phases have different behavior when the alloy is immersed into a solution for plating or chemical treatment. This means that the surface must be made equipotential by modifying the metallurgical structure before the coating process or by a specific surface pretreatment. Lunder reported that the corrosion potential of ┚-Mg17Al12 phase in 5% NaCl saturated with Mg(OH)2 is approximately -1.2 V vs. SCE, whereas the values for pure Mg and AZ91 are -1.66 V and -1.62 V vs SCE, respectively (Lunder et al., 1989). The heterogeneous structure of the alloys affects their corrosion resistance behavior and has a strong influence on the behavior of the alloy in the aqueous treatments processes, such as electroless and electrochemical depositions, and chemical conversions. A successful electroless or