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Plasma electrolytic oxidation (PEO) is a well-known surface treatment for Mg alloys, while high porosity is an issue to achieve long-term corrosion protection. Nevertheless, the presence of inherent porosity can be turned into advantage while used as nano/micro-structured reservoir of corrosion inhibitors or functional agents next to Mg surface providing an active corrosion protection and multi-functionality. For the first time, we report here the 3D internal architecture of PEO coated Mg via synchrotron-based microtomography. The pore characteristics have been qualitatively and quantitatively investigated to unveil the formation of the coating porosity and potential incorporation of functional elements in the pores.

Plasma electrolytic oxidation (PEO) processing has received considerable attention for improving the corrosion or wear resistance of magnesium and its alloys. However, it cannot provide high-barrier long-term protection due to its high porosity. A novel approach is to introduce reactive particles to PEO coatings, aiming to seal the porosity and to provide wider range of coating compositions. Experiments with nano- and micro-sized SiO2 particles and different concentrations of KOH in the electrolyte provide new insights into possible up-take and incorporation mechanisms of particles during PEO processing.

The influence of Ce additions to AM50 magnesium alloy prepared by high pressure die casting on the microstructure and corrosion behaviour has been investigated. The corrosion behaviour was evaluated by hydrogen evolution, salt spray, potentiodynamic polarization and electrochemical impedance experiments. The results show that Ce addition promotes the formation of Al 11Ce 3 phase and reduces the amount of β-phase (Mg 17Al 12). The corrosion resistance is improved with increasing Ce content which can be attributed to the formation of Al 11Ce 3 phase, reduction of β-phase (reduced galvanic effects) and higher purity of AME50X alloys (melt cleaning effects).

This paper describes the protection mechanism of polyacrylonitrile coatings on a hydrofluoric acid treated magnesium alloy. The protective performance of the adopted methodology was investigated by electrochemical impedance spectroscopy and immersion tests in 3.5 wt.% NaCl and in simulated body fluid (SBF). It was observed that interfacial reactions and interactions between polymer and substrate rendered long-term protection for the metal. A detailed characterization of the coating properties and of the interfacial processes is provided and the potential of the adopted methodology for biomedical application is discussed.

In this paper the interface of poly(vinylidene fluoride) coatings prepared by the dip coating method and HF-treated AZ31 magnesium alloy was evaluated. The best performance of this system in corrosion tests compared to ground, as-received and acetic acid cleaned substrates is related to an acid–base interaction at the interface and to interfacial reactions which resulted in a self-healing process. The protectiveness of the samples was investigated using impedance and immersion tests while the coating morphology and interface stability were investigated by scanning electron microscopy, X-ray photoelectron spectroscopy and adhesion tests.

In this study, investigations on the protectiveness of poly(ether imide) coatings against corrosion of magnesium AZ31 alloy sheets are performed. The coatings were prepared in different pre-treated substrates by the dip coating method using N′N′-dimethyl acetamide (DMAc) and N′-methyl pyrrolidone solutions. The optimal performance was obtained for hydrofluoric acid treated substrates coated using DMAc solution (coating thickness 13 μm) which showed impedances in the order of 107 Ω cm2 even after more than 3300 h of exposure to a 3.5 wt.% NaCl solution. This high performance is associated to an acid–base interaction at the interface as observed by X-ray photoelectron spectroscopy.

Surface contaminants as a result of thermo-mechanical processing of magnesium alloys, e.g. sheet rolling, can have a negative effect on the corrosion resistance of magnesium alloys. Especially contaminants such as Fe, Ni and Cu, left on the surface of magnesium alloys result in the formation of micro-galvanic couples and can therefore increase corrosion attack on these alloys. Due to this influence they should be removed to obtain good corrosion resistance. In this study, the effect of inorganic acid pickling on the corrosion behaviour of a commercial AZ31 magnesium alloy sheet was investigated. Sulphuric, nitric and phosphoric acids of different concentrations were used to clean the alloy for various pickling times. The surface morphology, composition and phases were elucidated using scanning electron microscopy, X-ray fluorescence analysis, spark discharge-optical emission spectroscopy, energy dispersive X-ray spectroscopy and infrared spectroscopy. The effect of surface cleaning on the corrosion properties was studied using salt spray test and electrochemical impedance spectroscopy. The experimental results show that acid pickling reduces the surface impurities and therefore enhances the corrosion resistance of the alloy. The cleaning efficiency of the three acids used and the corrosion protection mechanisms were found to be remarkably different. Best corrosion results were obtained with nitric acid, followed closely by phosphoric acid. Only the sulphuric acid failed more or less when cleaning the AZ31 sheet. However, to obtain reasonable corrosion resistance at least 5 μm of the surface of AZ31 magnesium alloy sheet have to be removed.

In the present study, the potential of poly(ether imide) as corrosion protective coating for magnesium alloys was evaluated using the spin coating technique. The influence of different parameters on the coating properties was evaluated and the corrosion behaviour of the coatings was investigated using electrochemical impedance spectroscopy. The best corrosion protection was obtained preparing the coatings under N2 atmosphere, using 15 wt.% solution in N′N′-dimethylacetamide (DMAc) which resulted in a coating of approximately 2 μm thickness, with an initial impedance of 109 Ω cm2 and of 105 Ω cm2 after 240 h of exposure to a 3.5% NaCl solution.

The microstructure of a friction stir welded magnesium alloy AM50 was examined by means of optical light microscopy. The chemical composition, particularly the iron content, and morphology of the oxide film were analyzed and discerned via auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Corrosion behaviour of the welds and base materials were investigated by virtue of neutral salt spray tests and potentiodynamic polarization measurements in conventional cells and in a mini cell. The results demonstrate that minor increases in iron concentration as might be speculated to occur as a consequence of tool/work piece interaction during the welding process on the corrosion resistance of the weld can be ignored. The corrosion morphology was predominantly influenced by the distribution of the Mg17Al12 phase. Here, it was also found that the corrosion resistance of the friction stir weld varied in response to changes in the joint microstructure.

In previous publications of the authors, good performance of poly(ether imide) as corrosion protective coatings for magnesium AZ31 alloy was reported. It was suggested that during the sample degradation magnesium hydroxide could react with the imide ring forming magnesium polyamate and polyamic acid, but this could not be experimentally confirmed. In the present letter, we confirm the occurrence of this reaction by infrared and X-ray photoelectron spectroscopy and discuss its influence in the corrosion behavior observed in electrochemical impedance spectroscopy tests.