The effect of current mode and discharge type on the corrosion resistance of plasma electrolytic oxidation (PEO) coated magnesium alloy AJ62

Magnesium alloys are increasingly being used as lightweight materials in the automotive, defense, electronics, biomaterial and aerospace industries. However, their inherently poor corrosion and wear resistance have, so far, limited their application. Plasma electrolytic oxidation (PEO) in an environmentally friendly aluminates electrolyte has been used to produce oxide coatings with thicknesses of similar to 80 mu m on an AJ62 magnesium alloy. Optical emission spectroscopy (OES) in the visible and near ultraviolet (NUV) band (285 nm-800 nm) was employed to characterize the PEO plasma. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to characterize the coated materials, and potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in a 3.5% NaCl solution were used to determine the corrosion behavior. It was found that the plasma discharge behavior significantly influenced the microstructure and the morphology of the oxide coatings and, hence the corrosion resistance. The corrosion resistance of the coated alloy was increased by changing the current mode from unipolar to bipolar, where the strong plasma discharges had been reduced or eliminated. (C) 2011 Elsevier B.V. All rights reserved

Hydrogen in Ti and Zr alloys: industrial perspective, failure modes and mechanistic understanding

Titanium is widely used in demanding applications, such as in aerospace. Its strength-to-weight ratio and corrosion resistance make it well suited to highly stressed rotating components. Zirconium has a no less critical application where its low neutron capture cross section and good corrosion resistance in hot water and steam make it well suited to reactor core use, including fuel cladding and structures. The similar metallurgical behaviour of these alloy systems make it alluring to compare and contrast their behaviour. This is rarely undertaken, mostly because the industrial and academic communities studying these alloys have little overlap. The similarities with respect to hydrogen are remarkable, albeit potentially unsurprising, and so this paper aims to provide an overview of the role hydrogen has to play through the material life cycle. This includes the relationship between alloy design and manufacturing process windows, the role of hydrogen in degradation and failure mechanisms and some of the underpinning metallurgy. The potential role of some advanced experimental and modelling techniques will also be explored to give a tentative view of potential for advances in this field in the next decade or so