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

Abstract

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~80 μm 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.

Plasma electrolytic oxidation (PEO) coatings on Mg-alloys for improved wear and corrosion resistance

“…The PEO coating system used to produce the oxide coatings consists basically of a container with the alkaline electrolyte and an electrical source, and is described in more detail in ref [5,12]. A stainless steel plate in the bath acts as a counter-electrode (cathode) with the Mg-alloy coupons as the anode.…”

“…Hence, a change of the process parameters, such as a current density and modes [5,14] and electrolyte composition and concentration [4], are important in controlling porosity levels to improve coating hardness. Coatings produced in silicate solutions generally [4].…”

“…Coatings produced in silicate solutions generally [4]. The use of sodium aluminate (NaAlO 2 ) in the electrolyte for AJ62, AM60B and AZ91D Mg-alloys was investigated using different current modes [5,6]. PEO coatings can increase the hardness of the magnesium alloys 5-9 times compared with the hardness of the base substrate [14].…”

“…CPE2 is the constant phase element for the double layer capacitance of the interface at, or near, the coating/substrate interface. R2 represents the polarization resistance which is the Faradic charge transfer resistance related to electrochemical reactions in the same coating/substrate interface region [5]. It is R2 that primarily controls the corrosion resistance of the coated alloy.…”

“…Such coatings should be uniform, pore free, relatively thick, hard, well adhered, flexible to withstand an overload, and insulating to minimize corrosion. Plasma electrolytic oxidation (PEO) fulfills most of these requirements and has been used for depositing a well adhered ceramic coating on light-weight metals (Mg, Al, Ti) that provides not only corrosion protection but also enhanced wear properties, hardness and toughness with better thermal stability and dielectric properties [4,5].…”