Spectroscopic study of plasma during electrolytic oxidation of magnesium- and aluminium-alloy

We present the results of an optical emission spectroscopy study of Plasma during Electrolytic Oxidation (PEO) of magnesium- and aluminum-alloy. Plasma electron number density N_e diagnostics is performed either from the H_β line shape or from the width or shift of non-hydrogenic ion lines of aluminum and magnesium. The line profile analysis of the H_β suggests presence of two PEO processes characterized by relatively low electron number densities N_e≈1.2×10¹⁵ cm^?3 and N_e≈2.3×10¹⁶  cm^?3. Apart from these two low N_e processes, there is the third one related to the ejection of evaporated anode material through micro-discharge channels. This process is characterized by larger electron density N_e=(1.2–1.6)10¹⁷ cm^?3, which is detected from the shape and shift of aluminum and magnesium singly charged ion lines. Two low N_e values detected from the H_β and large N_e measured from the widths and shift of ion lines suggest presence of three types of discharges during PEO with aluminum- and magnesium-alloy anode. On the basis of present and earlier results one can conclude that low N_e processes do not depend upon anode material or electrolyte composition.The electron temperature of 4000 K and 33,000 K are determined from relative intensities of Mg I and O II lines, respectively. The attention is drawn to the possibility of N_e application for T_e evaluation using Saha equation what is of importance for PEO metal plasma characterization. During the course of this study, difficulties in the analysis of spectral line shapes are encountered and the ways to overcome some of the obstacles are demonstrated.