Spectroscopic characterization of plasma during electrolytic oxidation (PEO) of aluminium

We present results of an optical emission spectroscopic study of Plasma during Electrolytic Oxidation (PEO) of aluminium in citric acid and in sodium–tungstate water solutions. The line shape analysis of the first two hydrogen Balmer lines indicates presence of two PEO processes characterized by relatively low electron number densities Ne ≈ 0.8 × 1015 cm− 3 and at 2.5 × 1016 cm− 3. Apart from these two Ne values, Ne ≈ 6.0 × 1016 cm− 3 is determined from the width of the Al II 704.2 nm line. Three considerably different Ne values imply presence of three types of discharge during PEO of aluminium. The electron temperature Te is determined from relative line intensities of the O II (Te ≈ 38,300 K) and W I (Te ≈ 3300 K) lines. The use of two databases of transition probabilities for O II lines introduces systematic difference of reported Te results.