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 N_e ≈ 0.8 × 10¹⁵ cm^− 3 and at 2.5 × 10¹⁶ cm^− 3. Apart from these two N_e values, N_e ≈ 6.0 × 10¹⁶ cm^− 3 is determined from the width of the Al II 704.2 nm line. Three considerably different N_e values imply presence of three types of discharge during PEO of aluminium. The electron temperature T_e is determined from relative line intensities of the O II (T_e ≈ 38,300 K) and W I (T_e ≈ 3300 K) lines. The use of two databases of transition probabilities for O II lines introduces systematic difference of reported T_e results.