  The growth of thin oxide coatings on the aluminum substrate in water-based
sodium tungstate electrolyte by plasma electrolytic oxidation (PEO) is
discussed and experimentally illustrated. The growth is carried out using a
distinctive ultra-low duty cycle pulsed direct current (DC) power supply.
During the PEO processing elements present in micro-discharges are identified
using standard optical emission spectroscopy (OES) technique. The spectral line
shape analysis of the first two hydrogen Balmer lines shows the presence of two
types of micro-discharges. Obtained coatings are also characterized with
respect to their morphology and chemical and phase composition. It is shown
that coatings are composed of Al, O, and W, featuring low roughness and
porosity. Partial crystallization of the coatings resulted in identification of
WO3, W3O8, and gamma-Al2O3 crystalline phases.
