Plasma electrolytic oxidation of titanium in heteropolytungstate acids

► Spatial density of microdischarges is the highest in the early stage of PEO process. ► Percentage of coatings area covered by active discharge sites decreases with time. ► Line shape analysis of hydrogen Hβ line, two electron number densities detected. ► Analysis of PEO emission spectra reveals H I, O I, O II and W I spectral lines. ► Oxide coatings are partly crystallized and mainly composed of WO3 and anatase.