Spectroscopic and real-time imaging investigation of tantalum plasma electrolytic oxidation (PEO)

► Spectroscopic study of the PEO process of tantalum was conducted. ► Line shape analysis of hydrogen Balmer lines, two electron number densities detected. ► New technique for Balmer lines self-absorption testing was demonstrated. ► Spatial density of microdischarges is the highest in the early stage of PEO process. ► Percentage of oxide coatings area covered by discharge sites decreases with PEO time.