Investigation of long-duration plasma electrolytic oxidation of aluminum by means of optical spectroscopy

The process of plasma electrolytic oxidation of aluminum was investigated by means of optical spectroscopy at a very long time interval. The change of morphology of obtained oxide coatings in the course of time was studied. Several hundreds of low-resolution emission optical spectra in the wavelength region between 250 nm and 850 nm, and higher-resolution spectra in the region from 230 nm to 290 nm were taken at the time interval up to 300 min from the beginning of the anodization. The spectra consisted of spectral lines and bands originating both from the anode material (Al, i.e. AlO) and the electrolyte (water solution of the boric acid and sodium tetraborate). Three pairs of spectral lines of aluminum were used to estimate the plasma temperature. It was found to be about 8000 K at the beginning of the process and as low as 3000–4000 K after roughly 1 h. The composition of the plasma containing aluminum, oxygen, hydrogen, and sodium was calculated in the temperature range up to 11,000 K under assumption of local thermal equilibrium, in order to explain the appearance of the observed spectral features.