Comparison of plasma electrolytic oxidation coatings on Al alloy produced in diluted and concentrated silicate electrolytes for space thermal control application

In this work, a bright white oxide coating was successfully prepared on 6061 Al alloy by PEO process from an environmentally friendly and low-cost concentrated silicate electrolyte as an alternative for commonly used fluozirconate electrolyte. The characteristics of PEO processes and resulting coatings both in diluted and concentrated silicate electrolytes were investigated by using Optical Emission Spectrometer (OES), Scanning Electron Microscope (SEM), X-ray Energy Dispersive Spectrometer (EDS) and X-ray Diffraction Spectrometer (XRD). The thermal control properties of the coatings were evaluated by UV-VIS-NIR spectrophotometer and infrared emissivity spectrometer, respectively. The corrosion protection performances of the coatings were investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests. Results show that the discharges in diluted silicate electrolyte are more pronounced than those in concentrated silicate electrolyte. The more intensive discharges in diluted silicate electrolyte are related to plasma electrochemical reactions at the material surface or at the metal-oxide interface, facilitating to a relatively dense structure and complex oxides including alpha-Al2O3, gamma-Al2O3 and mullite. The weaker discharges in concentrated silicate are associated with the fast silicate anions deposition process, resulting in the formation of amorphous SiO2 with a porous microstructure and high thickness. The coating prepared in concentrated silicate electrolyte is characterized by a bright white color in appearance and has much lower solar absorptance (alpha s) of 0.159 and higher emissivity (epsilon) of 0.90 than the coating prepared in diluted silicate electrolyte with alpha s of 0.429 and epsilon of 0.79. However, the coating prepared in concentrated silicate electrolyte has a little inferior corrosion protection performance compared with that prepared in diluted silicate electrolyte due to the more porous microstructure.