Comparison of plasma electrolytic oxidation coatings on Al alloy created in aqueous solution and molten salt electrolytes

Plasma electrolytic oxidation (PEO) is a surface treatment technology mostly applied to some metallic materials, such as Al, Ti or Mg. Generally, the process is conducted in aqueous electrolyte solution in parts of limited dimensions due to the heating-up the system. It was previously shown that this issue could be solved in the process takes place in molten salt. In the presented comparative research, the PEO process is conducted in silicate electrolyte and nitrate molten salt. Aluminum oxide coatings were obtained on Al 1050 alloy in both processes and examined by optical and electron microscopy, EDS, XRD, micro-hardness and corrosion resistance measurements. Obtained results illustrated that the created coating in the process conducted in molten salt consists of hard α-Al₂O₃ and ɣ-Al₂O₃, has no additional compounds and through pores and shows higher corrosion resistance than coating formed in silicate electrolyte. Coating formed in PEO in aqueous electrolyte solution consists of η-Al₂O₃ and amorphous SiO₂ in addition to hard α-Al₂O₃ and ɣ-Al₂O₃ phases, has through pores and shows lower corrosion resistance.