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.