Surface characterisation and corrosion behaviour of niobium treated in a Ca- and P-containing solution under sparking conditions

Highlights: • Plasma electrolytic oxidation of Nb in a Ca, Mg & P containing solution was studied. • Sparks characteristics and resulting surface features depended on applied potential. • PEO oxide coatings were enriched with all of the electrolyte elements. • Intensive sparking at high DC voltages provided a sealing effect to the coatings. - Abstract: The investigations of plasma electrolytic oxidation (PEO) process of Nb in a 1.5 mol dm⁻³ Ca(H₂PO₂)₂ and 0.5 mol dm⁻³ Mg(CH₃COO)₂ solution at a constant anodic current density of 150 mA cm⁻² up to several limiting voltages (200, 300, 400 and 500 V) were described. The resulting oxide layers were characterised in terms of their physicochemical properties and corrosion resistance (DC and AC techniques) in Ringer's solution at 37.0 ± 0.5 °C. The growth of the oxide layers during the PEO treatment was monitored using a digital camera and the sparks formed on the surface of niobium were analysed. The effect of the anodisation voltage on the oxide layer thickness (from less than 1 μm up to 150 μm), incorporation of the electrolyte solution species (the relative content of Nb in the coatings dropped from 36.5 to 0.7 at%), adhesion strength (the L_c2 increased from 6.31 ± 0.38 to 14.10 ± 0.47 N) and corrosion resistance (the R_p changed from 10.4 ± 2.7 to 60.9 ± 13.7 MΩ cm²) was described. Additionally, the phase and chemical compositions of the oxide layers were studied.