Influences of Sc(NO 3 ) 3 on characteristics of micro-arc oxidation coatings

Abstract

Micro-arc oxidation (MAO) coatings were fabricated on 2195 aluminium-lithium alloy substrate, using a silicate and phosphate electrolyte with variable scandium nitrate (Sc(NO₃)₃) concentrations. The properties of the coatings were analysed by scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical workstation. The comprehensive analysis demonstrates that adding proper amount of Sc(NO₃)₃ increased the oxidation voltage, which transformed the worm shape discharge micropores into round-hole shape, and the MAO coatings presented a double-layer structure. The primary phase composition of the coatings was γ-Al₂O₃, α-Al₂O₃, Sc₂O₃ and Al₃Sc. The optimum concentration of Sc(NO₃)₃ was 0.6 g L⁻¹. The hardness reached a maximum value of (1284 ± 43.5) HV, which was about 2.5 times higher than that without adding. The self-corrosion current density reduced from 138.00 × 10⁻⁷ A cm⁻² (substrate) to 3.65 × 10⁻⁷ A cm⁻², which was approximately two orders of magnitude lower, indicating that the corrosion resistance was improved.