Corrosion and Protection (Conversion Coating and Plasma Electrolytic Oxidation) of Ti₆Al₄V Processed by Powder Bed Fusion—Additive Manufacturing. EIS Study

Abstract

Green conversion coatings have been developed using oxyanion like permanganate additive (a conversion layer former) to an acidic medium (H₂SO₄). Reduction products of permanganate, mainly MnO₂, occur due to the high standard reduction potential (E⁰_red = 1.695 V) for the electrochemical reaction in acid solution. An anodic oxide film by anodization (plasma electrolytic oxidation) has been also performed. The structure obtained for these two protective coatings consists of an outer (porous) layer, observed at high frequencies, and an inner (barrier) layer, at low frequencies in the impedance diagram. An electrical equivalent circuit describing the coated alloy is proposed. Electrochemical corrosion results in the corroding electrolyte (NaCl 3%) of Ti₆Al₄V will be presented and compared to that of a rolled (Ti–0.15% Pd) foil. Finally, semiconductive properties of the passive film formed on the additive manufactured titanium alloy have been investigated. TiO_2−α is an n-type semiconductor having oxygen deficiency that provokes significant change in the electrical conductivity. Mott–Schottky plots (C⁻²_SC vs. E) have been drawn, and the donor concentration N_D has been determined from the linear part.