Guar gum-driven high-energy plasma electrolytic oxidation for concurrent improvements in the electrochemical and catalytic properties of Ti-15 Zr alloy

Abstract

A hybrid composite layer with optimal chemical stability and catalytic activity was formed on Ti-15 Zr alloy for the first time via high-energy plasma electrolytic oxidation driven by guar gum (GG). Here, the substrate was treated in a phosphate-based electrolyte containing GG (0, 1, 1.5, and 2 g/L), which might act as an electron donor during plasma-assisted electrochemical processes. The ability of GG molecules to form an adsorption layer on the substrate surface through either polar –OH or hetero-oxygen atoms would be responsible for the reduction in the intensity of plasma discharges observed at lower concentrations of GG (1 g/L). However, higher concentrations of GG would act as barriers to the electron avalanche by accumulating at the surface of the barrier passive layer developed on the substrate surface. As a result, the discharges in the 1.5GG and 2GG samples were more potent than those observed in other electrolytes to overcome the electron avalanche obstruction. In addition, the chemical incorporation of ZrO2 into the TiO2 layer would be affected by the GG additive, which would donate electrons to the oxygen bubble, acting as electron acceptors in the electrolyte. The chemical stability of the 1.5GG sample was superior to the other samples as it had the highest values of outer and inner layer resistances (1.79 × 107 and 4.81 × 107 Ω.cm2, respectively). The 1.5 GG sample achieved also a relatively good catalytic activity of 95.97% toward methylene blue. Thus, a multifunctional layer with high chemical stability and high catalytic activity was formed, which would be associated with the presence of rutile and ZrO2 in the coatings, respectively. The obtained results of the oxide layers were very high and comparable to other works.