For a Titanium Heat Rod Used to Heat a 5% Sodium Metavanadate Solution at 80°C, How Does the Vanadate Ion (VO₃⁻) Inhibit Titanium Corrosion by Forming a Protective Vanadium Oxide (V₂O₅) Layer That Reduces the Corrosion Rate to 0.005 mm/year?

In 5% sodium metavanadate at 80°C, the vanadate ion (VO₃⁻) reduces on the titanium surface to form a protective vanadium oxide (V₂O₅) layer. This layer is electronically conductive but chemically stable, reducing the titanium corrosion rate to 0.003-0.008 mm per year. The layer forms spontaneously at open circuit. Vanadate is an effective corrosion inhibitor for titanium in neutral and acidic solutions. The Mechanism of Vanadate Inhibition Vanadate ions are reduced: 2VO₃⁻ + 2H⁺ + 2e⁻ → V₂O₅ + H₂O. The V₂O₅ deposits on the titanium surface, forming a thin (10-50 nm) film. This film is insoluble in water and blocks further reaction. The inhibition efficiency is >90%. Quantitative Corrosion Rate as a Function of Vanadate Concentration NaVO₃ Concentration (%) Corrosion Rate (mm/year) Inhibition Efficiency (%) 0 0.01-0.03 0 0.1 0.005-0.010 60-80 0.5 0.004-0.008 70-85 1.0 0.003-0.006 80-90 5.0 0.002-0.005 85-95 Effect of pH on Vanadate Inhibition pH Vanadate Species Inhibition Efficiency (%) <2 VO₂⁺ 50-70 2-6 VO₃⁻ 80-95 6-8 V₂O₇⁴⁻ 70-85 >8 VO₄³⁻ 50-70 Making an Informed Specification For titanium heaters in corrosive environments, add 0.1-0.5% sodium metavanadate as a corrosion inhibitor. By forming a protective vanadium oxide layer, the engineer reduces the corrosion rate.