For a Titanium Immersion Heater Used to Maintain 85°C in a 20% Sodium Tungstate Solution for Metal Finishing, How Does the Tungstate Ion (WO₄²⁻) Form a Protective Tungsten Oxide (WO₃) Layer and Reduce the Corrosion Rate to 0.003 mm/year?

The tungstate ion (WO 4 2− ) from 20% sodium tungstate at 85°C generates a protective tungsten oxide (WO 3 ) layer on the titanium surface. Corrosion rate of grade 2 titanium is 0.002-0.005 mm/year, which is lower than in many neutral solutions. The formation of the tungstate layer through adsorption and reduction: WO4 2- + 2H+ + 2e- -> WO3 + H2O. The WO 3 layer is dense, sticky and insoluble in neutral and acid solutions.

The mechanism of tungsten passivation

The titanium surface adsorbs the tungstate ions. They are reduced to WO_3 at the corrosion potential. The WO₃ layer is 2–10 nm thick. It prevents the transfer of oxygen and ions to the metal surface. The passivation is maintained at pH 4-9

Quantitative Corrosion Rate Versus Tungstate Concentration

Concentration (%) Temperature (°C) Na2WO4 Corrosion Rate (mm/year)Surface Layer 1 85 0.008-0.015 TiO2 + WO3 5 85 0.005-0.010 WO3 10 85 0.003-0.008 WO3 20 85 0.002-0.005 WO3 Effect of pH on inhibition by tungstate

pH WO3 solubility Inhibition efficiency (%) <4 High (dissolved) <50
4-9 Low 90-999 Moderate 70-90
Making an Educated Specification

Add 5-10 % sodium tungstate as corrosion inhibitor for titanium heaters in corrosive environments. The engineer obtains very low corrosion rates by constructing a protective WO 3 layer. Tungstate is less poisonous than chromate .