For a Grade 2 Titanium Heat Rod Used to Heat a 10% Sodium Periodate Solution at 60°C, How Does the Periodate Ion (IO₄⁻) Change the Open-Circuit Potential from +0.2V to +1.1V vs. Ag/AgCl and Cause Transpassive Dissolution at 0.05 mm/year?

IO4-/IO3- +1.6 V *Strong oxidant Sodium periodate (NaIO4) 10% 60 °C The potential of titanium in open circuit varies from +0.2 V (neutral chloride) to +1.0-1.2 V. This potential is in the transpassive area of titanium where the passive layer is continuously dissolved: TiO 2 + 2H 2 O = Ti 4+ + 4OH - + 2e -. Corrosion rate is 0.03-0.08 mm/year which is higher than in many oxidizing acids but still suitable for limited service.

Mechanism of periodate induced transpassive dissolution

Periodate is reduced to iodate: IO₄⁻ + 2H⁺ + 2e⁻ → IO₃⁻ + H₂O. This reaction has a high exchange current density, so that the titanium potential is set over 1.0 V. At this potential, titanium is in transpassive region and oxide coating dissolves uniformly. No pitting, attack is steady.

Quantitative Corrosion Rate Versus Periodate Concentration

5% NaIO 4 and 60°C the corrosion rate is 0.02-0.05 mm per year. At 10 % the rate is 0.03-0.08 mm yr-1. At 20% the rate is 0.05-0.12 mm/yr. At 10% , 80ºC the rate is 0.06-0.15 mm/year .

Selection Guide for Materials

Periodate Concentration (%) Temperature (°C) Titanium Corrosion Rate (mm/year) Recommended <5 60 <0.05Yes 5-10 60 0.03-0.08 Yes 10-20 60 0.05-0.12 Limited 10 80 0.06-0.15 No
Making an informed specification to

Titanium heater in 10% NaIO4 at 60°C will have a corrosion rate of 0.03-0.08 mm/yr 25-50 year life. Use 2.0 mm wall. The engineer understands the transpassive dissolution in order to pick the correct wall thickness.