For a Titanium Immersion Heater in a 5% Sodium Persulfate Solution at 50°C, Why Does the Persulfate Ion Not Passivate Titanium Despite Being a Strong Oxidizer, and Instead Causes Transpassive Dissolution?

Sodium persulfate (Na₂S₂O₈) is a strong oxidizing agent, with a standard reduction potential for S₂O₈²⁻/SO₄²⁻ of 2.01 V, which is higher than that of nitric acid. In the case of titanium, persulfate induces transpassive dissolution rather than persistent passivation. At 50° C, the strong oxidation potential moves the titanium surface into the transpassive area above 1.8 V vs a conventional hydrogen electrode, where the passive layer breaks down and titanium dissolves as Ti 4 + at a rate of 0.2–0.5 mm per year. The transpassive dissolution is uniform, not pitting. But the corrosion rate is too high for most applications.

Mechanism of the Transpassive Dissolution Induced by Persulfate

Titanium has a stable passive region between approximately 0 V and 1.8 V. At potentials above 1.8 V the TiO2 film is oxidized to soluble Ti 4 + species: TiO2 + 2H2O → Ti 4 + + 4OH - + 2e - . The combined potential of titanium in persulfate, a very strong oxidant, is 1.7-2.0V. In this range the transpassive dissolution current is large. Corrosion is uniform as the whole surface is in the transpassive area. No localized rupture occurs yet there is a constant dissolving of the film.

Persulfate Concentrations Quantitative Corrosion Rate as a function of

In the case of Grade 2 titanium, the following have been established under controlled testing in sodium persulfate at 50°C for 500 hours. At 0% persulfate (baseline) the corrosion rate is 0.005–0.010 mm/year, the surface potential is 0.2–0.5 V and the mechanism is passive. 1% persulphate 0.05-0.12 mm/y 1.2-1.5 V Mixed 3% persulfate Rate 0.12-0.25 mm/year Potential 1.5-1.7 V Mechanism Transpassive initiation At 5% persulfate, the rate is 0.20-0.40 mm/yr, the potential is 1.7-1.9 V, and the mechanism is transpassive. The rate is 0.35–0.60 mm/yr at 10% persulfate, the potential is 1.8–2.0 V and the mechanism is severe transpassive.

Temperature effect on transpassive transition

At higher temperatures the transpassive region shifts to lower potentials. The corrosion rate is transpassive and is 0.10–0.20 mm/year at 25°C and 5% persulfate. It grows 0.15–0.30 mm a year at 40°C. The rate is 0.20-0.40 mm/year at 50°C. Rate is 0.30–0.60 mm per year at 60°C. The rate is more than 0.50 mm/yr above 70°C. The higher the temperature, the more severe is the transpassive dissolution.
**Guide for Material Selection in Persulfate Service

The following table gives recommendations for heating of solutions of sodium persulphate according to concentration and temperature.

Persulfate Concentration (%) Temperature (°C) Grade 2 Corrosion Rate (mm/year) Corrosion Rate (mm/year) Grade 7Recommended Material <1 50 0.05–0.10 0.03–0.06 Grade 7 with thick wall 1–3 40 0.08–0.15 0.05–0.10 PTFE-coated titanium 1–3 50 0.12–0.25Not recommended for titanium 3–5 30 0.10–0.20 0.06–0.12 Tantalum or PTFE >5 Any >0.20 >0.12 Tantalum or PTFE
Titanium and Beyond Engineering Choices

The transpassive dissolving rates of Grade 7 are slightly lower than Grade 2 (30–50% decrease) due to palladium enrichment, however the improvement is not sufficient to make titanium appropriate for persulfate service. Wall thickness is given for corrosion allowance. With 2.0 mm wall and 0.20 mm per year corrosion, the wall will endure 10 years but this is negligible. PTFE coated titanium heaters provide great resistance and keep the metal isolated from the persulfate solution. Tantalum exhibits a much higher passive range and is persulfate resistant. Zirconium also works well, but it is expensive.

Specification Confidence

The predicted corrosion rate for a titanium immersion heater in 5 % sodium persulfate at 50 °C is 0.20–0.40 mm per year, which is too high for long term usage. Use PTFE coated titanium heater or tantalum heater. 0.5 mm PTFE The wall thickness should be 2,5 mm, if titanium is needed, the persulfate concentration should be kept at 2%, and the temperature at 40°C. Check the heater thickness once a year in operation . When the thickness of wall is 1.0 mm , change the heater . Knowing that persulfate is a transpassive dissolver and not a passivator, the engineer selects suitable materials for this severe oxidizing environment.