In a Titanium Immersion Heater for a 20% Sodium Persulfate Solution at 50°C, Why Does the Persulfate Ion Cause a Higher Corrosion Rate (0.2 mm/year) Than an Equivalent Concentration of Hydrogen Peroxide (0.05 mm/year) Despite Both Being Strong Oxidizers?

The corrosion rate of Grade 2 titanium in 20% sodium persulfate at 50°C is 0.15-0.30 mm/year. At the same temperature, the corrosion rate in 20% hydrogen peroxide is 0.03-0.08 mm/year. The rate is higher with persulfate because it has a higher reduction potential (2.01 V) than H2O2 (1.78 V). The higher potential pushes the titanium further into the transpassive region where the passive film dissolves at a faster rate. Also persulfate degradation products (sulfate radicals) are more active than hydroxyl radicals from H2O2.

Mechanism of Persulfate Aggressiveness

S₂O₈²⁻ + 2e⁻ → 2SO₄²⁻ (E° = 2.01 V) Persulfate reduction. Reduction of hydrogen peroxide: E° = 1.78 V H2O2 + 2H + + 2e- → 2H2O The mixed potential of titanium in persulfate is 1.6-1.8 V and in H2O2 it is 1.3-1.5 V. The transpassive dissolution current grows exponentially with potential. The corrosion rate ratio (persulfate/H2O2) is 3–6× .

Quantitative Comparison of Corrosion Rate

Oxidizer Concentration % Temperature °C Corrosion Rate mm/year
H 2 O 2 20 50 0.03-0.08
Na₂S₂O₈ 5 50 0.10-0.20
Na2S2O8 10 50 0.15-0.25
Na 2 S 2 O 8 20 50 0.20-0.35 Material Selection Guide

Persulfate Concentration (%) Recommended Material Expected Corrosion Rate (mm/year)
<5 Grade 7 titanium 0.05-0.10 5-10 PTFE-coated titanium <0.01 >10 Tantalum 0.01
Detailing with the facts at hand

Avoid uncoated titanium for persulfate solutions above 5% concentration. Use titanium or tantalum with PTFE coating. Knowing the oxidizer strength is higher, the engineer chooses appropriate materials.