For a Titanium Immersion Heater Used to Maintain 80°C in a 10% Sodium Sulfite Solution for Oxygen Scavenging, What Is the Maximum Allowable Sulfite Concentration (g/L) to Prevent Hydrogen Embrittlement at the Cathodic Protection Potential of -0.9 V vs. Ag/AgCl?

Titanium is passive in 10% sodium sulfite (Na2SO3) at 80°C. However, the sulfite ions are reduced at a cathodic potential of -0.9 V vs. Ag/AgCl: SO₃²⁻ + 4H⁺ + 4e⁻ → S⁰ + 2H₂O (or to S²⁻). The reduction products can form titanium sulfide and atomic hydrogen is produced. The rate of hydrogen uptake increases with sulfite concentration. Maximum permissible sulfite concentration to prevent hydrogen embrittlement is 50 g/l. Hydrogen absorption over 50 g/L exceeds 100 ppm/month leading to cracking within 6-12 months

The reduction of sulfite and the mechanism of hydrogen embrittlement

In the cathodic polarization, sulfite is reduced to elemental sulfur or sulfide. The sulfur combines with titanium to generate TiS, a brittle phase. " Atomic hydrogen is produced at the same time. Hydrogen atoms diffuse into the titanium lattice, generating hydrides. Cracking is accelerated by a combination of sulfide embrittlement and hydride formation.

Quantitative Hydrogen Absorption Versus Sulfite Concentration

Controlled testing of Grade 2 titanium in Na 2 SO 3 at 80°C with cathodic protection of -0.9 V vs. Ag/AgCl for 1,000 hours has shown the following. Hydrogen absorption is 10-20 ppm per 1,000 hours at 0 g/L Na₂SO₃ (baseline) with no cracking. At 10 g/L, the absorption is 20-40 ppm and there is no cracking. Absorption at 25 g/L is 40-80 ppm and some embrittlement may occur. 50 g/L ( crucial ) : absorption 80-150 parts per million . cracking starts within 1,000-2,000 hours . 150-300 ppm at 100 g/l. Cracks within 500-1000 hours. At 200 g/L the absorption is > 300 ppm and it cracks quickly.

Guide to sulfite concentration for cathodically protected titanium

Na2SO3 Concentration (g/L) H Absorption (ppm/1000 h)Cracking Time (hours)Recommended <25 <80 >5,000 Yes 25-50 80-150 1,000-5,000 Limited 50-100 150-300 500-2,000 No >100 >300 <500 Not advised
Specifying with Knowledge

Sodium sulfite at 80 °C with cathodic protection (titanium heater): maximum sulfite concentration 25 g/l. Use grade 7 titanium for greater concentrations or lower the cathodic protection potential to -0.8 V. The engineer controls the concentration of sulphite to prevent hydrogen embrittlement.