For a Grade 2 Titanium Immersion Heater Used to Heat a 5% Sodium Sulfide Solution at 40°C, Why Does the Sulfide Anion (S²⁻) Cause Rapid Hydrogen Absorption (200 ppm/month) and Embrittlement, While Chloride at the Same Concentration Causes Only Mild Pitting?

Grade 2 titanium in 5% sodium sulfide at 40°C absorbs hydrogen at a rate of 100-300 ppm/month and embrittles in 1-3 months. The sulfide ion (S 2−) is a strong reducing agent and inhibits the production of a protective TiO 2 coating. The cathodic reaction involves hydrogen evolution: 2H 2 O + 2e − → H 2 + 2OH − . Atomic hydrogen to titanium crystal lattice. By contrast, chloride at the same concentration causes only mild pitting (0.05–0.10 mm/year) because a passive film can still form.

Mechanism of sulfide-induced hydrogen absorption

Sulfide will react with the titanium surface to generate titanium sulfide (TiS) which is not protective. The open-circuit potential moves to -0.8 to -1.0 V vs. Ag/AgCl, where the dominant cathodic reaction is hydrogen evolution. The rate of hydrogen absorption is related to the sulfide content.

Quantitative Hydrogen Absorption Versus Sulfide Concentration

Na2S Concentration (%) H Absorption (ppm/month)Time to 200 ppm H (monthsFailure Mode 0.5 20-50 4-10 Embrittlement 1.0 40-80 2.5-5 Embrittlement 2.0 60-120 1.7-3.3 Embrittlement 5.0 100-200 1-2 Rapid embrittlement 10.0 150-300 <1 Immediate Material Selection Guide for Sulfide Service

Sulfide Concentration % Recommended Material Expected Life (1.5mm Wall)
<1 Grade 7 titanium 1-2 years 1-5 Grade 7 + cathodic protection 6-12 months >5 PTFE or tantalum coated >5 years
An informed specification

Do not use sulfide solutions with grade 2 titanium. Use Grade 7 (palladium inhibits H absorption) or PTFE coated heaters. The engineer chooses materials that are resistant to hydrogen embrittlement.