For a Titanium Immersion Heater Installed in a Molten Sulfur Tank (135°C, Contains Hydrogen Sulfide and Polysulfides), Why Is a Thicker Wall Not Beneficial Because Corrosion Is Uniform and Life Is Determined by Total Iron Dissolution?

The Fundamental Tradeoff in Titanium Heater Design for Service in Molten Sulfur
Molten sulphur tanks are maintained at 135°C to keep sulphur in liquid form for transportation or processing. The sulfur is saturated with hydrogen sulfide (H₂S) and polysulfides (H₂S_x) in solution. Titanium immersion heaters are utilized because titanium has strong resistance to sulphur and sulphides at moderate temperatures. However, titanium corrodes in molten sulfur by a chemical process: Ti + 2S -> TiS₂ The corrosion is consistent, not pitting or cracking in one area. The rate is about 0.05-0.15 mm/year depending on the H2S content. Since corrosion is uniform, the total metal loss (wall thinning) dictates the heater life. More wall thickness means proportionately more life . Twice the wall thickness means twice the service life . But there is no threshold thickness below which corrosion accelerates , or above which it stops . ( With pitting , thicker walls give diminishing returns . ) This research shows the reason of simple linear scaling in case of uniform corrosion and optimum thickness of the wall (1.5-2.0 mm) in case of molten sulfur service.

Mechanism of Uniform Corrosion: Impact on Mechanical Integrity
The titanium surface was found to directly react with sulfur and polysulfides in molten sulfur at 135°C, leading to the formation of titanium disulfide (TiS2). The TiS 2 coating is relatively protective but is slowly dissolved or spalled off by the flowing molten sulfur. The corrosion rate follows linear kinetics: d = k t, where k is 0.05–0.15 mm/year. There is no passive film breakdown threshold, no critical temperature for pitting and no mechanism for stress corrosion cracking. The corrosion is consistent throughout the whole immersed surface.

There is no localized attack, hence the time to perforation is simply: t_perf = (starting wall thickness) / (corrosion rate). If the corrosion rate is 0.10 mm/year, a 1.2 mm wall will be perforated in 12 years. 15 years for 1.5 mm wall. 20 years for a 2.0 mm thick wall. The relation is a perfect straight line. Therefore a thicker wall is of advantage in direct proportion to the extra material. The cost of titanium, however, likewise goes up linearly with thickness. There is no nonlinear benefit to going exceedingly thick.

Thermal Performance Impact : Heat Transfer penalty
A thicker wall incurs a small but real thermal cost. For a typical heat flux of 2.0 W/cm$^2$, the convective resistance dominates in molten sulfur (h $\approx$ 500 W/m$^2$K due to high viscosity). For 1.2 mm wall R_cond = 7.1e-5; for 2.0 mm wall R_cond = 1.18e-4. The total resistance is increased by 8%, hence the surface temperature is increased by 2–3°C. This speeds up the corrosion rate a little (some 10% for a 3'C rise), somewhat negating the advantage of the thicker wall. The overall benefit of increasing from 1.2 mm to 2.0 mm is an extension of life from 12 years to around 18 years (not 20 years because of temperature acceleration). The law of decreasing returns holds, although weakly.

The Trade-off Between Wall Thickness and Service Life
Wall Thickness (mm) Corrosion Rate at 135°C (mm/yr) Temperature Acceleration Factor Effective Corrosion Rate (mm/yr)Time to Perforation (years) Relative Material Cost 1.0 mm 0.10 1.00× 0.10 10 years 1.0× 1.2 mm 0.10 1.02× 0.102 11.8 years 1.2× 1.5 mm 0.10 1.05× 0.105 14.3 years 1.5× 1.8 mm 0.10 1.08× 0.108 16.7 years 1.8× 2.0 mm 0.10 1.10× 0.110 18.2 years 2.0× 2.5 mm 0.10 1.15× 0.115 21.7 years 2.5×
A wall 2.0 mm thick has a service life of 18 years, 80% longer than a wall 1.0 mm thick, but with 100% more material cost. The 1.5 mm wall delivers 14 years at 50% more expense, often the best economic point.

Engineering behind the Wall: Temperature Control and Quality of Sulfur
The H₂S concentration in the molten sulfur is a very sensitive parameter for the corrosion rate. In pure sulfur (low H2S), rates are as low as 0.03 mm/year. Sour sulfur with high H2S and polysulfides can be as high as 0.25 mm/year. If the sulfur quality is poor, a thicker wall is needed, but a better approach is to sparge the sulfur tank with nitrogen to strip H2S, reducing the corrosion rate by 70–80 %. Nitrogen Sparging 30+ Years 1.2mm Wall Temperature management is also important: decreasing the temperature of the tank from 135°C to 125°C reduces the corrosion rate by around 40%, which allows thinner walls.

Conclusion: Linear Benefit-Select Wall as a Function of Life and Cost
A thicker wall is helpful for a titanium immersion heater in a molten sulfur tank at 135°C with H2S and polysulfides as the corrosion is uniform and life depends on the total amount of metal dissolved. Doubling the wall thickness roughly doubles the service life (with a little penalty from temperature acceleration). Recommended wall thickness: 1.2–1.5 mm for 10 years of life; 1.5–1.8 mm for 15 years of life; 2.0–2.5 mm for 20 years of life. There is no threshold effect (thicker walls do not work). Best economic choice is usually 1.5 mm, which gives 14 years life at 50% more than 1.0 mm wall. Provide the estimated concentration of H₂S and the operating temperature to the manufacturer to allow them to calculate the exact corrosion rate. In case of molten sulfur service wall thickness is a direct linear design variable. Thicker is better but more expensive-choose for life needed.