When a Titanium Immersion Heater Serves to Maintain 90°C in a Dipping Tank for Rubber Vulcanization (Contains Zinc Stearate and Sulfur), Does the Sulfur-Induced Corrosion at High Heat Flux Change the Optimal Wall Thickness Selection?When a Titanium Immersion Heater Serves to Maintain 90°C in a Dipping Tank for Rubber Vulcanization (Contains Zinc Stearate and Sulfur), Does the Sulfur-Induced Corrosion at High Heat Flux Change the Optimal Wall Thickness Selection?

Basic Trade-Offs in the Design of Titanium Heaters for Vulcanization Tanks
The rubber vulcanization dipping tanks are operated at 90°C with a formulation of zinc stearate (releasing agent) and sulfur (vulcanizing agent). Titanium immersion heaters sustain temperature for uniform vulcanization. The temperature of the titanium sheath surface at high heat flux (usually 2.5–3.5 W/cm^2) is 100–120°C. At these temperatures, sulfur interacts with titanium to generate titanium sulfide (TiS2) rather than the protective titanium dioxide layer. Titanium sulfide is non-passivating, porous and electrically conductive. This transforms the mechanism of corrosion from passive (negligible) to active (uniform or targeted attack). The wall thickness must be chosen to compensate for this sulfur attack corrosion which does not occur at lower heat fluxes or temperatures. The investigation evaluates if sulfur corrosion alters the optimum wall thickness in comparison to normal chloride or acid service.

Effect on Mechanical Integrity: Titanium Sulfide Growth
The reaction of titanium with sulfur at high temperatures is Ti + S2 -> TiS2. This reaction is kinetically important above 100 °C. The sulfide generation rate is minimal (<0.01 mm/year) with a titanium heater at 1.5 W/cm2 (surface temperature ≈ 95°C). At 2.5 Wcm-2 (surface temperature of ~115°C) the rate is 0.10-0.20 mm per year. At 3.5 W/cm2 (surface temperature ~ 135 degrees C) the rate is 0.5-1.0 mm/yr. The titanium sulphide layer does not shield the underlying metal. It is porous and spalls off exposing new titanium to additional reaction. This is a consistent thinning type corrosion, not a pitting.

Rubber vulcanization tanks usually contain 1 to 5 percent by weight sulfur . Zinc stearate is used as a releasing agent . Zinc stearate has no direct effect on corrosion but may trap sulfur against the heater surface and increase local concentration. Field data from rubber dipping lines indicate that titanium heaters with 1.2 mm walls operating at 2.5 W/cm2 have a thinning rate of 0.15 mm/year which equates to 8 years life. At 3.5 W/cm² the wall thickness decreases at 0.8 mm/year, giving only 1.5 years of life.

Control Variable: Heat Flux and Its Impact on Thermal Performance
The sulfur corrosion rate is exponentially related to the surface temperature, which is a function of wall thickness and heat flux. For a given bulk temperature (90°C) and convective coefficient (h ≈ 800 W/m2. K in agitated tank), the surface temperature is T_surface = 90 + q/h + ΔT_cond ΔT_cond = q x t / k_Ti A wall of 1.2 mm at 2.5 W/cm² yields T_surface ≈ 90 + 31.3 + 1.8 = 123°C. 1.5mm wall gives 90+31.3+2.2=123.5°C. minimal difference. For both thicknesses, at 3.5 W/cm 2 , T surface ≈ 90 + 43.8 + (2.5–3.0) = 136–137°C. Convective drop predominate and the influence of wall thickness on surface temperature is small. Hence, reducing the wall thickness does not have a substantial effect on lowering the rate of sulfur corrosion.

Trade-off Synthesis: optimal wall thickness and heat flux
Heat Flux (W/cm2) Surface Temperature (°C) Sulfur Corrosion Rate (mm/year)Wall Thickness for 5 Year Life (mm) Optimal Wall Thickness
1.5 W/cm² 95°C 0.01 0.5 mm 1.0 mm (mechanical minimum)
2.0 W/cm² 110°C 0.08 0.9 mm 1.2 mm 2.5 W/cm² 123°C 0.15 1.3 mm 1.5 mm 3.0 W/cm² 130°C 0.30 2.0 mm 2.0 mm 3.5 W/cm² 136°C 0.70 4.0 mm Not recommended (redesign process)
As the heat flux increases, the wall thickness required for a 5 year life increases correspondingly. The ideal wall thickness is shifted from 1.0 mm (low flux) to 1.5-2.0 mm (high flux). Sulfur corrosion is uniform thinning . Unlike chloride pitting where bigger walls have declining returns, the wall thickness gives a linear corrosion allowance.

Engineering Beyond the Wall: Surface Coatings and Heat Flux Reduction
The most efficient means of sulfur corrosion reduction is reducing the heat flux, not increasing the wall thickness. Operating at 2.0 W/cm² instead of 2.5 W/cm² can lower the needed wall thickness from 1.5 mm to 1.2 mm, while extending heater life. This means larger heating surface area (longer tubes or more tubes), but the trade off is worth it. Alternatively, a thin (2–5 µm) electroless nickel-phosphorus (ENP) layer on the titanium sheath prevents sulfur from contacting the titanium. ENP is resistant to sulphur attack up to 150°C. A 1.0 mm titanium wall with an ENP coating will last 10+ years at 3.5 W/cm2.

Conclusions: Dependence of wall thickness on heat flux in sulfur operation
A titanium immersion heater is used to maintain 90°C in a rubber vulcanization dipping tank with zinc stearate and sulfur, and the selection of the best wall thickness is modified by sulfur-induced corrosion. Standard wall of 1.0–1.2 mm at low heat flux (<=2.0 W/cm2) is enough. At high heat flux (2.5-3.0 W/cm 2 ) the sulfur corrosion rate increases to 0.15-0.30 mm/year needing 1.5-2.0 mm wall for 5-year life. When the power density is above 3.0 W/cm², wall thickness required is > 2.5 mm and a process redesign (reduced heat flux or coated sheath) is recommended. The optimum thickness of a wall for a certain application is decided by the needed heat flux and not by a standard thickness. For rubber vulcanization service, state to the heater manufacturer the expected heat flux (W/cm2) or surface temperature. Using this data, wall thickness can be computed as t = corrosion rate x desired life, with the corrosion rate being obtained using sulfur-temperature kinetics. Generally, a 1.5 mm titanium wall gives a strong 5-8 year service life for most rubber dipping tanks working at 2.0-2.5 W/cm2.