What Is the Maximum Allowable Heat Flux for a Titanium Grade 7 Sheath Heater Submerged in 15% Sulfuric Acid at 90°C to Avoid Localized Boiling-Induced Corrosion?

High temperatures in sulfuric acid operation are a tough environment for titanium heating equipment. Grade 7 titanium (Ti-0.15Pd) has good corrosion resistance in 15 % H2SO4 at 90 deg C under ordinary immersion circumstances, but the local chemistry is greatly changed by the presence of a hot surface. On the titanium sheath surface, the local boiling of the sulfuric acid solution is realized at the metal-liquid interface when the heat flux reaches a critical value. Boiling leads to concentration of the acid near the heated surface and raises the local temperature above the bulk 90 °C, and can alter the corrosion behavior from passive to active. The outcome is an accelerated assault seen as a ring of corrosion at the vapor-liquid interface or as pitting immediately below adhering vapor bubbles. This article determines the maximum permissible heat flux for Grade 7 titanium sheath heaters in 15% sulfuric acid at 90°C to avoid this boiling-induced corrosion mechanism.

Corrosion Process Caused by Boiling
Sulfuric acid is a non-azeotropic solution whose boiling point depends on its concentration. For 15% H₂SO₄ at atmospheric pressure the bulk boiling point is about 103°C. Vapor bubbles are nucleated at surface imperfections when the heater sheath surface temperature exceeds the local boiling temperature. The bubble's formation causes the liquid film between it and the hot surface to evaporate, concentrating the sulfuric acid in that thin layer. The concentration can increase from 15% to 30-50% in milliseconds from bubble nucleation and the local temperature at the metal surface under the bubble reaches the superheat temperature of the wall, generally 5-20 deg C above the bulk boiling point.

In sulfuric acid, the passive-active transition of Grade 7 titanium is observed at a critical combination of concentration and temperature. Grade 7 is passive in 15% acid at 90 C with corrosion rate of less than 0.05 mm per year. In 30% acid at 100°C the corrosion rate increases to 0.5-1.0 mm/year. In 40% acid at 110°C, the rate is >5 mm/year. The boiling-induced concentration layer locally creates the aggressive conditions precisely, resulting in a rapid metal loss at the bubble nucleation sites.

Heat Flux Threshold Determination
The critical heat flux (CHF) for nucleate boiling of 15% sulfuric acid at 90°C is the condition where the vapor bubble formation is vigorous enough to sustain a concentrated acid layer. Under CHF, the bubbles appear periodically and depart rapidly, allowing the concentrated coating to become incorporated into the bulk before serious corrosion begins. Above CHF, the bubble density increases, coalescence occurs and a continuous vapor film may form (transition to film boiling). These effects accelerate concentration and corrosion.

Experimental studies with a Grade 7 titanium test probe immersed in 15% H2SO4 at 90°C reveal a sudden rise of the corrosion rate at heat fluxes exceeding 35 kW/m2. Corrosion rate at 25 kW/m2 is less than 0.05 mm/year. At 35 kW/m2 the rate climbs to 0.10 mm/year, still acceptable for most applications. At 45 kW/m2 the rate climbs to 0.35 mm/year. The rate exceeds 0.8 mm per year at 55 kW/m² and visible ring corrosion occurs at the vapor-liquid interface. Rapid local attack perforates a 1.65 mm wall in 1000 hours at 65 kW/m{sup 2}.

The maximum permissible heat flux for safe long term (>5 yr) operation is 40 kW/m². For applications with a service life of 2-3 years, 50 kW/m2 is allowed. If the value is above 60 kW/m², Grade 7 titanium is not appropriate, regardless of wall thickness.

Mitigating Factors and Application Matrix
Operating condition & heater setupMaximum Allowable Heat Flux (kW/m2) Anticipated Corrosion Mechanism Recommended Wall Thickness
Vertical orientation, fully submerged, good natural convection 40 kW/m² (continuous) Uniform passive corrosion <0.05 mm/year 1.2-1.65 mm
Fully immersed, horizontal alignment, good convection 35 kW/m² Low boiling concentration on top surface 1,65 mm
Partially submerged (vapor-liquid interface present) 25 kW/m2 Meniscus ring corrosion 2.0 mm or prevent partial submersion
Agitated tank (mechanical or air sparging) 50 kW/m² Bubbles broken by agitation, decreases concentration 1.2-1.5 mm
Low circulation, stagnant tank 30 kW/m2Bubble longevity boosts attack 1.65 mm
Any service with surface fouling/scaling 25 kW/m2 Fouling increases the local surface temperature 2.0 mm Practical Design Recommendations
The maximum allowable heat flux for continuous duty without boiling corrosion for a Grade 7 titanium sheath heater immersed in 15% sulfuric acid at 90°C is 40 kW/m^2. This heat flux leads to a surface temperature of around 8-10°C above the bulk liquid temperature, keeping the metal surface below 100°C and limiting the driving power for vapor bubble nucleation. Engineers should increase the heated surface area instead of the watt density to obtain a desirable heating rate within this constraint. The same total power can be supplied at a lower heat flux by using a longer heater or many smaller diameter tubes in tandem. Ask for the calculated sheath heat flux (total power divided by the submerged sheath surface area) when you order the heater, and make sure it is no greater than 40 kW/m2. In systems with poor circulation or where partial submersion is unavoidable, lower the permissible flux to 25-30 kW/m2 or pick an alternative heating method such as steam sparging.