When a Titanium Sheath Heater Is Used to Boil 1% Hydrobromic Acid at 100°C, What Is the Maximum Allowable Bromide Concentration Gradient (Surface vs. Bulk) Before Localized Transpassive Dissolution Begins?

In boiling 1% hydrobromic acid (HBr) at 100°C, passivity of titanium is preserved by a stable oxide coating. But bromide ions (Br-) could trigger transpassive dissolving at high surface concentration. The risk is related to the concentration gradient between the surface of the tube (where HBr is concentrated via evaporation) and the bulk solution. Localised transpassive dissolution is initiated at a surface bromide concentration of 5× the bulk concentration.

Concentrational Gradients: Quantitative Limitations

For bulk 1% HBr (≈ 10 000 ppm Br−):

Surface/bulk ratio <3×: Passive. No transpassive dissolving.

Transition zone surf/bulk ratio 3-5x Metastable attack occasionally .

Surface/bulk ratio >5×: Transpassive dissolution, stable. Quick localised attack.

Gradient Control Reference

Bulk HBr Concentration (%) Max allowable surface/bulk ratioSuggested Action
0.5 8× Decrease heat flow
1.0 5X Normal Operation
2.0 3× Flow velocity increase
2× 3.0 Not suggested
Engineering tip

To keep the bromide concentration down, keep the tube surface temperature below 110°C (ΔT=10°C) during boiling 1% HBr. Flow velocity increased to 0.5 m/sec to lessen concentration gradient. The engineer controls the bromide concentration gradient to avoid transpassive dissolution.