For a Titanium Sheath Heater Used to Maintain 140°C in a 50% Sodium Hydroxide Solution Under 2 Bar Pressure, What Is the Maximum Allowable Tensile Stress (MPa) in the Tube Wall to Prevent Caustic Cracking for 5 Years?

Caustic cracking (stress corrosion cracking) of grade 2 titanium is caused by 50% sodium hydroxide at 140°C under 2 bar pressure. 5 years max allowed tensile stress to prevent cracking is 50 MPa. This is based on the threshold stress intensity factor of caustic cracking. The hoop stress caused by the internal pressure is σ = P × D / 2 t. For a 25 mm OD tube with 1.5 mm wall the hoop stress is 1.7 MPa at 2 bar far below the limit. But residual loads from bending or welding can be substantially higher.

Mechanism of Caustic Cracking in Ti

Titanium reacts with hot concentrated NaOH to form a layer of sodium titanate. Cracks begin at defects in this layer under tensile tension. The crack grows intergranularly. The threshold stress for fracture lowers with increasing temperature and concentration.

Quantitative threshold stress and its temperature dependence

Temperature (°C) NaOH Conc. (%) Stress Threshold for 5-Year Life (MPa) 120 50 100 130 50 75 140 50 50 150 50 30
Handbook of Stress Management

Source of Stress Normal Stress (MPa) Acceptable? Mitigation
Internal pressure (2 bar, 25x1.5 mm) 1.7 Yes No
Residual from bending (5D radius) 80-120 No Stress relief
Welding residual (as-welded) 150-250No Shine and stress relief
Thermal expansion stress 20-50 BorderlineFlexible Installation
Writing a Well-informed Specification

For titanium heater in 50% NaOH at 140°C: Stress relieve all welds and bends at 540°C for 30 minutes. Stress below 50 MPa. The engineer controls residual stress to prevent caustic cracking.