Grade 12 titanium (Ti-0.3Mo-0.8Ni) is used for heater tubes in chloride-containing environments due to its improved pitting resistance. When the tube is bent to a tight radius of 3 times the tube diameter (3D), the outer radius undergoes plastic tensile strain of 10–15%. This cold work creates residual tensile stress, increases dislocation density, and can align the alpha-phase grains. The pitting potential, which is the electrochemical potential at which stable pits initiate, decreases with increasing cold work. Stress-relief annealing at 540°C for 30 minutes recovers a portion of the strain, reduces residual stress, and allows some recrystallization. The pitting potential of a bent and stress-relieved tube returns to within 50–100 mV of the unbent condition, compared to a 150–250 mV depression for an as-bent tube without stress relief.

The Mechanism of Residual Strain Recovery and Pitting Potential Restoration

During plastic bending, dislocations multiply and accumulate at grain boundaries, creating high-energy sites that are preferential for pit initiation. Tensile residual stress at the outer radius further destabilizes the passive film. Stress-relief annealing at 540°C allows dislocation climb and annihilation, reducing the dislocation density by 60–80%. The process also reduces residual tensile stress from 150–250 MPa to 30–60 MPa. Molybdenum and nickel in Grade 12 redistribute during annealing, enriching the surface and improving passive film stability. The pitting potential is restored because the number of active sites for pit initiation is reduced, and the passive film becomes more uniform.

Quantitative Pitting Potential as a Function of Bending and Stress Relief

Controlled testing in 5% NaCl at 90°C has established pitting potentials for Grade 12 titanium tubes under various conditions. For an unbent, annealed tube (baseline), the pitting potential is +0.45 to +0.60 V vs. Ag/AgCl, with no pitting observed after 1,000 hours. For a tube bent to a 3D radius in the as-bent condition with no stress relief, the pitting potential drops to +0.20 to +0.35 V, and pitting initiates within 100–300 hours, producing a pit depth of 0.1–0.3 mm after 500 hours. For a tube bent to a 3D radius and stress-relieved at 540°C for 30 minutes, the pitting potential recovers to +0.35 to +0.50 V, pitting initiates after 500–1,500 hours, and pit depths are 0.05–0.15 mm after 500 hours. For a tube bent to a 5D radius (gentler bend) with stress relief, the pitting potential is +0.40 to +0.55 V, which is very close to the unbent baseline. For a tube bent to a 3D radius and stress-relieved at 480°C (insufficient temperature), the pitting potential is only +0.25 to +0.40 V, showing partial recovery.

Influence of Bend Radius and Stress Relief Temperature on Pitting Potential

The recovery of pitting potential depends on both the bend radius and the stress relief temperature. For a 3D bend, the as-bent pitting potential depression is 150–250 mV below the unbent value. Stress relief at 480°C recovers 30–50 mV, leaving a 120–200 mV depression. Stress relief at 540°C recovers 100–150 mV, leaving a 50–100 mV depression. Stress relief at 600°C recovers 150–200 mV, leaving a 20–50 mV depression, but risks alpha-case formation if not performed in an inert atmosphere. For a 5D bend, the as-bent depression is only 50–100 mV, and stress relief at 540°C recovers fully to the unbent level. For a 2D bend (tighter than 3D), the as-bent depression is 200–350 mV, and even stress relief at 540°C only recovers 80–120 mV, leaving a significant depression.

Bend Radius and Stress Relief Selection Guide for Grade 12 in Hot Chloride Service

The following table provides recommendations for bending Grade 12 titanium heater tubes for service in 5% NaCl at 90°C, based on bend radius, stress relief treatment, and desired pitting resistance.

Engineering Beyond Stress Relief Treatment

The titanium grade affects the initial pitting potential and the recovery after stress relief. Grade 7 has a baseline pitting potential 100–200 mV higher than Grade 12, so even a 3D as-bent Grade 7 tube (+0.30 to +0.45 V) may be acceptable for many applications. Wall thickness does not affect pitting potential but influences pit propagation time; a 2.0 mm wall with a 0.1 mm pit depth has 95% of its wall remaining. The stress relief atmosphere is critical; annealing in air at 540°C forms a thin oxide that does not harm pitting resistance, but annealing in argon or vacuum is preferred. The cooling rate after stress relief has no significant effect on pitting potential.

Making an Informed Specification

For a Grade 12 titanium heater tube requiring a 3D bend for service in 5% NaCl at 90°C, specify stress-relief annealing at 540°C for 30 minutes in an argon atmosphere after bending. Require verification of pitting potential by cyclic polarization testing on a sample from the bent and stress-relieved tube; the pitting potential should be within 100 mV of the unbent baseline value of +0.45 to +0.60 V. For bends tighter than 3D, consider using Grade 7 titanium or increasing the bend radius to 5D. If stress relief is not possible, accept a shorter service life and inspect the bend area quarterly for pitting. During operation, monitor the open-circuit potential of the heater; a drop below +0.20 V vs. Ag/AgCl indicates increased pitting risk. By stress-relieving at 540°C after bending, the engineer restores the pitting potential of Grade 12 titanium to within 50–100 mV of the unbent condition, ensuring reliable performance in hot chloride service.