Stress Corrosion Cracking Prevention of TA2 Pure Titanium Heating Tubes in Chloride Medium

TA2 industrial pure titanium boasts outstanding resistance to chloride-containing media such as seawater and brine, which makes it the preferred heating material for marine and desalination projects. Nevertheless, titanium is still susceptible to stress corrosion cracking (SCC) under the combined action of tensile residual stress, elevated temperature and concentrated chloride environment. Unlike stainless steel, titanium SCC has unique inducement conditions and expansion rules, and targeted process control can eliminate most cracking failures.

1. Formation Mechanism of Titanium Stress Corrosion Cracking

Titanium spontaneously forms a dense TiO₂ passive film in neutral and oxidizing chloride solutions. When tensile stress exists inside the pipe body, microcracks generate on the surface oxide film. Chloride ions permeate the crack tip, dissolving the fresh titanium matrix continuously. Stress concentrates at the crack front to drive further propagation of tiny cracks, finally developing into penetrating brittle cracks without obvious plastic deformation on the outer surface. This failure often happens suddenly during long-term stable operation, bringing risks of medium leakage and equipment shutdown.

2. Main Sources of Tensile Stress on Titanium Heating Tubes

(1) Cold Bending Forming Stress

U-shaped, L-shaped and coiled titanium heating tubes require cold bending processing. The outer arc of the bending area produces permanent tensile residual stress, which is the highest-risk position for SCC. Improper bending radius will amplify stress concentration sharply.

(2) Welding Residual Stress

Local high-temperature melting and rapid cooling during argon arc welding create uneven thermal shrinkage stress in the weld and heat-affected zone. Titanium welds without protective measures are prone to oxygen embrittlement, further reducing crack resistance.

(3) Assembly and Installation Stress

Forced alignment during flange locking, rigid clamping and pipeline pulling apply external assembly tensile stress to the tube body, superimposing with internal processing stress to greatly lower the critical SCC temperature threshold.

(4) Thermal Cycle Stress

Frequent temperature rise and fall of the heating tube produce alternating thermal stress, which accelerates the initiation and extension of microcracks.

3. Core Restrictive Service Conditions for Titanium SCC

Titanium SCC rarely occurs in room-temperature dilute chloride solution. The three necessary inducing factors are:

Persistent tensile stress (residual or external);

Medium temperature above 70℃;

Chloride ion concentration at a high level (seawater, saturated brine). Systems lacking any one condition basically avoid this cracking risk.

4. Full Process Prevention Technical Measures

① Standardize Bending Process Parameters

Specify the minimum bending radius no less than 3 times the outer diameter of the titanium tube to prevent excessive tensile deformation at the bend. After bending, carry out low-temperature stress relief annealing to eliminate forming residual stress.

② Strict Welding Protection & Post-Weld Treatment

Adopt double-sided full argon shielding welding to prevent weld oxidation and embrittlement. For heating tubes used in high-temperature brine, implement post-weld solution heat treatment to homogenize microstructure and eliminate welding stress.

③ Standardize Installation Specifications

Prohibit forced assembly and rigid fixed constraints; use flexible gaskets and loose mounting supports to avoid introducing additional external tensile stress.

④ Optimize Power Density to Control Surface Temperature

Limit surface power density to prevent local overheating of the pipe wall, keeping the working temperature below the sensitive temperature range of titanium SCC as much as possible.

⑤ Avoid Long-Term Static Medium Soaking

During equipment shutdown and standby, drain the brine inside the tank to prevent chloride ion enrichment and local temperature rise under static conditions.

5. Material Alternative Reference

If the working temperature is permanently higher than 120℃ with saturated brine and complex stress conditions, TA1 low-stress titanium or titanium alloy can be selected to improve anti-SCC performance. Fluoride-containing media are completely unsuitable for titanium regardless of stress conditions.

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Conclusion

TA2 titanium heating tubes do not have absolute immunity to stress corrosion cracking in high-temperature high-chloride environments. Cutting off stress sources from forming, welding and installation links is the most fundamental prevention method, which can give full play to titanium's long-life advantage in marine anti-corrosion heating scenarios.