Pitting corrosion is a typical localized destructive corrosion failure of titanium heating tubes. Chloride ions, fluoride ions and other halogen ions in the working medium break the integrity of the titanium dioxide passive film at tiny surface defects, forming independent micro-anode corrosion pits. Corrosion expands rapidly inward along the depth direction while the surrounding base metal remains intact. As pits continuously deepen and connect, they penetrate the pipe wall and cause medium leakage. This type of corrosion has inconspicuous macroscopic features, strong randomness and sudden onset, ranking as one of the most common unexpected failure forms of titanium heat exchange components.
1. Electrochemical Formation Mechanism of Pitting Corrosion
A dense inert TiO₂ protective film spontaneously forms on the titanium surface to isolate the matrix from corrosive media. When halogen ions adsorb on micro scratches, inclusion defects or weak areas of the passive layer, they replace oxygen in the oxide film and generate soluble titanium halide substances, partially destroying the protective passive structure. Corrosion products seal the pit opening, forming a closed microenvironment inside the pit where halogen ions keep concentrating and the pH value declines. The inner wall of the pit serves as the anode and dissolves continuously, while the large-area intact passive surface outside acts as the cathode to accelerate the anodic reaction inside the pit. Corrosion develops vertically downward rather than spreading horizontally, eventually piercing the tube wall and triggering equipment leakage.
2. Typical Positions Susceptible to Pitting Corrosion
Liquid level fluctuation zones where halogen-containing liquid evaporates and precipitates crystalline salt for ion enrichment;
Mechanical scratches, indentations and collision damage left during processing, transportation and installation;
Areas covered by sediment, scale and attachments leading to ion accumulation under deposits;
Welded areas with loose oxide layers and incomplete post-weld passivation repair;
dead water sections with long-term static medium and continuous accumulation of corrosive anions.
3. Core Factors Accelerating Titanium Pitting Corrosion
High concentration of chloride and fluoride ions in the circulating process medium;
Elevated operating temperature that boosts the chemical activity of halogen ions to erode the passive film;
Unremoved welding oxide skin and surface mechanical damage after machining;
Long-term shutdown with residual corrosive liquid attached without thorough flushing and drainage;
Thin, discontinuous native passive film on the titanium surface.
4. Multi-dimensional Whole-Link Prevention Technical Measures
① Strictly control halogen ion content in the medium
Install filtration and desalination facilities to reduce the concentration of fluorine and chlorine ions in the fluid system.
② Eliminate surface defects and standardize post-weld treatment
Grind off burrs and scratches; carry out pickling and passivation after welding to remove oxide scale and rebuild a uniform passive film.
③ Regularly clean sediment and empty residual liquid
Drain all medium during equipment downtime and flush pipelines to avoid salt crystallization and local ion concentration.
④ Strengthen surface passivation reinforcement
Adopt anodic oxidation to artificially thicken the TiO₂ film and improve resistance to ion penetration and film breakdown.
⑤ Set reasonable upper limit of operating temperature
Avoid long-term over-temperature service to lower the erosion capacity of corrosive anions against the protective layer.
5. Prevention Effect Comparison Table
表格
| Treatment & Operation Mode | Pitting Corrosion Risk | Recommended Optimization Plan |
|---|---|---|
| Unremoved welding oxide + high fluorine/chloride medium | Extremely high risk of rapid pinpoint perforation | Perform pickling passivation and optimize water quality immediately |
| Post-weld passivation + periodic cleaning maintenance | Effectively inhibit pit initiation and expansion | Standard production and operation specification |
| Anodic oxidation strengthening + medium purification | Minimum pitting corrosion tendency | Preferred scheme for harsh halogen-containing working conditions |
Conclusion
Pitting corrosion of titanium heating tubes arises from localized breakdown of the passive film by halogen ions and occluded accelerated electrochemical corrosion inside pits. The fundamental prevention strategies include reducing corrosive ion concentration in the medium, eliminating surface defect initiation points, repairing welding damage via passivation treatment, and preventing salt deposition from static residual liquid. Closed-loop control of finishing processing, surface protection and daily operation maintenance can thoroughly eliminate hidden leakage risks caused by pitting corrosion of titanium heating tubes.
