Pitting corrosion perforation failure refers to halogen ions, acid radical ions and solid particle scratches locally destroying the dense titanium dioxide passive film on the inner and outer walls of titanium heating pipes. Tiny anode corrosion pits form at damaged positions. A closed occluded microenvironment is generated inside pits, corrosive ions continuously accumulate and acidity rises, which accelerates unidirectional deep etching along the pipe wall thickness direction. Eventually, through-wall pinhole leakage takes place, while most areas of the titanium pipe surface still retain intact anti-corrosion passivation layer. This localized corrosion features strong concealment and sudden failure, being one of the most common failure forms of titanium pressure heating pipelines in chlorine-containing chemical working conditions.
1. Passive Film Breakdown and Self-Catalytic Pitting Expansion Mechanism
Titanium spontaneously forms a compact inert TiO₂ protective film in aerated neutral medium to block electrochemical corrosion. Chloride ions have strong adsorption and penetration ability, which replaces oxygen elements on the passive film and destroys the protective structure at micro defects. Exposed titanium matrix inside pits serves as anode and dissolves continuously; the surrounding complete passive area acts as cathode to form a galvanic corrosion loop. Corrosion products deposit at pit openings to seal the cavity, resulting in high enrichment of halogen ions and hydrogen ions inside the pit, forming a self-accelerating corrosion system. Corrosion develops inward vertically without lateral expansion, so only tiny leakage holes appear on the outer wall after penetration. Different from uniform thinning corrosion caused by overall medium erosion, pitting corrosion concentrates on discrete tiny points with extremely high penetration efficiency.
2. Typical Positions Prone to Pitting Damage
Bottom of horizontal pipelines where sediment and salt precipitates accumulate for long-term static immersion;
Surface scratch areas produced during transportation, hoisting and pipeline assembly construction;
Weld seams and heat-affected zones with uneven oxide film and metallurgical defects;
Blind pipes and dead legs with non-circulating medium leading to continuous ion concentration;
Outer wall insulation damaged parts where rainwater and condensate gather to form local corrosive environment.
3. Core Inducing Factors Accelerating Pitting Failure
Conveying medium contains chloride, bromide and other halogen corrosive components without filtration purification;
Welded titanium pipelines lack post-weld pickling and passivation treatment to repair damaged passive film;
Hard impurities in the fluid scratch the pipe wall and create initial defect points for pit initiation;
Long-term shutdown without draining residual liquid causes corrosive substances to precipitate and adhere to the inner wall;
Local overtemperature intensifies ion migration speed and electrochemical reaction, greatly shortening perforation cycle.
4. Full-Link Prevention and Control Technical Measures
① Install precision filtration and medium softening equipment at the front end of the system
Remove halogen ions and solid impurities to eliminate pitting inducing sources fundamentally.
② Conduct integral pickling, polishing and passivation treatment after welding and pipeline installation
Reconstruct uniform and stable anti-pitting passive film on titanium surface.
③ Optimize pipeline gradient layout and set regular blowdown drainage ports
Avoid medium stagnation and salt deposition in low-lying and dead-end pipe sections.
④ Standardize hoisting and construction operation to prevent hard object collision and surface scratching
Reduce mechanical defects that easily induce pit nucleation.
⑤ Arrange periodic wall thickness inspection and leakage monitoring for key vulnerable pipe segments
Detect shallow pits in advance before through-wall leakage occurs.
5. Prevention Effect Comparison Table
表格
| Treatment & Operation Mode | Pitting Perforation Risk | Application Suggestion |
|---|---|---|
| Unfiltered halogen-containing medium + no post-weld passivation + long-term liquid retention | Pinhole leakage occurs within medium-term operation cycle | Add medium purification device and implement overall passivation renovation |
| Medium impurity interception + standardized post-processing passivation + periodic blowdown maintenance | Effectively restrain passive film damage and self-catalytic pit expansion | Standard construction specification for titanium heating process pipelines |
| Enhanced super-passivation treatment + online medium ion monitoring + regular eddy current flaw detection | Extremely low hidden danger of sudden leakage and unplanned shutdown | Preferred scheme for long-term operation of chemical corrosive medium titanium heating pipe network |
Conclusion
Titanium heating tube pitting corrosion is triggered by local breakdown of the surface passive film under halogen ion erosion, forming a closed self-catalytic corrosion cell that penetrates the pipe wall to form pinhole leakage. Core prevention measures include purifying incoming medium to reduce corrosive components, completing pickling and passivation to reinforce the protective oxide layer, optimizing pipeline layout to prevent medium deposition, avoiding mechanical damage to the pipe surface and carrying out routine inspection and blowdown maintenance. Whole-process closed-loop management of pipeline processing, medium pretreatment and daily operation maintenance can eliminate sudden leakage and production interruption accidents caused by pitting perforation of titanium heating tubes.
