Pitting corrosion pinhole leakage failure refers to tiny defects, scratches and impurity inclusions on the surface passivation film of titanium heating pipes. Chloride, fluoride and other halogen ions in the process medium adsorb on these weak areas to break the TiO₂ protective layer locally. A tiny anode dissolution pit forms on the metal matrix, while the surrounding intact passivation area serves as the cathode. Inside the occluded pit, corrosive ions continuously enrich and acidify the solution, accelerating the pit to expand in depth rapidly. The corrosion hole penetrates the pipe wall to form pinhole leakage, and corrosive medium sprays out to erode surrounding equipment. This corrosion features small opening, deep hole body and strong concealment, which is one of the most common local failure modes of titanium pressure heating pipelines.
1. Passivation Film Breakdown and Pit Autocatalytic Expansion Mechanism
The complete titanium dioxide film can isolate electrochemical corrosion, yet micro defects make partial film lose protection. After halogen ions damage the passivation layer, the exposed titanium metal dissolves to form a small anode pit. To maintain charge balance, a large number of negative corrosive ions migrate into the pit, and hydrolysis reaction occurs inside the hole to generate hydrogen ions, further lowering the pH value. The acidic confined environment prevents the passive film from repairing itself, so the pit keeps expanding downward in an autocatalytic way. Different from crevice corrosion limited to assembly gaps, pitting corrosion initiates randomly on the pipe inner and outer surfaces without fixed structural slit conditions.
2. Typical Positions Prone to Pitting Damage
Inner and outer wall scratches left by mechanical collision, tool scraping and construction friction;
Inclusion defects on the base material surface and tiny pits formed during pickling treatment;
Horizontal pipeline bottom where sediment deposits and covers local pipe wall to form a closed microenvironment;
Weld surface oxidation areas and discontinuous passivation film positions after welding;
Low-flow dead leg pipelines where medium is static and corrosive ions continuously concentrate.
3. Core Inducing Factors Accelerating Pitting Failure
Medium contains chloride and fluoride ions exceeding the allowable concentration of titanium material;
No surface finishing and re-passivation treatment after pipeline cutting, bending and welding;
Pipeline lacks regular flushing and blowdown, solid dirt accumulates to cover the pipe wall;
External workshop acid mist and leaked corrosive liquid continuously erode the outer wall for a long time;
Improper pickling operation causes over-corrosion and residual tiny corrosion pits on the surface.
4. Full-Link Prevention and Control Technical Measures
① Install medium filtration and ion removal equipment at the system inlet to control halogen ion content
Cut off the core corrosive medium source from the front end of the process.
② Carry out integral polishing and re-passivation treatment on all processed and welded pipe sections
Repair damaged passivation film and eliminate surface vulnerable defect points.
③ Set low-point drain outlets and implement periodic online flushing to remove sediment deposits
Avoid local occlusion and ion enrichment caused by dirt covering.
④ Build workshop waste gas collection and outer wall anti-corrosion coating protection
Isolate external acid mist erosion to prevent outer wall pitting corrosion.
⑤ Regularly conduct eddy current inspection and wall thickness scanning on key pipeline sections
Detect hidden deep pitting holes before through-wall penetration and leakage.
5. Prevention Effect Comparison Table
表格
| Surface Treatment & Operation Mode | Pitting Corrosion Risk | Application Suggestion |
|---|---|---|
| Unpolished damaged surface + medium with high halogen ions + no regular blowdown | Random pinhole penetration leakage occurs during medium-term operation | Carry out overall passivation renovation and add medium purification facilities |
| Standard post-processing passivation + ion concentration control + periodic pipeline cleaning | Effectively restrain pit initiation and autocatalytic deep expansion | Standard acceptance specification for titanium heating pipeline prefabrication and operation maintenance |
| Enhanced anodic passivation treatment + real-time medium composition monitoring + annual full-line flaw detection | Extremely low random pinhole leakage and environmental pollution hidden danger | Preferred scheme for fine chemical high-corrosion medium long-cycle anti-corrosion heating pipe network projects |
Conclusion
Titanium heating tube pitting corrosion originates from local destruction of the surface passivation film by halogen ions, forming self-accelerating occluded corrosion pits that penetrate the pipe wall to cause leakage. Core prevention approaches include purifying the conveying medium to reduce corrosive ions, repairing surface protection film via polishing and passivation, removing pipeline sediment regularly, isolating external environmental corrosion and strengthening non-destructive inspection of hidden corrosion points. Whole-process closed-loop management of pipeline machining post-treatment, process medium control and daily maintenance inspection can eliminate unexpected shutdown and safety leakage accidents induced by pitting corrosion of titanium heating pressure pipelines.
