The service life and corrosion resistance of pure titanium heating tubes entirely depend on the integrity and compactness of the surface TiO₂ passive film. Under long-term alternating acid-base CIP cleaning, thermal cycle impact and microbial scaling, the passive film will continuously produce micro-defects, thinning and local peeling. Many workshops carry out regular passivation maintenance, but still face frequent pitting corrosion and potential attenuation, which is mostly caused by incomplete passive film repair. This article analyzes the core reasons for failed film repair and proposes standardized rectification measures to achieve stable and dense passive film regeneration.
Insufficient dissolved oxygen supply is the primary cause of incomplete passivation. Titanium passive film is an oxygen-dependent self-repair structure. After CIP acid cleaning, the tube surface oxide layer is stripped and thinned, requiring sufficient dissolved oxygen in the medium to rebuild a compact protective film. Most workshops adopt static water standing without aeration, resulting in local oxygen deficiency in dead zones such as tube upper arc and support gaps. The newly generated oxide film is loose and porous, unable to block the penetration of organic acid and chloride ions, leading to continuous corrosion of the titanium matrix.
Unclean residual scale and surface contaminants are key hidden obstacles for film formation. Residual carbonized sugar scale, inorganic salt precipitates and polishing dust cover local tube surfaces, isolating the contact between the titanium matrix and passivation solution. Even if the overall passivation process is completed, covered areas cannot form new passive film, forming numerous microscopic corrosion gaps. In subsequent fermentation production, these blank areas preferentially become pitting corrosion sources, gradually expanding into large-area corrosion defects.
Unreasonable temperature and time control during passivation also leads to unqualified film repair. Excessively high passivation temperature accelerates the rapid decomposition of hydrogen peroxide, resulting in insufficient active oxygen supply and incomplete film densification. Too low temperature or insufficient soaking time makes the new film thin and unstable, easy to peel off under thermal cycling. In addition, direct passivation without neutral water transition after strong alkali cleaning causes residual alkaline substances to neutralize passivation reagents, completely failing the film regeneration effect.
Local galvanic interference also hinders uniform passive film repair. PTFE insulation sleeve aging, exposed steel supports and unisolated metal fasteners form potential difference interference on the titanium tube surface. The current difference makes the passive film thickness uneven, with low-potential areas forming weak corrosion zones. Long-term potential imbalance will cause regional film failure, resulting in inconsistent aging speed of the whole tube surface.
To solve the problem of incomplete passive film repair, systematic standardized passivation procedures must be implemented. First, thoroughly remove surface scale and impurities before passivation, ensure full exposure of the titanium matrix. Second, maintain continuous aeration during passivation to keep dissolved oxygen above 6mg/L, eliminating oxygen-deficient dead zones. Strictly control the temperature at 35–45℃ and extend static maturation time to 12–24 hours, ensuring sufficient time for film densification.
Auxiliary anti-corrosion isolation measures should be matched synchronously. Regularly replace aging PTFE gaskets and insulation sleeves to avoid dissimilar metal contact interference. For tubes with long-term low potential, adopt offline full immersion hydrogen peroxide passivation to thoroughly repair defective films. Conduct potential testing after each maintenance to ensure the overall surface potential is higher than +150mV with uniform distribution.
In conclusion, incomplete passive film repair is a key chronic problem leading to early failure of titanium heating tubes. Oxygen deficiency, residual dirt, improper parameter control and galvanic interference are the four major root causes. Standardizing pre-cleaning, oxygen supplementation, temperature time parameters and isolation protection can effectively improve the compactness and stability of the passive film, fundamentally enhance the tube's anti-corrosion ability, and extend the overall service cycle of titanium heating equipment.
