How does dissolved oxygen content decide the self-repair efficiency of titanium passivation film?

# How does the self-repair efficacy of titanium passivation film depend on the dissolved oxygen content? In order to prevent chloride corrosion in fermentation media, Grade 2 pure titanium heating tubes are dependent on a thin TiO₂ passivation film. The primary raw material for the continuous self-repair of this protective film is dissolved oxygen in circulating cleansing water. Tiny scratches and etching spots on the tube surface will be unable to recover due to insufficient dissolved oxygen, resulting in the gradual development of large-area milky white corrosion layers. The following table illustrates the correlation between the passivation film repair effect and the dissolved oxygen concentration under standard CIP washing conditions. | Dissolved Oxygen Level | TiO₂ Film Repair Speed | Surface Condition of Titanium Tube | Corrosion Risk Level | Standard CIP Operation Adjustment | | ---- | ---- | ---- | ---- | ---- | | Below 4 mg/L | Almost no self-repair ability | Faint discolouration, scratch marks stay permanent | High risk | Start independent aeration pump for the whole cleaning cycle | | 4–7 mg/L | Slow incomplete repair | Shallow etching spots remain after cleaning | Medium risk | Extend aeration time by 15 minutes per CIP batch | | 8–12 mg/L | Fast complete self-repair | Smooth uniform film, no residual scratches | Low risk | Maintain existing aeration equipment parameters | | Above 12 mg/L | Stable ultra-fast repair | Dense thickened protective film, strong anti-chloride performance | Negligible risk | Keep current aeration without extra adjustment | The self-repair mechanism of titanium passivation film relies on electrochemical oxidation reactions between titanium metal and oxygen-containing water. Exposed fresh titanium metal will combine with dissolved oxygen and water molecules to generate new titanium dioxide, filling damaged areas and rebuilding a continuous anti-corrosion barrier, when the tube surface is scratched by solid particles or slightly etched by trace chloride. If dissolved oxygen is insufficient during CIP circulation, the chemical reaction is unable to proceed as intended, and the protection of damaged areas is lost. Metal substrates will undergo irreversible uniform thinning as chloride ions continue to infiltrate them. The connection between titanium heating tubes and aeration systems is frequently disregarded in fermentation workshops. Operators frequently disable aeration pumps during the CIP cleansing phase in order to reduce power consumption, resulting in a rapid decrease in dissolved oxygen levels below 4 mg/L. Despite the complete isolation of fluoride contamination, the absence of oxygen will still result in surface discolouration and corrosion within three months. Continuous declines in the electrochemical potential value of tube bundles will also result from long-term operation under low dissolved oxygen. This phenomenon can be readily observed during quarterly full-surface potential scanning inspections. A mandatory operating standard for production lines with a high chloride medium is constant dissolved oxygen levels exceeding 8 mg/L. In order to guarantee precise real-time readings, equipment technicians must calibrate dissolved oxygen detectors on a monthly basis. If the oxygen supply is inadequate due to the ageing of aeration equipment, it is imperative to promptly replace the aeration nozzles and increase the circulation flow. Before resuming fermentation, conduct a 30-minute oxygen-rich water circulation following each extended shutdown to fully restore the passivation film that was damaged during idle storage. In the long term, the inherent anti-corrosion advantage of titanium tubes can be maximised, the frequency of offline enhanced passivation maintenance can be reduced, and the cost of comprehensive equipment operation can be reduced by strict control of dissolved oxygen indicators.