Under-deposit pitting corrosion is the most common and destructive hidden failure of titanium heating tubes in long-term fermentation production. Different from uniform surface aging, this type of corrosion occurs under scale, biofilm and sediment coverage, developing silently without obvious surface signs. Most penetrating leakage and weld failure of titanium tubes are ultimately caused by long-term under-deposit pitting. This article systematically explains the formation mechanism, high-risk locations, influencing factors and complete prevention control schemes to guide on-site anti-corrosion management.
The core formation mechanism lies in the local micro-battery effect under scale coverage. Organic sugar carbon scale, bacterial biofilm and inorganic salt deposits tightly adhere to the titanium tube surface during heating operation. The covered area forms an oxygen-deficient anaerobic environment, while the surrounding clean tube surface fully contacts oxygen-rich fermentation liquid. This creates a significant oxygen concentration difference, turning the scale-covered area into an anode zone with continuous dissolution of the titanium matrix. Meanwhile, acid substances and chloride ions are continuously enriched under the scale, further lowering the local pH value and accelerating localized pitting expansion.
Heating temperature and power density are key inducing factors. High surface power density raises the tube wall temperature, accelerating sugar carbonization and salt crystallization to form compact hard scale that is difficult to remove by ordinary circulation scouring. High temperature also speeds up electrochemical reaction rates, making micro-pits expand rapidly along the grain boundary. This explains why high-load continuous fermentation tanks have far higher pitting failure rates than intermittent production tanks.
On-site statistics show three high-risk areas for under-deposit corrosion: weld heat-affected zones, upper tube bubble stagnation areas, and support contact gaps. The weld area has uneven surface and thin passive film, making scale adhesion easier. The upper tube arc easily accumulates micro-bubbles and forms scale-covered dead zones. Support gaps cannot be flushed by circulating liquid, leading to long-term sediment accumulation and concentrated pitting corrosion.
Improper CIP cleaning is the main human-induced cause of worsening pitting. Insufficient cleaning frequency, low flow velocity and unreasonable cleaning temperature leave residual scale on the tube wall after each maintenance cycle. Repeated accumulation forms layered hard scale, and the pitting pits generated in the early stage continue to deepen under subsequent production conditions, eventually penetrating the tube wall and causing medium leakage.
Targeted prevention measures must be implemented from design, operation and maintenance dimensions. In structural design, adopt low surface power density layout and inclined tube installation to reduce overheating and bubble retention. Optimize circulating flow velocity above 1.0 m/s to enhance surface scouring and inhibit scale adhesion. In daily operation, avoid long-term full-power heating and maintain stable aeration to reduce anaerobic microzone formation.
In terms of maintenance, formulate graded descaling cycles according to medium viscosity. High-sugar and high-viscosity fermentation media require biweekly deep composite cleaning to remove aged carbon scale. Conduct quarterly focused wall thickness measurement on high-risk areas to monitor pitting progression. After cleaning, complete oxygen-rich water passivation to repair damaged passive film and block ion invasion channels.
In summary, under-deposit pitting corrosion is the primary hidden danger restricting the service life of titanium heating tubes. It progresses covertly and has strong destructive power. Standardizing thermal load design, circulating flow control and scientific descaling passivation cycles can effectively eliminate sediment-induced localized corrosion, ensuring long-term safe and stable operation of titanium heating systems in fermentation workshops.
