Troubleshooting guide for common failure phenomena of titanium heating tubes in fermentation tanks

In organic acid fermentation containers for pharmaceuticals and food, titanium heating tubes are frequently employed. Various anomalous failure phenomena will result from long-term operation under high sugar broth, alternating acid-base CIP cleaning, and frequent thermal cycling. This article delineates the typical fault manifestations, fundamental corrosion/structural causes, rapid judgement methods, and targeted solutions for on-site maintenance personnel.

1. Phenomenon 1: The entire tube surface is uniformly matte grey, with no fissures or leakage.
Cause
The outer loose layer of the TiO₂ passive film is continuously thinned by long-term and repeated mild acid-base alternating erosion. The film is unable to maintain a bright, uniform state due to a slight insufficiency of medium dissolved oxygen. Passive film ageing is the sole occurrence; matrix erosion is not present.

Basis of Judgement
The electrochemical potential test is slightly lower than the standard +150mV threshold; wall thickness measurement does not indicate any apparent thinning.

Resolution
Full-tank circulation flushing with warm, aerated clean water for 24 hours to facilitate natural re-passivation.

For chemical passivation, soak the affected area in a dilute hydrogen peroxide solution if the discolouration is severe.

Ensure that the dissolved oxygen level in the tank remains above 6mg/L during daily operation by increasing the aeration.

2. Phenomenon 2: Local wall thickness loss, concentrated on welds and sediment coverage regions, scattered small deep pits
Cause
Under-deposit crevice corrosion and an anaerobic microenvironment under organic scale; the weld heat-affected zone has a naturally reduced passive film, resulting in priority corrosion hotspots.

Basis of Judgement
Local wall thinning at pits is detected through ultrasonic thickness measurement; pit sites are consistent with long-term sediment accumulation positions.

Resolution
Hydrogen peroxide-citric acid composite cleaning liquid is employed to eliminate compact organic carbon scale.

Utilise fluoride-free ultra-fine polishing paste to polish pit positions, followed by integral re-passivation.

Shorten the monthly online cleansing cycle and increase the circulating flow velocity to reduce scale adhesion.

3. Phenomenon 3: Localised accelerated corrosion in the flange contact area, resulting in flange pitting and thinning.
Cause
Galvanic corrosion is a result of the direct contact between titanium flanges and steel/galvanized brackets that lack PTFE insulating gaskets. The corrosion current density is accelerated by the small-area anode metal.

Basis of Judgement
Corrosion is exclusively observed in the metal contact spaces between the flange and support fittings, while the other tube surfaces remain unaffected.

Resolution
Cut conductive loops by disassembling all connections and adding complete PTFE isolation gaskets and insulating bolt sleeves.

Polish the surface of the corroded flange and administer a re-passivation treatment.

It is recommended that metal supports be replaced with either pure titanium or PTFE-coated supports for continued use.

4. Phenomenon 4: Tube surface dense micro-cracks, film flaking after frequent heating-shutdown cycles
Cause
The thermal expansion coefficients of the brittle TiO₂ passive film and the titanium substrate are mismatched. This results in cyclic thermal stress, which in turn causes film cracking and spalling due to the rapid rise and decline of temperature.

Basis of Judgement
The tube body is uniformly covered in cracks, with the most severe cracks occurring at locations with a significant temperature difference. The system does not contain any medium fluoride ions.

Resolution
Standardise operation: prevent instantaneous full-power startup and cold water quenching of hot tubes, regulate the heating/cooling rate to ≤5℃/min.

Quarterly maintenance is conducted to repair accumulated micro-cracks through the implementation of integral re-passivation.

A low surface power density heating design should be implemented for newly replaced tubes.

5. Phenomenon 5: A uniform thinning of the overall tube wall, a dull rough surface, and continuous matrix etching.
Cause
System medium comprises fluoride ions, or long-term boiling high-concentration strong alkali cleaning; fluoride/boiling alkali continuously dissolves TiO₂ film and evenly etches titanium substrate.

Basis of Judgement
The presence of fluoride ions is confirmed by water quality detection, or by the use of standard hot alkali CIP procedures that exceed the recommended level. The tube experiences a uniform loss of wall thickness.

Resolution
Adjust the process: stop boiling strong alkali cleaning and neutralise the medium to pH 11 to prevent fluoride corrosion.

In systems that are subject to fluoride ions that are unavoidable, it is necessary to replace titanium heating tubes with heaters that are entirely coated in PFA.

Reduce the frequency of wall thickness inspections to monthly and develop an advance replacement strategy.

6. Phenomenon 6: Sudden tube rupture without apparent surface corrosion marks, brittle fracture at welds
Cause
Hydrogen embrittlement damage is a result of long-term inorganic acid pickling. Atomic hydrogen penetrates the titanium matrix and accumulates at grain boundaries, causing brittle cracking that is triggered by thermal stress.

Basis of Judgement
The intergranular brittle morphology is evident in the fracture section, and the maintenance records indicate that frequent dilute hydrochloric/sulfuric acid descaling was performed without the need for post-pickling hydrogen removal baking.

Resolution
Substitute the inorganic acid pickling liquid with a mild citric acid organic descaling agent.

After any inorganic acid cleaning, it is mandatory to bake for hydrogen removal at a temperature of 120–150℃ for a duration of 2–3 hours.

Screen internal hydrogen defects on spare titanium tubes through ultrasonic flaw detection on a regular basis.

7. Phenomenon 7: The titanium tube body is unharmed, and there is a moderate amount of leakage at the flange joint gaps.
Cause
Gasket elastic failure is caused by frequent thermal cycle extrusion, which results in the formation of leakage channels at flange fissures, as well as ageing, swelling, or deformation of sealing gaskets.

Basis of Judgement
Leakage is limited to the flange sealing surfaces, with the tube body and welds remaining unaffected. Silicone ordinary rubber gaskets exhibit expansion deformation following prolonged acid-base immersion.

Resolution
Substitute all conventional rubber gaskets with virgin PTFE flat gaskets.

Before installing new gaskets, verify that the flange is planar and smooth any uneven sealing surfaces.

Establish a gasket replacement cycle that occurs every half-year for production lines that undergo frequent acid-base CIP.

Input:

Failure Phenomenon Core Root Cause Quick On-Site Remediation Measure
No pits, uniform grey discolouration throughout the entire tubeA minor oxygen deficiency, ageing thin passive filmRe-passivation is achieved by soaking warm aerated water, which also enhances the aeration of the tank.
Local pitting and thinning in the weld/sediment areaUnder-deposit anaerobic crevice corrosionLocal polishing, chemical passivation, and the removal of carbon scale
Concentrated corrosion at the location of the flange contactThere is a dearth of insulation in galvanic corrosion between dissimilar metalsInsert PTFE isolation gaskets between steel and titanium fittings.
Surface passive film microcracks and peelingThermal stress damage resulting from rapid thermal cyclingQuarterly re-passivation, slow temperature change operation
Overall, the wall etching and uneven surface are consistent.Boiling concentrated strong alkali / medium fluorideAlter the pH of the medium or transition to heating tubes that are coated with PFA.
Sudden brittle rupture of the weld without the presence of surface fissuresHydrogen embrittlement resulting from the pickling of inorganic acidsAdd post-cleaning hydrogen removal baking and utilise organic acid descaling.
Flange joint medium leakage, tube intactThe ageing of an ordinary sealing gasket due to growthSubstitute with PTFE gaskets that are resistant to chemicals.
Condensed Summary
The following are the six most common failures of titanium heating tubes in fermentation tanks: passive film ageing damage, local crevice/galvanic corrosion, thermal stress cracking, fluoride/alkali uniform etching, hydrogen embrittlement, and sealing accessory failure. Passivation repair, process parameter adjustment, and material replacement schemes, in conjunction with accurate judgement based on fault location and medium working conditions, can eradicate failure sources and extend the overall service life of titanium heating assemblies.