What inspection methods detect invisible fluoride etching on titanium tube surfaces?

#What methods of inspection are used to identify invisible fluoride etching on titanium tube surfaces? Once exposed to trace fluoride, Grade 2 titanium heating tubes undergo irreversible milky white etching. The shallow early etching marks are transparent and invisible to the naked eye during daily patrol. Uniform wall thinning will progress silently if concealed fluoride corrosion is not detected in time, resulting in through-wall leakage and full batch fermentation medium scrap. The table below compares the detection accuracy, operation difficulty, and applicable scenarios of multiple professional inspection means that can capture invisible fluoride damage. | Inspection Method | Detection Depth of Fluoride Etching | Operation Difficulty | On-site Realization Condition | Core Detection Advantage | | ---- | ---- | ---- | ---- | ---- | | Full surface electrochemical potential scanning | Capture micro film damage without visible traces | Medium | Portable potential tester, shutdown inspection | Quantify passivation film integrity, earliest warning signal | | High-power optical microscope observation | Find nanoscale etching pits on local sampling points | High | Need to disassemble tube bundle for sampling analysis | Observe tiny surface structural changes directly | | Long-term CIP cycle performance tracking | Judge hidden corrosion via daily dissolved oxygen matching data | Low | No disassembly, record routine production data | Real-time monitoring without stopping equipment operation | | Ultrasonic wall thickness full scanning | Measure subtle uniform thickness reduction of whole tube | Medium | Shutdown required, fixed point repeated measurement | Evaluate cumulative corrosion loss of tube wall | Titanium's anti-corrosion barrier is the TiO₂ passivation film. In the initial stage, fluoride ions dissolve the film at the molecular level, leaving no visible translucent white traces. Consequently, visual inspections are unable to identify any potential hazards. The primary screening method for invisible fluoride etching is electrochemical potential scanning, which is the most effective. The potential value of an intact titanium tube remains consistently high; however, the reading decreases significantly when fluoride degrades a portion of the film. Continuous low potential data confirms concealed etching pollution, even in the absence of surface discolouration. Equipment maintenance teams conduct quarterly full-tube potential scanning to identify abnormal areas for targeted re-inspection. After abnormal potential measurements, high-power microscope inspection is employed as a secondary verification tool. Staff members retrieve small tube samples from low-potential sections and analyse the surface morphology at a magnification of hundreds of times. The metal surface is uniformly covered with dense, small, concave pits that are the result of invisible fluoride erosion. These pits are entirely distinct from the scattered scratches that are the result of medium abrasion. This approach offers physical evidence of fluoride contamination for GMP equipment archives that is intuitive; however, it is not suitable for daily routine inspection due to the production downtime required for disassembly and sampling. The cumulative corrosion results are the primary focus of ultrasonic wall thickness scanning. The ultrasonic data remains unaltered at first due to the fact that early invisible fluoride etching only damages the surface film without causing apparent wall loss. Uniform thinning and a consistent decrease in thickness values at all measuring points are observed after 2–3 months of continuous fluoride contact. Technicians can differentiate fluoride uniform corrosion from local pitting caused by chloride when combined with prospective scanning results. A zero-cost auxiliary monitoring method is daily operation data tracking. Typically, titanium tubes with a complete passivation film maintain sustained operation when the concentration of dissolved oxygen exceeds 8 mg/L. The presence of invisible fluoride residues in the pipeline system is suggested by the necessity for operators to continuously increase the aeration volume in order to preserve surface stability. At this time, the entire tank and circulation loop require a complete flush to eliminate residual fluoride sources before the etching marks become visible. A multi-layered inspection system that integrates quarterly potential scanning, monthly wall thickness measurement, and daily ventilation data recording should be implemented by enterprises that have titanium heating systems. Immediately isolate the heating loop, flush all pipelines with oxygen-rich purified water, and strengthen raw material feeding isolation management to fundamentally cut off fluoride contamination sources, thereby avoiding irreversible uniform thinning damage to titanium tube bundles, once invisible fluoride etching signals are captured.