Standardized on-site chemical passivation operation process for pure titanium heating tubes

Title: Standardised on-site chemical passivation operation process for pure titanium heating tubes The TiO₂ passive film on titanium heating tubes will experience continuous thinning and damage as a result of mechanical abrasion, thermal cycling, bubble stagnation, and acid-base CIP cleansing. Chemical passivation is the primary maintenance procedure that is used to re-establish a compact, uniform anti-corrosion film. It is categorised into two types: online tank circulation passivation (routine monthly maintenance) and offline full immersion passivation (semi-annual overhaul disassembly). This document establishes the standards for reagent formula, temperature, time, operation sequence, post-treatment, and acceptability testing in order to prevent improper passivation that results in incomplete film formation or matrix over-corrosion. ## 1. Two scenarios that are applicable to the passivation process ### 1.1 Online circulation passivation (monthly routine maintenance, no tube disassembly) Applicable: Tube surface exhibits a uniform, minor grey discolouration, passive film potential is slightly lower than +150mV, and there is no deep pitting or large-area scratch damage. Advantages: Reduced labour costs, no need for tank emptying and tube disassembly, and can be completed following standard CIP cleaning. ### 1.2 Offline full immersion passivation (tube disassembled semi-annually) Relevant: Weld polishing repair, local deep flaws, large-area low-potential defects, and post-weld new titanium tubes prior to factory delivery. Advantages: The most effective anti-corrosion recovery effect, uniform full-surface film formation, and comprehensive repair of local damaged areas. ## 2. Formula for a safe passivation solution that is dedicated to titanium and is fluoride-free ### Main formula A: Universal standard for diluted hydrogen peroxide passivation liquid - Raw material ratio: 2% food-grade hydrogen peroxide and 0.5% food-grade citric acid, dissolved in aerated deionised water - Operating temperature: 35℃ to 45℃ - Fundamental mechanism: The continuous release of active oxygen by hydrogen peroxide facilitates dense The growth of the TiO₂ film is facilitated by trace citric acid, which eliminates surface free oxide residues without etching the titanium matrix. - Advantage: No hydrogen embrittlement risk, compatible with food/pharmaceutical sanitary standards, and no heavy metal residue. ### Formula B: Mild passivation with low concentrations of nitric acid (offline immersion is the only option for new tubes) - Solution: 5% dilute nitric acid in deionised water - Room temperature: 20℃ to 30℃ - Restriction: Residual nitric acid is strictly prohibited for online circulation in fermentation tanks, as it will destroy fermentation strains. It is used exclusively for offline pre-treatment of brand-new titanium tubes prior to installation. ### Reagents that are prohibited for passivation 1. Liquids that contain hydrofluoric acid and ammonium fluoride: Fluoride dissolves the titanium oxide film completely and induces irreversible matrix corrosion. 2. Hidden peril of hydrogen embrittlement: Mass hydrogen evolution resulting from high-concentration strong acids (hydrochloric acid, sulphuric acid). 3. Concentrated hot alkali at temperatures exceeding 80℃: Neutralises the passivation effect and uniformly thins the current passive film. ## 3. Monthly routine for online circulation passivation operation flow 1. Pre-treatment: Perform a standard CIP acid-base alternating cleaning sequence to completely remove inorganic mineral salt and organic carbon scale from the tube wall. Drain all cleaning fluid and flush with deionised water multiple times until the effluent pH is neutral. 2. Prepare the passivation liquid: Formula A hydrogen peroxide-citric acid solution should be mixed and entirely aerated for 30 minutes to achieve a dissolved oxygen level of ≥6mg/L. 3. Circulation passivation operation: Incorporate passivation liquid into the tank, ensure that all titanium heating tube surfaces are fully covered, and maintain a continuous circulation for 6 hours. Maintain the liquid temperature between 35 and 45℃. During circulation, activate micro-aeration to supplement oxygen. 4. Intermediate Inspection: After a three-hour circulation, obtain a liquid sample to determine the concentration of hydrogen peroxide. If the concentration falls below 1%, add reagent. 5. Post-passivation neutral flushing: After draining the passivation liquid completely, circulate warm aerated deionised water for 2 hours to remove any residual passivation reagent. 6. Static film maturation: Before feeding the fermentation medium, maintain the tank at a constant temperature and fill it with clean, oxygen-rich water. Allow the TiO₂ film to naturally densify for 12 to 24 hours. 4. Offline full immersion passivation process (disassembled tube overhaul) 1. Pre-cleaning: First, remove all carbon scale by soaking it in composite cleaning liquid. Then, use a fluoride-free 800 mesh ultra-fine polishing paste to polish deep scratches and weld discolouration areas. Finally, wipe off all oil, metal debris, and oxide residues. 2. Deionised water rinsing: A complete spray washing to eliminate polishing powder residue, with no surface dry spots permitted. 3. Immersion passivation: Submerge the entire titanium tube in the Formula. A passivation tank with a liquid completely submerging tube assembly, a constant temperature of 40℃, a static soak of 8 hours, and continuous air bubbling aeration at the tank's bottom. 4. Intermediate flipping: To prevent bubble stagnation and the formation of uneven films, the heating tube should be flipped once every 2 hours. This is necessary to prevent the formation of blind regions on the upper tube surface. 5. Multi-stage rinsing: Begin with a rough rinse using circulating deionised water, followed by a fine spray rinse for 30 minutes until no peroxide residual is detected on the surface. 6. Drying and maturation: Place the product in a clean, sanitary workshop for 24 hours to allow the natural film to densify before reinstallation. Avoid high-temperature baking and allow the product to air-dry naturally at room temperature. ## 5. Standards for post-passivation acceptance detection (qualification judgement premise) ### 5.1 Visual appearance standard A consistent lustre of the tube body and weld, a uniform matte silver-gray film on the tube surface, and the absence of dark black oxidation blotches, local colour differences, and residual white salt precipitates. ### 5.2 Electrochemical potential measurement (core quantitative index) Test with a saturated calomel electrode: Maximum potential difference of any two positions on the same tube is ≤30mV; surface stable potential is ≥+150mV. - Unqualified disposal: If the potential is consistently lower than +120mV, re-polish and perform full immersion passivation. If the potential is between +120mV and +150mV, re-circulate passivation for an additional 3 hours. 5.3 Rapid inspection of spot acid drops (non-destructive verification) Polish a small, spotless area and apply a micro-formul a passivation liquid; only a sluggish, faint bubble reaction within 10 seconds, no rapid violent corrosion, indicating a complete, compact passive film. ## 6. Critical operational risk control points to prevent passivation failure 1. Maintain strict temperature control: A passivation liquid temperature exceeding 50℃ accelerates hydrogen peroxide decomposition, resulting in a limp thin film due to insufficient oxygen supply. A temperature below 30℃ significantly prolongs the film formation process. 2. Ensure an adequate amount of dissolved oxygen: Oxygen is the primary raw material for the synthesis of TiO₂ films; static unaerated passivation will lead to an incomplete film and a low surface potential. 3. Complete pre-scale removal: The tube surface is covered by carbon scale and salt residues, which obstruct the formation of a film, resulting in the formation of local passivation blind areas and concentrated pitting zones. 4. No dry exposure during passivation: A partial tube surface that is exposed to air will form an inconsistent dry-wet alternate film that is easily peeled off during subsequent acid-base circulation. 5. Reagents that are consumed promptly are supplemented: The concentration of hydrogen peroxide is reduced, passivation capacity is lost, and a straightforward cleaning effect is achieved through prolonged circulation. ## 7. Troubleshooting of Common Passivation Unqualified Phenomena | Abnormal Phenomenon | Root Cause | Corrective Operation Scheme | | ---- | ---- | | | Local dark grey patchy discolouration on tube wall | Bubble stagnation, insufficient local dissolved oxygen | Increase aeration volume, flip tube bundle offline, extend passivation time by 2h | | Overall surface potential only +100+120mV | Hydrogen peroxide decomposed, reagent concentration insufficient | Supplement new hydrogen peroxide, r 4 hours | | Visible scratches remain dark after passivation | Polishing residue was not completely rinsed prior to passivation | Disassemble offline, re-polish, and perform a multi-stage fine rinse, followed by a full immersion passivation | | The weld area exhibits a clear colour difference and a low potential | The weld thick oxide layer was not removed during the pre-cleaning process | Local citric acid partial soak to remove weld oxide, integral secondary passivation | Summary of the Main Points The standard reagent for titanium heating tube passivation is a fluoride-free hydrogen peroxide-citric acid composite liquid. This process is divided into two phases: online circulation monthly maintenance and offline full immersion semi-annual overhaul. Achieving high-quality film formation necessitates consistent oxygen supply, thorough pre-descaling, and stable temperature control. The core qualified standard is a surface potential of ≥+150mV after passivation. A loose passive film, which is unable to resist long-term alternating acid-base CIP corrosion, may result from an improper reagent formula, insufficient aeration, or incomplete pre-cleaning. This can accelerate tube wall pitting and thinning failure.