Chemical passivation is the core maintenance process to rebuild compact TiO₂ protective film on titanium heating tubes, divided into offline full immersion passivation in independent passivation tanks and online circulation passivation inside fermentation tanks. Many production workshops only rely on online passivation for routine maintenance, ignoring its inherent limitations such as blind areas and uneven film formation, while excessive offline disassembly brings extra shutdown and mechanical scratch risks. This paper compares the two passivation modes from passivation uniformity, defect repair capacity, production loss, labor cost and film stability, and puts forward a matched periodic passivation system combining the two processes.
Online circulation passivation is the daily conventional scheme without tube disassembly. After completing standard CIP acid-base cleaning, hydrogen peroxide-citric acid passivation liquid circulates in the original tank loop to realize film repair without emptying the tank and removing tube bundles. Its prominent advantages lie in minimal production interruption, automatic pipeline circulation and low labor input. After passivation, the pipeline can directly switch to aerated clean water flushing, simplifying the whole maintenance process. For slight uniform passive film thinning after routine cleaning, online circulation can basically restore the surface potential above +150mV, which meets the operation requirements of low and medium-load fermentation tanks.
However, online passivation has unavoidable structural defects restricting film quality. Bubbles continuously gather at the upper arc of inclined titanium tubes, forming oxygen-deficient blind areas where passivation liquid cannot fully contact the metal matrix. Support gaps and weld concave areas are covered by static liquid film, leading to local incomplete film formation and persistent low-potential zones. Besides, residual trace sugar and salt impurities in the tank will contaminate the passivation liquid, reducing the activity of hydrogen peroxide and resulting in loose, thin oxide film with poor long-term stability. Online passivation cannot target deeply scratched and pitted positions for enhanced repair, so cumulative defects will worsen year by year.
Offline passivation tank immersion is the deep repair process for semi-annual overhaul. The whole titanium tube assembly is disassembled and transferred to a special fluoride-free passivation tank filled with standard passivation reagent. Operators flip the tube bundle regularly during static soaking to eliminate all bubble stagnation dead angles, ensuring uniform liquid contact on the upper and lower tube walls. Local polishing and spot cleaning can be carried out before immersion to remove weld oxide layers and deep scratch dirt, so the regenerated passive film is denser and more uniform, with the overall potential difference controlled within 30mV. For tubes with long-term low potential after multiple online treatments, offline immersion is the only effective repair method to restore complete anti-corrosion performance.
The biggest drawback of offline passivation is high comprehensive operation cost. Tank emptying, tube disassembly, transportation and reinstallation require 1 to 2 working days of full production halt, causing losses of abandoned fermentation broth and repeated sterilization energy consumption. Frequent hoisting and handling easily scratch the fragile passive film with hard tools or steel brackets, bringing new corrosion hidden dangers. In addition, enterprises need to invest in independent passivation tanks, supporting aeration equipment and a large amount of special passivation liquid, increasing the auxiliary material expenditure of each overhaul cycle.
Scientific matching of the two passivation modes can balance maintenance efficiency and anti-corrosion effect. Medium-load food fermentation tanks implement online passivation monthly and offline tank passivation once a year; high-frequency pharmaceutical sterile fermentation tanks adopt online passivation after each monthly descaling, with full offline immersion passivation every six months; high-corrosion industrial wastewater heating tubes shorten the offline passivation cycle to four months. After quarterly potential testing, tubes with local low-potential areas are arranged for offline enhanced repair in the next overhaul window.
In summary, online circulation passivation adapts to daily routine maintenance with low shutdown loss but limited defect repair capacity, while offline passivation tank immersion achieves thorough full-surface film reconstruction at the cost of high production and labor expenses. Formulating a layered mixed passivation system according to tank corrosion load can make up for the defects of single passivation process, keep the titanium tube passive film in a stable compact state all year round, and extend the overall service life of heating equipment.
