Why can’t ordinary carbon steel replace 316 stainless steel for fermentation heating loops?

# The impact of frequent offline passivation maintenance on the service life of 316 stainless steel heating tubes In the production of fermentation, 316 stainless steel is dependent on a thin passive film that is rich in chromium to resist chloride and acid-base corrosion. This protective film will be perpetually consumed and damaged by long-term medium scouring, solid particle abrasion, and high-temperature alkali cleaning. Inadequate maintenance frequency will result in rapid weld thinning and premature tube leakage, while regular offline pickling and passivation can re-establish a dense protective layer and delay pitting corrosion. The service life difference under various passivation maintenance cycles is illustrated in the table below. | Every 6 months | Continuous dense protective layer | 2.5–3 years | Corrosion Risk of Weld Seams | Average Service Life of Tube Bundle | Offline Passivation Frequency | Integrity of Passive Film | Matching Production Condition | | ---- | ---- | ---- | ---- | ---- | 1.8–2 years | Low hidden corrosion risk | Medium chloride concentration, 1–2 daily cleaning cycles | Every 24 months | Partial film loss at elbows and welds | Extremely low pitting probability | Daily alkali CIP cycles | High-chloride medium | Each 12 months | Severe local film breakdown | 1–1.2 years | High risk of deep pitting | Low-salt fermentation without strict chloride control | | No regular passivation at all | Fragmented incomplete film | Less than 1 year | Very high risk of through-wall leakage | Not recommended for any fermentation heating loop | Passivation maintenance removes rust, oxide layers, and chloride deposits on the tube surface with a special pickling liquid, then induces stainless steel to generate a new, uniform chromium oxide film. The grain structure of weld joints is unstable, and the natural passive film that forms during production is thinner than that of the main pipe body. Consequently, weld joints are the primary repair target during maintenance. The passivation treatment can only slow corrosion, not repair deep wall thinning that has already occurred, and the tiny pits formed by chloride erosion will expand continuously if the maintenance cycle is extended to 24 months or longer. The optimal standard for biopharmaceutical fermentation lines with chloride concentrations that are near to the 50 ppm critical threshold is half-year passivation. The original passive film has been subjected to repeated abrasion and high-temperature damage after six months of cyclic production. Offline treatment is capable of thoroughly cleaning residual organic sediments that have been trapped in weld crevices. Localised corrosion cells are formed by these sediments in the absence of opportune passivation, which accelerates pit expansion under heating conditions. In order to maintain equipment stability and minimise maintenance costs, numerous food fermentation workshops implement a 12-month cycle. However, this approach is only feasible when the cleansing alkali temperature is rigorously maintained at or below 60 ppm and medium solid particles are effectively filtered. By extending the passivation cycle to two years, there will be clear and apparent concealed hazards. Continuous thickness reduction at all welding positions will be detected by ultrasonic wall thickness scanning, even if no visual leakage occurs. Once the tube wall is penetrated by pits, the entire tube bundle must be scrapped, and an emergency shutdown will result in a significant loss of fermentation broth, violating the continuous production management requirements of GMP. It is also important to consider that the base metal of the tube wall will be continuously consumed by excessive frequent passivation (less than 4 months), resulting in a gradual reduction in the overall structural strength and a reduction in the total service life. Enterprises should develop passivation strategies that are tailored to the composition of their respective mediums. Semi-annual maintenance is required for production lines that require frequent alkali cleansing and contain high salt levels, while annual passivation is permissible for low-salt food fermentation. Record the results of the surface inspection and the thickness of the weld in the full life cycle equipment archives to ensure traceability after each passivation. A low-cost and efficient approach to extending the service life of 316 stainless steel heating tubes and reducing the risk of abrupt leakage in production is to rationally arrange the frequency of offline passivation.