The hazards of improper pH value in fermentation medium to different heating tubes

# The risks associated with the improper pH value of the fermentation medium in various heating tubes Determining the ion activity and chemical reaction rate of corrosion is directly dependent on the pH of the fermentation broth and CIP cleaning fluid. The protective structure of heating tubes will be perpetually destroyed by long-term operation in extreme acidic or alkaline conditions. Even a minor deviation from the optimal safe pH range, when combined with heating, static retention, and biofilm, will significantly accelerate corrosion, coating blistering, and surface etching. The following table organises the safe pH range, targeted control schemes, and pH damage characteristics of four common heating tube materials. | Heating Tube Material | Safe Long-Term Operating pH Range | Corrosion Risk Mechanism Under Abnormal pH | Typical Failure Manifestation | pH Adjustment & Control Measures | | ---- | ---- | ---- | ---- | ---- | | 316 Stainless Steel | pH 5.5–8.0 | Strong acid dissolves chromium passive film; pH＞9 hot alkali strips oxide layer; H⁺/OH⁻ promote chloride penetration into welds | Online pH interlock alarm; neutralise over-acid/over-alkali medium before circulation | | Grade 2 Titanium | pH 4.0–9.0 | Excess acid erodes TiO₂ film; high pH reduces dissolved oxygen film regeneration efficiency; alkaline environment amplifies fluoride etching | Local milky white etching fog, continuous electrochemical potential drop | Maintain medium neutrality; add oxygen-rich circulation under high pH conditions | | PFA Coated Heater | pH 2.0–12.0 (limited by temperature) | | High-temperature strong alkali penetrates coating scratches to form interlayer blisters; strong acid accelerates ageing and embrittlement of coating surface | Coating bulging, peeling, and rust contamination of medium | Limit hot alkali temperature to ≤85℃; complete multi-stage rinsing to remove residual strong base | Quartz Glass | pH 2.0–7.0 | Any pH⼞7 alkaline liquid breaks Si-O covalent bonds; higher pH and temperature speed up frosting reaction | Immediately neutralise inadvertent alkaline leakage; strictly isolate all alkaline pipelines; matte frosted tube wall, reduced structural strength, thermal cracking risk | ## 1. The general principle of pH corrosion Hydroxide ions and hydrogen ions are the driving forces behind corrosion reactions: 1. Low pH (acidic environment): The protective layer of metal oxide passive films is dissolved by excess H⁺, revealing the exposed substrate. In the meantime, the hydrogen evolution reaction creates minute gaps between the metal and oxide, which accelerates ion infiltration. 2. Alkaline environment (high pH): OH⁻ decomposes amphoteric metal oxide coatings; soluble corrosion products are produced by the continuous reaction of hydroxide with the material matrix during heating. 3. The charge state of tube wall surfaces is also altered by extreme pH, which facilitates the adsorption of biofilm, heavy metal particles, and corrosive ions. This results in the formation of localised high-corrosion micro-zones beneath deposits. ## 2. Progressive pH-induced degradation to individual materials ### 316 Stainless Steel The chromium oxide passive film is only stable in weak acid/weak alkali solutions that are near neutral. - Continuously below medium pH 5.5: The protective film on welds is dissolved by organic acid from fermentation or inorganic acid supplementation. Deep pitting is rapidly formed when chloride ions easily penetrate bare metal. - In the event of a medium pH exceeding 8.5 and heating above 55℃, the continuous chromium oxide film is stripped by hot alkali. If proper passivation is not performed, uniform rust will cover the heating sections within 2–3 months. Over-alkaline corrosion is the result of operators adding an excessive amount of base to regulate pH, which poses a risk of acidic deviation during microbial metabolism in the majority of fermentation lines. ### Grade 2 Titanium Titanium dioxide film has a greater acid-base tolerance than stainless steel, but it still has clear limitations. - Long-term pH: 4: The TiO₂ protective layer is slowly dissolved by high concentration hydrogen ions; minor wounds are unable to self-heal even with standard dissolved oxygen. - Long-term pH: 9: The solubility of oxygen is diminished and the regeneration of the film is impeded by a high concentration of hydroxide. High pH will synergistically accelerate uniform milky etching on the tube wall if trace fluoride is present in the system. Titanium is capable of withstanding brief periods of CIP with strong acids; however, it will accumulate irreversible corrosion damage as a result of prolonged fermentation at an extreme pH. ### Heater with PFA Coating At room temperature, PFA fluoroplastic is chemically resistant to the full pH range. However, pH damage is primarily caused by high temperatures in conjunction with coating scratches. - High-temperature strong alkali (pH＞11, T＞85℃): The alkali liquid seeps into scratch gaps; after repeated heating cycles, the trapped alkali vaporises to produce internal pressure and concealed blisters. Surface coating micro-embrittlement is gradually induced by long-term, strong acid soaking, which increases the susceptibility to scratches during particulate scouring. pH deviation will not result in the rapid failure of an intact PFA lining, provided that the temperature is maintained and residual strong alkali is thoroughly rinsed. ### Quartz Glass Quartz silicon dioxide is acid-resistant; however, it is highly susceptible to alkali, which is its fatal pH vulnerability. - Quartz is susceptible to OH⁻ attacks when exposed to an alkaline medium or residual alkali cleaning solvent. Si-O bonds to produce soluble silicate, which results in the formation of rough, frosted layers on the tube wall. A faster etching rate is associated with greater pH and temperature. - After numerous heating cycles, even a weak alkaline medium with a pH of 7.5 will result in visible matte discolouration. In a vicious cycle, the formation of frosting results in the adhesion of additional biofilm and alkali residues, which further exacerbates corrosion. Quartz loops must be designed to prevent any contact with alkaline fluids; any inadvertent cross-contamination necessitates immediate full acid circulation neutralisation. ## 3. Common on-site operations that result in anomalous pH levels 1. The long-term medium pH deviates significantly from neutrality due to the excessive acid/alkali supplementation required for the rapid pH adjustment of the fermentation broth. 2. Residual strong acid or alkali retained in pipeline dead zones due to incomplete neutralisation following acid/alkali CIP circulation. 3. Failure to monitor pH levels during the late fermentation phase results in the accumulation of microbial metabolic acids, which can result in a persistently low pH level for several hours. 4. Severe frosting etching was initiated by the shared pipeline mixing of alkaline process fluid and quartz heating loop. 4. Standardised full-process pH anti-corrosion control 1. Implement online pH real-time monitoring with an automatic interlock alarm. This will initiate a warning and feeding interlock when the pH exceeds the material's safe range. 2. Improve pH adjustment operation: Use slow, segmented dosing to prevent sharp pH overshoot and maintain the medium within a neutral safe range for long-term fermentation storage. 3. Standardise the judgement of the CIP endpoint: Prior to entering the subsequent production stage, rinse until the effluent pH reaches neutral in order to eradicate any remaining acid or alkali. 4. Material zoning management: High-alkali production processes utilise independent PFA or stainless steel loops, while fluoride/acid processes utilise dedicated titanium or quartz conduits with no cross-connection. 5. Targeted regular maintenance: The offline passivation cycle is shortened in stainless steel due to long-term acidic operation, while quartz equipment undergoes monthly acid flushing to remove trace alkali frosting. ## Executive Summary The heating tube's surface protective structure is disrupted by a pH deviation in the cleansing fluid and medium. Acid dissolves metal oxide films, alkali erodes titanium and quartz, and high-temperature strong alkali compromises the integrity of PFA coatings. Strict online pH monitoring, standardised dispensing adjustment, and complete neutralised rinsing can fundamentally reduce pH-induced accelerated corrosion of heating tube bundles by maintaining the system within the safe pH interval.