# Fermentation Heating Tube Anti-Corrosion Passivation Treatment & Passive Film Integrity Protection Specification ## Preface This specification (Doc.158) is a core technical supporting document of the anti-corrosion management system from Doc.157 backward to Doc.15. It standardizes the pre-passivation surface pretreatment, pickling passivation process parameters, passivation liquid formula management, post-treatment flushing verification, passive film integrity inspection and daily protective maintenance for stainless steel and titanium heating tubes. A dense and intact passive film is the core natural barrier against electrochemical corrosion, pitting corrosion and crevice corrosion of metal heating pipelines. Improper passivation operation will lead to incomplete film formation, local film damage or film aging failure, which greatly accelerates pipeline thinning and leakage risks. This specification unifies the full-process passivation technical standards to stabilize the inherent anti-corrosion performance of base metal materials. ## 1. Pre-Passivation Surface Pretreatment Requirements Before formal passivation construction, the pipeline loop must complete thorough CIP alkaline cleaning and purified water flushing to completely remove organic residues, biofilm, grease and loose scale attached to the inner wall. Welding joints, bent pipes and disassembled flange sections shall be inspected to eliminate welding oxide skin, burrs and surface mechanical scratches. Severe local rust spots or mechanical damage must be polished and repaired first; otherwise, passive film cannot form uniformly on the defective area, forming high-risk corrosion weak points. The pipeline shall be fully drained without residual stagnant water to avoid diluting passivation solution and affecting film-forming effect. ## 2. Standard Pickling and Passivation Process Parameter Control Classified passivation processes are formulated for stainless steel and titanium alloy heating tubes respectively. Strictly control passivation agent concentration, circulating temperature, flow velocity and constant temperature holding time according to approved process sheets. Too short holding time results in thin and incomplete passive film; excessive temperature and overlong soaking will cause over-corrosion on the metal surface and destroy the original base material. Operators are prohibited from arbitrarily shortening passivation cycles or diluting passivation liquid to save construction time and consumables. All passivation reagents must use approved qualified batches, and mixing different brands of passivation agents is forbidden to prevent invalid film formation. ## 3. Post-Passivation Flushing and Residual Safety Verification After the constant-temperature passivation cycle is completed, the pipeline shall be flushed continuously with purified water until the pH value of outlet drainage returns to the neutral range. Sampling inspection is required to confirm no acid passivation agent residue remains inside the loop. Residual passivation waste liquid shall be collected centrally and treated in compliance with environmental protection specifications before discharge to avoid water pollution. All flushing parameters, reagent batch numbers, construction time and inspection results shall be recorded into the equipment full-life archive as important anti-corrosion original technical data. ## 4. Passive Film Integrity Inspection and Effect Evaluation Two inspection methods are adopted to verify passivation quality: chemical drop test and surface potential detection. The chemical spot test can quickly judge whether local passive film is missing or incomplete; potential testing quantitatively evaluates the compactness and stability of the overall passive film. For newly installed, overhauled and locally replaced pipeline sections, 100% passivation effect inspection is mandatory. If the detection result is unqualified, the pipeline must be re-pretreated and passivated again until reaching the acceptance standard before being put into production. ## 5. Daily Passive Film Protection and Repassivation Management Mechanical scratch, excessive acid cleaning, long-term chloride enrichment and high-temperature dry burning are the main causes of passive film damage. Daily anti-corrosion operation must strictly limit over-concentration pickling and local medium stagnation. Heating pipelines that have undergone long-term high-risk operating cycles, frequent maintenance disassembly or local leakage repair shall be arranged for regular repassivation treatment to repair slightly aged passive films. Once the passive film is confirmed to be damaged in local areas, enhanced protection measures such as adjusting circulating water corrosion inhibitor dosage shall be implemented to avoid rapid pitting corrosion expansion. ## 6. System Value This specification locks in the intrinsic anti-corrosion advantage of metal base materials through standardized passivation process control, builds a dense protective barrier on the inner and outer surfaces of heating tubes, effectively reduces the probability of electrochemical corrosion and local pitting defects, forms the fundamental technical guarantee for long-term stable operation of heating equipment, and further improves the full-process refined anti-corrosion closed-loop management system.
