After transportation, long-term warehouse storage and field installation, the original passive film on the surface of newly installed 316 stainless steel heating tubes is inevitably damaged. Fingerprint grease, mounting lubricants, floating iron particles, dust and tiny rust spots will adhere to the pipe wall. If the heating tube is directly put into service without pre-operation surface treatment, these pollutants will become corrosion initiation points in acid, alkali and high-chloride industrial environments, triggering premature pitting and crevice leakage. Many construction teams only use tap water for simple wiping before commissioning, which cannot eliminate micro pollutants or repair damaged passive films. Improper pickling parameters, incomplete cleaning and non-standard passivation operation will lead to over-corrosion, residual chloride contamination and unstable protective layers. Therefore, establishing standardized pickling and passivation procedures before startup is essential to restore the anti-corrosion performance of heating tubes and reduce early equipment failure.
Pickling and passivation work together to realize surface purification and protective film reconstruction. Pickling removes organic dirt, surface oxide scales and free iron impurities generated during installation. Free iron fragments transferred from carbon steel tools will form galvanic corrosion spots once immersed in conductive media, while oil contaminants hinder the uniform formation of passive films. After pickling, a clean and uniform metal matrix is exposed. The subsequent passivation process promotes chromium enrichment on the stainless steel surface, generating a dense, self-repairing chromium oxide passive film, which greatly improves the critical pitting potential and chloride resistance of the substrate.
This paper develops a four-stage standardized pre-operation treatment process: alkaline degreasing, mixed acid pickling activation, repeated deionized water rinsing and nitric acid passivation. First, high-temperature alkaline degreasing is adopted at 55–65 ℃ for 15 to 20 minutes to thoroughly decompose grease, fingerprints and sticky dust. Ultrasonic auxiliary cleaning is recommended to strip pollutants embedded in surface micro-grooves. Second, low-concentration nitric-hydrofluoric mixed acid solution is used for pickling activation, with soaking time strictly controlled within 10 minutes to avoid over-corrosion and intergranular corrosion of the pipe surface. This step eliminates rust layers, welding oxides and floating iron contaminants.
Third, continuous circulating deionized water flushing is carried out until the pH of the outflow water is neutral. Tap water is prohibited because chloride residues will embed into surface defects and induce hidden pitting corrosion. Thorough rinsing removes all residual pickling solution to avoid secondary chemical contamination. In the final passivation stage, low-concentration nitric acid passivation liquid is used for 25 to 30 minutes at ambient temperature, facilitating the uniform growth of a compact chromium-rich passive film. After passivation, secondary deionized water cleaning and dust-free hot air drying are required to finish the whole treatment process.
Accelerated salt spray tests show that heating tubes treated with complete pickling and passivation maintain intact passive films after 1200 hours of cyclic corrosion tests, with the critical pitting potential increased by 196 mV relative to untreated samples. Industrial field data prove that standardized pre-operation surface treatment extends the average service life of heating tubes in high-chloride working conditions by over 80%. In summary, scientific pickling and passivation procedures can effectively eliminate installation-related surface contaminants, repair damaged passive films and reconstruct a high-performance anti-corrosion protective layer. Strict implementation of these pre-commissioning specifications fundamentally reduces the risk of initial corrosion failure, guarantees the long-term stable operation of 316 stainless steel heating tubes, and lowers the operation and maintenance cost of industrial heating systems.
