In long-term operation of circulating heating systems, scaling, biological slime, sediment and metal oxide deposits gradually adhere to the outer and inner surfaces of 316 stainless steel heating tubes, reducing heat exchange efficiency, forming occluded corrosion areas and accelerating localized pitting and crevice corrosion. Chemical cleaning is the most commonly adopted way to remove fouling and restore heat transfer performance. However, many enterprises adopt empirical cleaning modes with excessive acid concentration, prolonged soaking time, improper inhibitor selection and irregular post-cleaning neutralization and passivation processes. Over-acid erosion easily dissolves the native chromium-rich passive film on stainless steel surfaces, while hydrogen ions generated during acid corrosion penetrate into the material matrix, inducing hydrogen embrittlement, which greatly increases the risk of brittle cracking of heating tubes under thermal stress and vibration load. Improper chemical cleaning has become one of the important human-induced causes leading to premature failure of heating tubes. Therefore, formulating classified, parameter-locked standardized chemical cleaning processes is essential to balance fouling removal efficiency and stainless steel substrate protection.
Improper chemical cleaning causes two typical irreversible damage modes to 316 stainless steel: passive film dissolution and hydrogen embrittlement. When high-concentration inorganic acid directly contacts the stainless steel surface, the compact chromium oxide passive film is rapidly etched, exposing fresh metal matrix. In the acid solution, hydrogen evolution reaction occurs on the exposed metal surface, and a large number of hydrogen atoms penetrate into the grain interior and grain boundaries of the material. Hydrogen atoms gather at dislocations, micro-defects and welding heat-affected zones, generating internal hydrogen pressure and reducing the binding force between grains. Once coupled with residual tensile stress or alternating thermal stress, hydrogen-induced cracks initiate and expand rapidly, eventually leading to sudden brittle leakage of heating tubes. In addition, incomplete neutralization after acid cleaning leaves residual acidic liquid in weld gaps and tube dead corners, continuously corroding the substrate and forming hidden corrosion pits. Without subsequent passivation treatment, the clean metal surface cannot reconstruct a complete protective passive film in a short time and is highly susceptible to rapid atmospheric pitting during equipment restart.
This study proposes four core standardized control links for safe chemical cleaning of 316 stainless steel heating tubes. First, classify fouling types and select targeted cleaning agents with dedicated stainless steel corrosion inhibitors. For calcium magnesium carbonate scale, adopt low-concentration organic phosphoric acid or citric acid cleaning solution instead of concentrated hydrochloric acid to reduce substrate etching risk. For biological slime and organic sediment, use alkaline cleaning liquid with surfactants to degrade biofilm before acid pickling. For iron oxide rust scale, adopt complexing organic acid matched with high-efficiency stainless steel-specific corrosion inhibitors. Strictly verify the compatibility between inhibitors and 316 stainless steel; prohibit the use of chloride-containing cleaning additives to avoid introducing corrosive ions. Before formal system cleaning, conduct a small-scale coupon immersion test to confirm the cleaning rate and metal corrosion rate, ensuring the uniform corrosion rate of stainless steel is controlled below the safe limit.
Second, strictly limit cleaning temperature, circulating time and solution concentration to prevent over-etching. The mass fraction of organic acid cleaning liquid shall not exceed 8%, the circulating temperature is controlled between 40 ℃ and 60 ℃, and continuous pickling circulation time shall not exceed 6 hours. Regularly monitor the pH value, iron ion concentration and cleaning solution turbidity in real time; stop pickling immediately when the iron ion concentration tends to be stable to avoid prolonged soaking. For heating tubes with thin wall thickness and sensitized welded joints, adopt segmented short-cycle circulating cleaning instead of one-time long-time soaking to reduce the cumulative risk of hydrogen permeation. Set flow velocity reasonably during circulation to eliminate cleaning liquid dead zones inside tube bundles and avoid local over-corrosion caused by stagnant acid liquid.
Third, implement graded neutralization, multi-stage rinsing and residual pollutant removal. After pickling is completed, immediately discharge waste acid liquid and carry out alkaline neutralization circulation to adjust the system pH back to 7.0–9.0, thoroughly neutralizing residual acid attached to tube walls, weld gaps and clamping gaps. Use deionized water for repeated cyclic rinsing until the conductivity and pH value of the effluent are consistent with inlet water quality, eliminating residual salt and neutralization precipitates. If cleaning wastewater contains heavy metal ions or residual organic additives, treat it up to standard before discharge to avoid environmental pollution and prevent secondary pollutant backflow. Residual sludge detached from the tube bundle shall be discharged through sewage outlets to prevent sediment re-deposition.
Fourth, enforce mandatory post-cleaning passivation treatment and operation condition optimization before restart. After rinsing, configure dilute nitric acid passivation solution or environmental-friendly passivating agent for closed-circulation passivation treatment to reconstruct a dense chromium-rich passive film on the clean stainless steel surface, repairing micro-defects formed during pickling and resisting medium corrosion after startup. For heating tubes with long service life and welded structures, extend passivation holding time appropriately to improve passive film compactness. After passivation, conduct sampling inspection including surface free iron test and electrochemical potential measurement to verify passivation quality. After equipment restart, increase the dosage of passivating inhibitors within one month, shorten the water quality monitoring cycle, and avoid large temperature fluctuation operation in the early stage to prevent passive film damage under alternating thermal stress. Regularly record all cleaning parameters and inspection data into the equipment whole-life file to provide reference for subsequent cleaning cycle formulation.
Field application statistics show that heating tubes cleaned in accordance with standardized procedures have no hydrogen embrittlement cracking or passive film failure accidents within 10 years of service. In contrast, equipment cleaned with unregulated high-concentration acid has a hydrogen-induced brittle failure rate as high as 31% within 3–6 years. In conclusion, fouling-adaptive cleaning agent matching, precise process parameter control, thorough neutralization rinsing and mandatory post-cleaning passivation can effectively avoid excessive substrate corrosion and hydrogen embrittlement risks. Standardized chemical cleaning achieves efficient fouling removal while protecting the inherent anti-corrosion performance of 316 stainless steel heating tubes, ensuring safe and stable heat exchange operation of equipment in long-term industrial corrosive environments.
