Post-Cleaning Passivation Treatment for Industrial Equipment

1. Introduction

The purpose of chemical cleaning is to remove fouling and corrosion products from equipment inner walls, but during the pickling process, the metal surface becomes activated — the original oxide film is dissolved, exposing fresh metal substrate directly. If passivation treatment is not performed after cleaning, the activated metal will rapidly undergo secondary oxidation in air (commonly known as "flash rust"). Carbon steel equipment exposed to humid air after pickling can develop visible rust spots within just a few hours. Although stainless steel equipment has better corrosion resistance, once the chromium-rich oxide layer on the surface is removed by pickling, its pitting resistance drops significantly. The essence of passivation treatment is to rebuild a dense, stable protective oxide film on the clean metal surface, transforming activated metal into a passive state, thereby restoring its corrosion resistance before the equipment is recommissioned. This article systematically describes the process principles, chemical selection, operational key points, and quality inspection methods for passivation treatment after chemical cleaning.

2. Chemical Principles of Passivation

2.1 Metal Passivation Theory

Passivation is an electrochemical process in which metal transitions from an active dissolution state to a highly corrosion-resistant state. Taking the passivation of carbon steel in NaNO₂ solution as an example: NO₂⁻ is reduced on the metal surface, and the generated oxygen atoms react with iron atoms to form a γ-Fe₂O₃ passive film. The simplified reaction equation is: 2Fe + NaNO₂ + H₂O → Fe₂O₃ + NaOH + NH₃↑. This passive film typically has a thickness of 3~10nm with a dense structure, effectively blocking contact between corrosive media and the metal substrate. Stainless steel passivation relies on the formation of a chromium-rich oxide layer (Cr₂O₃) on the surface; after pickling removes the chromium-depleted layer, this protective film must be rebuilt in an oxidizing environment.

2.2 Differences in Passivation Mechanisms by Material

The passive film on carbon steel (γ-Fe₂O₃) is an "exogenous" protective film — it requires a passivator (such as NaNO₂) to continuously provide oxidizing conditions for formation and maintenance, and is easily damaged in non-oxidizing acids. The passive film on stainless steel (Cr₂O₃) is "self-healing" — as long as trace oxygen is present in the environment, Cr elements can spontaneously form an oxide layer. This means stainless steel passivation is relatively easy (only requiring an oxidizing environment), but it is essential to ensure sufficient surface chromium content (pickling must not cause severe selective corrosion). Aluminum and aluminum alloys form an Al₂O₃ passive film in HNO₃, while titanium materials rely on self-passivation characteristics to naturally form a TiO₂ film in air.

3. Common Passivator Systems

4. Typical Passivation Process Flows

4.1 NaNO₂ Passivation for Carbon Steel Equipment

After acid cleaning is complete and rinsed to pH 5~6, carbon steel heat exchangers, pipelines, and storage tanks are immediately transferred to the passivation process. Formula: NaNO₂ 1.5% + Na₂CO₃ 1.0% (adjust pH to 9~10), temperature 40~50℃, circulation time 4~6 hours. During passivation, samples must be taken every 30 minutes to test pH and NaNO₂ concentration, ensuring they remain within the effective range. After passivation, drain the passivation solution and flush with deionized water until the effluent pH matches the influent pH. Finally, purge with dry compressed air or hot air to rapidly dewater the equipment inner walls. Note: NaNO₂ waste liquid contains nitrites and must undergo oxidation treatment (e.g., adding NaClO to oxidize NO₂⁻ to NO₃⁻) before compliant discharge.

4.2 Citric Acid Passivation for Stainless Steel Equipment

Stainless steel equipment (304/316L) undergoes Citric Acid passivation after pickling, combining environmental friendliness with process simplicity. Formula: Citric Acid 4%~8% (food grade), temperature 55~65℃, circulation or immersion 30~60 minutes. The passivation mechanism of Citric Acid is twofold: first, it provides a weakly acidic environment to dissolve residual free iron on the surface; second, it chelates iron ions to prevent redeposition on the surface, allowing chromium elements to naturally oxidize and form a chromium-rich passive film. After passivation, thoroughly rinse with deionized water until conductivity <50 μS/cm (to prevent Cl⁻ residue-induced pitting). After drying, avoid direct hand contact with the passivated surface. A pharmaceutical enterprise's stainless steel reactor (316L, 8000L) using this protocol achieved a 100% pass rate on blue spot testing, with no rust spots appearing within 3 months of commissioning.

4.3 Integrated Pickling-Passivation Process

For stainless steel equipment with light scaling, an integrated pickling-passivation process can be used, completing descaling and passivation in a single step. Formula: Citric Acid 3%~5% + Sulfamic Acid 2%~3% (descaling) + H₂O₂ 1%~2% (oxidative passivation), temperature 50~60℃, circulation 2~3 hours. In the pickling phase, Sulfamic Acid and Citric Acid dissolve CaCO₃ scale and iron oxides; in the later phase, Citric Acid and H₂O₂ work synergistically to form a passive film on the clean surface. The advantage of this process is shortened duration (eliminating the separate passivation step), but it is only applicable to equipment with thin scale layers (<0.5mm) and materials confirmed free of intergranular corrosion risk. For equipment with severe scaling or material defects, separate-step pickling and passivation should still be performed.

5. Passive Film Quality Inspection

5.1 Blue Spot Test (Potassium Ferricyanide Method)

The blue spot test is the classic method for detecting free iron contamination and passive film integrity on stainless steel surfaces. Test solution formula: K₃[Fe(CN)₆] 1g + HNO₃ (65%) 3mL + deionized water 97mL. Apply the test solution dropwise onto the surface to be tested or apply with filter paper. If deep blue spots appear within 30 seconds, it indicates the presence of free iron at that location — the passive film is incomplete or has been damaged. No blue spots indicates a pass. Note: The blue spot test solution is mildly corrosive to stainless steel; after testing, the surface must be thoroughly rinsed with deionized water. This test should be conducted after passivation and before equipment drying.

5.2 Humidity Test (Carbon Steel Passive Film Inspection)

Carbon steel passive film inspection commonly uses the humid heat exposure method: place the passivated test coupon or equipment section in a sealed environment with relative humidity >95% and temperature 40±2℃, and inspect after 8 hours. No rust spots on the surface indicates a pass; rust spot area >5% indicates failed passivation requiring re-passivation. For rapid on-site testing, the copper sulfate spot test can be used: apply CuSO₄ solution (CuSO₄·5H₂O 3% + HCl 0.5%) dropwise onto the test surface; no displaced copper precipitation (no red spots) within 20 seconds indicates a dense passive film.

5.3 Electrochemical Testing

For equipment with stringent requirements (such as GMP pharmaceutical equipment and nuclear power equipment), electrochemical methods can quantitatively evaluate passive film quality. By measuring the open circuit potential (OCP) and pitting potential of the passivated surface: the more positive the OCP, the more stable the passive film; the larger the difference between pitting potential and OCP, the stronger the pitting resistance. Typical 304 stainless steel after Citric Acid passivation exhibits a pitting potential of +250~+350 mV (vs. SCE) in 3.5% NaCl solution, significantly higher than the +50~+100 mV of unpassivated material. Additionally, electrochemical impedance spectroscopy (EIS) can assess film density by measuring the polarization resistance (Rp) of the passive film — high-quality passive films typically have Rp values in the 10⁵~10⁶ Ω·cm² range, 2~3 orders of magnitude higher than activated surfaces. It is recommended that for critical equipment, test coupons be sampled after each batch of passivation for electrochemical testing, with data incorporated into equipment records as a long-term traceability basis for passivation quality.

6. Common Problems and Precautions

Flash rust after passivation: The most common causes include — insufficient passivator concentration or excessively low temperature resulting in incomplete film formation; rinse water Cl⁻ content exceeding limits (should be <50mg/L); delayed drying after passivation, leaving the metal surface in a moist state for extended periods. Solution: Strictly control passivation parameters, use deionized water for rinsing, and complete drying within 15 minutes after passivation.

Blue spots at stainless steel weld zones: The weld heat-affected zone experiences surface chromium depletion due to high-temperature oxidation, and blue spot tests often fail in this area after passivation. Solution: Perform pickling paste treatment on welds after welding to remove weld discoloration, then perform overall passivation. In severe cases, solution annealing may be required to restore corrosion resistance.

Carbon steel passive film service life: NaNO₂ passive films can last for months in dry environments, but the protection period is significantly shortened in humid or Cl⁻-containing environments. If equipment is not immediately commissioned after passivation, it should be nitrogen-blanketed or have desiccant placed. After commissioning, corrosion inhibitors in the process medium can help maintain the passive state.

7. Conclusion

Passivation treatment is an indispensable finishing step in chemical cleaning — equipment without passivation is like an open wound, and secondary corrosion is often more rapid and severe than the original corrosion. From NaNO₂ alkaline passivation for carbon steel to Citric Acid environmentally friendly passivation for stainless steel, from the classic blue spot test to modern electrochemical evaluation — passivation technology has developed mature process systems and inspection standards. In industrial practice, the most suitable passivation protocol should be selected based on equipment material, process medium, and operating environment, and passivation should be included as a mandatory inspection item in cleaning project acceptance criteria. Only by combining "thorough cleaning + standardized passivation" can the engineering goal of equipment corrosion protection be truly achieved.