Stainless Steel Coil Tube — Nitric/Citric Acid Passivation Treatment (External Surface)
1. Product Overview
The Stainless Steel Coil Tube (Coil Tube) supplied by Wenzhou Jixing Metal Materials Co., Ltd. undergoes a rigorous Passivation Treatment on its external surface to ensure maximum corrosion resistance and long-term service integrity. This post-manufacturing surface treatment process is a critical step in the metallurgical finishing of austenitic stainless steel coil tubing, designed to remove non-metallic residues and catalytically promote the formation of a protective chromium oxide passive film (Cr₂O₃ passive film) on the tube's outer surface.
The passivation treatment is classified under the Manufacturing Process → Surface Treatment → External Surface category and represents a fundamental quality-assurance step that distinguishes industrial-grade coil tubing from raw, untreated product.
2. Material & Properties
The coil tubes subject to this passivation process are typically manufactured from austenitic stainless steel grades, including but not limited to:
- 304 (UNS S30400) — General-purpose austenitic grade with excellent corrosion resistance
- 316 (UNS S31600) — Molybdenum-enhanced grade for aggressive chemical environments
- 321 (UNS S32100) — Titanium-stabilized grade for high-temperature applications
- 316L (UNS S31603) — Low-carbon variant for weldability and high-purity applications
The passivation treatment specifically targets the external surface of the coil tube and is designed to:
- Remove free iron (free Fe) contamination introduced during cold drawing, forming, machining, or handling operations
- Promote the formation of Cr₂O₃ passive film — a dense, adherent chromium trioxide layer that acts as a thermodynamically stable barrier against electrochemical corrosion
- Restore the natural passive state of the stainless steel surface to its as-annealed condition or better
3. Specifications
| Parameter | Specification / Requirement | Test Standard |
|---|---|---|
| Treatment Method | Nitric Acid / Citric Acid Immersion (硝酸/柠檬酸浸泡) | ASTM A380 / ASTM A967 |
| Acid Concentration (Nitric) | 20–30% HNO₃ by volume | — |
| Acid Concentration (Citric) | 10–20% C₆H₈O₇ by weight | — |
| Treatment Temperature | 50–80 °C (ambient to controlled heating) | — |
| Immersion Duration | 5–30 minutes (dependent on grade and surface condition) | — |
| Post-Treatment Rinse | Deionized (DI) water rinse, pH 6.5–8.5 | — |
| Target Passive Film | Cr₂O₃ (Chromium Trioxide) — thickness 2–5 nm | ASTM G15 / ASTM G142 |
| Free Iron Removal | ≤ 50 ppm (Copper Sulfate Dip Test) | ASTM A967 / ASTM G15 |
| Surface Finish (Post-Treatment) | Passivated, corrosion-free, uniform appearance | — |
| Applicable Tube OD Range | 3.0 mm – 50.8 mm (varies by grade) | — |
| Applicable Wall Thickness | 0.3 mm – 6.0 mm | — |
4. Manufacturing Process
The passivation treatment follows a well-defined, multi-stage process to ensure consistent and reliable results:
- Pre-Cleaning / Degreasing — The coil tube external surface is cleaned to remove oils, greases, and organic contaminants, typically via alkaline detergent immersion or solvent cleaning, ensuring acid contact with the bare metal surface.
- Acid Immersion (Passivation) — The cleaned coil tube is immersed in a controlled bath of nitric acid (HNO₃) or citric acid (C₆H₈O₇) solution at the specified concentration and temperature. Nitric acid is a strong oxidizing agent that effectively dissolves free iron and iron oxide deposits while simultaneously oxidizing the surface chromium to form a Cr₂O₃ passive layer. Citric acid, a chelating organic acid, serves as a gentler alternative particularly suitable for sensitized or thin-walled tubing, providing effective iron removal without excessive surface attack.
- Neutralization & Rinse — After immersion, the coil tube is immediately rinsed in deionized water to halt acid action and neutralize residual acid on the surface. A controlled rinse pH of 6.5–8.5 is maintained to prevent re-contamination.
- Drying — The passivated surface is dried using clean compressed air or a controlled drying process to prevent water-spot formation or localized corrosion initiation.
- Verification Testing — The passivated surface is verified using a Copper Sulfate Dip Test (ASTM G15) or Blue Spot Test (ASTM G142) to confirm the absence of free iron contamination and the integrity of the passive film.
4.1 Mechanism of Action
Nitric Acid Passivation: HNO₃ acts as both a reductive cleaner (dissolving free Fe and Fe₂O₃/Fe₃O₄) and an oxidizing agent (converting surface Cr to Cr³⁺ and subsequently Cr₂O₃). The reaction can be summarized as: 3Cr + 4HNO₃ → Cr₂O₃ + 4NO + 2H₂O (simplified). The resulting Cr₂O₃ film is thermodynamically stable in oxidizing environments, providing a self-healing passive barrier.
Citric Acid Passivation: C₆H₈O₇ chelates dissolved iron ions (Fe²⁺/Fe³⁺), preventing their re-deposition on the surface, while the mildly acidic environment promotes selective chromium oxidation without aggressive surface etching.
5. Applications
Coil tubes with passivated external surfaces are widely deployed across demanding industries where corrosion resistance is paramount:
- Food & Beverage Processing — Dairy, brewing, and pharmaceutical pipelines requiring hygienic, corrosion-free surfaces
- Chemical Processing — Transfer of aggressive acids, alkalis, and solvents
- Oil & Gas Industry — Downstream processing equipment and heat exchanger tubing
- Medical & Pharmaceutical — Autoclavable components and sterile fluid transfer systems
- Automotive & Aerospace — Exhaust systems, fuel lines, and hydraulic circuits
- Power Generation — Boiler feedwater systems and nuclear-grade applications
- Marine & Offshore — Seawater cooling systems and desalination plant components
6. Quality Certifications & Compliance
Wenzhou Jixing Metal Materials Co., Ltd. ensures that the passivation treatment and the resulting coil tube product comply with internationally recognized standards:
- ASTM A380 — Standard Specification for Chemical Cleaning and Passivation of Stainless Steel Parts
- ASTM A967 — Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts
- ASTM G15 — Standard Test Method for Evaluating the Effectiveness of Chemical Passivation Treatments for Stainless Steel
- ASTM G142 — Standard Test Method for Evaluating the Effectiveness of Chemical Passivation Treatments for Stainless Steel (Blue Spot Method)
- ISO 9223 — Corrosion of Metals and Alloys — Corrosivity of Atmospheres
- EN 10204 — Metallic Products — Types of Inspection Documents
Each batch of passivated coil tubing is accompanied by a Mill Test Certificate (MTC) and/or EN 10204 3.1 Certificate, documenting chemical composition, mechanical properties, and passivation verification results.
7. Unique Selling Points
- Dual-Acid Flexibility — The availability of both nitric acid and citric acid passivation options allows customization based on grade sensitivity, environmental regulations, and end-use requirements.
- Verified Passive Film Integrity — Every batch undergoes ASTM G15/G142 verification, guaranteeing a minimum Cr₂O₃ passive film quality with free iron content ≤ 50 ppm.
- Enhanced Service Life — The passivated external surface extends the operational lifespan of coil tubing by 3–5× compared to untreated surfaces in aggressive environments.
- Environmental Compliance — Citric acid passivation offers an eco-friendly, non-toxic alternative to nitric acid, aligning with RoHS and REACH regulatory frameworks.
- Coil-Specific Processing — The treatment is specifically optimized for coil tube geometry, ensuring uniform acid contact and consistent passivation quality along the entire coil length without the handling constraints of straight lengths.