Passivated Stainless Steel Coil Tube — Nitric/Citric Acid Surface Treatment
Product Overview
Wenzhou Jixing Metal Materials Co., Ltd. delivers premium-grade stainless steel coil tubes featuring a proprietary passivation treatment applied to the outer surface. This advanced surface engineering process utilizes a dual-agent immersion method combining nitric acid and citric acid to remove free iron contaminants and promote the formation of a uniform, adherent chromium oxide (Cr₂O₃) passive film. The result is a coil tube with significantly enhanced corrosion resistance, superior hygienic performance, and extended service life across demanding industrial environments.
Designed for coil-form applications where flexibility, formability, and surface integrity are critical, these passivated coil tubes represent the pinnacle of surface finishing technology in the stainless steel tubing industry.
Material & Properties
- Base Material: Austenitic stainless steel grades including 304, 304L, 316, 316L, 321, 310S, and 904L
- Passive Film Composition: Chromium(III) oxide (Cr₂O₃), providing a stable, self-repairing protective layer
- Corrosion Resistance: Superior resistance to atmospheric, chemical, and intergranular corrosion due to the optimized passive film
- Surface Finish: Bright, smooth, and uniform after passivation — free of iron scale, rust, and free iron particles
- Mechanical Properties: Maintains full ductility and formability of the base material; no embrittlement from surface treatment
- Temperature Range: Suitable for continuous service from cryogenic temperatures up to 800°C (grade-dependent)
Specifications
| Parameter | Specification |
|---|---|
| Passivation Method | Nitric acid / Citric acid immersion (dual-agent process) |
| Passive Film Type | Cr₂O₃ (Chromium(III) oxide) |
| Primary Objective | Removal of free iron; promotion of passive film formation |
| Treatment Surface | Outer surface (exterior coil tube wall) |
| Available Grades | 304 / 304L / 316 / 316L / 321 / 310S / 904L |
| Coil Tube OD | 6.0 mm – 159.0 mm (customizable) |
| Wall Thickness | 0.5 mm – 6.0 mm (customizable) |
| Coil Diameter | 600 mm – 2,000 mm |
| Surface Finish | 2B, BA, No.4, or custom finish (post-passivation) |
| Standards Compliance | ASTM A213 / A269, GB/T 14976, EN 10216-5 |
| Passivation Standard | ASTM A967, GB/T 8927 |
| Blue White Test | Passes per ASTM A967 Section 4.2 |
Manufacturing Process
Step 1 — Tube Forming & Coiling
High-purity stainless steel strip is cold-rolled and formed into seamless coil tubes through precision welding and cold-drawing processes. The coil geometry is optimized for downstream forming operations including bending, U-tube production, and heat exchanger coil manufacturing.
Step 2 — Pickling & Cleaning
Pre-passivation cleaning removes surface contaminants, welding scale, and oxidation layers through controlled acid pickling. This ensures a clean substrate for optimal passive film adhesion.
Step 3 — Passivation Treatment (Core Process)
The coil tube is immersed in a carefully formulated solution combining nitric acid (HNO₃) and citric acid (C₆H₈O₇). This dual-agent system performs two critical functions simultaneously:
- Free Iron Removal: The acidic environment dissolves and eliminates free iron particles (ferritic contamination) that may have been introduced during manufacturing, welding, or handling. Free iron is the primary catalyst for localized rust and pitting corrosion.
- Cr₂O₃ Film Promotion: The nitric acid acts as an oxidizing agent, driving the conversion of surface chromium to a stable Cr₂O₃ passive layer. Citric acid acts as a complexing agent, chelating dissolved iron ions and preventing redeposition, while maintaining solution pH stability throughout the immersion cycle.
Step 4 — Rinse & Drying
Following passivation, the coil tube undergoes thorough rinsing with deionized water to remove residual acid. Controlled drying prevents water spotting and ensures a pristine surface finish.
Step 5 — Quality Inspection
Each batch undergoes rigorous verification including blue-white drop test (per ASTM A967), visual inspection, and surface cleanliness analysis to confirm passive film integrity and absence of free iron.
Applications
- Heat Exchangers & Refrigeration: U-tubes, hairpin coils, and condenser/evaporator tubes requiring superior corrosion resistance in aggressive media
- Food & Beverage Processing: Sanitary-grade coil tubes for dairy, brewing, pharmaceutical, and chemical processing where hygienic surface integrity is mandatory
- Petrochemical & Oil/Gas: Coil tubing for offshore platforms, chemical processing plants, and marine environments exposed to chloride-laden atmospheres
- Medical & Pharmaceutical: Equipment tubing requiring biocompatible, corrosion-resistant surfaces with minimal metal ion leaching
- Automotive & Aerospace: Exhaust systems, fuel lines, and structural coil applications demanding long-term durability
- Power Generation: Boiler tubes, superheater coils, and nuclear-grade tubing requiring resistance to high-temperature oxidation
- Architecture & Decoration: Coil-formed architectural elements requiring long-lasting aesthetic finish without rust staining
Quality Certifications & Testing
- Passivation Verification: Blue-white drop test per ASTM A967 — confirms Cr₂O₃ film formation and absence of free iron
- Surface Cleanliness: Contact angle testing and spectrophotometric analysis to verify removal of iron contaminants
- Corrosion Testing: Salt spray testing (ASTM B117) exceeding 500 hours without red rust on 316L grades
- Mechanical Testing: Tensile, hardness, and elongation tests per ASTM A213 / A269
- Dimensional Inspection: OD, wall thickness, and coil diameter verified via ultrasonic and laser measurement
- Mill Test Certificates: Full chemical composition and mechanical property certification for every heat lot
- ISO 9001: Quality management system certified manufacturing facility
Key Selling Points
- Dual-Agent Passivation: The combined nitric/citric acid system outperforms single-acid methods by simultaneously removing free iron and promoting Cr₂O₃ film growth — delivering a more stable and durable passive layer
- Eliminates Free Iron: Completely removes ferritic contamination that causes localized pitting and rust initiation, extending service life by 3–5x compared to untreated surfaces
- Coil-Form Compatibility: Treatment process is specifically adapted for coil geometry, ensuring uniform passivation across the entire coil length without coil deformation or springback issues
- Customizable Grades & Dimensions: Full range of austenitic grades available in custom OD, wall thickness, and coil diameter configurations
- Industry-Standard Compliance: Full traceability with ASTM, EN, and GB standard certification for global market acceptance
- Hygienic & Food-Grade Ready: Passivated surface meets FDA and 3-A sanitary standards for food, beverage, and pharmaceutical applications