Stainless Steel Coil Tube — External Surface Passivation Treatment (Nitric Acid / Citric Acid Immersion)

1. Product Overview

The passivation treatment is a critical post-processing surface engineering step applied to the external surface of stainless steel coil tubes manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This process is classified under the manufacturing process category, specifically within external surface treatment procedures. The primary objective of passivation is to chemically remove free iron (Fe) contaminants that may be introduced during mechanical forming, cold-drawing, or handling operations, and to promote the formation of a thermodynamically stable chromium oxide (Cr2O3) passive film on the tube surface. This passive layer dramatically enhances the corrosion resistance of the stainless steel, extending service life in aggressive chemical and environmental conditions.

Coiltubes — also referred to as coil tubing or continuous-length tubing — are supplied in coiled form to facilitate downstream processing such as bending, welding, and forming without the constraints of straight-length joints. The passivation treatment ensures that the coiled geometry, which may accumulate iron particles in inter-coil contact zones, is uniformly restored to a corrosion-resistant state.

2. Material & Properties

2.1 Base Material

Passivation treatment is applicable to austenitic, duplex, and super-austenitic stainless steel coil tubes, including but not limited to:

2.2 Passive Film Characteristics

The Cr2O3 passive film formed through proper passivation treatment typically exhibits the following characteristics:

2.3 Chemical Composition of Passive Film

The ideal passive film on austenitic stainless steel is predominantly composed of:

3. Specifications

Parameter Specification Test Method / Standard
Treatment Process Nitric Acid (HNO3) / Citric Acid (C6H8O7) Immersion —
Nitric Acid Concentration 10%–20% (by volume) in deionized water ASTM A967
Citric Acid Concentration 5%–10% (by weight) in deionized water ASTM A967 (Type 3)
Treatment Temperature 50°C–80°C (122°F–176°F) ASTM A967
Immersion Duration 10–30 minutes (depending on grade and contamination level) Manufacturer's procedure
Post-Treatment Rinse Deionized water rinse (conductivity < 1 µS/cm) ASTM A967
Residual Free Iron Content < 50 ppm (post-passivation surface) Blue Drop Test (ASTM A967 Annex A)
Passive Film Integrity Continuous, uniform Cr2O3 layer confirmed by XPS/SEM-EDS XPS / SEM-EDS Analysis
Surface Roughness (Ra) ≤ 0.4 µm (for bright annealed coil tube) ASTM B95
Chloride Stress Corrosion Cracking Resistance Passivated surface demonstrates > 1000 h without SCC in 6% FeCl3 at 60°C ASTM G48 Practice A
Applicable Tube OD Range 3 mm – 114.3 mm (1/8" – 4") ASME B36.19M / EN 10217
Coil Length Custom, typically 50 m – 300 m per coil —

4. Manufacturing Process

4.1 Process Flow

  1. Pre-Cleaning (Degreasing): The coil tube is immersed in an alkaline detergent solution (pH 11–13, 60°C–80°C) for 5–10 minutes to remove oils, lubricants, and organic contaminants introduced during cold-drawing or coiling operations.
  2. Rinsing: Thorough rinse with deionized water to remove all alkaline residue. Residual pH must be neutralized to 6–8 before passivation.
  3. Acid Pickling (Optional Pre-Step): For heavily contaminated surfaces, a preliminary pickling step using a mixed acid solution (HNO3 + H2SO4 or HNO3 + HF) may be performed to remove scale and heavily oxidized layers.
  4. Passivation — Nitric Acid Immersion: The coil tube is immersed in a 10%–20% HNO3 solution at 50°C–80°C for 10–20 minutes. The nitric acid acts as both a mild oxidizer and a cleaning agent, dissolving free iron particles and promoting chromium enrichment at the surface.
  5. Passivation — Citric Acid Immersion (Alternative or Sequential): Alternatively or sequentially, the coil tube is immersed in a 5%–10% citric acid solution at 50°C–80°C for 15–30 minutes. Citric acid is a chelating agent that complexes with Fe2+ and Fe3+ ions, effectively removing them without aggressive etching. This method is preferred for environmentally sensitive applications and grades susceptible to intergranular corrosion (e.g., 304, 321).
  6. Final Rinse: Immediate and thorough rinse with deionized water (conductivity < 1 µS/cm) to remove all acid residues. A final rinse with distilled water is recommended.
  7. Drying: Air drying or nitrogen purge to prevent water spot formation. For critical applications, a controlled drying atmosphere (dew point < −40°C) is maintained.
  8. Quality Verification: Blue Drop Test (Ferric Chloride Spot Test) to confirm absence of free iron; optional potentiodynamic polarization testing to verify passive film quality.
  9. Coiling & Packaging: The passivated coil tube is re-coiled and packaged with protective wrapping (plastic film, VCI paper) to prevent re-contamination during storage and transportation.

4.2 Chemical Reactions

The fundamental electrochemical reactions during passivation include:

Free iron dissolution:

Fe + 4HNO3 → Fe(NO3)3 + NO↑ + 2H2O
Fe + C6H8O7 → [Fe(C6H5O7)]− + 3H+

Chromium enrichment and passive film formation:

2Cr + 3O2 → 2Cr2O3 (passive film)
2Cr + 6HNO3 → 2Cr(NO3)3 + 3H2O (surface dissolution exposing Cr-rich layer)

4.3 Nitric Acid vs. Citric Acid — Comparative Analysis

Criterion Nitric Acid (HNO3) Citric Acid (C6H8O7)
Environmental Impact Produces NOx emissions; requires fume extraction Biodegradable; minimal emissions; eco-friendly
Effectiveness on Heavy Contamination Excellent — strong oxidizing power Moderate — chelation-based removal
Risk of Intergranular Corrosion Low (non-halide acid) Very low (non-halide, non-oxidizing)
Processing Temperature 50°C–80°C 50°C–80°C
Processing Time 10–20 minutes 15–30 minutes
Waste Disposal Requires neutralization and NOx scrubbing Biodegradable; simpler disposal
Preferred Application Heavy industrial environments, heavily contaminated surfaces Food-grade, pharmaceutical, environmentally regulated facilities

5. Applications

Coil tubes with externally passivated surfaces are deployed in demanding industries where corrosion resistance and surface integrity are paramount:

6. Quality Certifications & Standards Compliance

The passivation treatment process and resulting coil tube products comply with the following international standards and certifications: