Stainless Steel Coil Tube with Electrolytic Polishing (EP) Surface Finish
Product Overview
The Stainless Steel Coil Tube with Electrolytic Polishing (EP) Surface Finish, manufactured by Wenzhou Jixing Metal Materials Co., Ltd., represents the pinnacle of surface treatment technology in the stainless steel tubing industry. This product leverages advanced electrochemical polishing processes to achieve an ultra-smooth external surface with a surface roughness of Ra ≤ 0.25 μm, eliminating embedded abrasives and delivering the highest achievable cleanliness grade for industrial-grade stainless steel coil tubing.
Unlike conventional mechanical polishing methods that leave residual scratches, embedded particles, and micro-deformations on the surface, electrolytic polishing employs controlled electrochemical dissolution to remove microscopic convexities (micro-peaks) from the tube's outer surface. This process fundamentally alters the surface topology at the micro-scale, producing a passive, chemically inert finish that is superior to any mechanically achieved surface condition.
Material & Properties
Base Material
- Material Grades: 304, 304L, 316, 316L, 321, 347, 904L, and custom alloy specifications available
- Form: Cold-rolled stainless steel coil tube (Coil Tube / Coil Tubing)
- Delivery Form: Continuous coil for downstream processing, forming, and fabrication
Surface Properties After Electrolytic Polishing
- Surface Roughness: Ra ≤ 0.25 μm (equivalent to mirror-like finish)
- Embedded Abrasives: None — completely free of residual abrasive particles
- Cleanliness Grade: Highest achievable (free of surface contaminants, oils, and particulates)
- Passivation: Enhanced passive chromium oxide layer formed during the EP process
- Corrosion Resistance: Superior to mechanically polished surfaces due to stress-free, chemically homogeneous surface
Mechanical Properties
- Tensile Strength: ≥ 520 MPa (typical for austenitic grades)
- Yield Strength: ≥ 205 MPa
- Elongation: ≥ 40%
- Hardness: ≤ 92 HRB (soft/annealed condition)
Specifications
| Parameter | Specification |
|---|---|
| Surface Roughness (Ra) | ≤ 0.25 μm |
| Surface Roughness (Rz) | ≤ 1.0 μm |
| Embedded Abrasive Particles | None (0 detected) |
| Cleanliness Level | Maximum (electrochemically cleaned) |
| Surface Treatment | Electrolytic Polishing (EP) — External Surface |
| Process Mechanism | Electrochemical dissolution of micro-convexities |
| Electrolyte | Phosphoric acid / sulfuric acid solution (proprietary formulation) |
| Passive Layer Thickness | 2–5 nm (enhanced Cr₂O₃ passive film) |
| Applicable Tube OD Range | Φ3 mm – Φ300 mm (standard); custom upon request |
| Applicable Wall Thickness | 0.3 mm – 10.0 mm |
| Coil Length | Up to 1000 m per coil (standard); custom lengths available |
| Applicable Standards | ASTM A269, ASTM A213, ASTM A554, EN 10217, ISO 28512 |
Manufacturing Process
Electrolytic Polishing (EP) — Process Description
Electrolytic Polishing is an electrochemical process in which the stainless steel tube surface acts as the anode in an electrolytic cell. When a controlled current is applied, the metal surface undergoes preferential dissolution at microscopic peaks and convexities. The fundamental principle relies on the fact that micro-convexities on the surface have a higher current density and therefore dissolve faster than the surrounding valleys, resulting in a progressively smoothed surface.
Process Steps
- Pre-Treatment / Degreasing: The coil tube is subjected to alkaline degreasing to remove oils, lubricants, and surface contaminants. This ensures uniform current distribution during the EP process.
- Acid Pickling: Immersion in a mixed acid solution (typically HNO₃ + H₂SO₄) to remove scale and oxide layers, exposing the bare metal surface.
- Rinsing: Thorough deionized water rinsing to eliminate residual acid from the pickling step.
- Electrolytic Polishing: The tube is immersed in a heated phosphoric acid–sulfuric acid electrolyte bath. A DC current is applied with the tube as the anode. The micro-convexities dissolve preferentially, producing a smooth, mirror-like surface. Typical parameters include:
- Current density: 20–50 A/dm²
- Bath temperature: 50–70 °C
- Processing time: 5–20 minutes (depending on tube geometry and desired finish)
- Electrolyte composition: H₃PO₄ (60–75%) + H₂SO₄ (5–15%) + H₂O
- Post-Rinsing: Multiple stages of deionized water rinsing to remove electrolyte residue.
- Neutralization: Treatment with a neutralizing solution (e.g., sodium bicarbonate) to eliminate residual acidity.
- Final Rinse & Drying: Pure water rinse followed by controlled drying to prevent water spotting.
- Quality Inspection: Surface roughness measurement (Ra/Rz), visual inspection for embedded particles, and cleanliness verification.
- Coiling & Packaging: The treated tube is recoiled under controlled tension and packaged in protective film or wooden crates.
Key Technical Advantages of EP vs. Mechanical Polishing
| Characteristic | Electrolytic Polishing (EP) | Mechanical Polishing |
|---|---|---|
| Surface Roughness | Ra ≤ 0.25 μm | Ra 0.4–1.6 μm (typical) |
| Embedded Abrasives | None | Possible residual particles |
| Surface Stress | Stress-free (dissolution removes work-hardened layer) | Residual compressive/tensile stress |
| Passive Layer | Enhanced, uniform Cr₂O₃ film | Disrupted, locally thinned |
| Corrosion Resistance | Superior | Standard |
| Contamination Risk | Minimal | Potential (abrasive particles, oils) |
Applications
The stainless steel coil tube with electrolytic polishing surface finish is specified for the most demanding applications where surface integrity, cleanliness, and corrosion resistance are critical:
- Pharmaceutical Industry: Processing equipment, CIP/SIP systems, bioreactor piping, and drug transfer lines where sterility and surface cleanliness are paramount (FDA-compliant)
- Food & Beverage Industry: Dairy processing, brewery piping, juice and beverage transfer lines requiring high hygiene standards
- Semiconductor Manufacturing: Ultra-pure fluid (UPF) delivery systems, chemical distribution lines, and process gas lines
- Medical Devices: Implant manufacturing, surgical instrument tubing, and medical gas lines
- Chemical Processing: Corrosive fluid transfer, reactor piping, and high-purity chemical distribution systems
- Electroplating Industry: Anode support tubes and fluid distribution in plating baths
- Aerospace: Hydraulic lines, fuel system tubing, and high-purity fluid conveyance
- Optical & Precision Instrument: Mirror-like surface finish for light-guiding tubes and precision components
Quality Certifications & Standards
- ISO 9001:2015 — Quality Management System
- ISO 14001:2015 — Environmental Management System
- ISO 28512 — Stainless Steel Tubing for Medical Devices
- ASTM A269 — Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubes for Chemical and Similar Severities of Service
- ASTM A554 — Standard Specification for Seamless and Welded Austenitic Stainless Steel Tube for Boilers, Heat Exchangers, and Similar Heat-Transfer Applications
- EN 10217-7 — Stainless Steel Tubes for Technical Purposes
- 3-A Sanitary Standards — For food-grade applications
- GMP Compliance — Good Manufacturing Practice for pharmaceutical applications
- Surface Roughness Verification: Confirmed by profilometry (Ra ≤ 0.25 μm) per ISO 4287
- Cleanliness Testing: Verified per ASTM F86 / ISO 14644-1
Unique Selling Points
Wenzhou Jixing Metal Materials Co., Ltd. delivers an industry-leading electrolytic polishing capability for stainless steel coil tubing, achieving surface roughness of Ra ≤ 0.25 μm with zero embedded abrasives and maximum cleanliness — a standard unmatched by mechanical surface treatment alternatives.
- Superior Surface Integrity: Electrochemical dissolution eliminates micro-convexities without introducing mechanical stress, residual particles, or work-hardening — producing a fundamentally cleaner and more corrosion-resistant surface than any mechanical method.
- Highest Cleanliness Grade: The EP process inherently removes surface contaminants, oils, and particulates, achieving a cleanliness level that meets or exceeds pharmaceutical and semiconductor industry requirements without additional post-treatment.
- Enhanced Passive Layer: The process thickens and uniformizes the chromium oxide passive film, providing superior long-term corrosion resistance in aggressive chemical environments.
- Continuous Coil Processing: Available in coil form for seamless integration into downstream manufacturing lines, reducing cutting waste and enabling high-volume, automated fabrication.
- Zero Embedded Abrasives: Unlike mechanical polishing, which can embed abrasive particles into the surface (a critical contamination risk in pharmaceutical and food applications), EP produces a chemically clean surface with no residual foreign material.