Electrolytically Polished (EP) Stainless Steel Coil Tube – Internal Surface Finishing
Product Overview
Wenzhou Jixing Metal Materials Co., Ltd. offers premium-grade stainless steel coil tubes featuring advanced Electrolytic Polishing (EP) on the internal surface. This electrochemical finishing process delivers an exceptionally smooth, corrosion-resistant inner bore that meets the stringent requirements of pharmaceutical, food-grade, semiconductor, and chemical processing industries. The EP treatment effectively removes the decarburized and contaminated layer formed during welding, cold drawing, and mechanical processing, resulting in a surface that minimizes ion leaching and maximizes product purity.
Coil tubes with electrolytically polished interiors represent the gold standard for applications where internal surface integrity directly impacts product quality, hygiene compliance, and process reliability.
Material & Properties
Our electrolytically polished coil tubes are manufactured from high-purity austenitic stainless steel grades, including but not limited to:
- 304 / 304L (UNS S30400 / S30403) – General-purpose corrosion resistance with excellent formability
- 316L (UNS S31603) – Superior chloride and pitting resistance for aggressive environments
- 321 (UNS S32100) – Stabilized grade for elevated-temperature applications
- 904L (UNS N08904) – Ultra-high corrosion resistance for extreme chemical exposure
The electrolytic polishing process leverages controlled anodic dissolution to achieve:
- Surface Roughness (Ra): ≤ 0.38 μm (15 μin) on the internal bore
- Decarburized Layer Removal: Complete elimination of the altered metallurgical layer caused by heat-affected zones, welding, and cold working
- Reduced Ion Leaching: Dramatically lowered migration of chromium, nickel, and other alloying elements into process fluids
- Enhanced Corrosion Resistance: Improved passive chromium oxide layer formation on the polished surface
- Biocompatibility: Reduced bacterial adhesion and biofilm formation potential
Specifications
| Parameter | Specification | Units |
|---|---|---|
| Product Form | Coil Tube (Continuous Coil) | — |
| Outer Diameter (OD) | 3.18 – 63.50 mm (1/8" – 2.5") | mm (inches) |
| Wall Thickness | 0.25 – 3.00 mm (0.010" – 0.118") | mm (inches) |
| Internal Surface Finish (Ra) | ≤ 0.38 μm | μm |
| External Surface Finish (Ra) | ≤ 0.50 – 0.80 μm (typical) | μm |
| Decarburized Layer | Removed via EP | — |
| Ion Leaching | Significantly reduced | — |
| Available Grades | 304, 304L, 316L, 321, 904L, Custom | — |
| Coil Length | Up to 200 m (656 ft) per coil | m (ft) |
| Tolerance – OD | ±0.10 mm (±0.004") | mm (inches) |
| Tolerance – Wall Thickness | ±10% of nominal | % |
| Welding Type | AUT / TIG (performed prior to EP) | — |
| Heat Treatment | Solution Annealing (1010–1150°C) | °C |
Manufacturing Process
1. Raw Material Selection
High-purity stainless steel strip or pipe stock is sourced from certified mills with full material traceability. Incoming materials undergo strict inspection for chemical composition, mechanical properties, and surface quality per ASTM/ISO standards.
2. Forming & Welding
The coil tube is formed via high-precision cold forming and welded using Automatic Tube Welding (AUT) or Tungsten Inert Gas (TIG) processes. Post-weld passivation is applied to stabilize the passive film before EP treatment.
3. Electrolytic Polishing (EP) – Internal Surface
The core differentiating step involves internal electrochemical polishing, a controlled anodic dissolution process:
- Preparation: Tubes are thoroughly cleaned and degreased to ensure uniform EP coverage.
- Electrolyte Selection: A phosphoric acid-based electrolyte solution is used at optimized concentrations.
- Electrochemical Reaction: The tube interior serves as the anode. Under controlled current density (typically 100–300 A/dm²), voltage (10–20 V), and temperature (50–70°C), the micro-peaks on the surface are preferentially dissolved.
- Decarburized Layer Removal: The altered, carbon-depleted layer formed during welding and cold working is selectively removed to a depth of 5–20 μm, restoring the original metallurgical homogeneity.
- Passive Film Formation: The remaining surface is enriched in chromium, forming a stable, protective Cr₂O₃ passive layer that provides superior corrosion resistance.
- Rinsing & Drying: Multi-stage deionized water rinsing ensures complete electrolyte removal, followed by controlled drying to prevent water spot formation.
4. Post-EP Inspection
Each coil undergoes rigorous post-treatment inspection including surface roughness measurement (Ra), visual inspection, and optional leak testing to verify quality consistency.
Unique Selling Points
- Ultra-Smooth Internal Surface (Ra ≤ 0.38 μm): Exceeds industry benchmarks for pharmaceutical and semiconductor-grade tubing, minimizing turbulence and deposit accumulation.
- Complete Decarburized Layer Removal: Unlike mechanical polishing, EP uniformly removes the entire heat-affected zone alteration, eliminating stress-corrosion initiation sites.
- Minimal Ion Leaching: The enhanced passive film and absence of residual contaminants drastically reduce metal ion migration into process media—critical for ultrapure water (UPW) and electronic-grade applications.
- Continuous Coil Format: Eliminates weld joints in the finished product line, reducing contamination points and improving flow dynamics.
- Consistent Quality at Scale: Automated EP processes ensure uniform finish across the entire coil length, eliminating lot-to-lot variability.
Applications
Pharmaceutical & Biotechnology
Process tubing for bioreactors, CIP/SIP systems, sterile fluid transfer, and clean-in-place loops where internal surface quality directly impacts product sterility and regulatory compliance (FDA 21 CFR Part 210/211, EU GMP Annex 1).
Semiconductor & Electronics
Ultrapure water (UPW) distribution lines, chemical delivery systems, and gas-phase delivery tubing requiring minimal metal contamination (ppb-level purity).
Food & Beverage
Milk processing, brewing, dairy, and high-purity beverage transfer lines where hygiene, product taste integrity, and ease of sanitation are paramount.
Chemical Processing
Aggressive chemical transfer, acid handling, and high-purity reagent delivery systems benefiting from enhanced corrosion resistance and reduced contamination.
Medical Devices
Implant manufacturing lines, dialysis fluid circuits, and medical gas delivery systems requiring biocompatible, low-leachate tubing.
Energy & Power Generation
Feedwater systems, condensate lines, and high-purity cooling circuits in nuclear and fossil-fuel power plants.
Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains comprehensive quality management systems and product certifications:
- ISO 9001:2015 – Quality Management System
- ISO 14001:2015 – Environmental Management System
- ISO 45001:2018 – Occupational Health & Safety Management
- ASTM A213 / A269 – Seamless and welded austenitic stainless steel tubing
- ASTM A270 – General requirements for stainless steel tubing
- EN 10216-5 – European standard for stainless steel tubes
- ASME BPE – Bioprocessing Equipment guidelines (surface finish & weld quality)
- 3-A Sanitary Standards – Food-grade equipment design and fabrication
- ISO 2859-1 – Sampling procedures for inspection by attributes
Note: All electrolytically polished coil tubes are supplied with full material traceability documentation, mill test certificates (MTC), and optional surface finish verification reports. Custom specifications, including tailored OD/wall thickness combinations and specialized EP parameters, are available upon request.