Electrolytically Polished (EP) Stainless Steel Coil Tube — Inner Surface Treatment Specification
1. Product Overview
The Electrolytically Polished (EP) Stainless Steel Coil Tube is a precision-engineered tubing product manufactured by Wenzhou Jixing Metal Materials Co., Ltd., featuring advanced electrochemical polishing applied to the inner surface (ID — Inner Diameter). This treatment represents the highest level of surface finishing achievable for stainless steel tubing, delivering an exceptionally smooth, corrosion-resistant interior that meets the demanding requirements of pharmaceutical, food-grade, semiconductor, and chemical processing industries.
Electrolytic polishing is an electrochemical process in which the stainless steel tube acts as the anode in an electrolyte solution. Through controlled dissolution of the metal surface, microscopic asperities are preferentially removed, resulting in a surface roughness of Ra ≤ 0.38 μm. This process simultaneously removes the decarburized layer (decarburization zone) and heat-affected zone (HAZ) formed during prior welding or forming operations, thereby significantly reducing the risk of metallic ion leaching (ion migration) into the conveyed fluid or medium.
2. Material & Properties
2.1 Base Material
- Grade: 304, 304L, 316, 316L, 321, 347 (AISI/ASTM designations); custom grades available upon request
- Standards: ASTM A213, ASTM A269, EN 10216-5, GB/T 14976, ASME B36.19M
- Condition: Solution annealed and pickled (AP — Annealed and Pickled) prior to EP treatment
2.2 Post-EP Surface Characteristics
- Surface Roughness (Inner Wall): Ra ≤ 0.38 μm (equivalent to approximately 15 μin RMS or better)
- Surface Finish Classification: Passivated, chemically clean, free of scale, oxide, and mechanical damage
- Crystalline Structure: Retained austenitic structure with no martensitic transformation on the surface
- Chromium Oxide Film: Enhanced passive layer (Cr₂O₃) thickness and uniformity post-EP
2.3 Mechanical Properties (Typical — Grade 316L)
| Property | Specification | Test Method |
|---|---|---|
| Tensile Strength (Rm) | ≥ 485 MPa (70 ksi) | ASTM E8 |
| Yield Strength (Rp0.2) | ≥ 170 MPa (25 ksi) | ASTM E8 |
| Elongation (A) | ≥ 40% | ASTM E8 |
| Hardness (HBW) | ≤ 210 | ASTM E10 |
| Corrosion Resistance (PWHT) | ≥ 500 h (no red rust) | ASTM A967 |
3. Technical Specifications
| Parameter | Specification / Range | Remark |
|---|---|---|
| Product Type | Coil Tube (盘管) | Supplied in coil form for downstream fabrication |
| Surface Treatment | Electrolytic Polishing (EP) — Inner Surface | Electrochemical process on ID |
| Surface Roughness (ID) | Ra ≤ 0.38 μm | Measured per ISO 4287 / ASME B46.1 |
| Decarburized Layer | Removed (depth ≥ 0.05 mm) | Eliminated via controlled anodic dissolution |
| Ion Leaching | Significantly reduced | Lower Ni, Cr, Fe release into fluid |
| Available OD (Outer Diameter) | Φ6.00 mm – Φ159.00 mm | Custom sizes available |
| Available Wall Thickness | 0.50 mm – 6.00 mm | Dependent on OD and grade |
| Coil Length | Up to 100 m per coil | Custom cutting available |
| Coil Inner Diameter | 150 mm / 300 mm / 500 mm | Configurable per customer requirement |
| Electrolyte | Orthophosphoric acid (H₃PO₄) based | Non-fluoride formulation available |
| Operating Temperature (Process) | 60°C – 90°C | Controlled during EP operation |
| Current Density | 15 – 40 A/dm² | Optimized per tube geometry |
| Post-EP Rinse | Deionized (DI) water, conductivity ≤ 1 μS/cm | Triple-rinse protocol |
| Packaging | Protective caps, VCI wrapping, wooden crate | Moisture-proof, contamination-free |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Verification: Incoming stainless steel coil/tube undergoes spectroscopic analysis (PMI — Positive Material Identification) and dimensional inspection to confirm grade conformance.
- Pretreatment — Degreasing: Alkaline degreasing or solvent cleaning to remove oils, lubricants, and surface contaminants.
- Pretreatment — Pickling: Acid pickling (HNO₃ + H₂SO₄ blend) to remove oxide scale and mill finish residues.
- Pretreatment — Passivation: Nitric acid passivation to establish a baseline chromium oxide passive film.
- Electrolytic Polishing (EP): The tube is loaded into the EP tank with the inner surface serving as the anode. The electrolyte (typically 70–80% orthophosphoric acid) is circulated through the tube bore. Under controlled DC current and temperature, preferential dissolution removes surface peaks, decarburized layers, and welding HAZ residues.
- Post-EP Rinse: Multi-stage rinse with progressively purer water, concluding with deionized water to prevent electrolyte residue.
- Post-EP Passivation (Optional): Additional passivation treatment to maximize passive film integrity.
- Final Inspection: Surface roughness measurement (contact profilometry or laser profilometry), visual inspection, and dimensional verification.
- Packaging & Dispatch: Protected coiling with end caps, labeling, and documentation preparation.
4.2 Mechanism of Electrolytic Polishing
The EP process operates on the principle of electrochemical leveling. During anodic dissolution, the rate of metal removal is highest at surface asperities (peaks) where current density is locally concentrated. This preferential dissolution progressively smooths the surface. Simultaneously, a viscous electrolyte film forms on the anode surface, creating a diffusion-limited environment that further enhances the smoothing effect. The process depth is typically controlled to remove 0.02–0.10 mm of material, which is sufficient to eliminate the decarburized layer and any residual welding defects while preserving dimensional tolerances.
5. Unique Selling Points
- Superior Surface Integrity: Ra ≤ 0.38 μm on the inner surface — a level of smoothness that minimizes fluid turbulence, prevents biofilm formation, and reduces particulate generation.
- Decarburized Layer Removal: The EP process chemically eliminates the carbon-depleted zone formed during welding or hot forming, restoring full corrosion resistance and mechanical properties at the surface.
- Reduced Ion Leaching: By removing surface contaminants and homogenizing the passive film, EP-treated tubes exhibit significantly lower rates of nickel (Ni), chromium (Cr), and iron (Fe) release — critical for pharmaceutical and semiconductor applications.
- Coil Form Supply: Delivered in coil configuration, enabling customers to fabricate complex geometries (heat exchangers, condensers, reactors) without butt welds, thereby maintaining EP surface continuity.
- Non-Mechanical Process: Unlike mechanical polishing (tubing, brushing), EP introduces no mechanical stress, no embedded abrasive particles, and no surface work-hardening.
- Reproducibility: Process parameters are precisely controlled (current density, temperature, time, electrolyte composition), ensuring batch-to-batch consistency.
6. Applications
- Pharmaceutical Industry: CIP (Clean-In-Place) systems, SIP (Sterilize-In-Place) loops, sterile fluid transfer lines, bioreactor internal tubing, and sanitary process connections.
- Food & Beverage Industry: Dairy processing, beverage transfer, juice and wine handling systems, and hygienic CIP circuits.
- Semiconductor & Electronics: Ultra-pure water (UPW) delivery systems, chemical vapor deposition (CVD) gas lines, and high-purity chemical transport.
- Chemical Processing: Transfer of aggressive media (acids, solvents), reactor internals, and heat exchanger bundles.
- Medical Devices: Dialysis tubing, infusion systems, and implant manufacturing feedstock.
- Heat Exchangers & Condensers: Condenser tubes, evaporator coils, and refrigeration system tubing where internal surface cleanliness is critical.
- Cosmetics & Personal Care: High-purity ingredient transfer and manufacturing lines.
7. Quality Certifications & Compliance
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System — Wenzhou Jixing Metal Materials Co., Ltd. |
| ISO 14001:2015 | Environmental Management System |
| ASME BPE (Biomédica de Procesos) | Pharmaceutical tubing surface finish and weld quality |
| 3-A Sanitary Standards | Food-grade surface requirements |
| EN 10204 3.1 | Material test certificates (MTC) with inspection report |
| ASTM A967 | Standard Practice for Chemical Cleaning and Passivation of Stainless Steel Parts |
| ASTM A270 / A269 | Austenitic stainless steel tubing specifications |
| GB/T 14976 | Chinese national standard for seamless stainless steel tubes |
| EU PED 2014/68/EU | Pressure Equipment Directive compliance (where applicable) |
8. Inspection & Testing Protocol
- Surface Roughness: Measured using a contact type roughness tester (e.g., Mitutoyo SJ-410) or white light interferometry; minimum 3 measurement points per sample length.
- Visual Inspection: Magnified examination (10× minimum) of inner surface for uniformity, absence of pitting, and color consistency.
- Dimensional Verification: OD, wall thickness, and coil diameter measured per ASTM A213/A269 tolerances.
- Chemical Analysis: Optical emission spectrometry (OES) or XRF for grade confirmation; grain boundary analysis for intergranular corrosion resistance (ASTM A240).
- Hydrostatic Testing: Performed at 2× working pressure for 10 seconds minimum, per ASTM A269.
- Leachate Testing (Optional): ICP-MS analysis of metal ion content after simulated fluid contact, confirming reduced Ni, Cr, Fe release.
Technical Note: Electrolytic polishing is not merely a cosmetic surface treatment — it is a metallurgical process that fundamentally improves the functional performance of the tube's inner surface. The removal of the decarburized layer restores the full alloy composition at the surface, the enhanced passive film provides superior long-term corrosion resistance, and the ultra-smooth finish minimizes the potential for microbial adhesion and particulate generation. These properties collectively ensure product integrity and process reliability in critical applications.
9. Ordering Information
When placing an order for Electrolytically Polished (EP) Stainless Steel Coil Tube, customers should specify:
- Material grade (e.g., 316L, 304L)
- Outer diameter (OD) and wall thickness
- Required surface roughness (Ra value)
- Coil length and coil inner diameter