Electropolishing
Also: electrolytic polishing, electrochemical polishing, electro-polishing, electropolish, EP finish, anodic polishing, ASTM B912, reverse plating, electropolished stainless
Electropolishing removes a thin, controlled layer from stainless steel in an acid bath under current, leaving a bright, smooth, clean and passive surface.
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Electropolishing is electroplating in reverse. The part is the anode in a heated electrolyte, usually a mixture of phosphoric and sulfuric acids, and a DC current dissolves metal from its surface. The current concentrates on microscopic peaks while a viscous layer of dissolved metal settles in the valleys, so the peaks go first and the surface levels and brightens. A typical cycle removes 5-40 µm (0.0002-0.0015") per surface, and takes micro-burrs, embedded iron, heat tint and the smeared, cold-worked skin left by machining with it.
What is left is a smooth, reflective surface with a chromium-rich passive film, more corrosion resistant than chemical passivation alone produces. ASTM B912 covers passivation of stainless steel by electropolishing, and ASME BPE sets Ra limits for electropolished product-contact surfaces in biopharma equipment. The process is standard on medical instruments and implants, pharmaceutical and biotech process equipment, food and dairy equipment, semiconductor gas-system tubing and vacuum components, because the surface cleans easily, sheds particles and bacteria, and outgasses less.
Electropolishing improves a surface; it does not repair one. Ra typically drops by up to about half, but scratches, pits, tool marks and weld undercut deeper than the removal stay visible and can look worse on a bright surface. The starting finish sets the final one, so a mirror result starts from a mechanically polished part. Edges and thread crests lose more metal than flats, and deep bores and recesses lose less unless an internal cathode is used. Free-machining 303 and inclusion-rich or porous castings pit, because the sulfide stringers and pores are etched out.
At a glance
| Typical tolerances | Removal is set by current and time, commonly 5-40 µm (0.0002-0.0015") per surface, and repeatable within a few microns on external surfaces for a given part and rack. A diameter changes by twice the removal: a pin loses 10-80 µm (0.0004-0.003") on diameter. Sharp edges and thread crests lose more, blind holes and deep bores less. Dimension tight fits after electropolishing, or state a removal allowance on the drawing so the machinist leaves stock. |
|---|---|
| Size limits | Job-shop tanks commonly take parts up to about 1.5-3 m (5-10 ft); tube and pipe in 6 m (20 ft) lengths are electropolished on the ID with an internal cathode. Large vessels and tanks are electropolished in place by specialists with portable equipment. Every part needs a solid electrical contact, which leaves a small mark, and small parts are racked individually. |
| Surface finish | Bright, reflective and smooth, with micro-burrs gone and edges slightly rounded. Ra typically improves by up to about 50%: a 0.8 µm Ra (32 µin) machined or 2B surface comes out near 0.4-0.5 µm Ra, and a mechanically polished 0.4 µm Ra surface reaches about 0.2-0.25 µm Ra (8-10 µin). No coating, no colour change beyond brightening, no change in hardness. The passive film has a higher chromium-to-iron ratio than chemical passivation gives, and the parts are verified with the passivation tests the spec calls for. |
| Lead time | 3-7 business days at a job shop; 1-2 weeks for medical or biopharma work with validated processes, cleanroom packaging and full documentation. In-place electropolishing of vessels is scheduled as a site job. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Austenitic stainless, 316L and 304 above all, electropolishes best. 17-4 PH, 15-5 PH, 2205 duplex and the martensitic grades 410, 420 and 440C work but come out less bright. Free-machining 303 and 416 pit because their sulfide inclusions dissolve. Titanium, nitinol, nickel alloys, copper, brass and aluminum can be electropolished in different electrolytes at fewer shops. Carbon steel can be electropolished but gains nothing lasting, since it has no passive film to enrich.
What drives the cost
- Racking and fixturing: every part needs a secure electrical contact, and small parts are racked one by one
- Internal surfaces: tube IDs, bores and cavities need custom internal cathodes
- Starting finish: mechanical polishing ahead of electropolishing to reach a low Ra is often the bigger cost
- Removal target and cycle time, and masking of surfaces that must not change
- Documentation: Ra reports, ASTM B912 passivation tests, validated process records, lot traceability for medical work
- Cleaning and packaging: particulate-free cleaning, cleanroom bagging, capped and purged tubing
- Quantity: small lots pay a minimum charge; racks of hundreds drop the unit price
When to use it
- Medical instruments, implants and components that must be smooth, clean and corrosion resistant
- Pharmaceutical, biotech, food and dairy equipment whose product-contact surfaces must clean in place and resist bacteria
- Semiconductor gas-system and vacuum components that must shed few particles and outgas little
- Micro-deburring small precision stainless parts: springs, needles, stents, Swiss-turned parts, fine stampings
- Stainless parts that need better corrosion resistance than chemical passivation gives, or heat tint removed after welding
- A bright cosmetic finish on stainless hardware without mechanical buffing
When not to
- Removing deep scratches, tool marks, weld spatter or pits; grind and polish first
- Free-machining 303 and 416, and porous castings, which pit or trap acid
- Features whose sharp edges or thread crests must stay exactly to size
- Assemblies with crevices, press fits, lap joints or blind cavities that trap electrolyte
- Carbon steel parts that need corrosion protection: electropolishing leaves no protective layer on them
Design tips
- Call out "Electropolish per ASTM B912" with either a removal allowance ("remove 10-20 µm per surface") or a target Ra, not both unless you know they agree.
- Specify 316L or 304 rather than 303 when electropolishing is planned, and make the change before the drawing is released.
- State the starting finish and whether mechanical polishing is included, since the final Ra depends on it.
- Show where the contact mark may go, and which edges must stay sharp.
- Grind welds flush and remove heat tint first if the surface must look uniform across the joint.
- Avoid crevices, lap joints and blind threaded holes in electropolished assemblies; electropolish components before assembly where possible.
- For tube or bore IDs, specify the ID Ra separately and ask the shop how they will reach it.
Frequently asked questions
- What is the difference between electropolishing and passivation?
- Passivation is a chemical dip that removes free iron and lets the chromium oxide film re-form; it removes almost no metal and does not change the finish. Electropolishing removes a controlled layer of metal, smooths and brightens the surface, and leaves a more chromium-rich passive film. An electropolished part is also passivated.
- How much material does electropolishing remove?
- Typically 5-40 µm (0.0002-0.0015") per surface, set by the current and the time. Brightening uses the low end; deburring and heavy smoothing the high end. A diameter changes by twice the removal.
- Can 303 stainless be electropolished?
- Poorly. The sulfide inclusions that make 303 free-machining dissolve first and leave pits, and the surface looks frosted. Use 304 or 316L for parts that must be electropolished.
- Does electropolishing improve Ra?
- Yes, typically by up to about half. A 0.8 µm Ra (32 µin) machined surface comes out near 0.4-0.5 µm Ra. It smooths microscopic peaks but not deep scratches, so a mirror finish starts from a mechanically polished part.
- Is electropolishing better than mechanical polishing?
- They do different things and are often used together. Mechanical polishing removes scratches and sets the overall finish, but it smears metal and can embed abrasive. Electropolishing removes that smeared layer, deburrs and passivates, and reaches recesses a buff cannot. For sanitary and medical surfaces, polish, then electropolish.
