Electropolishing Services: Mirror Finish and Corrosion Resistance for Stainless

Electropolishing services reverse the plating process. Instead of depositing metal onto a part, an electrochemical cell dissolves a controlled layer off it, and because current density concentrates on peaks and edges, the high points dissolve faster than the valleys. The surface levels itself. A part that goes in at 0.8 µm Ra can come out at 0.2 µm with a mirror lustre, deburred, decontaminated, and passivated, all in a single operation lasting a few minutes. For stainless steel components in medical, pharmaceutical, semiconductor, and food processing service, electropolishing is not a cosmetic upgrade. It is a functional requirement that determines whether the part cleans properly, resists corrosion, and passes validation.

How Electropolishing Works

The part is submerged in a heated electrolyte, typically a viscous mixture of phosphoric and sulphuric acid, and connected as the anode. A cathode completes the circuit and direct current flows. Metal atoms at the part surface give up electrons, enter solution as ions, and are carried away.

The levelling effect comes from a viscous boundary layer that forms at the surface. This film is thinner over protrusions and thicker in recesses, so electrical resistance is lower at the peaks. Current concentrates there and material is removed preferentially from high points. Microscopic burrs, machining marks, and grain boundary ridges disappear first.

Typical removal is 10 to 50 µm from the surface, though this can be tuned from a light brightening pass of a few microns to aggressive deburring exceeding 75 µm. Removal rate depends on current density, electrolyte temperature and composition, and time, and it is not perfectly uniform across a complex part, which has direct implications for tolerancing.

Electropolishing vs Mechanical Polishing

Mechanical polishing, buffing, and tumbling remove material by abrasion. They work on virtually any metal, cost little for simple flat or cylindrical shapes, and can achieve very high gloss. Our polishing services handle this class of work well.

But abrasion has drawbacks that matter in critical applications. It smears metal across the surface, creating a disturbed Beilby layer that can hide subsurface defects and trap contaminants. It embeds abrasive particles and polishing compound in the surface. It cannot reach internal bores, blind features, or complex internal geometry. And it induces tensile residual stress in the worked layer.

Electropolishing does none of these things. It removes material atom by atom, so there is no smearing, no embedded media, and no mechanical stress. It reaches every surface the electrolyte contacts, including internal passages, provided cathode placement is arranged to give reasonable current distribution. Critically, it exposes rather than conceals subsurface defects, which is why it is used as a pre-inspection step on aerospace and medical components.

The two are often combined. Mechanical work removes gross weld spatter, scale, or deep tool marks, sometimes assisted by sandblasting, then electropolishing produces the final surface. Electropolishing amplifies whatever is beneath it; it will make a good surface excellent and a poor surface shiny but still poor.

Which Alloys Respond Well

Austenitic stainless steels are the ideal substrate. Grades 304, 316, and 316L electropolish beautifully and account for the overwhelming majority of production work. The 316L grade, with its low carbon content and molybdenum addition, is the standard for pharmaceutical and medical use, and electropolishing raises its already good corrosion resistance substantially.

Duplex stainless steels electropolish reasonably, though the two-phase microstructure can etch at slightly different rates. Ferritic and martensitic grades such as 410 and 17-4 PH work but give less dramatic results. Nickel alloys including Inconel and Hastelloy respond well with appropriate electrolytes. Titanium, aluminium, copper, and brass can all be electropolished using alloy-specific chemistry, though the process windows are narrower.

Materials that do not work include cast iron with high graphite content, heavily leaded free-machining alloys, and any part with mixed metals in electrical contact, since galvanic effects cause uneven attack. Machining alloy selection therefore matters upstream, and the considerations are covered in our stainless steel machining guide.

Material Removal, Tolerances, and Design Rules

Because electropolishing removes real material, dimensions change and this must be planned rather than discovered.

Expect roughly 10 to 50 µm off each exposed surface for a standard cycle. On a shaft, the diameter reduces by twice that figure. On a bore, the diameter increases. If a part has features held to ±0.02 mm, either machine them oversize or undersize to compensate, or mask them. Communicate the tolerance intent to the finisher explicitly at quoting stage.

Removal is not perfectly even. Sharp external edges and corners see the highest current density and lose the most material, which is exactly what makes electropolishing an excellent deburring process but also means fine edges become noticeably radiused. Deep recesses and internal corners see lower current density and polish less. Where a part has an unavoidable mix of both, a preliminary mechanical deburring pass improves consistency.

Design guidance follows naturally. Avoid deep blind holes that trap electrolyte and cause staining. Provide drainage for tubular parts. Allow a contact point where the electrical connection can be made, since that spot will not polish and should be placed on a non-critical surface. Where a precision face must remain dimensionally exact, finish it by surface grinding after electropolishing rather than before. Parts arriving from CNC milling should be free of cutting fluid residue, since organic contamination causes patchy results.

Corrosion Resistance and Passivation

The most valuable outcome is often invisible. Stainless steel resists corrosion because chromium forms a self-healing oxide film. Machining, grinding, and handling smear iron across the surface and deplete chromium in the worked layer, leaving free iron that rusts and initiates pitting.

Electropolishing dissolves that contaminated layer entirely and preferentially removes iron relative to chromium, so the finished surface is chromium-enriched. The resulting passive film is thicker and more uniform than that achieved by chemical passivation alone. Salt spray performance typically improves several-fold, and pitting resistance rises measurably.

Electropolishing satisfies the requirements of ASTM A967 and ASTM B912 for passivation, so a separate passivation step is generally unnecessary afterward. Where parts are only cleaned rather than polished, standalone treatment as described in our passivation services guide remains the appropriate route, and the wider comparison of finishing options is set out in our metal surface treatment guide.

Frequently Asked Questions

How much material does electropolishing remove? Typically 10 to 50 µm per surface for a standard cycle, adjustable from a few microns for brightening to over 75 µm for heavy deburring. Always account for this in your machining dimensions.

Will electropolishing fix deep scratches? No. It levels micro-roughness, not macro defects. A deep scratch will become a smooth, shiny scratch. Remove gross defects mechanically first.

What surface finish can I expect? Roughly a 50 percent reduction in Ra is typical. A 0.8 µm Ra machined surface usually finishes around 0.3 to 0.4 µm. Achieving 0.2 µm or better requires a well-prepared starting surface.

Does it meet pharmaceutical requirements? Yes. Electropolished 316L to a specified Ra is the standard for ASME BPE bioprocessing equipment, and documentation including Ra measurements and material certificates can be supplied.

Can welded assemblies be electropolished? Yes, and it is common practice, since it removes heat tint and restores corrosion resistance in the heat-affected zone. Welds should be ground flush first, as electropolishing will not level a proud bead.

Requesting an Electropolishing Quote

Send a drawing with the target surface finish in Ra, the alloy grade and condition, the incoming surface finish from machining, any dimensions that must be held after finishing, and areas requiring masking. State the applicable standard if one governs, such as ASTM B912 or ASME BPE, and whether you need documented Ra measurements or corrosion test reports.

Part size, quantity, and geometry all affect fixturing and therefore cost, so include overall dimensions and batch quantity. If you are currently seeing rust bloom, pitting, or cleaning validation failures on machined stainless parts, send a sample along with the drawing. Those symptoms almost always trace back to free iron and a disturbed surface layer, and electropolishing addresses the root cause rather than the symptom.