Electropolishing: Process, Ra 0.1-0.2 µm & When to Use It

Electropolishing is an electrochemical finishing process that removes a thin, controlled layer of metal from a part. The part runs as the anode in a bath of concentrated sulfuric and phosphoric acid. It is the reverse of electroplating: instead of building metal onto the surface, the current dissolves metal off it. A typical treatment takes off about 5 to 25µm (0.0002 to 0.001in) and leaves a bright, smooth, passive surface. On suitably prepared parts the finish reaches mirror quality, around Ra 0.1 to 0.2µm, finer than most machining or abrasive operations produce. Within surface finishing, electropolishing is the process specified when a stainless part needs cleanability, corrosion resistance, and a fine microfinish at the same time. That combination is why it is common on medical, food, pharmaceutical, and semiconductor hardware.

The process earns that position because it does three jobs in one dip. It smooths the surface by dissolving microscopic peaks faster than valleys. It deburrs edges and micro-burrs the same way. It also leaves stainless steel passivated, because the dissolution strips iron from the surface and enriches it in chromium. Nothing is added, so there is no coating to chip or peel. The finished surface is the base metal itself, with its damaged outer layer gone.

The trade is dimensional, and it frames every decision on this page. The same dissolution that produces the finish removes stock from every treated surface, so the 5 to 25µm removal has to be planned like a machining allowance. A part that only needs restored corrosion resistance can skip that planning: passivation removes no measurable stock. A part that must clean, resist corrosion aggressively, or hit a low Ra needs electropolishing, and its dimensions need the budget described below.

How electropolishing works

The part is fixtured to a rack and connected to the positive terminal of a DC rectifier. It is submerged in the heated electrolyte alongside a cathode, typically stainless steel or zirconium, connected to the negative terminal. Current flows through the bath, and at the part surface metal atoms oxidize and dissolve into the electrolyte. Hydrogen gas evolves at the cathode while oxygen evolves at the part. That split matters later, for hydrogen embrittlement and for the streaking defects gas bubbles can leave behind.

Why peaks dissolve first

The smoothing comes from how current distributes over a rough surface. A microscopic peak stands closer to the cathode and presents a smaller area to the same current. Its local current density is higher, so it dissolves faster than the valley beside it. The viscous, concentrated electrolyte also forms a thin boundary layer over the surface. Peaks push through this layer and dissolve readily. Recesses sit partly shielded. The two effects together are called anodic leveling, and they turn a machined surface into a smooth, bright one without any tool touching the part.

Preferential dissolution has a second consequence that matters to designers. Edges and burrs concentrate current the same way peaks do, so they erode more quickly than any flat region. That is what makes electropolishing an effective micro-deburring step. It is also what makes removal uneven on sharp-featured parts, a point the tolerance section returns to.

What the finish looks like

No abrasive touches the part, so the surface carries no directional polish lines. The result is a uniform, diffuse brightness that follows the part’s geometry, including bores, internal channels, and other features abrasive methods cannot reach. On stainless, the dissolved iron leaves the surface enriched in chromium and nickel. The enrichment thickens the passive oxide film that forms on exposure to air, so the part leaves the bath both smoother and more corrosion resistant than a chemically passivated equivalent.

Process specification and control

The table below collects the operating band a stainless electropolishing line runs in. The values are ranges, not constants. Within the band, the operator trades current density against time to hit a target removal, and every parameter interacts with the alloy and the part geometry.

The electrolyte is the heart of the control problem. It is a concentrated, viscous mixture of sulfuric and phosphoric acid, phosphoric-rich for the austenitic grades that make up most electropolished parts. The bath is heated, held roughly between 60 and 82°C (140 and 180°F), and its condition drifts as it absorbs dissolved metal. Shops monitor specific gravity and metal content, decant and refresh portions of the bath, and keep cathodes clear of the sludge that dissolution produces. A drifted bath produces dull or stained finishes no matter how well the current is set.

The cycle, start to finish

A compliant cycle has three phases. The part is first cleaned, usually degreased and alkaline cleaned, then rinsed. An acid pickle is added when weld tint, laser scale, or heat discoloration must come off. The part is then racked and electropolished, anywhere from about ten seconds in flash cycles that only strip oxide, up to around twenty minutes for full smoothing. Finally it receives a post-treatment dip, nitric or citric acid, followed by a multistage rinse that ends in deionized water. The post-dip and rinsing matter as much as the polish: residual electrolyte left on the part causes staining and later corrosion.

What the process cannot do

Electropolishing dissolves microscopic peaks, and that is all it does. It will not remove visible scratches, deep tool marks, or grinding damage, because the removal depth of a normal cycle is smaller than those defects. It will not conceal non-metallic inclusions. It tends to expose them as tiny pits as the surrounding metal recedes. It cannot bring a coarse surface to a mirror finish in one step, since it improves the starting Ra by a fraction rather than resetting it. Parts that need a fine, defect-free mirror are mechanically polished first and electropolished after, in that order.

Stainless grades and the finish comparison

Electropolishing is a stainless steel process first. The 300-series austenitic grades, 304 and 316 prominent among them, electropolish best and account for most medical, food, and pharmaceutical work. The 400-series martensitic and ferritic grades electropolish successfully and gain corrosion resistance they otherwise lack. The precipitation-hardening grades, 17-4, 15-5, 13-8, and 17-7, electropolish to bright finishes while keeping their heat-treated properties, which is why aerospace hardware uses the combination. The honest exceptions: free-machining grades with high sulfur, 303 being the common one, polish poorly because sulfide inclusions interfere. Castings with heavy silicon or carbon content give limited results. Grade behavior is covered further in the stainless steel guide.

Other metals electropolish under different chemistries. Aluminum, titanium and its medical alloys, brass, copper, and the nickel alloys all have working processes. Titanium and nitinol are electropolished at scale for implants and devices. None of these have the depth of standard support stainless has, and this page’s numbers, like the governing ASTM B912 specification, are stainless numbers. Aluminum parts that need finishing usually take anodizing instead, since anodize grows a hard coating that electropolishing cannot.

Electropolishing versus passivation and mechanical polish

The table frames the decision. The short version: passivation is the low-cost, zero-dimensional-change choice when corrosion resistance is the only requirement. Mechanical polishing cuts visible tool marks and reaches low Ra on simple, accessible surfaces. Electropolishing is the choice when the surface must clean, resist corrosion aggressively, or finish features abrasives cannot reach.

Passivation, governed by ASTM A967, dips the part in nitric or citric acid to dissolve free iron from the surface and let the passive chromium oxide film rebuild. It changes no dimension, no texture, and no appearance. Electropolishing, governed by ASTM B912, removes surface metal under current and is itself accepted as a passivation route, since properly run electropolishing leaves the surface more passive than chemical treatment alone. Published salt-spray comparisons consistently show electropolished parts far outlasting passivated-only ones. Study results range from hundreds to thousands of hours against much shorter lives for passivated-only samples, and exact hours vary with alloy and test.

Mechanical polishing and electropolishing are complements more than rivals. Abrasive polishing removes the macroscopic defects electropolishing cannot touch. But it works line of sight only, it leaves directional lines, and it can smear surface metal over inclusions or embed abrasive in the face. All three matter on a part that must clean or corrode slowly. On critical surfaces the sequence is mechanical first, electropolish second. The same logic orders electropolishing against other treatments: for a surface that must grip coating or look matte, bead blasting is the better call. For carbon steel that needs sacrificial protection, plating or powder coating does what electropolishing cannot, as the anodizing versus powder coating comparison discusses for aluminum.

Tolerances and the dimensional budget

Electropolishing removes stock, and the removal has to be planned. The verified band is 5 to 25µm per treatment, per surface, and the control a processor holds on that removal is on the order of plus or minus a few micrometres on simple geometry. That places the process between passivation, which removes nothing, and a grinding pass, which removes a lot. Fine work can hold tight tolerances through the bath, but only if the removal is budgeted before machining, and the budgeting must respect the part’s geometry.

Budgeting the removal

The arithmetic works per surface. On a diameter, the change is double the per-surface removal, because both sides of the diameter recede. For example, a 10.000mm shaft machined oversize and electropolished with 10µm removed per surface finishes near 9.980mm. A bore under the same cycle opens by the same 20µm. Three planning rules follow. Machine critical external features oversize and internal features undersize by the agreed removal plus its variation. Keep the agreed removal constant across the project by fixing bath time and current density with the processor. And measure finished parts on the features that matter, using the methods covered under quality inspection and metrology, rather than trusting the cycle alone.

The edge effect

Removal is not uniform, and geometry is the reason. Current concentrates on sharp edges, burrs, and points, so those features lose material at the highest rate on the part. On knife edges the rate easily reaches several times the flat-surface rate. A thread, a knife-edged orifice, or a fine point can change shape, not just size. That is why micro-deburring works, and why delicate features need protection. Designers respond in three ways: add radii in place of knife edges, keep the drawing’s edge callouts realistic about what survives the bath, and flag fragile features for masking or for finishing after electropolishing. Complex geometry also distorts current distribution across the part, so the flatter and simpler the part, the more predictable the removal. The general tolerance planning background is in the CNC tolerances reference table.

What stays safe

Contrast the finishes that remove nothing. Passivation removes no dimensional stock, so a tolerance-critical stainless part whose only problem is corrosion can be passivated at zero dimensional risk. Electropolishing sits at the other end of the finishing scale from additive processes. Powder coating and plating build material on, electropolishing takes it off, and anodizing both consumes and builds. Ra values by operation, for comparison across finishing routes, are tabulated in the Ra reference table.

When electropolishing is the wrong choice

The process fails in predictable ways, and most of them are visible before the part reaches a bath.

• Visible defects. Scratches, deep tool marks, and grind lines survive electropolishing, and non-metallic inclusions surface as pits. If the part has cosmetic defects, abrasive or mechanical finishing comes first.
• Wrong alloy. Free-machining stainless with high sulfur polishes poorly, and heavily cast structures give dull, uneven results. A part in 303 that needs a mirror finish is a redesign conversation, not a finishing one.
• Corrosion is the only requirement. If the part needs nothing beyond a restored passive film, passivation does the job for roughly a third of the cost in secondary estimates, with zero dimensional effect.
• No dimensional room. A feature already at its limit, with no oversize stock and no masking option, will not survive the bath on paper. Finish it after electropolishing instead.
• Tight quantity, tight tolerance, untested geometry. Low-quantity work leaves no parts to burn on process development, and complex geometry makes removal hard to predict. In that case, either prove the finishing on a prototype or keep machining and finishing under one roof that owns the dimensional outcome.

What a buyer should verify with any processor: the agreed removal per surface and the tolerance held on it, the standard the line runs to (ASTM B912 for passivation by electropolishing), the alloy’s suitability, and the inspection performed on finished parts. A line that cannot state its removal control cannot support a tolerance budget.

Design and specification rules

The rules below turn the trade-offs into drawing callouts. They follow the part from design review to finished inspection.

• Specify the standard and the outcome, not the process alone. Call out ASTM B912 where the part needs passivation by electropolishing. State the required Ra where finish matters. Name the alloy, because the alloy sets the result.
• Machine critical features for the removal. External features are cut oversize and shrink toward nominal in the bath; internal features are cut undersize and open toward it. Agree the number with the processor before releasing the drawing.
• Prepare the surface the process assumes. Electropolishing improves a starting finish by a fraction. Specify a pre-polish or fine machined finish where the part must reach Ra 0.1 to 0.2µm, and fix visible defects before the bath.
• Prefer radii over knife edges. Edges erode before flats do, so radius the edges that must survive the bath, and flag delicate points, fine threads, and orifice edges for masking or post-finishing.
• Design for the bath. Parts rack on fixtures that carry current, so include a rack point if the geometry allows. Deep recesses and blind features need bath and current access to polish evenly, and mixed-geometry parts polish unevenly by nature.
• Plan the sequence with the other finishes. Mechanical polish before electropolish on mirrors. Weld before electropolish so heat tint comes off in the cycle. Paint or coat after, since the bath strips contamination the coating must not sit over.
• Agree the verification. State which dimensions and which Ra measurements the finished parts are inspected on, and hold first articles to them. That is the only way the dimensional budget closes.

The common thread is communication. The machinist holds the tolerance and the processor holds the bath, and the budget closes only when both work to the same removal number. Machinists’ forums return to that point again and again on electropolished work.

Worked examples

Two examples show the budget closing on real parts.

Example: a 316L surgical instrument

A machined 316L instrument with a fine internal channel needs a surface that sterilizes cleanly and a part that resists corrosion through repeated autoclave cycles. Electropolishing is specified to ASTM B912 with a target finish of Ra 0.2µm or better. The process strips the free iron and smeared metal that machining left, smooths a channel an abrasive cannot reach, and leaves the surface passive. The pivot pin hole is reamed undersize by 20µm on diameter, with 10µm removal per surface agreed with the processor, so the finished hole lands in tolerance after the bath. The knife edge on the working tip is radiused on the drawing, since a true knife edge would erode past print. The part leaves the bath bright, clean, in tolerance, and more corrosion resistant than a passivated-only equivalent.

Example: a food-contact hopper surface

A fabricated 304 stainless hopper for a food line must meet the 3-A limit of Ra 0.8µm (32µin) on product-contact surfaces. The welds are made, then the interior is mechanically polished to remove weld discoloration and tool marks, which electropolishing alone would not remove. The whole interior is electropolished next. A moderate removal near the low end of the 5 to 25µm band is enough, since the pre-polish already carried most of the Ra reduction. The electropolish pass closes the remaining roughness, removes embedded abrasive and smeared metal from the polishing, and leaves a surface with documented lower bacterial adhesion in food-engineering studies. The thin sheet gauge is checked against the removal before the cycle runs: 25µm off each face of a 1mm wall is five percent of the wall, and a heavy-handed cycle would take it without the part visibly changing to the eye.

Where electropolished parts show up

The applications follow the finish’s properties. Medical and dental instruments, implants, and needles use electropolishing for cleanability, biocompatibility, and micro-deburring. Food, beverage, dairy, and pharmaceutical equipment use it on product-contact surfaces that must drain clean and resist the chemicals that sanitize them, with ASME BPE surface designations specifying the finish grades those industries buy to. Semiconductor and vacuum hardware uses it because a smooth, low-contamination surface outgasses less and holds fewer particles. Aerospace uses it on PH-grade fasteners, springs, and flight hardware, where removing surface stress concentrators matters as much as the corrosion gain. Published studies report sizeable fatigue-life improvements from exactly that mechanism. Consumer and architectural parts take the finish for its uniform brightness.

For a part that needs none of those properties, electropolishing is an expensive way to make stainless shiny. For a part that needs any of them, it is usually the single step that delivers the finish, the passivation, and the deburr together. That holds provided the 5 to 25µm it takes off every surface was budgeted from the first drawing revision.