Passivation vs Electropolishing for Stainless Steel Parts

Updated October 14, 2026

The short answer, side by side

Both processes leave stainless steel passive, but only electropolishing removes base metal. That one difference drives the dimensional allowance, the finish you get, the cost and which specification goes on the drawing. Passivation as a general finish callout is already covered on the surface finishing page; this page is about choosing between the two.

Removal and roughness figures above are typical values published by one processor, not specification minimums. Your processor should confirm the removal it can hold on your alloy and geometry before you release the drawing.

What each process does to the surface

Passivation cleans; electropolishing machines the surface on a microscopic scale and cleans it at the same time.

Passivation in nitric or citric acid dissolves free iron picked up from cutting tools, fixtures and shop handling, along with other less noble surface contaminants, and leaves the chromium oxide film to form evenly. SAE describes AMS 2700 as a process to assure removal of free iron or other less noble contaminants from corrosion resistant steel surfaces. It does not touch the roughness profile, burrs, heat tint or embedded scale, which is why a passivated part looks the same as it did before treatment.

Electropolishing makes the part the anode in an electrolytic cell. Current density is higher on peaks than in valleys, so peaks dissolve first and the profile flattens. The ASTM B912 scope states that passivation occurs at the same time as electropolishing under proper operating conditions, that free iron is removed, that surface smoothing improves corrosion resistance and that heat tint and oxide scale are removed. Because the process follows current distribution, edges and outside corners see more removal than deep bores and recesses, which matters for internal passages and blind features.

The practical result: a passivated part keeps its machined finish and dimensions, and an electropolished part comes back brighter, smoother, slightly smaller and with sharp micro burrs rounded off.

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The specification map

Passivation has three common documents and electropolishing has one; know which one your drawing actually names before you read a cert.

Two points trip up buyers. First, A380 is a practice, so a drawing that says only "passivate per ASTM A380" leaves the treatment and the acceptance test open; the 2017 edition sends readers to A967 for tests with acceptance criteria. Second, A967 includes an electrochemical treatment, so a processor can electropolish and certify to A967, but that is not the same document as B912. If the drawing says B912, the cert should say B912.

PRC-5002G is a useful model of how a prime tightens AMS 2700: JSC allows only Method 1 nitric, requires all corrosion testing to meet Class 4, prohibits descaling before passivation unless the drawing directs it, and requires weekly bath analysis. Your program may have its own equivalent, and it overrides the general specification.

Dimensional change and surface finish

Passivation has no dimensional allowance to plan; electropolishing needs one on the drawing.

Able Electropolishing publishes removal by application: 0.0001 to 0.0005 in per surface for micro finishing, 0.00025 to 0.0015 in for endurance finishing and 0.0005 to 0.0015 in for deburring, with size removal set per job. The same page says electropolishing may not suit parts that cannot accept at least 0.0002 in of removal, and that removal can be held to plus or minus 0.0001 to 0.0005 in depending on alloy and application. Treat those as one processor's typical capability, not a guarantee.

• Diameters move twice. Removal is per surface, so a 0.0003 in removal changes an outside diameter by about 0.0006 in and opens a bore by the same amount.
• Threads and seal grooves. Electropolishing rounds thread crests and groove edges. Mask threads, or machine them to a pre finish size agreed with engineering, and confirm O-ring gland dimensions after finishing.
• Edges. Removal concentrates at edges, so a knife edge or a sharp sealing land can lose definition. Call the edge condition on the drawing if it matters.
• Roughness. State the finished Ra on the drawing rather than relying on a percentage improvement. Able reports up to 50 percent Ra improvement depending on alloy; a part that starts rough will still be rougher than one that started smooth.

Passivation, by contrast, can go on a finished part at its final size. The trade is that it leaves the as machined surface, including tool marks and micro burrs, exactly as it was.

Cleanliness and oxygen service

Neither process is a cleanliness level. Oxygen, propellant and pneumatic hardware still needs precision cleaning and verification to the level the program specifies.

Passivation and electropolishing change the chemistry and texture of the surface; a precision cleaning specification controls particles and nonvolatile residue and requires them to be measured. The precision and oxygen cleaning page covers IEST-STD-CC1246 levels, ASTM G93 and the NASA center specifications, and notes that passivation of stainless parts is usually done before precision cleaning, not after. Sequence the finish first, then clean, then package.

Where electropolishing helps is upstream of cleaning. A smoother surface with fewer burrs and no heat tint has fewer places to hold particles and residue, which is the reason processors cite it for process piping and pharmaceutical components; Able states that it electropolishes pharmaceutical components to ASTM B912 and ASME BPE. Whether that matters for your part is an engineering call: a valve body with internal passages may benefit, while a bracket that only needs corrosion resistance does not.

For oxygen hardware, confirm that the electropolishing electrolyte, rinse and drying steps are acceptable to the materials and processes engineer, and that nothing used after finishing reintroduces hydrocarbons before the precision clean.

Verification tests compared

Both processes are proven with the same family of corrosion and free iron tests; the drawing or specification decides which one, and the cert must name it.

A967 calls for testing on each lot, and B912 allows one or more of its listed procedures. PRC-5002G requires each lot to pass one of four AMS 2700 tests and requires the method and result on the certificate of conformance. If your drawing names a specific test, for example salt spray for parts that will see a marine launch site environment, write it into the callout; otherwise the processor will pick the test that is fastest for them.

Visual criteria matter too. PRC-5002G requires passivated surfaces to show no etching, pitting or frosting. Frosting on a passivated part usually points at the wrong bath for the alloy; on an electropolished part it points at bath or current control. Either is a reason to reject the lot, not to accept it on a passing corrosion test.

Choose passivation when, choose electropolishing when

When to specify both

Specify both when a part is electropolished as a finishing step and then sees further fabrication, such as welding, machining a feature, or assembly with steel tools, before delivery. Any operation after the final passivating step can reintroduce free iron, so the last chemical step should follow the last fabrication step. Some programs also require an AMS 2700 or prime process spec cert on every stainless part regardless of finish; in that case electropolish to B912 for the surface and passivate to the program document for the paperwork, with engineering confirming the order. For precipitation hardening grades, passivate after final aging, as noted on the stainless and specialty steels page; the 17-4 vs 15-5 PH comparison covers the condition choices that sit upstream of the finish.

Processing notes by alloy

The alloy decides the passivation type far more than the buyer does, and it limits how well a part electropolishes.

• Type selection. PRC-5002G Table 1 assigns AMS 2700 nitric types by alloy. For example, it lists 303, 416 and 440C under Type II, lists 316L and 321 under Type VI, and lists 17-4PH and A286 under both. It marks Types VII and VIII as not recommended for new design. Unless a drawing names a type, JSC lets the shop choose from that table after coordination with materials and processes engineering.
• Chloride control. PRC-5002G requires monthly chloride checks below 1,000 ppm when Type VI is used on certain precipitation hardening alloys. Ask your processor whether its bath control matches the program document.
• Descaling. PRC-5002G prohibits descaling or pickling before passivation unless the drawing directs it. If a part has scale or heat tint, that is a design decision, and electropolishing or a directed pickle is the route, not a stronger passivation bath.
• Electropolishing response. B912 covers 200, 300 and 400 series and precipitation hardening grades, and states that results depend on the steel type, the electrolyte and the operating conditions. Because response varies by grade, ask for a sample or a first article on the actual alloy and heat treat condition before committing a production lot, especially for free machining and martensitic grades.
• Racking. Every electropolished part needs an electrical contact point, which leaves a small witness mark. Show acceptable contact locations on the drawing for cosmetic or sealing surfaces.

Cost and availability drivers

Passivation is usually the lower cost, wider availability choice; electropolishing costs more per part because of racking, current control and dimensional planning. No price is quoted here because both depend heavily on part size, quantity and paperwork.

• Batch versus racked processing. Small passivated parts can often be processed in baskets. Electropolished parts generally need individual electrical contact, so handling time scales with part count.
• Masking and fixturing. Masking threads, bores and sealing lands adds labor to both, more so where a removal allowance has to be protected.
• Engineering time. Electropolishing needs a removal allowance, a finished Ra and sometimes a first article to confirm size, which adds lead time to the first lot.
• Testing and certs. Each lot test, and any witness or retained specimen requirement, adds cost to either process.
• Processor approvals. If your prime or program requires an approved or accredited chemical processor, the list of eligible shops shrinks and lead time can grow. Read the approval scope on the processor's certificate.

Able Electropolishing argues that electropolishing can lower cost per part by combining deburring, finishing, cleaning and passivation into one step. That is a processor's claim; it can hold on parts that would otherwise be hand deburred, and it does not hold on simple parts that only need a passivation cert.

Callout examples

These are example formats only. Engineering must confirm the current revision on ASSIST or SAE, or with ASTM for ASTM documents, and select the type, class, test and removal for the actual part.

• Passivation, prime process spec: PASSIVATE PER NASA/JSC PRC-5002, TYPE VI. This is the example drawing note given in PRC-5002G.
• Passivation, AMS: PASSIVATE PER AMS 2700, METHOD 1, TYPE [per alloy], CLASS [per program]. VERIFY EACH LOT BY [named test]. CERT TO STATE METHOD, TYPE, TEST AND RESULT.
• Passivation, commercial: PASSIVATE PER ASTM A967/A967M, [nitric or citric treatment]. VERIFY EACH LOT BY [named test].
• Electropolish: ELECTROPOLISH PER ASTM B912 ALL SURFACES EXCEPT AS NOTED. REMOVE [x] TO [y] IN PER SURFACE. FINISHED RA [value] MAX ON SURFACES MARKED [symbol]. MASK THREADS. CONTACT POINTS IN ZONES SHOWN. VERIFY BY [named test].
• Both: ELECTROPOLISH PER ASTM B912 AS ABOVE, THEN AFTER FINAL FABRICATION PASSIVATE PER [program document]. PRECISION CLEAN PER [cleanliness specification and level] AFTER PASSIVATION.

What to put on the PO

• Process specification and revision, with method, type and class for AMS 2700, or treatment for A967
• For electropolishing: removal per surface, finished Ra, masked features and allowed contact locations
• Alloy, heat treat condition and whether parts are welded
• Required verification test, sample size and whether results must appear on the cert
• Sequence relative to machining, welding, aging, marking and precision cleaning
• Processor approval or accreditation required by your prime or program
• Certificate content: specification, revision, method or treatment, lot, test and result, and processor identification
• Packaging after finishing, especially for parts going straight to precision cleaning

Put the same items in the RFQ so the quote reflects masking and testing. The RFQ builder helps structure finish requirements, and the coatings library covers the other finishes that often share a drawing with stainless parts.

Get passivation or electropolishing sourced with the right cert

Send the drawing, alloy, finish callout and any program process specification through the special process request form. We source the work from processors that hold the approvals your callout needs, flow down the specification, removal allowance, masking and test requirements, and check the certificate against the drawing before parts ship. A person replies within one business day, and if the parts also need machining or precision cleaning we quote the whole sequence on one PO. NDAs come before files; see confidentiality.

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Part, quantity and date are enough to start. Drawings and the mutual NDA follow by reply, and controlled programs never send technical data through this site.

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Questions

Related

• Aerospace Surface Finishing: Specs, Callouts and Pitfalls
• Precision Cleaning and Oxygen Service Cleaning
• Stainless and Specialty Steels for Aerospace Parts
• 17-4 PH vs 15-5 PH Stainless Steel

Sources

• SAE, AMS2700G Passivation of Corrosion Resistant Steels
• ANSI Webstore, SAE AMS2700G (2025) listing
• ASTM, B912 Standard Specification for Passivation of Stainless Steels Using Electropolishing
• SNV, ASTM A967/A967M-25 listing
• Nimonik, ASTM A967/A967M summary
• BSI Knowledge, ASTM A380/A380M Standard Practice for Cleaning, Descaling, Pickling and Passivation
• ASTM, A380/A380M-17 (historical) scope
• NASA JSC, PRC-5002G Process Specification for Passivation and Pickling of Metallic Materials (January 2021)
• Able Electropolishing, Passivation vs. Pickling vs. Electropolishing compared
• Able Electropolishing, ISO 15730 electropolishing standard page