Passivation and Electropolishing: What Each One Does

September 21, 2026

On a medical, food or pharmaceutical part the finishing route decides as much as the cutting does. Two processes come up constantly and they are not alternatives to each other. This is what each one actually does.

The problem both of them solve

Stainless resists corrosion because of a thin chromium oxide film that forms by itself in air. Machining damages that arrangement in two ways.

First, the tooling smears free iron across the surface. Carbide inserts, steel fixtures, grinding wheels and even the swarf in the machine all leave iron particles embedded in the part. Iron rusts. The alloy underneath is fine, but the part shows rust spots within days of shipping and the buyer reasonably assumes the wrong grade was used.

Second, machining leaves a rough surface at the microscopic level. Peaks, valleys and folded metal are harder to clean and give bacteria and chemicals places to sit.

Passivation deals with the first problem. Electropolishing deals with both.

Passivation

Passivation is a chemical bath, not a coating. It dissolves free iron from the surface and leaves the chromium behind, so the oxide film rebuilds evenly across the whole part. Nothing is added, and the dimensions do not change in any way you can measure. The general chemistry of passivation is the same idea applied to many metals.

The standard to name is ASTM A967, or AMS 2700 in aerospace work. Both cover two chemistries:

• Citric acid. Gentler on the part, much safer to handle and easier to dispose of. It has become the usual choice for medical and food work.
• Nitric acid. Long established, still written into many customer standards. More aggressive, and with some free-machining and high carbon grades it can attack the surface rather than just clean it.

The standards also cover the tests: copper sulphate, high humidity, salt spray and free iron checks. Say which test you want, because the test is what produces the certificate.

Order of operations matters. The part has to be clean before it goes in. Cutting oil, marker pen and fingerprints all shield the surface, and passivating a dirty part locks the contamination underneath. Deburring comes first as well, since a burr that breaks off later exposes fresh unpassivated metal.

Electropolishing

Electropolishing is the reverse of plating. The part goes into an electrolyte as the anode and a controlled current dissolves metal from the surface. It removes more from the peaks than from the valleys, so the surface flattens out as material comes off.

The standard is ASTM B912. What you get:

• Lower Ra. A typical improvement is around half the starting roughness. It does not rescue a badly machined surface, it improves an already good one.
• A cleanable surface. Fewer places for residue and bacteria, which is why it is standard on pharmaceutical and biotech contact surfaces.
• A passive surface as a by-product. The process removes free iron along with everything else, so it leaves the part passivated.
• Brightness. A visible mirror effect, which is often why someone asks for it without needing the rest.

What you pay:

• Material comes off. Typically 5 to 25 micrometres, sometimes more. On a tight tolerance feature that has to be allowed for before machining.
• Edges round over. Sharp edges and thread crests lose definition. Good for deburring, bad for a sealing edge.
• Current has to reach the surface. Deep bores, blind holes and internal passages polish unevenly without a custom cathode.
• Cost and lead time. Usually an outside process with its own queue.

Which one does your part need?

Passivation only. Most machined stainless parts that have to resist corrosion. Valve bodies, fittings, shafts, brackets, instrument components. It is the default and it is cheap.

Electropolishing as well. Parts where cleanability, a surface finish number or appearance is specified. Pharmaceutical and biotech contact surfaces, high purity vacuum parts, some surgical instruments, visible cosmetic parts.

Neither. Parts that stay dry indoors, in a grade that is not asked to do corrosion work. Do not pay for a finishing process the part does not need.

If you are unsure, pick passivation and say so on the drawing. Adding electropolishing later is a second trip to a second supplier, and it changes the size.

What to write on the drawing

A finishing callout that a shop can quote from looks like this:

Passivate per ASTM A967, citric, Method 1. Test per copper sulphate. Certificate required.

or

Electropolish per ASTM B912. Ra 0.4 micrometre maximum on internal surfaces. Surface finish report required.

Three things make it quotable: the standard, the specific variant and the documentation. Leave any of the three out and the quote you get back is a guess, or you get three quotes that cannot be compared because each shop assumed something different.

If the finish spec is what makes your part difficult, that is worth saying in the RFQ. It usually narrows the list of suitable shops more than the tolerance does. The grades and their finishing behaviour and the buyer guide cover what else goes on the print.

Frequently asked questions

Figures in this guide are published specification limits or published typical values for the alloy, not measurements of any part. Machinability is rated against free-machining carbon steel B1112 at 100 and PREN is chromium plus 3.3 times molybdenum plus 16 times nitrogen. The conventions are set out in thereference section and the editorial standards.

Keep reading

• Choosing a Passivation Method Under ASTM A967
• Weld Then Machine, or Machine Then Weld?