// FINISHING — PASSIVATION
Stainless Steel Passivation
Nitric or citric acid passivation for stainless steel — free iron removed, chromium-oxide passive layer restored, ASTM A967 and AMS 2700.
Passivation is a chemical cleaning process for stainless steel that removes free iron and contaminants introduced during machining, forming, and handling, then allows the chromium-rich passive oxide layer to regenerate. That passive layer is what makes stainless steel stainless — passivation restores and maximizes it. Machining, grinding, and shop handling all deposit or expose free iron that degrades corrosion resistance until the part is properly passivated. Rigid Concepts coordinates passivation through our trusted finishing partner NorTex Metal Finishing.
Why machined stainless steel needs passivation
The cutting tools, fixtures, and abrasives used to machine stainless steel deposit free iron on the surface. This contamination, invisible to the naked eye, can cause rust spots and premature corrosion even on a high-grade stainless alloy — a phenomenon sometimes called “tea staining” or “rouge.” Passivation dissolves those free iron deposits in an acid bath and allows the chromium in the alloy to re-oxidize into the protective passive film. The result is a stainless surface performing as its alloy intends.
- Removes free iron and metallic contamination from machining
- Restores and maximizes the chromium-oxide passive layer
- Eliminates surface rust risk on freshly machined parts
- Required or recommended by many aerospace, medical, and food-processing specs
- No dimensional change — purely a surface chemistry treatment
Nitric vs. citric acid process
Nitric acid passivation is the traditional process and is specified by many legacy aerospace and defense standards. Citric acid passivation is a newer, environmentally preferred alternative that is equally effective at removing free iron and is approved by current versions of ASTM A967 and AMS 2700. Both are available; your drawing call-out or application drives the choice.
Material and size capability
Passivation is a stainless steel process. Most austenitic (300 series), ferritic, martensitic, and precipitation-hardening stainless alloys respond well. NorTex handles parts up to 20" × 20". Contact us for larger assemblies.
| Process | Nitric acid · Citric acid (per ASTM A967 / AMS 2700) |
|---|---|
| Substrate | Stainless steel (300 series, 400 series, PH grades) |
| Max part size | 20" × 20" |
| Dimensional impact | None — surface chemistry only |
| Standards | ASTM A967 · AMS 2700 |
Passivation is a natural companion to our stainless steel CNC machining. For stainless parts that also need a black matte finish, black oxide coating is available. For electroless nickel over stainless, see electroless nickel plating.
// FAQ
Frequently asked questions
Not always — but it is best practice, and many aerospace, medical, and food-processing specifications require it. Any machined stainless part that will be used in a corrosion-sensitive environment or that contacts a regulated product or fluid should be passivated. It is inexpensive relative to the cost of a part failing in service.
Both processes remove free iron and restore the chromium-oxide passive layer. Nitric acid is the traditional method, specified by legacy standards. Citric acid is an environmentally preferred alternative approved by current ASTM A967 and AMS 2700 revisions. Performance is equivalent for most applications; your drawing or specification drives the choice.
No. Passivation removes surface contamination chemically and adds no material. Dimensions are unchanged, and the appearance of the stainless surface is unaffected — the part looks the same but corrosion-resists as intended.
ASTM A967 (Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts) and AMS 2700 (Passivation of Corrosion Resistant Steels) are the primary references. Both cover nitric and citric acid processes.
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