Passivation

Passivation chemically removes free iron and enhances a stable oxide layer on stainless steel, improving corrosion resistance without changing part dimensions or finish.

Overview

Passivation is a chemical surface treatment—most commonly for stainless steel—that removes free iron and shop contamination from machining, grinding, or handling. The process promotes a uniform chromium-rich oxide layer, improving corrosion resistance without adding thickness like a coating.

Choose passivation when corrosion performance matters and you want to keep the as-machined geometry, tolerances, and surface texture. It’s common after machining, welding, bead blasting, or electropolishing, and for parts going into medical, food, marine, or semiconductor environments.

Tradeoffs: passivation won’t hide scratches, improve flatness, or fix poor surface finish. It also won’t protect carbon steel and won’t stop galvanic issues caused by dissimilar-metal contact. Chemistry selection (citric vs nitric), cleanliness, and rinse/dry control drive results; certification and testing (e.g., ASTM A967/AMS2700) can add lead time and cost.

Common Materials

  • Stainless Steel 304
  • Stainless Steel 316
  • Stainless Steel 17-4 PH
  • Stainless Steel 410
  • Stainless Steel 440C

Tolerances

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Applications

  • Medical device housings and brackets
  • Food and beverage sanitary fittings
  • Semiconductor vacuum hardware
  • Marine fasteners and hardware
  • Pharmaceutical processing components
  • Hydraulic manifolds and valve bodies

When to Choose Passivation

Choose passivation for stainless parts that must resist corrosion after machining, welding, blasting, or heavy handling. It’s a good fit for tight-tolerance components where adding coating thickness is unacceptable and the functional surface finish must remain unchanged.

vs Machined Surface Finishing

Choose passivation when the primary risk is corrosion from embedded/free iron rather than surface roughness or tool marks. It preserves the machined geometry while addressing contamination that machining alone can leave behind.

vs Polishing

Choose passivation when you need corrosion resistance improvement without changing part geometry or aggressively altering the surface. Polishing can improve cleanability and appearance, but passivation targets chemical cleanliness and oxide stability.

vs Coatings

Choose passivation when you need corrosion resistance without added thickness, masking complexity, or coating adhesion risk. Coatings can provide barrier protection and color, but they can chip, wear, or interfere with fits and electrical/thermal requirements.

vs Painting

Choose passivation when parts must remain metallic, clean, and dimensionally unchanged, or when paint films would compromise fits, sealing, or cleanliness. Painting provides cosmetic and barrier protection but typically needs more masking and ongoing damage control.

vs Heat Treatment

Choose passivation when material strength is already correct and the requirement is surface corrosion performance after fabrication steps. Heat treatment changes bulk properties; passivation targets surface condition and contamination removal.

Design Considerations

  • Call out the passivation specification and acceptance criteria (e.g., ASTM A967 or AMS2700, citric vs nitric) on the drawing or PO
  • Identify any areas that cannot be exposed to passivation chemistry and specify masking requirements up front
  • Specify post-passivation cleanliness needs (water break test, copper sulfate test, salt spray, particle limits) only if required—testing drives cost and lead time
  • Avoid trapped volumes and blind cavities without drain/vent paths to prevent chemical entrapment and staining
  • Specify allowable surface condition after passivation (no discoloration limits, cosmetic zones) since heat tint and weld scale may require pre-cleaning
  • Separate stainless from carbon steel in fabrication/handling to reduce cross-contamination that passivation may not fully recover