Passivation

Passivation chemically removes free iron and contaminants to form a stable oxide layer that boosts corrosion resistance without changing part dimensions.

Overview

Passivation is a chemical surface treatment, typically using nitric or citric acid solutions, that strips free iron and contaminants from metal surfaces and promotes a stable oxide layer. It targets the surface chemistry, not the geometry, so dimensions and surface finish remain essentially unchanged.

Use passivation after machining, forming, or welding stainless and corrosion-resistant alloys when you need reliable, repeatable corrosion performance or to meet standards like ASTM A967 or A380. It’s common in regulated industries where cleanliness, corrosion resistance, and documentation matter more than appearance alone. Tradeoffs: parts must be clean and properly fixtured, some alloys require specific chemistries, and you need clear masking instructions for areas that must not see acid. Passivation will not fix poor base material, rough surfaces, or improper heat treatment, but it is a low-cost, high-impact final step to extend service life and reduce rust-related field failures.

Common Materials

  • Stainless Steel 304
  • Stainless Steel 316
  • Stainless Steel 17-4PH
  • Stainless Steel 410
  • Titanium Grade 5

Tolerances

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Applications

  • Surgical instruments and implants
  • Food processing and dairy equipment
  • Pharmaceutical and biotech process piping
  • Hydraulic and pneumatic fittings
  • Marine fasteners and hardware
  • Semiconductor and vacuum tooling

When to Choose Passivation

Specify passivation when corrosion resistance and cleanliness of stainless or titanium parts are critical, especially in wet, hygienic, or chloride-rich environments. It fits best as a final step after machining, welding, and cleaning, for low to high volumes where you need consistent, certifiable surface condition. It’s especially useful when customers or standards call out specific passivation specs or documented corrosion performance.

vs Machined Surface Finishing

Choose passivation when the surface chemistry and corrosion resistance matter more than tool marks or roughness alone. Machined finishes control Ra and appearance, but any embedded iron or shop contamination can still drive rust; passivation removes that contamination while leaving the machined geometry and texture intact.

vs Polishing

Pick passivation when you need corrosion resistance and cleanliness, not a cosmetic mirror finish. Polishing improves Ra and visual quality but can smear free iron and contaminants into the surface. Passivation is faster and cheaper for functional parts where microscopic surface condition and rust prevention outweigh aesthetics.

vs Coatings

Prefer passivation when you want bare-metal corrosion resistance without added thickness, peel risk, or ongoing coating maintenance. Coatings create a barrier layer, but they change dimensions and can chip or wear off; passivation keeps the part dimensionally true and relies on the alloy’s own passive film for long-term protection.

vs Painting

Choose passivation when you need clean, corrosion-resistant stainless surfaces that remain metallic and weldable. Painting is better for color coding or hiding cosmetic defects but adds thickness, can interfere with fits, and may fail at edges or fasteners; passivation treats the entire exposed surface uniformly without altering clearances.

vs Heat Treatment

Select passivation when the issue is surface contamination and rust, not strength or hardness. Heat treatment modifies bulk properties and can even degrade corrosion resistance if done poorly; passivation comes afterward to clean the surface and restore or enhance the passive film without affecting mechanical properties.

Design Considerations

  • Call out the exact alloy and required standard (e.g., ASTM A967, A380) on the drawing so shops can select the correct chemistry and method
  • Avoid deep blind holes, tight crevices, and unvented cavities where solutions can trap; add vent holes or design for good solution flow and draining
  • Specify which surfaces must be masked from acid (threads, critical bores, identification areas) using clear notes and marked drawings
  • Define required final surface finish range before passivation; very rough or heavily machined surfaces may still rust even after treatment
  • Ensure welds are fully cleaned and any heat tint removed before passivation to get consistent corrosion performance across base metal and welds
  • Ask for sample coupons or first-article testing when corrosion performance is critical, especially for new alloys or mixed weld materials