Passivation of Stainless Steel Springs: Why "Stainless" Still Needs a Bath
Stainless springs can arrive rusty — not because the alloy is wrong, but because manufacturing contaminated the surface. Passivation is the cleanup step that lets stainless be stainless.
Where the free iron comes from
Coiling tools, grinding wheels, steel conveyors, and shared tumbling media all smear microscopic carbon-steel particles onto stainless surfaces. Those particles rust, staining the part and potentially seeding crevice attack. Heat tint from stress relieving also locally depletes chromium.
What passivation does
An acid bath — citric (ASTM A967 preferred, safer) or nitric (traditional) — dissolves free iron and surface contamination without attacking the bulk alloy. The chromium-rich surface then re-forms its passive oxide film in air within hours. The result: uniform corrosion resistance at the level the alloy promises.
When to require it
- Always: medical, food-contact, pharmaceutical, marine
- Recommended: visible parts, humid service, any 316 application (you paid for chloride resistance — collect it)
- Skippable: dry indoor mechanisms where minor tea-staining is acceptable
Specify: "Passivate per ASTM A967 (citric), verify per water immersion or copper sulfate test."
Related but different treatments
- Electropolishing: removes a surface layer electrochemically — brighter, smoother, and inherently passivated; the premium option for medical
- Pickling: stronger acid attack that removes heat scale — for welded assemblies more than springs
- Plating: for carbon steel; stainless springs need passivation, not plating
Most Formosa Spring stainless springs ship passivated as standard; confirm the requirement on your RFQ and we include the verification report.
Frequently Asked Questions
My stainless springs show brown spots after a few weeks. Are they fake stainless?
Almost certainly real stainless with free-iron contamination — classic missing passivation. A citric passivation bath usually restores them; specify passivation for future lots.
Citric or nitric passivation — does it matter?
Both meet ASTM A967. Citric is safer, more environmental, and equally effective for spring grades; nitric remains common for legacy specs. Accept either unless your industry mandates one.
Does passivation change dimensions or spring properties?
No — it removes only nanometer-to-micron scale contamination. Loads, dimensions, and fatigue behavior are unaffected. It is purely a surface-chemistry reset.