Invar Passivation: What It Does and When It's Worth Specifying
Passivation gets mentioned alongside plating and coating options for Invar parts, but it is a different kind of process with a different purpose, and understanding what it actually does helps you decide whether it belongs on your drawing or whether a bare or plated surface serves your part better.
What passivation actually does
Passivation is a chemical treatment that removes surface contamination and free iron from a metal surface, then promotes the formation of a thin, stable, protective oxide layer. It does not add a separate coating layer with its own thickness the way plating does. Instead, it improves the quality and consistency of the oxide layer the metal would naturally form on its own, making that layer more uniform and more protective than what happens through uncontrolled exposure to air.
This distinction matters directly for dimensional purposes. A plated coating, covered in surface finish and coating options for machined Invar, adds a measurable thickness that has to be accounted for on critical dimensions. Passivation does not add a meaningful thickness, so it does not introduce the same before-or-after-coating tolerance question that plating does.
Why passivation comes up for Invar and Kovar
Invar’s corrosion resistance, driven by its roughly 36 percent nickel content, is better than plain carbon steel but below stainless steel, a gap covered in surface finish and coating options for machined Invar. Passivation is one option for improving that resistance modestly without adding a coating layer, useful for parts that need some improvement over bare, uncontrolled surface condition but do not need the fuller protection a plated coating provides.
Kovar parts destined for glass or ceramic sealing are also commonly passivated or otherwise carefully cleaned before sealing, since surface oxide condition affects how reliably the metal bonds to the glass or ceramic during the sealing process, a point also touched on in Invar 36 vs Kovar vs Super Invar. In this context passivation, or a related surface preparation step, is less about long-term corrosion resistance and more about achieving a clean, consistent surface for a reliable seal.
Passivation versus plating: which to specify
| Passivation | Plating (e.g. electroless nickel) | |
|---|---|---|
| Adds coating thickness | No | Yes, needs to be accounted for in tolerances |
| Corrosion improvement | Modest | More substantial |
| Cost | Lower | Higher |
| Typical use | General improvement over bare metal, pre-seal surface prep | Parts needing meaningful corrosion protection or specific appearance |
For parts used indoors in a controlled, dry environment, bare uncoated Invar is often sufficient and neither treatment is strictly necessary. For parts in humid or less controlled environments where some improvement is warranted but a full plated coating is not justified by cost or dimensional constraints, passivation is a reasonable middle option. For parts needing substantial, reliable corrosion protection, plating remains the more robust choice.
Passivation chemistry specific to Invar
Passivation processes are often associated with stainless steel, where the chemistry is well established around chromium oxide formation. Invar’s composition, roughly 64 percent iron and 36 percent nickel with no significant chromium, is different, and a passivation process appropriate for stainless steel is not automatically the correct one for Invar. A shop specifying passivation for an Invar or Kovar part should use a process and chemistry validated for that specific alloy’s composition, not simply apply a generic stainless steel passivation specification without adjustment.
Specifying passivation on your drawing
- State explicitly that passivation, not plating, is the intended treatment, since the two are sometimes conflated in casual conversation but specified very differently
- Reference an appropriate specification if your program requires a specific documented process, and confirm with the shop or a surface treatment supplier that the referenced spec is validated for Invar or Kovar rather than only for stainless steel
- Note whether the passivation is primarily for corrosion resistance or as surface preparation ahead of a sealing or bonding operation, since this affects which process parameters matter most
Getting a quote
If your part’s surface treatment requirement is not yet finalized, describing the operating environment and any sealing or bonding requirements on your RFQ lets the shop recommend whether passivation, plating or a bare finish is the right fit, rather than guessing from a drawing note alone.
Send your drawing through our RFQ form with your surface treatment requirements noted, and we match you with shops that can specify and apply the right process for the alloy.
Frequently asked questions
Does passivation change Invar's dimensions the way plating can?
No. Passivation is a chemical surface treatment that modifies the existing oxide layer rather than adding a measurable coating thickness, so it does not need to be accounted for in critical dimensions the way an electroless nickel or other plating thickness does.
Is passivation the same process for Invar as it is for stainless steel?
The underlying goal, improving the natural protective oxide layer, is similar, but the specific chemistry and process parameters differ because Invar's composition is not the same as stainless steel's. A shop should use a process appropriate to Invar's specific chemistry, not simply apply a stainless steel passivation spec without adjustment.
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