Surface treatment is where a good stamped part is most often turned into a rejected one. It is also the step most frequently outsourced without the buyer knowing — which is exactly why it should be specified, controlled and declared.
Passivation, electropolishing or plating — which should you specify?
Choose passivation for stainless steel parts where the goal is to restore corrosion resistance by removing free iron from the surface after forming and machining. Choose electropolishing when stainless steel also needs a bright, burr-free surface with improved micro-finish, accepting that material removal changes the final dimension. Choose plating — zinc, zinc-nickel, nickel or chrome — for carbon and low-alloy steel parts that need corrosion protection or a decorative finish, accepting that plating adds thickness on edges, may create hydrogen embrittlement risk on high-strength parts, and requires the plating chemistry to be declared for RoHS and IMDS. Phosphating suits parts that need a conversion coating before painting or to aid sliding; powder coating and painting are for appearance or broader chemical resistance rather than dimensional surfaces.
Compare the options
| Treatment | Best for | Watch out for |
|---|---|---|
| Passivation | Stainless parts after forming or machining | Does not hide marks; requires clean surface |
| Electropolishing | Stainless parts needing bright, burr-free finish | Removes material — affects tolerances |
| Zinc plating | Carbon steel corrosion protection | Thickness on edges, hydrogen embrittlement risk |
| Zinc-nickel plating | Higher corrosion resistance, automotive | Cost; still requires adhesion control |
| Nickel / chrome | Decorative or wear surfaces | Chemistry declaration, masking requirements |
| Phosphating | Pre-paint conversion coating, sliding surfaces | Not a final appearance finish |
| Powder coating / paint | Appearance and chemical resistance | Thickness, edge coverage, masking |
The part buyers forget: declaration
Every plating and passivation layer has a chemical composition that belongs in the material data declaration. Zinc-nickel plating, chromate conversion layers and phosphating all need to be declared in IMDS, and RoHS exposure is usually a plating chemistry question — hexavalent chromium in particular. A supplier who cannot say who performs the treatment, to which standard, and with which chemistry cannot support an automotive declaration.
Sequence matters as much as choice
- Deburr before coating: a burr under a coating becomes a coating defect
- Machine before plating if the feature has a tight tolerance, since plating adds thickness uniformly rather than selectively
- Avoid welding after plating where the coating affects weld quality
- Control masking: threads, press fits and electrical contact areas often need to be masked
- Define the corrosion test — a salt spray requirement turns an aesthetic choice into a measurable one
Key point
Specify three things with any surface treatment: the standard, the corrosion test and the declaration. Without them, the treatment becomes the least controlled step in the programme.
Frequently asked questions
Does passivation change the dimensions of a stainless part?
No, passivation removes free iron from the surface rather than bulk material, so dimensions are effectively unchanged. Electropolishing, by contrast, does remove material.
Why is zinc-nickel plating common in automotive?
Because it provides substantially better corrosion resistance than plain zinc at a thickness that still suits stamped parts, and it can be specified with a defined salt spray requirement.
Who should declare the plating chemistry?
The supplier that manages the treatment. If plating is outsourced, the composition data must come from the subcontractor and be kept current in IMDS.