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Keeping Bare Copper Bright: Passivation, Coatings and Packaging

September 26, 2026

Bare copper tarnishes. It is not a defect, no shop can stop it permanently, and it causes rejected shipments every week because nobody agreed what the part should look like on arrival. This is what the options are and what to write.

What tarnish is

A thin oxide and sulphide layer that forms in air, faster with humidity, handling and sulphur in the atmosphere. It changes the colour from bright salmon to brown. It does not change a dimension. On a contact face it raises resistance, which is why contact faces are plated rather than relying on bare copper.

The options, from lightest to heaviest

Do nothing and say so. Acceptable for parts that are plated, brazed, assembled or machined further after delivery. Write “tarnish acceptable” on the drawing and the argument never happens.

Chromate free passivation. A thin conversion film, often a benzotriazole based treatment, applied by dip after cleaning. Slows tarnish for storage and shipping. Nearly invisible, does not affect dimensions, and solderable in most cases. This is the usual answer for “arrive bright”.

Clear lacquer or acrylic. A visible coating that protects for longer. Used on decorative and architectural copper. Not suitable where the part is soldered, plated or used as a contact, because the coating has to be removed first.

Inhibitor packaging. Vapour phase corrosion inhibitor paper or bags. Protects bare copper in transit and storage without touching the part. Often used with a passivation, and it is the correct answer for any part that cannot carry a film.

Plating. The only long term answer for a contact surface. See tin, nickel and silver plating.

Vacuum parts are the exception

Anything going into a vacuum system is cleaned, not coated. A passivation film outgasses. Specify the cleaning process, gloves, and a clean bag, and write “no surface treatment” so a helpful plater does not add one. See oxygen free copper for vacuum.

What to write

Pick one and put it on the drawing.

Surface: as machined. Tarnish acceptable.

Surface: as machined, chromate free passivation,
solderable. Ship in VCI bag.

Surface: as machined, no coating or passivation.
Ultrasonic clean, glove handling, sealed bag.

Then add the acceptance criterion, because it is what incoming inspection will use: “uniform colour, no fingerprints, no water marks” is checkable. “Bright” is not.

The conversation to have before the first shipment

If your incoming inspection has ever rejected copper for being brown, decide now which of the above you want and what it may cost. A passivation and a VCI bag are cheap. A rejected batch and a return shipment are not.

Copper surface finish covers the machined finish underneath, which is a separate specification from the tarnish state.

Frequently asked questions

Keep reading

• How to Clean and Prepare Machined Copper for Vacuum Service
• Plating Thickness on Copper: What to Write and Where