Key takeaways
- Passivation on copper and brass is usually a thin conversion or organic layer, not the same process used on stainless steel.
- A passivation film can protect tarnish and still allow a low contact resistance if the chemistry and thickness are controlled.
- The cleaning step decides the result. Oil, oxide and machining residue under a passivation layer cause patches and failures.
- Specify the chemistry, the thickness or coating weight, the contact resistance limit and the test method, not just the word passivated.
What passivation means on copper and brass
On stainless steel, passivation is a chemical treatment that removes free iron from the surface and lets the chromium oxide layer form. On copper and brass, the word is used more loosely. It can mean a chromate conversion coating, an azole-based tarnish inhibitor, a clean and dry oxide control or a thin organic seal.
The ambiguity matters because the processes behave differently. A chromate conversion coating gives a visible film and a defined corrosion resistance, but it can raise contact resistance and may be restricted for some applications. An azole inhibitor is often thin and colourless, and it protects against tarnish while leaving the metal available for contact.
Ask the supplier what chemistry is used and what the film is expected to do. If the drawing only says passivated, two suppliers can quote two different processes and both claim to comply.
Why machined surfaces behave differently
A machined copper or brass part has a surface that has been cut, rubbed and heated. It carries micro-scratches, a deformed layer and sometimes a film of coolant or tapping oil. The surface is not the same as a rolled strip, and the passivation response is not the same either.
Edges, recesses and tapped holes are the difficult areas. A brushed or tumbled surface may clean evenly, but a blind hole or a tight corner can trap oil and resist the cleaning step. The result is a patchy film, a stain or a spot that fails a contact resistance check.
The machining process also leaves different amounts of exposed zinc in brass. A high-zinc brass can dezincify if the surface is not protected, while a low-lead or lead-free grade may behave differently in the same bath. State the alloy, not only the word brass.
The cleaner matters as much as the passivation
A passivation layer over a contaminated surface is a cover, not a treatment. Ask for the cleaning sequence and the rinse quality, not only the passivation chemistry.
The process sequence
A typical line starts with alkaline cleaning to remove oil and machining residue, followed by a rinse. A pickling or bright dip step removes oxide and scale. The part is rinsed again, then enters the passivation or inhibitor bath, rinses and dries.
The rinse quality is critical. If the rinse water carries contamination from the previous bath, the film will be uneven. If the part is dried slowly, water spots can form and the surface may stain before it reaches the customer. Demineralised water and a controlled dry are common controls.
For a part with blind holes, the line needs a drain and a dry step that actually reaches the inside. A part that looks dry on the outside can hold a drop in a tapped hole, and that drop becomes a corrosion site or a stain.
Contact resistance and solderability
For an electrical part, the passivation film is part of the electrical path. A thin azole film can be displaced by contact force and may have little effect on a bolted joint. A thicker conversion coating can add resistance and should not be used on a contact face unless the design allows it.
Specify the contact resistance limit and the test method. A millivolt drop test at a defined current, or a contact resistance test at a defined force, gives a result that can be compared. Do not rely on a visual check for an electrical requirement.
Solderability is a separate question. A tarnish inhibitor that protects the surface may also make it harder to wet with solder. If the part is soldered or welded after passivation, confirm that the process is compatible, or mask the joint area and passivate the rest.
Appearance, tarnish and handling
Copper and brass change colour when they oxidise. A passivation film slows that change, but it does not make the part permanently bright. The customer may still see a colour shift over time, especially in a humid or sulphur-bearing environment.
Agree the appearance standard before production. A clear film, a slightly iridescent chromate and a dyed coating look different, and a colour chip is more useful than a written description. If the part is visible, the cosmetic requirement may drive the process more than the corrosion requirement.
Handling is part of the finish. Fingerprints, packaging paper and cardboard can leave marks or accelerate tarnish. Use gloves, clean packing and a suitable interleaf. A part that passes the line can fail after a week in the wrong bag.
Testing and acceptance
The test should match the risk. For tarnish, an accelerated test in a humid, sulphur-bearing atmosphere gives a comparative result. For corrosion, a salt spray or a mixed flowing gas test may be appropriate, depending on the service environment. For electrical performance, a contact resistance or millivolt drop measurement is more direct.
A salt spray result is not a prediction of every service life. It is a comparative test under defined conditions. State the hours, the concentration, the pH and the inspection criteria. A claim that a part passes salt spray without those details cannot be checked.
For a first article, record the chemistry, the bath parameters, the film thickness or coating weight and the test result. Keep a sample of the finished surface for comparison. If the process changes, the sample is the reference that shows what changed.
Common failures
The most common failure is a surface that was not clean before passivation. Oil or polish residue produces a patchy film, a water break or a spot that fails the corrosion test. The fix is in the cleaning line, not in a longer passivation time.
The second is an over-thick film. A heavy chromate or organic layer can raise contact resistance, change the colour or interfere with a press fit. The thickness should be specified as a range, not a minimum only.
The third is a process that is not controlled. Bath concentration, pH, temperature and dwell time all move the result. A supplier who can show the bath log and the replenishment record is easier to hold to a specification than one who reports only the final appearance.
Specifying it on the drawing
Name the process or the performance, not only the word. For a tarnish inhibitor, say the chemistry family and the thickness or coating weight. For a conversion coating, say the type and the colour. For an electrical contact, add the contact resistance limit and the test method.
Say which areas are functional. A contact face, a solder joint and a sealing surface may need masking or a different treatment. A general note to passivate the whole part can create a conflict with a note that the contact face must be bare copper.
Add the acceptance test and the sample size. If the part is critical, ask for a coupon or a test piece that goes through the same line. That gives a result without damaging the part and makes the comparison between suppliers fair.
| Stage | Purpose | What to control |
|---|---|---|
| Cleaning | Remove oil and machining residue | Bath strength, temperature, time |
| Rinse | Stop carry-over | Water quality, flow, drain time |
| Pickle or bright dip | Remove oxide and scale | Concentration, dwell, alloy compatibility |
| Passivation | Form the protective film | Chemistry, pH, temperature, dwell |
| Final rinse | Remove bath residue | Demineralised water, conductivity |
| Dry | Prevent spots and stains | Air quality, temperature, blind holes |
| Inspection | Prove appearance and function | Colour, thickness, contact resistance |
The exact stages depend on the alloy and the chemistry. A brass with a high zinc content may need a different pickle from a high-copper grade.
Frequently asked questions
Is passivation the same for copper and stainless steel?
No. Stainless passivation removes free iron and relies on a chromium oxide layer. Copper and brass treatments are usually conversion coatings or organic tarnish inhibitors. The chemistry, the appearance and the test methods are different.
Does passivation increase contact resistance?
It can if the film is thick or if it covers the contact face. A thin inhibitor may have little effect under contact force, but the safe approach is to specify a resistance limit and test the actual finish.
How long does a passivation film last?
It depends on the chemistry, the thickness, the environment and the handling. A tarnish inhibitor slows oxidation but does not stop it forever. State the service environment and use an accelerated test for comparison.
Can passivated brass still be soldered?
It depends on the inhibitor and the soldering process. Some films are designed to be solderable; others interfere with wetting. Confirm with a trial, or mask the joint area and passivate the rest of the part.
Sources and further reading
- ISO 9227, Corrosion tests in artificial atmospheres, salt spray tests
- ASTM B117, Operating salt spray apparatus
- IEC 60068-2-60, Flowing mixed gas corrosion test
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Specifying a passivation finish on copper or brass?
Send the drawing, the alloy and the electrical requirement. The engineering desk will reply with the chemistry question, the masking call and the test to put on the first article.
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