Black zinc
Alkaline zinc with black trivalent passivation. It is the lowest-cost black finish — and a different coating from black zinc-nickel.
Black zinc is the alkaline zinc bath with black trivalent passivation. The deposition is the same as white zinc and yellow zinc; what changes is the passivator in the final step, and with it the color.
The difference from black zinc-nickel is not one of finish, it is one of coating. Here the deposit is pure zinc; there it is an alloy with 12 to 16% nickel. Without an organic finish, electrolytic zinc sits between 200 and 300 h of salt spray and the black zinc-nickel base runs from 900 to 1,000 h. The color is identical on the part, and the performance is not.
That makes black zinc the right choice where the requirement is appearance and protection in a sheltered atmosphere, in volume — and the wrong choice where there is salt, permanent humidity or a European OEM specification. When the drawing says only "black", the question to ask is which of the two, because the difference is an order of magnitude.
Trivalent passivation replaced hexavalent by legal requirement, and the alkaline bath distributes the layer more uniformly than the acid one on parts of irregular geometry — holes, recesses and corners get a thickness closer to that of the flat.
Technical data
- Deposit
- Alkaline zinc
- Passivation
- Black trivalent (Cr³⁺)
- Other bath colors
- White · yellow
- Salt spray · without finish
- 200 to 300 h
The applications that come here.
Part in plain sight in a sheltered atmosphere
Component assembled where a dark finish is a requirement and exposure is indoor or protected.
Production volume
Series lot with irregular geometry, where the uniformity of the alkaline bath weighs more than the shine of the acid one.
Identification by color
When black tells one lot or specification from another at the customer’s goods-in, without changing the coating.
What specifiers usually ask.
What is the difference between black zinc and black zinc-nickel?
The metal deposited, not the finish. Black zinc is pure zinc with black trivalent passivation and sits between 200 and 300 hours of salt spray. Black zinc-nickel is an alloy with 12 to 16% nickel to ASTM B841, and the base alone runs from 900 to 1,000 hours. On the part both are black and visually similar — the difference shows up in the field.
In which colors is alkaline zinc passivated?
Black, white and yellow, in trivalent passivation, to the OEM’s or the customer’s specification. Color does not change protection: it comes from the thickness of the zinc and the sealer applied.
When is black zinc not enough?
In marine, aggressive industrial or permanent weather-exposure environments. In those cases the way forward is not to thicken the zinc — which eats thread tolerance — but to swap the alloy for black zinc-nickel, which delivers the same finish with protection of another order.