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Phosphating of Nickel Zinc electroplating

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Q. Hello Ted and everyone,
Does anyone know anything about the phosphating of chrome III passivated nickel zinc plating?
A customer has requested this via a spec.
Is it used as a key for further organic coatings?
Is it a different process to the standard zinc phosphating of steel?

Mark Lees
- A sunny rock in the Irish sea
July 2, 2024

A. Hi Mark.

By any chance is the work to be processed tubular? As we all know, you will normally get no plating inside a tube, which tends to cause rust stains. So I have seen zinc plated tubes go on to be immersed in a phosphatizing solution in order to offer at least a little rust prevention to the innards.

If it is not tubular work, then sorry, I've never heard of phosphatizing zinc-nickel and tri-chromated parts. Although, yes, zinc plating (without the chromate) can be phosphatized as a pretreatment for organic coatings. Zinc-nickel is not the same thing as zinc and I'm not sure whether or not conventional zinc phosphatizing solutions will react well on it, but they will certainly not react properly on top of a trivalent chromate and topcoat.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

A. Both Zinc Phosphating and Trivalent Passivation of Zinc are conversion coatings - in other words, they convert the zinc to another chemical. Applying a zinc phosphate to a trivalent - passivated zinc plated article would necessarily involve dissolving the passivate and part of the zinc plating. So why bother passivating? The same would be true vice versa. (IMHO sometimes you encounter technical specifications that make no sense.)

Tom Rochester
CTO - Jackson, Michigan, USA
Plating Systems & Technologies, Inc.
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thumbs up sign Hi Ted and Tom.
Thanks for your comments.
Ted, some of the parts are plated internally, with the assistance of internal anodes.
But they are not painted internally.
looking again at the spec it does seem at least to imply that the phosphating is on the plating and not a Cr III passivation. So that is useful. I may try some experiments with a zinc phosphating solution to assess its performance. Additionally, i have asked the customer to clarify the nature of the phosphating process they want.
Anyway, thanks again.

Mark Lees
- Ballasalla

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