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Inconsistent appearance of zinc plated with black chromate parts

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Q. The shop I work at is getting pieces rejected for bad black zinc finishes. Some research tells me that it's normal, it's a functional finish, not decorative. I'm hopeful a picture here can help us out. Thanks ahead of time for any insight.

Greg Graham
- Butler Pennsylvania
August 23, 2022

A. Hi Greg. I get what you're saying about zinc being a functional finish -- it is true that zinc is a sacrificial coating rather than a barrier layer plating, and that there are nicer looking, more expensive, finishes like black nickel and black chrome.

But zinc plating can be very good looking, and we mustn't go overboard in the other direction and claim that the customer should not expect good-looking parts either. I remember decades ago when bright acid zinc plating was developed, some bicycle manufacturers switched their wheel plating from nickel-chrome to acid zinc plus lacquer -- and while acid zinc is not as bright as nickel-chrome, it obviously had to be consistently attractive.

Although people tend to hate sample boards ("these are acceptable parts / these are non-acceptable parts"), sometimes they are the only way of resolving questions like yours.

Luck & Regards,

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

⇩ Closely related postings, oldest first ⇩

Q. We are an automotive seatbelt component manufacturer that ships parts to a company that manufactures seatbelt assemblies. A part that we have been having problems with requires zinc plate .008 mm (min) plus Black Trivalent Chromate surface treatment. These parts are barrel plated, and at times will have an inconsistent appearance. I have attached pictures of the part.

58244-1 58244-2

The customer calls it contamination. What can we do to help our plating processor with this problem?

Timothy Morris
Director of Quality - Dolton, Illinois, USA
February 1, 2012

A. Hi, Timothy.

Maybe someone with more experience in this type of coating on this type of part will contradict me, but for now it looks to me like water stains caused by poor rinsing and/or drying. My second guess would be poor coverage with chromate because of something funny in the black chromate tank, like the plating barrel not rotating properly, so that parts are properly exposed to the chemistry. I'd say these are on the plater and there's not much you can do from your end.


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

A. From the photographs of parts you have posted, it appears that the problem is related to pretreatment and at the rinsing stages prior to plating. Please talk to your plater or try your parts at some other place.

Kishore Phadnis
- Mumbai, Maha, India

A. Hi Mr Morris,

You can never hope to plate stampings and black passivate them in a barrel process particularly these type of parts that nestle into one another.

A rack process is the way to go.

Khozem Vahaanwala
Khozem Vahaanwala
Saify Ind
supporting advertiser
Bengaluru, Karnataka, India
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A. For Black Trivalent use zinc-iron plating instead of zinc.

sara michaeli
sara michaeli signature
Sara Michaeli
Tel-Aviv-Yafo, Israel
March 20, 2012

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