Inconsistent appearance of zinc plated with black chromate parts

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,

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.

Regards,

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.

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.

A. For Black Trivalent use zinc-iron plating instead of zinc.