Hazy white tips on long pins... alkaline zinc plating

Q. We have been having an issue with zinc plating pins on a rack line. They are approx. 3/8" dia and 16" long. The very tips on some (not the entire rack) have a matte, silvery-white grainy appearance. Zinc content is 1.9 oz/gal and caustic is 16.8 oz/gal.

I do not believe it is a bath quality issue. We have perfectly fine parts right next to bad ones.

61694-1

I am new to the plating world and trying to learn as much as I can. Thanks in advice to any replies.

Mike A
- Pennsylvania
July 29, 2024

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A. Hi Mike,

Because you believe your bath is okay, and
because the defect is on tips which are the high current density area of the pins, and
because it's a matte, grainy appearance

My first suspicion would be 'burning', i.e., an attempt to plate at an unsustainable current density which results in excessive hydrogen evolution at that point, combined with the attempt to deposit zinc so rapidly that it can't form proper crystals. I would guess that the defective tips are closer to the anodes than the tips which are not burned; or the ones that are okay are more closely packed and sharing the load, or shielded in some way.

But if you're trying to learn as much as you can, please search the site for "Hull Cell" to learn how this can give you a better idea whether it is a bath quality issue.

Luck & Regards,

ted_yosem

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

A. Alkaline zinc bath,
I saw the picture, looks like HCD burning. Reduce the current or put the bleeder around tips.

Popatbhai B Patel
Electroplating consultant - Sterling Heights

We added more base and eliminated the inner rows from the rack and the problem seems to be gone. Looked like a combination of bath chemistry and rack design/loading.

MikeA [returning]
Lab Tech - Pennsylvania

Q. When we lowered the voltage, the LCD area (mid shank) would not plate properly. We could not find a voltage that was the sweet spot for these longer pins. We have much less issue running the shorter pins.

MikeA
Lab Tech - Pennsylvania
August 1, 2024

A. Hi again,

As you recognized and as Popat suggested, the current density at those points is too high for proper plating. If reducing it causes improper plating in the low current density areas, there seem to be only two solutions:

1. Extreme agitation to allow higher current density, probably impractical in this case (you would need some sort of jets to create high velocity impingement of solution onto the tip area).
2. Reducing the ratio of high to low current density by racking differently or by using current grabbing thieves or current diverting shields of some sort. I personally think plastic shields are more practical than plating out onto thieves.

If the problem is very complicated, there is electrochemical modeling software available for purchase. I haven't used it but it apparently does something similar to finite element analysis to predict the current density at the various locations when you input your racking situation.

Luck & Regards,

ted_yosem
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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