Zinc Alloy Plating on carbonitrided steel is "chipping" during plating and drying

A. The chipping of the plated Zinc/Cobalt coating that you describe is coming from part on part action in any of the final finishing operations. You mention the dryer. I believe that dryers should have a "soft-start" mechanism and also be thermostatically controlled.

The soft start will allow the parts to dry in your centrifugal dryer without any part on part chatter, or at least minimized chatter. If your dryer cannot be retro-fitted, you can jockey the start switch off and on until the parts are all comfortably seated. Then allow them to dry. Also, I do not like drying over 140 °F. Higher temperatures will desiccate or dry out the chromate film that you have just applied, reducing the corrosion resistance markedly.

Finally, if the plated deposit in your opinion is brittle, you might wish to contact your vendor and address that problem with their Sales/Technical staff for a possible remedy.

A. Hello Tim!

First of all, what are other stamped parts? Please specify as much you can to help us help you! And are you using a zinc plating bath, or other kind of bath? Alkaline or acid?

If you can clarify these questions above, we may help you with your issue in barrel plating.

Regards :)

A. Hello Tim,

Nuts and bolts usually are heat hardened, so they are somehow more difficult to plate than non hardened parts. This problem usually appears in alkaline zinc baths, and not in acid ones. If you have problems with chipping in bolts and nuts, I suggest:

FIRST TALK TO YOUR BRIGHTENER VENDOR, HE MUST ASSIST YOU!!

Then, there are some things you can try...

- Lower the brightener concentration, or do not add brighteners until you see in Hull cell (or in the parts if you can't do a Hull test) some brightener problem.
- Lower the tension, so the current will more leveled and the zinc deposit will be less brittle. You may have to increase the time in order to achieve the same thickness!
- Make the total chlorides (potassium or ammonium chloride) go up to the maximum concentration that your vendor puts in the technical data sheet (135 grams per liter of total chloride is a good concentration for bolts, nuts and screws). This will grant you more level in the process current.

I hope some of this works! And welcome to the plating world!

Regards,

A. Hi Tim!

Please tell me what is for you "fine" in Hull cell, as I have seen many Hull cell tests validated as "OK" and not being OK for me...

If the brighteners are OK, Hull cell test OK, zinc in 28 g/l (seems high but some acid zinc systems work in that value, I work in that value for barrel plating and works fine) and no organic contamination (Hull cell test should consider this), you should see the potential and current parameters in the zinc tank for this part: maybe you should lower the V (potential) in your process, so lowering the current and diminishing the chipping problem.

This change in your process parameters could "hide" some other problem in your process, please ask your vendor for assistance to get concentration of zinc and brighteners to normal :)

Good luck and best wishes!

A. Hi Geetha. As you'll read on this and related threads, alkaline zinc alloy plating of carbonitrided parts probably can be done, but is quite difficult to do reliably. In the old days, platers would do a cadmium strike before cyanide zinc plating. Today, a cadmium strike is usually not environmentally acceptable. So your choices are probably to tear your hair out trying to achieve a robust alkaline zinc-nickel plating direct on the carbonitrided surface, or start with a Wood's Nickel strike :-)

Other input is welcomed; I am not a plater and most of my suggestions are just book knowledge.

Regards,