Chrome plating of zinc die castings -- Problems & Solutions

A. Hi Ilyas
Although your cyanide copper preceding your acid copper may not be thick enough, the most likely problem is probably that the diecastings are simply not plateable quality. The plating will not be satisfactory if you can see cold shuts or porosity in the castings.
Luck & Regards,


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

A. Dear Mr. Bomy Dabhoiwala,

Here are suggestions that may help...

1. Vibratory finishing is the possible solution to uniform base metal finishing before plating.This will remove die casting flash & burrs and also even out the surface to provide a uniform base.
2. After good cleaning cycle the parts need to be plated with Cyanide Copper to minimum 4-6 microns.
3. Thereafter following good rinsing and 3% sulfuric activation the parts are plated in superior High leveling and Ductile Acid Copper process to a thickness not less than 12.5-15 microns.
4. This is followed by good rinsing and activation in 3% sulfuric followed by Bright Nickel plating to 10 microns using High leveling ductile process followed by rinsing and Chrome plating to 0.25 microns.

our job plater should know the basics of plating and you should be able to obtain a beautiful finish.

A. Rais,
The metallurgist is right and the manufacturer is wrong. Zn die castings are porous, period. The best way I know is to look at the parts under high magnification or a SEM (scanning electron microscope). I would do this at three stages. Incoming inspection, after cleaning, and after plating. I would assume that you are putting down a layer of copper under the nickel. Copper strike and copper plate. Good luck to you.

A. Die casters will always claim they produce castings without porosity, but they would, wouldn't they. In the absence of photographic evidence, I am quite prepared to believe that the castings are porous and this is the source of your problems. The best way to overcome porosity is to plate the casting with a high throwing power deposit. One of the more common ones is cyanide copper and this will fill in the holes and allow deposits with lower throwing power to be plated successfully

A. Rais,
I think your metallurgist is right and I also think that your manufacturer of casting claims that they do not have base metal porosity is wrong.Please let your manufacturer know that pitting (blisters) happens during or after plating is the result of hydrogen gas generated by the reaction between zinc and plating sol'n (or other corrosive solutions) entrapped in pits.So if there is no pits in casting, then there is no blisters after plating.Gone are the days that any body can get away by just washing of their hands with false claims.
But Mr.Rais, Zinc die castings have to have pores! Unless it is cast with foolproof techniques which is practically humanly impossible.
Then what to do- For that you need expert plating techniques.This forum does not allow me to impart full information in a platter for you as this technique somebodies may be the lifetime achievement as result of hard work of some(few) expert platers all over the world.
Just to give you some tips, take care in buffing- use soft cloths in final buff (smoothening, good soak cleaning, electrocleaning (anodic and NOT cathodic) and ... Go for a cyanide copper strike plating before going to Nickel.
Good luck!

A. You are correct.
If he rebels at the bake, let him know that you will gladly send him the parts, as plated, and that you will charge for the strip and replate each time they are returned.
His other option is to go to investment castings. Expensive!
If you lose him as a customer the world probably will be more pleasant.
Don't you love people that have the opinion of "do not bother me with facts, my mind is already made up."

A. Hello all,

It's the same with even high carbon steel parts, the carbon migration initiated by pickling and plating can be baked prior to plating, the pickling step removes the carbon from the surface, and resolves blistering issues in the same way as your baking removes entrapped air.

But the customer won't agree; you see it's a commercial issue here: if he does, then he has to pay you for the extra step and also pay for the plated parts that form blisters; by denying the problem exists, he can continue to pick holes in your process, and dump the liability of process rejects onto you.

Welcome to the ways of the world.

Insightful response, KV!

Regards,

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

A. Hi Amir. Although I am unfamiliar with buffing parts between nickel plating and chrome plating, I have no reason to believe it is unworkable. So I see nothing wrong offhand with your plating cycle. But it brings back the question of what is the condition of the die-castings? Is there good inspection of them before you attempt to plate them? If they have visible cold shuts or porosity they are not reliably plateable. Good luck.

Regards,

A. Hi.
I would spend some time at incoming store to check on the zinc die casting material. Bet you will find those porosity and bubble problem in the material itself. Yes, the bubbles will look like plating blisters...
For me, after showing these defects to the customer, that the casting itself is not defect free, we have now come to an understanding and certain % defect and limit samples are accepted. Hope it works out the same for you. Good Luck.

cheah

A. I have little experience in zamak plating compared to those who often post answers here.
There is one possible cause that is worthwhile to be analyzed --

1) Zamak is very sensitive to acids (Zinc, Aluminum, Magnesium and Kupfer[copper])
2) Porosity can carry solutions from one tank to another

If you combine both factors you have that if you have for example:

- Too strong acid dip -> You will create small pits in the substrate that carry solution from (for example) Alkaline cleaner or alkaline copper to watts nickel bath.

-Cyanide copper layer too low (due to little time, unbalanced bath chemistry, etc.) this will allow acid baths to corrode substrate creating small pits on it.

The problems with pits is that they carry solution and after the nickel plate they create a lack of adhesion (Blisters)

The same happens when the substrate is corroded or defective or for example with very old parts which are very difficult to be plated because you must remove all the pits that can hide solution inside.

By the way you can try if it is the case to use hot water rinses alternating with cold water rinses, this is going to help solution entrapped to leave the pits.

But now in this point this is becoming too complicated, I suggest check rinses (5% sulfuric acid) please readers tell me if I am wrong, check bath chemistry of copper, and also check the parts before entering to the plating line for corrosion.

I have little knowledge about zamak plating but when corroded zinc diecast is somewhat difficult to plate for me, I used to grind and polish it very well again before plating.

I hope this helps.

I will gladly receive any corrections, recommendations and feedback to learn more about this subject.

A. Also the acid attack of the zamak substrate alters the surface and produces the lack of adhesion.

A. Hi Chuck. I doubt that the metal itself "ages" in any significant way under these condition, but Safranek & Brooman .
make the point that buffing compounds are always difficult to remove but get even more so with age. For this reason they suggest considering vibratory finishing in lieu of your buffing if that is workable for your situation. Good luck.

Regards,

A. Hi Parmod. Unfortunately, even when the readers have a clear description and understanding of the problem, it can be tough to help solve the problem. But I doubt that anyone can offer much based on this verbal description :-(

It happens only on the high current density edges and corners where burning could occur, but it is "very smooth"? Why is it objectionable then -- is it the wrong color? What do you mean by "passive problem" -- is something not adhering? Please send photos of the defect, your process cycle, and a few paragraphs of detail about the defect and I think someone will be able to help.

Regards,

A. Hi Jeremy. Are the pits semi-hemispherical and bright when examined at low power magnification such as with a jeweler's loupe? If so, these are "gas pits" / "hydrogen pits". Widespread pitting which does not seem related to defects in the casting may be such gas pitting. Although a number of things can cause it, including poor buffing, the first step is to check the wetters and agitation in your nickel bath because any hydrogen bubbles that adhere too long will cause these pits.

Regards,

A. Hi again. The shiny semi-hemispherical pits are definitely "gas pits" (formed by a small bubble of hydrogen adhering to the surface as the plating builds around it). Again there can be a few causes, but the first thing to check is the surface tension in the nickel plating tanks. We have several threads on that: search the site for "surface tension nickel" or "pitting nickel".

I'm not sure about the bright hemispherical bumps, but they sound more like nodularity than roughness. Are the nickel plating baths commercial proprietary processes where you can get technical data sheets and analysis of the additives from the supplier?

Although modern nickel plating baths can level and bring reflectivity to a roughly polished steel surface, I'm not sure that that translates in any way to diecastings. To my limited knowledge of plating of diecastings, any visible porosity or cold shut will cause a plating defect.

Regards,

Ed. note: for further info on this subject, please also see letter 3778, "Problems in Nickel Chrome plating on Zinc Diecast Parts"

A. Hi Jeremy,
Letter 1000 mentions details regarding etching of zinc die-cast parts. Perhaps I missed it...is there an etch step in the existing process? According to my understanding, etching can help knock down micro-peaks formed during the casting process. As Ted pointed out, an improved buffing process may need to be investigated.

A. Hi VK. On a couple of assignments I acquired enough experience to be able to fairly reliably separate zinc diecastings and aluminum diecastings into "plateable" and "non-plateable", but there are many people with far greater experience & skill at it than I acquired in my short stints. In fact, William Safranek wrote whole chapters about inspection of diecastings before plating in several books :-)

But my experience was that porous surfaces (sometimes created by mechanical removal of swarf) are not plateable and "cold shuts" (tiny hairline cracks where the flow came together from two directions but was no longer hot enough to fuse) are not plateable.

Although you probably know much more about diecasting than I'll ever know, if you have a major contract, I think you should meet with your best plater and inspect a hundred diecastings or so and have him/her mark those that s/he thinks are and aren't plateable, then review the results after plating so you get a good feel for what is plateable and what isn't. On one of my jobs we powder coated the castings that we felt weren't good enough for plating but were too good for re-melting, further showing that there are degrees to "good" diecastings.

Regards,

A. Hi cousin Jesus. After the die-casting has been mechanically treated sufficiently for plateability you will probably need to electroclean, acid activate, cyanide copper strike, possibly nickel plate, and then chrome plate. Please start with our "Introduction to Chrome Plating", but recognize that there are whole books about plating of die-castings, and whole books about hard chrome plating, so your posting is quite open-ended and cannot be fully answered.

But be sure of what you want the plating to do -- it can improve wear resistance, but it's unlikely to have any good effect otherwise, just as an egg shell or a sheet of glass is very limited in its ability to protect an egg from much :-)

Luck & Regards,