Chromate carburetors/diecastings: is zinc plating required first

A. Hi Michael. I don't think anyone on this thread said it wasn't 'safe', although strong & regulated chemicals are surely involved. But they said the likelihood of successfully re-chromating a carburetor without first zinc electroplating it was low.

Yes, there are spray paints intended to look like the gold/yellow chromate coating on O.E.M. carburetors. Eastwood has a 3-step system where it looks like you paint the whole thing with their gold color, then touch up with little streaks of red and green to give the iridescent look. But I can't say how well it works.

Luck & Regards,

Thanks for the very complete explanation.

Luck & Regards,

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A. I have seen zinc plating of zinc diecastings, but in my limited experience it seemed to be that it was done to impart a uniform & shiny surface (see thread 35697 "Carburetor re-plating / re-coloring"). But sometimes diecastings are successfully directly chromated without first zinc plating them.

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P.S., March 2015: Note that no one on the "Carburetor..." thread I referred to offered any corrosion testing reports. And after seeing the knowledgeable input from Tim, Khozem, Robert, Tom, and others below, I don't think direct chromating of zinc diecastings is a good strategy :-)

A. The older zinc-alloy plating systems consist of a zinc-alloy plating followed by hexavalent chrome passivation. The zinc-alloy protects the base metal from red rust. The passivate protects the zinc-alloy from white corrosion.

In the new system, you have the same zinc-alloy plating followed by a trivalent chrome passivation. Since the trivalent chrome is not as good as the hexavalent, you sometimes need an additional layer of something after the passivation. However, you definitely still need the zinc-alloy.

Passivation with no plating does not make much sense. It is like trying to build a wooden house with paint and no wood.

A. Aptly said Tim!

I have Salt Spray tested parts with passivation over Zinc without plate and there is only about 1 hour difference between bare castings and passivated ones to white rust.

However these parts are for two wheeler manufacturers facing extinction from imaginary Chinese competition, hence cost reduction is the issue, not corrosion protection, so choose wisely before changing specs.

By the way we did a good job on the chromating and also tested parts for the presence of the chromate film. As an undercoat to paint or lacquer, chromate alone is okay, but not as a final finish.

A. Zinc dissolves in a cyanide bath, so although I've never tried to do it, I guess the zinc alloy substrate can be attacked before deposition begins and the adhesion could be affected. Obviously zinc alloys are affected by acid zinc baths as well.

A. You can zinc plate over zinc die cast with an alkaline zinc bath. What do you mean by zinc alloy? You can plate zinc over aluminum, but must utilize a zincate process (does incorporate a few process tanks). Suggest you contact a local proprietary vendor for help. Several support this web site nationally.

A. Hundreds of times I have recommended and implemented the concept of improving the quality of yellow chromate on zinc die cast (the surface of which is about 7% aluminum) by first zinc electroplating a purer top coat of zinc upon which the chromate forms a better gel.

On one occasion I successfully improved the quality of zinc electroplate on rivets by first plating with alkaline zinc to get "throw" under the head and then top coating with acid chloride zinc to get a brighter appearance.

One rather unique way to plate zinc over aluminum that has not yet found a market niche is to clean the aluminum by removing organic soil and then deoxidizing then placing in a alkaline zinc-cobalt or alkaline zinc-nickel or alkaline zinc-iron or any alkaline zinc 30 seconds before turning on the current, then turn on the current and increase very slowly up to about 15 ASF, then plate for thickness. You see, the matrix of the alkaline zinc systems solution is also the matrix of a "zincate" therefore you apply the zincate right in the solution before you start plating.

Just note that zinc die cast is NOT pure zinc. Aluminum die cast is NOT pure aluminum therefore all I have said must be developed around the particular alloy.

A. Yellows do typically go up to 96 hours. However you must realise that they are usually applied to a freshly plated item, that is one with a nice even corrosion free & activated surface.

Perhaps you could get 96 hours out of yellow chromating diecast parts, but I bet surface prep would play in important role in this.

A. Blisters can come from many different things. Typical is...
1) too much brightener or organics in the solution
2) bad pretreatment

It would be helpful to get more information on this problem.

1) What type of zinc solution are you using?
2) Is the zinc layer chromated or passivated?
3) Where on the parts is the blistering occurring?

A. Look for a pattern in the porosity, most likely it is a hot spot in the die casting process or fill related. Try to raise the die temperature and increase the pressure on the injection during solidification. Look at the die condition, you may find that there is a loss of pressure there. Try some of the die coating products... polish the die. Look at process control, often so much is based on visual criteria. Some times it can be plating solution entrapment, but that is only a expedited symptom since casting is full of pores anyway.

What it is.. there are micro pores in the casting from the solidification of the metal, these fill with solution ( even water will do this )often plate over and then bleed out, peeling and or blistering later on.

Pending on the economics you may need to impregnate the part to fill the voids. One other trick is to vibromill the parts, not to remove the surface but to peen over the surface to close the pores ( part on part, or a polish treatment.)

A. Hi Christopher, I don't think you should either chromate or plate it at all, if you must then CED paint is the acknowledged best coating solution for zinc diecastings than any other coating process.

A. Hi Christopher.
Interesting question.
Zamak 5 is zinc diecasting. The conventional method of plating on Zinc diecasting, as stipulated in ASTM B252 - 'Standard guide for preparation of zinc alloy die casting for electroplating and conversion coating', involves a cyanide copper strike followed by the subsequent required plating. There are no zinc plate involved here.

A lesser known method and to move away from cyanide, which is more harmful to the environment, is to plate a zinc or zinc alloy (preferably an alkaline zinc bath followed by the subsequent required plating. Enthone OMI has a 1996 patent on this method (US Patent 5578187). In this case, the zinc plate is mandatory.
Good Luck.

A. Hi Christopher. I learned something from Cheah's answer, but I'm not certain if that was actually your question. I think your question may have been whether zinc diecastings are sufficiently corrosion resistant for interior use without further surface treatment.

If you have no specific standard to meet, we are into the realm of personal opinion about what is good enough and what isn't.

Based on a corrosion test project which finishing.com completed for the IZA last year, I agree with Khozem that CED performed quite well in the corrosion tests.

Regards,

Thank you for your responses, it was my customer's requirement to Zinc plate Zinc die casting components.
They have requested a Zn 5 µm coating with clear passivation, due to possible exposure to external weather.
A 5 µm coating does not provide adequate protection, as per ISO or ASTM requirements, so I was asking does Zinc Die casting really need to be coated to prevent corrosion.

From all your responses the answer is yes.

Thank you

A. Hi Teja,
We appended your inquiry to an earlier thread on the same subject. It seems that direct chromate conversion coating is problematical and not really advised. In this age of trivalent chromating, using more finicky, proprietary, processes on substrates for which they were not intended is even more questionable.

Hi Ravneet,
We appended your inquiry to a thread where Teja claims 72 hours was achievable. Still this process doesn't sound robust. And if you want QA traceability you will need to find a statement from the trivalent chromate conversion chemistry supplier that their process is designed for use directly on Zamak castings (and you probably won't). Good luck.

Regards,

A. The reason that Zamak presents a problem for conversion coatings is that there are a number of Zamak alloys (2, 3, 5, and 7 being the most common) and within each numbered alloy there is variation in metal content. So it would be uneconomical for a metal finishing supplier to develop a large number of passivates that would be specifically targeted to a single alloy, of which there are probably dozens. And not only would it take time to develop them, and they would have to stock small quantities. That's why the most common commercial practice is to plate with zinc and passivate the zinc, as many formulations and suppliers are available. So the optimal coating for Zamak would be one that does not react with the substrate. So the best bet would be a topcoat of a paint like Doerken, Magni, Zintek, Geomet or the like. A single coat should give 200+ hours, but check with your supplier. No-rinse or dry-in-place trivalent passivates are available in the USA but not generally worldwide.

A. Hi Robert. The rectifier wouldn't be my first guess. But what is not clear to me yet is whether this is an established process which started acting up, or whether you have not yet obtained reliable robust plating on these components.

My experience with zinc plating of diecastings is limited, but it looks to me like the castings in the top photo have received no mechanical pretreatment. And while we appreciate you not naming proprietary treatment chemistry, if any, your description of your process is general enough that I'm not really learning much from it.

One thing I have seen on barrel lines is one barrel not quite meshing with the rotation drive on an important tank like your electrodegreasing tank, and leaving the parts dirty such that they blister like this. Make sure you "walk the line" to find out what really goes on, not just what the paperwork says :-)

Hopefully between the entries already on this page, and the anticipated responses, you will get some useful input.

Luck & Regards,

• Topic 0338 "Zinc plating & chromating of Carburetors"

• Topic 11847 "Restoring gold Iridite color to diecast parts"

• Topic 33779 "Clean and brighten carburetor bodies"

• Topic 35697 "Carburetor re-plating / re-coloring"

• Topic 37255 "Restoring the yellow dichromate finish on automobile carburetors"