Gold plating of aluminum

A. Hi Jeff. I think most maskants can withstand a hard gold plating cycle and wouldn't expect much trouble there. But even if you already know it, we should mention for other readers following the discussion that the electroless nickel will probably passivate during the masking operation, so a Wood's Nickel Strike will probably be required before the gold plating.

Luck & Regards,

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A. Hi again. You should normally clean before & after masking. Many electroless nickel baths are quite hot and aggressive, so reliable masking may be more difficult, but yes, you can mask before any plating.

Yes a gold strike is very useful not just for activation but to prevent immersion plating and as a sewer to protect your main gold tank from contamination. It's probably not as powerful an activating agent as Wood's Nickel, but probably more than adequate if you're going there right after electroless nickel rather than stopping to mask.

I'm not a gold plating expert, but I understand that you can get good hardness from any of the three, but you need a little (probably less than 1/2%) of cobalt or nickel for good hardness. Book knowledge says acid golds are easiest to operate and most robust.

Luck & Regards,

A. That is one nasty job. So tiny and so many in a relatively small area.

I would check to see if any vendor makes a "pull plug" out of soft silicone. A 0.030 would probably be a proper size.

You can get taper plugs that small, but they are not easy to install.

You can take vinyl tubing, about .030 OD, cut the ends on a taper at appropriate lengths and pull these thru. They work, very well, but are more time consuming than a pull plug.

Do you absolutely have to plug them, Could 0.001 oversize and plated do the same thing? I assume that they cannot be plated and chased afterwards because of edge burrs.

Q. I don't have to plug the aluminum block before gold plating.

So, can you advise a gold type filler to fill the holes after the plate has been gold plated?

thanks, Brent

A. Sorry, I don't understand what you are looking for. Please explain what you mean by "I need to plug up 200 of the holes prior to hard gold plating" but "I don't have to plug the aluminum block before gold plating". I guess you're saying that 200 of the holes have to be permanently "blocked" and after reconsideration it really doesn't matter that it be done before plating? Maybe a reader who has done something similar will have more insight :-)

I'm a little lost, Brent. Sorry :-(

A. Me too.

A. Hi Theo. This should not be a problem. The aluminum can be zincated, then copper plated, then gold plated. Although gold and copper will interdiffuse at high temperature over time, if the part is not hot the plating should meet your needs. Good luck.

A. High phosphorus electroless nickel deposits are non-magnetic unless heated to over 250 °C.

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A. Exposed aluminum immediately forms an oxide layer that impedes further plating or coating. EN also forms an oxide layer that does the same thing. Both have to be activated to receive a gold plate but the sequence and chemicals are different. It is better to EN plate after drilling, then you'd have a single metal to plate onto.

? You need to clarify. Gold Acid Copper? Do you mean acid gold and/or acid copper solution?

A. Hello Gerry,
I understood part of the question, so I'll answer that. Plating gold directly on zincated aluminum is a bad idea. The zincate will migrate through the gold in no time and affect the appearance of the part. If gold is to be the final finish you will need a barrier plate such as Ni. Without Ni or a sufficient diffusion barrier even the copper will migrate through the gold layer. Acid copper is widely used in electroforming, but you may want to consider a copper strike before acid copper either Cn or alkaline to ensure good adhesion. Good Luck!

Q. My English was a little confusing there :-)

To clarify, I'd be starting with an Alum. mandrel which is eventually etched out of the finished piece.

We always zincate the Alum. before further plating. In the past we would zincate the mandrel, CuCN flash, plate a few hundred microinches of gold, CuCN flash over the gold, and then go into our acid Cu to build up copper for the bulk of the part.

After the finished piece is machined we etch out the alum-zn-cu, leaving the gold plate as the interior working surface of a microwave waveguide structure. (alum-zn come out with HCl or NaOH, little HNO3 to get the copper).

Recently we've experimented with: Alum mandrel - zincate - EN - gold - straight into acid copper. This seems to work generally OK, but I think I've had trouble etching out the nickel, EN. (it's an interior surface, hard to inspect)

My thought was that if I could gold plate directly on Zn then my etch out would just be aluminum-zinc. IF the Zinc-Gold dissolving happens fairly slowly I could live with it, since the Zn will be etched out within 3-4 weeks anyhow. I realize the Zn-gold may have "fingered" together some and when the Zn is etched away I'll have roughened the gold surface some, but how much? I don't need a mirror, just a decent gold surface to finish the waveguide interior.

Maybe I should bite the bullet and just set up a CuCN bath, but the acid copper seems to plate fine directly onto gold, so if I can just get a plateable but easily dissolved flash over the alum or zinc we'll be in business.

Thanks for the replies.

Did a quick experiment re plating gold directly on zincate.

I took a thin alum. plate, few square inches, zincated it on one side (other side masked with plastic).

I went into my CN-gold with that and plated on .0005" of gold.

Good news is, it plated nicely from the first second, no signs of any strangeness at all. Went in hot, hope I'm not contaminating the bath with zinc at all. But the plated gold came out nice as anything.

I then etched the Alum-zinc away with HCl to get a rare look at the "inside" of some plating. That inside surface (was against the zinc layer) looked nice, but had a "brassy" look. So possibly after just 2-3 hours in contact with zinc, at bath and ambient temps, we've had enough zinc go into the gold to change it's appearance.

I might take this a little further, let the next test piece set at room temperature for 3-4 weeks and try elevated temp also.

A. Hi, Tony. Atotech offers (or used to offer) their Alstan system for metallizing aluminum in lieu of zincating. The trade name is a play on "all tin". But I can't guarantee that there is no zinc or phosphates or danger of sublimation; you'd need to contact them. Good luck.

Regards,

A. Steve, there is some more information needed. First of all, I would recommend reading MIL-G-45204 [link is to free spec spec at Defense Logistics Agency, dla.mil] and MIL-C-26074 [on DLA]. You may find all of your questions answered. Is there any chance the process is overloaded? The step plating should have triggered an investigation into overloading. Next, you'll need a process map to understand C&E and inputs. Is the current process electrolytic or electroless gold? Is there any embrittlement relief or intermediate steps before the gold but after the nickel? Your message almost made it sound as if there is nothing wrong with the new EN and all you are seeing is voids in the gold. If that is the case you can look into drag-out and rinsing. The 20 minute step you describe is not a step I am familiar with. All signs seem to point to overloading but when you are using such a thin gold layer it could be something else, take a look at Mil-G-45204 and you'll see what I mean. I would think a good process would be EN, drag-out, rinse (fast), gold strike, gold. That is the process I am familiar with. One big problem is when people plate the nickel and then let the nickel dry. If the nickel dries just a tiny bit, it can cause spots. This could be causing your problem. The nickel must not dry if you want to use the process I described. Another thing you need to do is verify the EN coating is good. You can do that by destructive testing. There's no sense in curing a gold problem if you really have a nickel or pre-treatment problem. One last thing is to blow dry the parts with dry N2 immediately after gold and rinsing. You can also use IPA rinses and hot rinses after the gold, it's highly recommended, but based on your message it sounds like the gold not being dried properly is not the problem here but it is certainly worth looking into because if you don't dry the parts quickly and properly after gold, you will see a similar spotting condition. Good luck.

A. Hi Bharathi. Unfortunately, one must acquire a good understanding both of finishing processes & the needs of the specific product in order to do what you'd like. There isn't a "standard" type of plating; rather, the plating is designed and specified for the particular application.

0.25 microns of gold is rather little, so the underlying nickel plating will definitely be functional and critical. Some of the first things you need to determine include the function of the plating, and the environment to which it will be subjected. Is the gold for decorative purposes or for low contact resistance electronic application? Then, what type of gold plating do you need (hard gold, soft, solderable, etc.). Then what kind of nickel plating. Yes, there are certainly ASM, ASTM, ISO, and other plating specs which you'll want to reference -- but you can't choose them until you've clearly delineated the general requirements of the plating. Good luck.

Regards,

A. Hi Glenn. Yes, it can be done. The anodizing must be stripped, then the aluminum zincated, plated with something compatible with zincate (possibly cyanide copper), then nickel plated, then chrome plated.

If you're going to gold plate or chrome plate something though, it's better to not make it out of aluminum if practical. You would have been better off starting with steel or zinc diecast parts if available. Happy restoration!

If you've got the money for full restoration, Hammer & Dolly in Traverse City Michigan just finished a beautiful 1956 Nomad per their Facebook page. Owner Adam Hammer is the son of Chris Hammer, president of Hill-Cross Company [a finishing.com supporting advertiser] and grandson of Herm Hammer -- co-founder of Hill-Cross, along with (small world) Donald Wood, the inventor of the Wood's Nickel Strike process.

Regards,

A. Hi Glenn.
It can be done. But if you intend to use the car rather than stand it in a showroom you should know that roads have one of the most corrosive environments known to plating. Thin gold is porous and will rapidly fail. Sufficiently thick gold say 5+ microns is going to cost serious money.

Docker Daimlers

Back in the 1950s Daimler produced a gold plated model for Lady Docker.
At that time gold was about $35/oz
It's not now!

Most anodizing shops can offer a very good looking gold finish and a clear lacquer can provide the shine.

A. For perhaps the first time in my memory I have to disagree with my super learned colleague, Geoff. Nickel is not expensive and it is the nickel that provides most of the corrosion resistance in a decorative nickel-chrome or nickel-gold plating process. But again, steel or diecast zinc is a better starting point for gold plating than aluminum.

Regards,

A. Hi Maarten. Most anything is possible, but I'll mention a couple of things which you may want to consider...

If corrosion resistance is an issue, it's hard to afford enough gold plating to reliably prevent it -- nickel plating often offers the bulk of the corrosion resistance; it's hard to get gold "bright, mirror-like" without an underlayer of something bright and mirror-like, although cobalt plating or white bronze might do; diffusion of copper into gold is an issue if there is no barrier layer between them, although the super low temperatures may greatly slow it; there might be a coefficient of expansion issue that causes cracking or adhesion problems at very low temperatures.

Your question becomes more interesting and people can be more helpful if you tell us more details on what you're doing -- for example, why you want to avoid nickel plating because if you're doing it to avoid skin allergies, it's possible that cobalt plating may prove just as bad. Plus, if that's the only reason to avoid nickel, you could probably electroless nickel plate the zincate and thereby preclude the use of cyanide in the copper plating. Does "very low temperature" imply liquid gas temperatures? Thanks.

Luck & Regards,

A. Thanks for the details, Maarten -- sounds like very interesting work!

I think the sequence: zincate, cyanide copper strike, heavy bright acid copper plate, heavy gold plate sounds promising. Strictly theoretical possibility: If it still results in cracking due to thermal expansion issues, it might be possible to either anneal the copper then bright dip it before the gold plating or deposit dull soft copper and bright dip it before the gold plating.

Luck & Regards,

A. Hi Maarten,
Wonder if you have considered sputtering or thermal evaporation for depositing gold on aluminum? It is pretty straightforward. Adhesion may not be a problem. If it is, a thin - less than a micron- layer of Ti or Cr would solve it. Good reflectivity could be obtained if aluminum is polished to required brightness before depositing as PVD coatings are conformal. You may have to tackle the geometry of the Al box.
Your experiment at zero B and near zero T should be something special and interesting. Good luck!