Problems in Plating on Aluminum (ADC-12)

A. Hi Milosz,
The easiest way to look at it is that aluminum processing works on aluminum, rather that's its alloying ingredients, so generally the high silicon alloys like most castings are troublesome, as are the high copper alloys. Although electroplating on aluminum is arguably more difficult than anodizing it, at least the surface will be that other metal, whereas when anodizing aluminum you are trying to make the aluminum alloy itself somewhat attractive looking and it's difficult or impractical on some castings.

I would guess than 6061 is the most commonly anodized and plated alloy and probably the first choice. High strength alloys 2xxx and 7xxx often won't look as good and/or will be more troublesome. Luck & Regards,


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

A. Hi Vishal.

Obtaining a complete process sequence in exacting detail would require retaining a plating consultant, although your process supplier might be willing to process a few sample parts for you in return for your business. But we appended your inquiry to a thread which already addresses a lot of it. I assume you will also want a clearcoat over the antique brass? Please study the thread, introduce yourself and your situation so we know what plating lingo, steps, & procedures you are already generally familiar with, then propose a treatment sequence, and pose specific questions -- and I'm sure you will get good answers. Thanks!

Luck & Regards,


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

A. I have no idea what an aluminum sealer dip is. Without a zincate step, I am surprised that you do not have an adhesion problem. Zincate followed by an alkaline electroless nickel is an excellent process for aluminum.

A. The preplate cycle seems to be wrong. If the Aluminum is a casting, then your nitric acid is not good enough. You have to add fluorides as well and for some castings should include also sulfuric acid!

Aluminum sealer - not familiar with this very odd term. It should be a zincate! On top of it, one should use a double zincate, meaning the first dip 60-90 sec, a dip in CLEAN 50% nitric acid and then a second dip in the zincate for 15-25 sec.

Neutral Nickel - what is it? It should be a neutral electroless nickel or a cyanide copper. There are some special zincates that enable direct nickel plating but it is not always safe.

A. Process seems strange with so many nickels.

Also color buffed aluminum has oil in it, that may come out in plating process, but normally shows as bad adhesion.

Light blisters are normally a sign of zinc.

Pitting may be excessive brightener, excessive air, or tank contamination.

Whiteness may be burn if it is on high spots and outer edges nearest anodes.

Blisters from air are normally on undersides. If all over check brighteners.

A. Edmund, in ADC-12 aluminium alloy there is a high content of silicon; therefore the job shop must use ammonium fluoride to dissolve the contaminants after the alkali pickling. I can see also that he uses nitric acid; that's nice. He also uses a sealer which I believe is a zincate. I think the problem can be solved by a dip in ammonium bifluoride solution.

A. Hi,

Edmund, we have similar problem with decorative plating on aluminum parts (small aluminum cast). We apply instead cyanide copper electro nickel (pH 5,0 2-3 A/dm2). Probably our Ni-bath is like yours, neutral Ni-bath.

Our process

1.clean
2.rinse
3.alkali pickling (temperature 80 °C, NaOH )
4.rinse
5.Nitric acid + 10%HF
6.zincate bath
7.rinse
8.nitric acid wo/HF
9.zincate bath
10.rinse
11.Ni bath pH5
12. rinse
13.gloss Cu bath
14.rinse
15.gloss Ni bath
16.rinse
17.Cr bath

We had a problem with pitting on finished parts . Too deeply polished parts (inside, the casting had different structure) and too high pH in the first Ni bath were the reasons of pitting. Furthermore we changed the air stir in the first Ni-bath. Air strongly mixed the bath and exactly flows around the parts.

Best regards

Hull Cells

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Amazon

(affil links)

A. Dear Farmar,

I hope sealing of aluminium means zincating.
Plating Aluminium which consists silicon requires a very precise degreasing and rinsing. First see that your cleaners should be non silicated. Be sure of even your desmutting and re-zincating. See that your cleaning chemicals are not etching the die castings. I agree with other platers, you can sure some etchants with nitric dip, only for proper bonding of zinc on your job.

There are many platers who are plating direct Semi Bright Nickel over zincating, but your metal seems to be not fit for this process. Some metals are difficult to plate but not impossible. I suggest, use Copper Cyanide and Bright acid copper after zincating. Since these Copper has a good adhesion and good leveling properties. I think your metal requires uniform leveling and uniform throw all over the current densities after zincating.

For pitting, if your hull cell and other analysis of you baths are ok, then I recommend please concentrate on your pretreatment, quality of rinsing water, proper maintenance of your electrolytes, timely clear out contaminations, organic as well as inorganic and filtration. And sometimes your metal supplier may filling the pinholes, jalla and sori defects by impregnation which gets clear when you degrease them or dip in acid.

All The Best!

A. Hi again,

I read Edmund's posting. He is zincating on casting aluminum two times. That will build a bridge on casting aluminum and involves bad rinsing water and contamination from the pretreatment steps in the aluminium substrate. Pure aluminium is an alloy; there you can use 2 and 3 zincating steps, but never on casting alloy.

Regards

A. Hi,

I think the ultrasonic cleaning can do that as the plating can build a bridge over the aluminium area, and in the aluminium substrate some pretreatment will stay left.

Regards

A. Hi Mike,

Cast aluminium should only be zincated one time, and also take care when you etch the parts. Both etching and zincating will make some pores in casting aluminium. And in the pores it will involve dirty contamination from the pretreatment steps. Also the pretreatment steps are important because of what is in the alloy in the aluminium casting.

Try to translate on the library here on finishing.com what I have written about aluminium.

Regards,

A. It may be porosity in the casting that is causing your problem. A possible solution is to try a double zincate process. Or, if it is porosity related, then baking the parts at low temperature before plating, or an anaerobic sealant may help.

A. Cast aluminium is a problem to electroplate and needs to be carefully treated, because solutions can get trapped in the porous structure of the casting. Firstly, you need to treat the casting with a zincate treatment and then I would suggest you fill the pores and bridge any surface spaces with a high throwing power copper. The thickness of the copper will in part depend on the casting porosity. I am not convinced that nickel will have adequate throwing power and if you are using a nickel strike, it will probably be based on chloride, which is a potential disaster area for uncoated aluminium. You should also ensure the parts are extremely well washed between these initial stages and as Anders says, ultrasonics may be beneficial.

A. Hi Joe. Sorry, I have no experience with this, and can't find reference to it in my texts or on-line. But we appended your inquiry to this thread because I think the process will start with zincating and then copper plating of the aluminum, followed by electroless nickel on everything. Copper is not catalytic to electroless nickel, so you will have to "spark" it first (apply electricity for a second or two).

I'm sure you well know that plating or chemically processing an assembly of different materials is invariably a bad idea -- but if there is no way around it, I think it can be done. Good luck.

Regards,

A. Hi, Kurt. A plating shop is supposed to do good plating, not discolored and sticky plating, so our first thought must be that the plating shop probably did something wrong. If you can give us some facts like what you did and what you asked the plating shop to do, a less superficial answer may be possible.

Luck and Regards,

Hi Raghunandana. Either of us can look up the word "coverage" in a dictionary, but it doesn't help, does it? Sometimes it might refer to a requirement that there must be visible zinc plating everywhere on the extrusion, with no bare spots; sometimes it might dictate a replenishment schedule of how much zincate to add for every 1000 sq. foot of extrusion processed -- just as you might read on a quart paint can that it offers 100 square foot of coverage.

Unfortunately it is nearly impossible to have any assurance what some third party meant when we hear two words transcribed out of context like this. At this point neither of us knows what the requirement is, so it's not possible to say whether an EDXRF is of any value in determining compliance. Please try to figure out and explain the exact situation. Thanks.

Regards,

A. Hi Gary. What function or process do you have in mind when you say "coated with clear zinc"? Even without language translation difficulties, "coated with clear zinc" sounds like it might have led to misunderstandings :-(

When *I* hear that phrase, I envision electroplating with zinc (which is silvery metallic in color, not clear) followed by a light blue or "clear" chromate conversion coating so the luster of the zinc plating is retained. Zinc electroplating and conversion coating is not a high temperature process, but others might envision something else.

I also have a little bit of a problem interpreting what you mean by "matched up with". Do you mean that you want the "coated with a clear zinc" part to look similar to the anodized part (it won't), or do you mean they will be connected together (they are not very galvanically compatible).

Regards,

A. Hi Gary_. The drawing note doubtless means, as I described, zinc electroplating followed by RoHS-compliant clear trivalent chromate conversion coating. You can get a copy of ASTM B633 to understand all this better (although that spec actually covers zinc plating of steel rather than aluminum), and you will probably then interpret light, medium, and heavy as 5, 8, and 13 micron thick coatings.

However, it's possible that your customer is also a neophyte and is hoping for a voice of knowledge & experience to chime in before the parts are plated. It doesn't sound like your manufacturer in China will be offering anything remotely like that (perhaps you're dealing with a broker of some sort rather than a knowledgable plater). Further, although it is certainly possible to zinc plate aluminum, it is a rather unusual finish. So it might be a good idea to phone your customer and understand each other a bit better. At the risk of being called jingoistic, I might also suggest that you consider getting those samples plated in California, where you are and can easily visit shops, and where every plating shop already understands that drawing note and knows what to do :-)

Luck and Regards,

 Thank you Ted. That is good advice and helps a lot.
Gary Taverrite [returning]
- Sacramento California USA

? Hi Jason. Sorry but I have no idea what that pic is supposed to illustrate :-(

If the pits are small but varying sizes, shiny, and hemispherical, they are "gas pits" (hydrogen bubbles growing on the surface and becoming dislodged only after they are big enough that their buoyancy floats them off, leaving behind that hemispherical indentation where the plating grew around them). Common causes of gas pits include insufficient wetting agent in the nickel plating tank, insufficient air agitation, and organic contamination of some sort on the surface of the part.

But aluminum castings must also be totally free of porosity and cold shuts in order to plate pit-free. Good luck.

Regards,

A. Hi Jason. I guess I am looking at a reflection of a camera phone and a chicken wire type pattern on your shiny surface? Your picture is not clear enough nor large enough to see the actual appearance of the pitting, but the close placements of the pits could be hydrogen pitting. And if the pits are shiny hemispheres, that's what they are.

Is it possible that the way the parts are racked, no air agitation hits this area of the part during nickel plating. Please ask your supplier what the surface tension of the nickel plating bath is supposed to be and how to measure it. Lack of wetter and/or poor agitation is one, but not the only possible, cause of hydrogen pits though.

You are confident that the parts were free of cold shuts and porosity before polishing?

Regards,