Problems in Plating on Aluminum
A discussion started in 2005 but continuing through 20182005
Q. Our plating supplier is having a difficult time plating a decorative aluminum part (die-cast ADC-12 material, part size 2.4" H x 5.4" W x 9.1" L, nominal wall thickness 3-5 mm, part weight 570 grams). The final plated finish is an antiqued Brass or Nickel, but we are rejecting 31% of finished goods in IQC inspection due to some pitting and white "chalky" material build-up in corners of the parts. The plating supplier is blaming the pitting and chalk build-up on the die-cast suppliers materials, saying that the material is not pure. The die-cast supplier claims that the material is ADC-12 (not recycled) and the parts arrive at the platers with a mirror polish/buff finish (no visible pitting). The problem is I don't know enough about the plating process to solve the problem ... is it the material, is it the plating process, or a combination of both? According to the plater here is the process for the finish on the aluminum part:
1. Clean (ultrasonic) 2. Rinse x 2 3. Nitric Acid bath 4. Rinse x 2 5. Aluminum Sealer bath (quick dip, chemical bath, no current) 6. Rinse x 3 7. Neutral Nickel bath 8. Rinse x 3 9. Semi-gloss Nickel bath 10. Rinse x 1 11. Gloss Nickel bath 12. Rinse x 3 13. Brass bath 14. Rinse x 3 15. Black Nickel bath 16. Rinse x 2 17. Color Sealer bath 18. Rinse x 2 19. Pure Water Final Rinse
I questioned why the plater used Nickel, not Copper, as the first base metal plating bath (my experience in the past has always been Copper first). He said that 'Neutral Nickel' is best in this application. Based on the above process, is there anything that stands out as a potential culprit regarding the pitting & chalking issue? For now the work-around is to use a heavy Copper bath, post polish, then proceed with the Brass plating process.Edmund Farmer
lamp mfgr. - Stamford, Connecticut, USA
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.James Watts
- Navarre, Florida
Second of two simultaneous responses -- 2005
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 casing 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.
chemical process supplier
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 brighter,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.Steve Clark
polishing - Belfast, Maine, USA
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 use 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.
3rd Generation in Plating
Consultant - Arvika, Sweden
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.
3.alkali pickling (temperature 80 °C, NaOH )
5.Nitric acid + 10%HF
8.nitric acid wo/HF
11.Ni bath pH5
13.gloss Cu bath
15.gloss Ni 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 regardsWaldemar Tomankiewicz
- Krakow, Poland
September 20, 2008
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!
metal coating shop
Dubai, United Arab Emirates
April 3, 2013
A. Hi again,
I read Edmund's letter. 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.
Sundman & Nylander AB
March 14, 2013 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread
Q. Hi there, I have an aluminium high pressure die cast part that is currently electroplated and I'm getting some random issues with surface finish. It looks like there is some chemical reaction happening under the surface coating. Sometimes it looks like pitting other times it just looks like corrosion. It can be wiped off, but sometimes reappears when it is outdoors for a long time.
The alloy is ADC10 and the process we are currently using is:
- Ultrasonic Wave Cleaning
- Zincate (displacement Reaction) 0.5 - 1µ
- Ni Plating 5 - 10µ
- Cu Plating 20 - 30 µ
- Ni Plating 5 - 10 µ
- SnCo Plating 0.05 - 0.1 µ
- Cr3 Plating 0.15 - 0.2 µ
I was wondering if there was a process to put the cast parts through before the plating process.
Do you have any advice to prevent this type of surface defect?
- Sydney, Australia
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.
Sundman & Nylander AB
April 8, 2013
Q. Hi Anders,
Thanks for the response. Do you have any suggestions as to how to eliminate this issue. It's actually very similar to the chalky effect that Edmund references in his first post.
Any help would be greatly appreciated.
- Sydney, Australia
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.
Sundman & Nylander AB
April 10, 2013
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.Lyle Kirman
consultant - Cleveland, Ohio
April 18, 2013
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.
R&D practical scientist
Chesham, Bucks, UK
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