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Preparation Process, Anodizing and Sealing A356 and Cast Aluminum Alloys

Q. I'm having a problem trying to anodize various cast aluminum alloys. Actually, they appear to anodize fine, but they are not sealing (DI water seal, seal quality determined by admittance). The alloys in question are A356 and 535 (Al-7Mg).

Scott Dickerson
- Fremont, California
2000

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Q. We are currently specifying some lightweight high strength Al parts that will need a high degree of corrosion protection. When anodized, the surface appearance of gravity cast or high-pressure cast A356 alloys is mottled or blotchy. Does squeeze casting with its near zero porosity and smaller grain size produce a visually more acceptable anodized surface finish?

Mike Putnam
- San Diego, California
2003

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A. I cannot address "squeeze" casting, but I can address 356.

First, you "anodize" "aluminum". All that silicon and other junk in the recipe does not anodize and comes out a grey color.

Also, in the case of thin walls in die castings and sand castings (again, I cannot address squeeze), the silicon concentrates even more on the surface.

Whether lesser or more porous, the industry standard for increasing the quantity of aluminum on the surface is a dip in 100% Nitric Acid with 2 pounds per gallon ammonium bifluoride.

All of this information was recently transferred in a half day seminar I presented in SanDiego for all the anodizers that I knew to contact in that area..

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2003

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Multiple threads were merged: please forgive repetition, chronology errors, or disrespect towards other postings [they weren't on the same page] :-)

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Q. Hi, I am currently working for a company that sells high voltage cable clamps that are sand cast (die casting will be considered) of the A356 alloy. For most customers this alloy provides sufficient corrosion resistance. However, we do receive some requests for an anodized clamp. I know this alloy is difficult to anodize and am therefore putting much thought into preparing a process that will result in a good quality anodize. Most of the knowledge I have obtained is from an old ASM metals handbook (8th ed.) and is likely out of date.
Please advise me on any parts of my plan that should be altered. Clearly I have not yet decided on most of the chemicals to be used. I have left out rinsing.

Vibratory finishing(smooth surface) … solvent clean(remove oils) … inhibited alkaline clean(remove soils) … acid clean(remove oxides) … acid etch(nitric acid with fluorides - remove precipitate and desmut) … anodize.

I would also like to know how important the mechanical finish is, must the surface be smooth to anodize well or just clean? In my mind it seems a good anodize requires a clean surface with minimal precipitate and small grain size, is this correct? Also hould I be considering grain refiners?

Thank you very much, any help is greatly appreciated.

James Endersbee
product designer - Melbourne, Victoria, Australia
January 6, 2008

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January 9, 2008

Q. After more searching through the earlier posts on this site I have made a little more progress on the plan:

1. Trichloroethylene clean
2. Acid clean
3. Caustic etch
4. HF/nitric desmut
5. anodize

My theory is this:
Trichloroethylene removes all oils. Acid clean removes soils and oxide. Caustic etch evens out the surface but reveals more silicon. HF/nitric acid removes silicon. Surface is then clean and ready to anodize.

I removed the tumbling step as I am not sure it is necessary.

I am still very uncertain and am only stating my plan as a base to work with.

Thank you for any suggestions.

James Endersbee [returning]
- Melbourne, Victoria, Australia

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A. Aluminum fines can cause a serious problem in a vapor degreaser using trike. Notice it is "can" , not "will". There are some conditions that go with it, but many people do not properly maintain their degreasers. Check your literature on the degreaser and possibly the internet. James Watts - Navarre, Florida January 10, 2008 January 10, 2008 Q. Ok. Thank you for your help. I hate to ask questions that are too simple to be of any interest to the people on this site, but I have been researching this for over a week and still have not found a pretreatment process for these casts. Anyway, Ted, is this the plan you suggest?.. 1. Trichloroethylene or ultrasonic (depending on which is easiest). 2. Non-etching alkaline clean. 3. Caustic etch. 4. HF/Nitric desmut. 5. Anodize 6. Seal. Please tell me for my understanding; is the purpose of the alkaline clean to remove oxides so the etch will be even? Also I have heard many people suggest ammonium bifluoride as an alternative to the dangerous HF/nitric mixture.After researching ammonium bifluoride I have gathered that this substance separates into ammonium fluoride and HF and is not much safer? James Endersbee [returning] - Melbourne, Victoria, Australia

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A. Stay out of alkali etch, it does the opposite of what you want. You want to decrease the area of silicon on the surface and increase the area of aluminum (which is what you "anodize"). Alkali etching chews away the aluminum and increases the area of silicon, you do not want to do that. Clean in a non-etch cleaner to remove organic soil. Treat in 100% Nitric Acid + 2 pounds per gallon ammonium bifluoride until the part turns "frothy white" all over, about 20 to 30 seconds, hurriedly rinse two times and anodize. Robert H Probert Robert H Probert Technical Services Garner, North Carolina January 11, 2008 A. You are adding the ammonium bifluoride to get the fluoride ion. Ammonium bifluoride is extremely safe to handle compared to many other materials if the barest of minimum safety procedures are used. HF on the other hand, even 48% is mean wicked and nasty, but is a relative pussycat compared to 70% HF. What you end up with is a nitric acid/HF solution that is minorly worse than a plain nitric acid solution. If you use procedures for safely handling the nitric, you will be fine with the mixed acid. Again, HF acid, by itself is NASTY! James Watts - Navarre, Florida January 11, 2008

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January 15, 2008

Q. Thank you all for your help.

The clamp is composed of 2 parts, top and bottom, with a bolt on each side to pull each part together and clamp onto the cables.

The strange result from the anodize is that each part turns out a different color. The bottom part turns out with little color change and the anodize is more easily scratched off. The top part turns out darker gray, which I understand is due to the silicon, but the anodize is harder to scratch off. The bottom part also appears to have been polished with a coarse belt on some surfaces (~20% of total surface area).

The shop has not opened after the new year yet so I cannot ask them their process.
Is it likely that while they polished the bottom part mechanically they instead used a stronger etch on the top part, revealing more silicon leading to the darker appearance? I am confused that if this is the case, the part with greater silicon surface content seems to have anodized better (harder to scratch off).

Robert, do I not need the caustic etch to smooth the surface? It is quite rough after sand casting.
Should I used vibratory tumbling (or some other method) to smooth it? I am still unclear whether a smooth surface is required for a good anodize. Also, does the acid treatment remove oxide?

Now, regarding HF/Nitric vs Ammonium Bifluoride. If ammonium bifluoride is the safer option, does this mean HF/nitric is the more effective option?
I think that I'm being told that HF/nitric is only slightly more dangerous, but pure HF is extremely dangerous (Ted and James?).

Anyway, thanks again for all the help.

James Endersbee [returning]
- Melbourne, Victoria, Australia

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January 24, 2008

Q. Ok, I think I'm sorted then.

1. Ultrasonic clean
2. Non-etching alkaline clean
3. Nitric + Ammonium bifluoride
4. Anodize
5. Seal
(rinsing not shown)

Thank you all. Hopefully I will have some samples done soon, in which case I will let you know how they turn out.

Hemant Kumar, while I appreciate your help, unfortunately I don't have the power to affect the outcome of the test! Let's see how it unfolds.

Can I not skip the ultrasonic clean? Isn't the alkaline clean effective enough?

James Endersbee [returning]
- Melbourne, Victoria, Australia

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Q. What is the balanced equation for adding ammonium bifluoride to nitric acid and water? Chuck Ritchard - Grand Rapids, Michigan, USA April 19, 2013 April 23, 2013 A. Hi Chuck. I don't think 'balanced equation' is exactly the right term; it's a mixture, not a compound. I think you are asking in what ratio these chemicals should be mixed. I believe it depends on the application but that Bob Probert has offered one very good answer. Good luck! Regards, Ted Mooney, P.E. Striving to live Aloha finishing.com - Pine Beach, New Jersey

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Q. My question regarding the balanced equation was to understand what the by-products were when mixing Ammonium Bifluoride and Nitric Acid and whether or not they could be reclaimed through a filtration process.

Charles Ritchard [returning]
- Grand Rapids, Michigan, USA
April 24, 2013

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Pretreatment of A356 aluminum before anodizing

June 3, 2020

Q. I am currently processing a356 aluminum for anodizing using the following pretreatment process:

1) non-etch cleaner
2) rinse
3) alkaline etch
4) rinse
5) acid desmut - HNO3 + fluoride salts
6) rinse
7) proprietary iron-based, nitric-based deoxidizer
8) rinse
9) sulfuric anodize

The reason I use an alkaline etch is because if I do not, I get a non-uniform appearance following anodizing. I have recently ran tests using the following process.

1) non-etch cleaner
2) rinse
3) proprietary deoxidizer - Fe and HNO3 based
4) rinse
5) anodize

     

These parts came out with a non-uniform appearance. I would like to ultimately remove the need for the homemade acid desmut (HNO3 & fluoride salts) because of the fumes produced. I would also prefer to not use the alkaline etch as I always thought you should never etch a casting.

Any feedback would be much appreciated.

D Shoaf
- Brunswick, Ohio, USA

June 4, 2020 A. Hi D! Run #1 looks successful under your listed conditions. This process listing is quite similar to what we do for a certain job that is A356 castings with dual processing; that is, both conversion coating and black hardcoat on the same part. But with one BIG exception, that I'll get to. It is true that etching a casting type alloy is going to expose 'things that are not aluminum' and degrade the quality of the ultimate finish. But because these parts are dual-process, and the hardcoat is the second process, we absolutely MUST etch it to remove the conversion coating from the surfaces to be hardcoated. IT's not even a choice. But the trick is to leave it in the etch for the absolute bare minimum of time to see the reaction and then plop it in the rinse immediately before any part of the parts have a chance to dry off. A caveat is that these parts, while yes, they are castings, have already had the raw finish milled off. So it goes FAST! A casting with the raw surface would automatically be flash etched by our operator; again, just to the point of reaction (tiny bubbles uniformly appearing on the part surface) to 'break through the raw' as he calls it. Castings with no 'raw' and only milled surfaces get ZERO etch. Because, as you brought up, etching casting alloys simply isn't desirable... it's a tool to be used when circumstances require it. And yours might, as shown by your second, etchless, test. The big difference is we don't use the fluoride desmut at all- for the same reasons you want to remove it. And I think you might be able to! I'm not sure if we are using the same nitric-ferric deox, but I use one that is a proprietary sulfuric-nitric-ferric (not sure if Moderator will allow product mention but if it's okay, it's MAcdermid-Enthone Isoprep 184, and check out the SDS and TDS to see how it compares to yours; it was developed for 6xxx going into conversion coatings but has proven versatile) and it does a good job of removing the etch smut from castings and preparing them for hardcoat in a single step. It's a little spicy, though, and can give a scorchy appearance to the finished product if left in much past the removal of the smut. Part of that is, no doubt, that the part was ever etched to begin with- but the nitric based deox needs close attention paid to it regardless. A skilled and observant operator will use his instincts to sense when enough is enough, in both of these tanks. The only pain with our process is that casting alloys foul the desmut/deox tank relatively quickly with dissolved and suspended dissolved and suspended Things That Are Not Aluminum , so filtration and regular dumps are required. However, it functions as it should, at ambient temperatures, and doesn't have that HF component which we, too, just wanted out of the building. Our previous desmut required an actual HF additive but still wasn't as effective as the nitric-ferric we use now. I think it is absolutely worth some time testing again, maybe on scrap parts if you can get your hands on some, to see how necessary the fluoride desmut is to your process and whether you can remove smut right in the deox. It's the third variable. And you will have to determine if it adds value. Rachel Mackintosh - Greenfield, Vermont A. The reason to avoid alkali etching is because the alkali decreases the exposed area of aluminum and increases the exposed area of silicon, exactly opposite of what you want to do. On 356, change your acid to 100% commercial nitric acid with 2#/gal ammonium bifluoride, then after about 30-40 seconds jerk it out and double rise and enter the anodize. You do not need the wrought alloy deox on 356 sand castings. Robert H Probert Robert H Probert Technical Services Garner, North Carolina June 4, 2020

Thank you Rachel and Robert for your timely response to my post. I forgot to mention that these castings are being cleaned prior to anodizing or chem-film. I was really certain that using a ferric/nitric-based deoxidizer would be enough to obtain a clean and uniform surface. Perhaps the condition of these parts from the manufacturer require at least, a slight etch in order to provide some uniformity to the surface. As you can see from the original pictures, the non-etch parts do not possess a uniform appearance similar to the etched parts. It is possible that I setup my process to include a quick etch (long enough for the reaction to begin), followed by a few minutes in the proprietary deoxidizer. This would at least remove the use of the secondary, highly concentrated nitric desmut. Eliminating the fume from that solution is my first priority.

Q. If you have any additional feedback, I would enjoy reading it. Thank you both and stay safe.

D Shoaf [returning]
- Brunswick, Ohio, USA
June 5, 2020