Anodizing Cast Aluminum

A. Hi Shubham. We have about a dozen threads in this forum about anodizing castings and their high silicon content, so we attached your inquiry to one of several which seem to answer your questions well...
It is best to not etch (although some knowledgable readers say you can if you must), then you must use a nitric acid plus aluminum bifluoride desmut as described by Robert Probert.

And don't have high expectations for aesthetics :-)

Luck & Regards,

A. You are very probably looking for "two step anodizing". I doubt if regular dyed anodizing will have the fade resistance you are looking for. Find a job shop that does that type of work. There are not many, but there are several. Cost is a fair bit higher. Wrought alloy anodizes beautifully, extrusions are from good to bad and castings are from fair to very bad.

A. Powder coating may be an option. If the parts can be brought to a high luster, the powder coat can be applied with a translucent effect that is very attractive and this can be done in a large variety of colors.

A. Hi, Craig and readers.

The internet is a gigantic one-room schoolhouse; so sometimes it's hard to find the information you're looking for at the level you wish. But here's a quick explanation about why it's hard to anodize certain types of aluminum: only the aluminum per se can be anodized, not the other stuff that may be in an alloy or casting like the silicon, copper, zinc, magnesium, etc.

Some grades of aluminum (and all castings) have a lot of that other stuff. That other stuff doesn't etch away easily, doesn't get converted to glassy looking aluminum oxide, but just turns into a gray to black smut which may be porous or speckled, won't be bright or smooth, and won't dye to the colors you'd like. So the more of that other stuff, the less satisfactory the anodizing, and that's just a lot of that garbage in castings.

The next point, that James Watts mentioned, is that even if you do dye it, most dyes are organic and subject to degradation from high heat. So you probably need a special inorganic coloring for an under-hood application. Those inorganic dyes, which are also used for architectural anodizing are applied by methods called "integral color anodizing" or "two step anodizing" but tend to be champagne to bronze to brown in color. Purple sounds unlikely for an inorganic dye; before the days of synthetic inorganic dyes, purple was so rare and difficult that it was reserved for royalty :-)

So, an attractive "purple mag lite" anodized finish for an under-hood application on cast aluminum doesn't sound very do-able to me, and would be a development project rather than something a consumer purchases. I think a a purple ceramic sounds like a more promising approach. Good luck!

Regards,

A. Hi Sam. I'm not sure if you're speaking of anodizing/plating the innards or the outside surface but the innards may not be possible considering the way those processes work.

Luck & Regards,


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

A. Hi Tom. We appended your inquiry to one of our prior threads on this subject. Cast aluminum contains a lot of garbage that isn't aluminum and will not anodize. You can slightly improve the situation by de-smutting before anodizing, with acids tailored to the specific contaminants (like nitric acid for the copper, fluoride-bearing acids for the silicon), but you're unlikely to get good aesthetics from anodized cast aluminum.

Regards,

A. Hi Tom,
In addition to Ted's comment which is of course spot on, what is your temperature during anodizing? If it is low enough to approach Type III / Hardcoat range, that also will darken your end result.
The best you can do with a gnarly casting IMO (our standard protocol here) is hot caustic etch the part, deox/desmut it very well, using a bath that has both Nitric acid and Sulfuric Acid with ferric chloride [on eBay or Amazon] and LOTS of bubbles, and maintain your Type II temperature range at 70 °F +/- 2. And then cross your fingers and hope it comes out marginally attractive ... That bit doesn't show up in the routers ;)
Black dyed parts aren't the worst thing to ever happen. But make sure you are using a black dye that will stand up to engine temperatures, fluid leaks/spills, and UV exposure over time.

A. Both of above are right, but I take difference on alkaline etching. caustic soda [affil links] etching removes aluminum and exposes more silicon - exactly opposite of what you really want, so stay out of the alkaline etch. After non-etch soak cleaning, then 100% Nitric acid with 2#/gallon ammonium bifluoride, the parts will turn "frothy white" in about 15 seconds, take them out, rinse two times hurriedly, do not stop to look, anodize - that is the best you can get with that particular casting. After a few runs it will take more than 15 seconds. The above is for sand cast, some of those motorcycle parts are die cast in which case you can use only 50% nitric with one #/gal ammonium bifluoride. Anodize the castings at about 22 volts.

A. In my experience, many automotive parts are clear coated. This includes motorcycle engines, and especially polished mag wheels. Do a scratch test in an inconspicuous area and see if it flakes.

A. Hi Mike. Very nice pictures, thanks. But I have to apologize that I have little idea what your questions & comments refer to. It seems like you are saying you did a bright hammer finish on some aluminum castings (and that's what it looks like). Now I'm guessing you want to anodize them? But as people have said previously, it's not very promising; if you want to preserve the look, a clear coat seems like a much better idea. Good luck.

Regards,

A. Hi Qwee. I think you will find that it anodizes far too gray and irregularly to please you. But you could cut a cast aluminum wheel into a bunch of pieces and experiment with them, or just accept Robert Probert's advice on the best approach.

Luck & Regards,

A. Hi Ben. We added your inquiry to one of several threads on anodizing of cast aluminum. I think you already understand that the basic problem is that aluminum castings have a high percentage of non-aluminum material in them. and only the aluminum portion anodizes -- so discoloration and blotchiness is inescapable. It is possible to anodize an outer layer of aluminum plating, provided it's thick enough, but aluminum plating is a difficult proposition with (to my knowledge) only one shop in America offering it.

Luck & Regards,

A. The best "white" finish you can get depends on the alloy quality of the casting and the surface preparation. Do NOT alkaline etch. Dip in 100% commercial nitric acid with 2#/gal ammonium bifluoride for the approx. 15 seconds it takes to get "frothy white" all over, rinse twice and anodize, and that is the best you can get with that particular casting. The fumes eat holes in the air sacs of your lungs.

A. If you alkaline etch die castings alloy you decrease the exposed aluminum and increase the exposure of non-anodizable silicon. This is opposite of what you need to do. Do not alkaline etch castings. For die castings, deox in 50% commercial nitric acid with 1 pound per gallon ammonium bifluoride, takes about 20 seconds, rinse twice and go to anodizing quickly. ONLY fluoride will remove all or part of the exposed silicon and show more aluminum area. You anodize "aluminum", you only darken the silicon that is on the surface.

A. Hi Riccardo. With Robert's wide experience, hopefully he can predict.

But for the benefit of other readers trying to better understand metal finishing I should take the time to not mislead them by noting that, while our subject is science-based, it relies very greatly on empirical knowledge. Questions like: "what will be the corrosion resistance of AlSi9Cu3 aluminum subjected to a sulfuric acid + peroxide dip before 10 µm anodizing" cannot be answered by 'first principles' at the current state of the art, but only by conjecture based on similar experience. And where there is not significant prior experience there is not a lot of empirical knowledge :-(

Luck & Regards,

A. Hi Robert. That's not exactly what we said, or at least it's not what we were trying to say :-)

The '2-step process' (actually just slang) refers to a method of dyeing (usually reserved for architectural work) which, instead of using organic dyes, 'implants' metals into the anodizing pores to give colors with exceptional fade resistance and UV resistance. If it could give a good copper color it's still not going to resolve the issue of the unsuitability of castings for decorative, aesthetic, anodizing which is a problem related to the impurities and general crudiness inherent in castings. Your headers will probably not look good if anodized and dyed a copper color whether with organic dyes or a two-step inorganic dyeing process.

You could copper plate the headers but I don't know how you would prevent oxidation and keep them looking good. I think high-temperature paint in more promising.

Luck & Regards,

  Hi Bill. Thank you the kind thoughts :-)
Regards,


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