Small scale aluminum anodizing

A. Hi Maria. I've visited your lovely town twice this year.

Rectifiers are usually specified by their voltage & amperage, rather than by the Watts they consume. 18 volts sounds good. You will probably anodize at about 12 ASF, so I doubt that you will need 400 Amps; 150 Amps sounds more reasonable. So I think you'll want a 150 Amp 18 Volt rectifier.

When you say "air cooled" I think you mean no cooling except for what little is provided by air agitation. Unfortunately, your anodizing tank will heat up quickly. I think the best you can do cheaply is to use a cooling coil in your anodizing tank and recirculate water from a chilled water tank through it, chilling the water with ice or dry ice when needed.

Luck & Regards,


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

A. Eddie, You did not say just how big the parts are that you need to plate or if by color you mean the parts are dyed. With that said I will put in my options for you.

One option would be to brush anodize the parts that you need done using hand held electrodes. The brush plating process meets the requirements of MIL-A-8625 / MIL-PRF-8625 [on DLA], but cannot be dyed to different colors (black, red, green, etc.). The brush plating process uses a lot less solution than tank plating; you work with a liter of solution instead of gallons.

A. Greetings Mr. Wheeler and everyone else that's contributed so far.

I've helped guys do this stuff out of their homes for something which they just want to look a decent color other than bare aluminum. It's not hard in that regard, but it is messy, it can be dangerous to work with in regards to your health, and it is difficult to dispose of the wastes. If you have 1 liter of solution, you may have to dispose more frequently than a larger operation.

Have you checked the job shop listings on finishing.com for a shop in your area you maybe didn't know about? What size parts are you talking anyway? I, being a very small shop myself compared to some of the others roaming around here can generally meet your needs on small quantity, small part requirements. But I'm not here to make a sale, I'm here to help you find someone to do it first and foremost, who is local to you.

I suggest checking those resources, then scanning over the Thomas Regional website and seeing if you can locate someone that way. Basically unless you're trying to deal with the biggest of the big in your area (I'm talking size of the company, not the size of your parts) then you "should" be able to find someone.

Good Luck to you.

A. The most practical book that I owned was "Light Metals" by David Montgomery and Charlie Grubbs. Written in very plain language by two very highly experienced anodizers. As books go, it is cheap. Comes in a notebook. Contact AESF (www.nasf.org) to get one. I shortened the title by several words, but the authors are the same. Newer versions have the authors in reverse order.

Personal feelings, 2 inch spacing is inadequate -- 4" is better. You will have a lot less problems if you use mild oil-free air agitation. Your acid range is too great. Settle on a book value. The stronger the acid the more it can eat the oxide formed. Current control is a nice way to anodize but people have generated good product by plain voltage control. Temperature control is critical and you did not mention it. This can be your entire problem. If you are only doing a small amount, you could get by with a larger volume tank which will hold the temp rise down and would help your spacing out. Buy the suggested book. It is unusually practical. I have met a couple of the British authors of your books and their seminars are as boring as their books :-)

A. Patrick, you mentioned you have no agitation. This is a critical step in the anodize bath. Agitation will produce an even anodize finish and maintain a consistent bath temperature. You will need a chiller to cool the bath if you start to run parts with a large surface area; this will also maintain the temp of the bath 65-70 F. Running 17 Volts for 55 min for black & 15 volts for 25 min for clear produces quality parts for us. This process is for a 6061 aluminum other substrates will not give you the same results, especially 2024.

A. Casey, the parameters that you suggest will produce a very heavy anodize coating that will certainly have no problem meeting the coating weight test for Type 2.

Ahhhh ... fish tank pump ... good idea, James!

A. Personally, I would not use the ziplock bags more than once until you have a great idea of how many times that it might be used. You can do this by reusing only one baggie until it does not look very good. It is a good idea for the small user. I had done it with heavy duty plastic bottles tied to the side of the tank with fair luck, but you will have a much better heat transfer thru the thinner bag. I gravitated to the pump thru the seamless stainless tubing and then went first class with a chiller and a store bought heat exchanger. Warning, 316SS is attacked by the 15% acid as you get above 75 °F, so if you go that route, be prepared to take it out of the acid when the chilling device is not on.

A. Hi Dave. The short answer is that, no, you don't need to go below 50 °F is you are using a proper additive system for Type 2-1/2 anodizing, and you probably need to anodize in the 24-36 ASF range depending on the alloy. Letter 52350, "Hardcoat Anodizing Voltage" should be helpful.

But I think faster progress will be made if you take a couple of sentences to introduce yourself and describe your situation. The problem I'm having, which is fairly common in this forum, is we don't know your experience level. If you have never done any successful anodizing previously, then starting with Type 2-1/2 without any guidance from a vendor is a tall order. We'd probably like to see you enjoy some success with Type 2 first because, if you come back and tell us the Type 2-1/2 is not working, we're in a situation where we've brought in too many variables to simultaneously tame. One tiger at a time, please :-)

Good luck.

Regards,

A. Hi again Dave. The temperature needs to be 28 to 32 °F unless you are using proprietary additives; then it might be around 50 °F as suggested by the vendor. Please see letter 27828, "Setting up hard anodizing on 7075 aluminum alloys" and letter 40535, "Problems and Issues Hard Anodizing of 7075 Aluminum" for the dissolved aluminum, current density, etc.

But let me say the usual: if the parts are important to you then you should send them to an anodizing shop.

Nothing is impossible, but experience is the best teacher ... and never having anodized anything before, you are trying to:
1. do type III or 2-1/2 hard anodizing (which is significantly more difficult than standard class II anodizing),
2. on a tricky alloy (7075 sometimes develops segregation at the grain boundaries and has other problems),
3. working live with your precious real parts rather than spending a few months learning with scrap parts,
4. while doing it in a home setup without professional cooling & agitation to prevent burning,
5. with no professional instrumentation to gauge how you are progressing.

Again, nothing is impossible, but I think the odds of you not ruining parts on your first try is slim. Best of luck, but we'd like to see you gain some experience on scrap before risking your parts.

Regards,

A. Hi Nicholas. The first thing you might need to know is that voltage and current cannot be independently controlled. Ohm's Law says Current = Voltage / Resistance. I think you are misreading or mis-setting something because if you were truly holding your current down to 0.1 Amp/in2 when you placed the parts into the solution, the voltage would hold at only enough volts to deliver that current (probably less than 6 volts at the beginning of the cycle).

Near the end of the cycle, depending on the alloy, it might take 30 volts to keep anodizing at 0.1 Amp /in2. This change is because: when you put bare aluminum into the solution, lots of current can flow into it without the need for much voltage, but as the highly insulative anodized coating builds in thickness, it keeps taking more and more voltage to keep the current flowing against that increasing resistance.

If your power supply is capable of "current control" and is set right, what should happen is you just set it for 0.1 Amp/in2, and it will start out at low voltage and the voltage will gradually increase to maintain the current. Alternately, although more work, you can control by voltage: start at about 3 volts and gradually increase the voltage so as to maintain the 0.1 Amp/in2 that you are shooting for.

I would urge you and other newcomers to experiment on scrap before trying to work with valuable parts. Good luck.

Regards,

A. Hi again; apparently I partially misunderstood your situation. If you've been managing to keep the voltage slowly rising at a fixed current of 0.1 Amps/in2, but it eventually rises to 30V without building the required thickness, and your rectifier can do no more, yes, the rectifier is too low in voltage.

If you wish to process 1000 square inches, even at 0.1 Amp/in2 you would need 100 Amps, whereas you probably should be anodizing at nearly twice that current density. For 1000 sq. in. (6.94 ft2) of parts at 24 ASF, your amperage should be 167 Amps. Voltage depends on alloy, but could easily be 60-75 volts, maybe even more.

Although I would thus think that your rectifier should perhaps be 175 Amps at 75 Volts rather than 50 Amps at 30 Volts, I'd urge you to learn as much as you can from the equipment at hand, rather than trying to do ultimate fine tuning of parameters by the book instead of from practical experience. Good luck.

Regards,