Anodizing aluminum

Tricks of the trade

Anodizing aluminum

Postby Doug Coulter » Sun Aug 29, 2010 8:34 pm

Not sure where this best fits in the org scheme, but here's how I do it, and it's great for vacuum parts as well as decorative stuff. I made all the window frames in this place and colored them nicely using this technique.

Anodizing makes a honeycomb oxide layer on the aluminum. You can control the size of the pores in it by process variables, and the results you get for a given application require changing to suit what you want. If you're working for decoration and dyeing, then you want mid sized pores -- hold enough dye to see, and hold it tight. Too big and they dye easy, but lose it too quick with time. Too tiny, and the dye molecules don't fit in the pores pre-sealing and the piece won't take the dye well. But that latter is perfect for vacuum use, and gives really good insulation which tends to cut sputtering (aluminum is already good there, this makes it really good) too.
AnodizeEM.gif
Electron microscope picture of anodized aluminum

Since this picture isn't that great (scan of a scan, a couple generations) here are some drawings too.
AnodizeEMDrawings.gif
Drawings of the results



The really key variables are current density, and temperature. For small pores, current is high, and temperature low, and controlling that is a pain -- as the power heats things up and it takes tons of ice or a proper solution cooler to keep things cold. For decorative purposes, room temperature is fine, and lower current density -- just don't let it heat to 80 or 90F and you'll be fine.
During the process, bubbles are formed, and if they are allowed to stick to the workpiece, it won't anodize there anymore. The best way to agitate them off is an aquarium air pump and a plastic "air stone". Of course, if it's working, this means a fizz and some tiny droplets coming out of the tank -- so do it someplace where that's not going to be a problem, or just deal with that some other way. Taking the part in and out of solution can work, but it can also burn the anodize coat when that first little bit is put back in under power and gets far too high current density, so it's only for someone who has had a lot of practice doing this, and with small parts.

The workpiece has to be really clean. Bake to remove oil, say 450f for a couple hours, then re-clean. Solvents help. If you buff the aluminum you're going to have one heck of a time getting all that wax from the buffing off it, and the resulting cleaning will make it not a mirror anyway, so there's not much point. I use a weak lye solution as the final cleaning before putting the work in the anodizing tank. Just enough to see it bubble a little -- don't overdo this, and rinse well so as not to drag lye into the working tank. I use an old windex bottle full of distilled water for that. The key word here is old -- you have to get rid of all the windex atoms and all plasticisers for this to be pure enough to work well.

If your workpiece is clean enough, it will pass the "water break test" which is used all over industry for testing for oil contamination. If water beads at all, rather than forming a perfect thin film, you're not done cleaning yet. One caveat is that the tinyiest amount of lye will make things appear to pass this test when they aren't really clean, so watch for that. Just once, get some on you (hard to avoid anyway) and then see how much rinsing and scrubbing it takes to get your hands back to not-slippery -- it's educational and intimidating, believe me. Just dipping a part isn't going to get it off well enough, as this easily convinces one. Don't make a habit of this, it's real hard on your skin and you kinda lose a layer each time it happens.

I use a weak sulfuric acid solution for anodizing. Back in the day they'd use a stronger one, and with chromic acid added, but that's far too nasty to be around unless you're really hardcore and know the dangers (considerable, chromic acid is poison, corrosive, and a carcinogen). Even weak sulfuric acid, in amounts you'd not normally notice (doesn't burn you and you don't have to have dripping hands) wiped on your clothes and they'll fall to pieces in the next wash, if not sooner. You might have noticed in pictures of me that I wear these patched up clown suits -- that's why they needed patching. Since people wipe their hands on their pants without thinking about it often enough -- pick some clothing you don't care about to do this. Some syntetics have an even more drastic reaction -- they look like they were hit with a blowtorch and just hole instantly -- some older floor rugs are like that too.

You really want to have a decent constant current supply for this, as it lets you know when you've reached the max anodize thickness, no guessing about when it's done. During the process, the thickness initially builds up, but then thins back as the solution eats it some. You want to pull the part when it's peak thick -- too short or too long isn't as good for results, and this varies with the amps/surface area a lot. You might think calculating surface area is a trivial exercise, but it isn't -- most things have a lot more than simple geometry would indicate, as surface roughness can easily double (or more) the actual area. So having a way to know what's going on is a very good thing, and after doing this a lot, I stuck with the more expensive supply as the results got a lot better with a lot less testing and guessing.

Post anodizing (and any dyeing) you need to seal the anodize. This is actually a chemical reaction that changes the pore material composition and closes the ends off. For parts to be used in vacuum, you use no dye, and the sealing process is just boiling in degassed water for half an hour or so. In other words, you get the water boiling first, not after putting the work in, so it's gas free at the time. For dying applications, a lot of people do this in a weak solution of Nickel acetate and they claim that's better. I haven't been able to tell except with red dye, which is always the hardest one to get right anyway. I've not tried it for parts for vacuum, on the theory that anything but aluminum oxide in there is going to be trouble. I haven't noticed any big increase in outgassing, but then I do seal my anodize for that really well, so I don't have all the microscopic open end pores.

Solution Makeup:


I use battery acid as a source of very pure sulfuric acid here. Yes, it's diluted with water already, but it's otherwise very very pure. For this, you dilute it even more. I use one gallon of battery acid (from any NAPA store, it's cheap) and 3 gallons of distilled water as a recipe. Although in this case, it's not a big deal, always add acid to water, not the other way around, as exotherms will then spit the water, not the concentrated acid. This really only matters if the acid wasn't dilute to begin with, but it's a good habit. Even this diluted solution is pure evil on clothing and wood -- pick a good place to do this work. It won't burn your skin so's you'd notice so you might think it's tamer than it really is. Given time to work it will take most things to pieces. I keep some sodium bicarbonate around and dose up any spills with that till it stops fizzing.

Tank cathode electrode:

Can be either lead (pure please) or aluminum, should be larger then the workpiece and set up so no part of the piece is really closer or farther from it than any other. If you do this in a plastic bucket or cut piece of PVC pipe (cut lengthwise so you have an open top with pipe cap ends for long pieces, you don't need so many gallons to fill it), just wrapping the inside will do.
Sadly, this is a case where you really can't leave the electrodes in the solution when not in use, which means you have to have a way to get them apart without spilling acid all over. I use all-plastic valves in my tanks to drain them into some other container, then rinse.

Parts preparation and hanging:

You can't use most materials here. You can hang parts with pure Al ground wire (which I've found can be tapped with a 10/32 die and then screwed into a tapped hole in the part), or Ti wire. The Al wire will have to be stripped in a lye solution each time, as it gets anodized, and therefore insulating too. You can't really use lead as the anode, screwy things happen as you make a kind of lousy lead-acid battery. All parts must pass the water break test or it's not going to work well. Note that lye makes anything seem to pass the test, so get that off real rigorously first -- rinsing and scrubbing. I apply the test with an old spray bottle (rinsed out maybe 20 times) with distilled water, it's a quite sensitive indicator that there's oil or other contamination -- if the water beads at all -- you're not done with cleaning yet. If it sheets into a thin invisible layer -- you're there. If you use Ti wire, you can use it over and over without having to strip anodize off it every time. Remember in any case that you need a really good connection, as the currents involved are high, and you don't want heat or the connection itself to get anodized and become an insulator. Tapped holes are best, with a tight fit. If you need hardware to get a good connection -- use Al or plastic screws, not SS as it will get into the solution and ruin it -- nearly everything will, this is critical.

Running the process:


This is where you adjust things depending on what you want. Colder is almost always better, right up to almost freezing the stuff. This is because the anti-anodizing dissolution reaction is slowed without messing up the process going in the desired direction, and you can get a better coat with smaller pores in it. For parts to be dyed, it's not as important, and you don't want super tiny pores anyway -- the good dye molecules are big! Of course, you can also make the pores so big the dye won't stay in them long enough for sealing too, which is never what you want.
Big pores and thin coats come from low currents and high temperatures -- and here we are dumping enough power into the process to make noticeable heating happen in a lot of cases.
I have used a lot of "blue ice" in this game, and all that fits in a freezer easily can be barely enough for one run. Other people resort to things like modified air conditioners and solution coolers, using Al pipes in the solution. It's an issue.

Note that you shouldn't expect perfection on the first run -- this seems to need a little Al sulfate in the solution to really work best, so do a few practice runs until it works right, then you're good to go for a long time after that with a given batch of the solution.

For dyeing, I use room temperature (cool, not summertime) and about 4.5 amps per sq foot of stuff (31.25 ma/sq in). Remember that surface roughness will affect the actual surface area a lot, so you'll just have to practice a little to get a good sense for this. For dyeing to work (especially the troublesome reds) you need to really really rinse the part well before the dye, or the residual acid takes that molecule apart -- it's still in the pore, but not red anymore, and at that point it's a do-over from the beginning.

For vacuum parts, I get the solution as cold as I can, and use more like 7 amps/sq foot or even a little more. This makes for tiny pores and a very dense coat, easy to seal so it's not an outgassing issue later on. Again, really rinse it well -- you don't want any of the solution components in a vacuum tank.

Here's the big secret. Parts reach what one guy calls "peak anodic resistance" at a certain time in the process, and that's when it's the best. After that, the resistance goes down again, and the coating is gradually destroyed by a competing chemical reaction. This is where your constant current supply comes in super handy -- you can watch the volts rise, then begin to fall again, and know when to remove the part right on the good spot.

Note, this curve is messed up, as is the coating, by letting bubbles sit on the part -- they are produced by electrolysis, and under a bubble there's no current flowing and the process is stopped there, while the current density goes up everywhere else as the surface area is lessened. You can just shake the parts, or use an aquarium air pump and plastic stone. But whatever you do, make sure it works. Dye some practice parts and you'll see what happens if you don't get this right -- it's ugly.

Here is a curve one guy got on one setup. Here I find the volts don't go quite as high, or the peaks come as fast -- that's life when duplicating an empirical process, but this is in the ballpark. If in doubt, leave the part in about 20% longer -- as you can see from the curve shapes, you're not going to lose much if you overdo this just a little bit.
AnodizePAR.gif
Curves for current, volts, and time

The main trouble you have is that bubbles affect the measurement, as does any dead time with power off, and probably the phase of the moon. Temperature is critical and can make the volts drop before you're really done if it's going up -- and it always is some -- you're putting power into the solution that comes out as heat.

Dyeing:


The fun part for some people -- you can do all kinds of crazy things as the dye "takes" slowly, so people fade from color to color by dipping parts and pulling them out slowly with all sorts of fancy rigs and get some really pretty things done. Me, I'm into function more, and don't do it much. The best dyes are the ones from Caswell plating, but you know, about anything water soluble works... I've used rit clothing dye, plotting ink, sharpies (work really well) -- you name it. The Casewell dyes do work about the best, and are much more color fast in the sun than most of the other alternatives. And by the way, they sell whole setups for doing this, and are a great outfit -- I recommend them heartily.
As soon as you take the part out of the tank, the pores start to close up, and heat makes it happen quicker. This affects how well it will take dye and so on, so pay attention.
As noted, there's no reason to not rinse it until it seems stupid -- cleaner is better, and high pressure water jet is a nice way. I use a thing adapted to an air blow gun off my shop air to really blast things clean here.

Sealing:


This is the money part, the thing that has to be right, and luckily it's not very hard. Just pre boil some water, and put in the part(s) and boil them for half an hour to an hour. It works best with degassed water and you're going to lose some in a rolling boil, so start with too much unless you want to pre boil additions in another pan -- dissolved gas messes this up. This closes off the pores and makes the dye stay in there, or makes the part not a hideous outgassing thing in a vacuum system. One good way to tell if you're getting this right is to mark the part with a sharpie (any color). If you seal the part well, you can no longer wipe that mark off with acetone -- before, it's easy to remove. I've tried the Nickel acetate and other things -- I can't tell the difference particularly on un-dyed or black dyed parts here. I'm pretty sure it might be better for long sun exposure, but probably not great in a vacuum -- acetate would seem to be something that would wind up as a gas at some point with heat or charged particle bombardment.

Well, that's about what I know on this topic. For vacuum, cold, high current, no dye. For decorations, play around with less current, and various dyes.

I recently did this on an aluminum (6061) rod I was using as part of a feedthrough I made and it really made a huge difference on "artifacts" like little sputtering spots and such, Seems it's good for a few kV worth of insulation all by itself, and makes punch through of the main insulator a lot less of a problem. I will do all of them like this from now on.

Here are Caswells words on this, some better than mine (depends on the reader I think), some not so much -- I hope they don't mind.
lcd_ano.pdf
Caswell Plating instructions for their anodizing kit.
(1.91 MiB) Downloaded 584 times

Remember, they are more into the decorative than the mil spec vacuum kinds of things...I use most of their plating kits here all the time, it's the good stuff once you get a handle on it all, mostly cleaning has to be so insanely good most just can't believe how hard it is -- and never get the good results. Just taking too long to move a part from tank to tank can spoil it.

Hint -- like a lot of things, plating and anodizing are "all prep". The "baking in the oven" part is easy. And most cheap customers think they can do the prep and save money. They can't -- 30 seconds from prep finish to process start is too long, for one thing. Don't let them think they can, as most people's idea of "clean" is so far off, you'll wind up polluting your plating solutions, and it won't work for them (the plate will flake off) or for you and your reputation at putting out quality work.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Anodizing aluminum

Postby Jerry » Mon Aug 30, 2010 2:39 am

From what I have been told the reason your hands feel slick after handling lye is because the stuff is literally dissolving the fats right out of your skin. One of the reasons your hands get so dry if you came into contact with the stuff. Ugh...
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Re: Anodizing aluminum

Postby Doug Coulter » Mon Aug 30, 2010 8:39 am

Yup, that's about the size of it, but it makes nitrile gloves slick too so there must be more to it -- it's also a surfactant. The stuff would be ideal for a safecracker who thought that super thin sensitive fingertips were the key (they aren't, see Feynman and others on that topic). I use pretty weak solutions (10% and on down -- well down), but yes, ugh. At least it doesn't eat clothing so much. I find that minor exposure isn't terrible, just annoying. The same stuff is used as developer when making PC boards, and it's hard to get one out of the tank without scratching it or dinging it and not get a little on you once in awhile. The effects are much more severe than the usual solvent or freon degreaser, I believe because the stuff takes apart the lipids that are the cell walls, where those only get the grease -- no cell walls, the skin just falls off, disintegrated.

There was a big flap in the safe/lock business about year back, reported on Bruce Schneier's security blog. Turns out the common bank safe combo lock hasn't been redesigned in many decades, and it doesn't take much to find the numbers due to simple imperfections in the mechanicals -- you can feel them fine as they go by (as it's impossible to make several moving parts all exactly the same height as things wipe over the top, so you can feel them going past), and all you have to sweat is the order of the numbers after that, making such locks easy to pop so fast it just looks like you had a bad day or are hung over at the time. Some university published the pictures of the internals, the trick and the how to, and it set off a firestorm in the lock business -- they evidently think they are entitled to a lifetime of good pay for doing zero innovation. Well, the whole story is long and off topic here, but I like Bruce's blog myself, so consider this an advert for it.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Anodizing aluminum

Postby Joe Jarski » Fri Sep 17, 2010 1:04 am

Here's a good link about anodizing aluminum that helped me started. About halfway down the page, past the kits & supplies, there's a good rundown on the process.

http://www.focuser.com/anodize.html
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Re: Anodizing aluminum

Postby Doug Coulter » Fri Sep 17, 2010 8:12 am

What a co-incidence (or maybe not) -- I found this guy early on myself, and even still have some of the kit I bought from him in-hand. At that time he was agonizing about cooling his anodize solution to get a harder coat, but I see that's gone from his site now. For what it's worth, I went down that path myself, and a cooler solution does work better for many things. Maybe not so much for dyeing, as it tends to make the pores smaller, but for a real good hardcoat, nothing beats doing it cold. I use encapsulated "blue ice" in summer, and in winter just move the rig outdoors. For that matter, any 'smithing I do I try to schedule for winter as well -- more fun to be around a really hot fire when it's cold.

Main issue I've had with colored parts is light fastness -- even some of the pro dyes fade in a couple years when used as window parts. I don't think thats a sealing problem. I did all the windowsill trim on my shop in various colors here, and the ones on the south side of course faded a lot worse than the ones on the north side.
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Re: Anodizing aluminum

Postby Joe Jarski » Tue Apr 26, 2011 8:10 pm

Doug Coulter wrote:The conductive parts of this are 6061 aluminum, which will be anodized before use. I've found that this adds a few KV worth of goodness, and prevents high current density sparks to a point on the surface. I'd suppose a coating of carbon black might do that about as well, but anodizing is easy and more permanent.

Well, this has popped up a few times and the quote is from another thread and topic, but I figured that it would be better to ask here.

Do you think hardcoat anodizing is really worth a few kV of insulation? Alumina dielectric strength is ~400 V/mil and hardcoat anodizing is 3-5 mils so, in other words, would you trust it to be the only insulator up to, say 1 kV? I'm trying to figure out a cheaper alternative (I need a lot of parts) to things like pure alumina that might survive in a fusor for a little while if it's somewhat protected from direct high speed D-ion hits.

A combination of the two may be the best way to go and keep the ceramic work to simple shapes.
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Re: Anodizing aluminum

Postby Doug Coulter » Tue Apr 26, 2011 10:02 pm

I'm not sure I have a perfect answer, and there's an "it depends" component to it at any rate - don'tcha hate those?

What I had was that side feedthrough on my tank, a 3/4" quartz tube with the Al inside. It was marginal both at the end and where it went through the tank. I turned it thinner where it went through the tank (it had been filling the quartz before, touching all round) and polished it, and that helped that part. But the exposed end was attracting high current density arcs still, and the quartz there was getting coated with Al on the inside as a result of that. Turned that down some too, so the quartz was a little farther away, but still had the sparking behavior. When I anodized it (got the coating as thick as I could -- I went for peak anodic resistance in the solution) the sparking went away, and was replaced by "sparkling" at the end, which didn't seem to hurt anything.
And in fact that FT, though used a little less brutally than the main one, has lasted longer than any other. It almost seems like a self-healing coating with a lot of resistance per square, more so than an insulator (but it measures insulating with the ohmmeter). So it can't do high current densities -- it seems to almost quench arcs before they get going.

And suddenly, I had a ton of margin on the thru-tank part too -- 10-15 more kv with no hint of problems than before, which I know doesn't add up -- it's just what I observed. It was definitely a good move, and I replicated it in the new one I made for the upstairs fusor too, and that one's working well also. The only thing I can figure is that the anodizing knocked off any sharp conducting points and reduced field emission of electrons as well as being a high resistance.

I can only speculate that there's some magic going on with Al hydroxide being able to regenerate or something here (all this done in D of course).

I don't claim to have "the answer" to this -- the FT is one of the big ones I don't feel is really solved yet, but each try is getting better and I'm getting longer life each time. I very much agree with the approach of making the hard to get stuff (quartz or alumina) as simple and cheap as possible, as well as easy to switch out if it fails. What I seem to be finding out is that the field near the end of the insulator is key -- and keeping the ion hits off that part is the game. This might mean that a grounded cover near the end actually helps, or it's what I'm going to try next time at any rate. This would use the rather weird fact that at the pressures a fusor runs at, Paschens law lets short gaps not draw current and things don't take the shortest path, real counter intuitive. The idea would be to create a field shape there that would tend not to attract D+'s to the critical part of the insulator. Don't know how that would work out in various geometries, this is just "tried this, now try that" turf for me.

Who knows, I've tried about everything else...

FWIW, I'm using moderate current density, about 7 amps/sq foot or a little more, and watching on a constant current supply. Voltage reaches a peak, then starts back down -- that's when you stop, the coating is as thick as it can get. The Caswell manual gives expected times for this and they are fairly close. They also mention that the "pore" size is larger for lower current densities, and smaller for higher, and I've not tried a range in this application yet, so there might be more to be had varying that one.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Anodizing aluminum

Postby Joe Jarski » Wed Apr 27, 2011 7:57 pm

Yeah, that seems to fall in line with everything that I read too. There really isn't a definitive answer. I'll have to do a little testing an give it a try. These are just for my suppression grid so a failure isn't going to ruin any high dollar items, maybe just a CCFL inverter in the worst case.

That's interesting that one FT would affect the other. Of course, there's a lot going on inside a seemingly simple device.

That is a really moderate current density that you're using. Thanks for the info it's always good to know the boundaries of what works for everyone else.
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Re: Anodizing aluminum

Postby Doug Coulter » Wed Apr 27, 2011 8:34 pm

You might well bake a CCFL and not due to that alone -- funny things happen in these on occasion. Nothing you can't explain in hindsight, but the main supply can store a lotta joules and that can take out a lot of things...So can a big cloud of charged particles store some juice, and let it go in ways that aren't obvious till after you see it happen.

I used the low current density because it was easiest to do on the bench in a beaker without it getting too hot and messing up my measurement of max anodic R, that's all. As far as I know, a higher one would be as good or better -- just haven't tried it in this context. (self) Heating during the process messes things up -- and I didn't have a big tank or active cooling readily available at the moment I needed to do it.
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Re: Anodizing aluminum

Postby JonathanH13 » Thu Apr 28, 2011 5:30 pm

Fantastic thread here - I wish I had some of this information a few years back! When I tried to do this I found some of the details really tricky and a 'trade secret' attitude out there.

This is 'wood cut' that I did with photoshop for a friend:

clean.jpg
An aluminium sheet is cleaned and polished. The sheet is washed in a dilute sodium hydroxide solution to remove the top layer of aluminium. It is then washed in dilute fuming nitric acid to remove any metal in the sheet that is not aluminium.


sun.jpg
It is then washed in distilled water, coated with photosensitive ink (aerosol) and baked in the dark.
The aluminium sheet is covered by a negative of the image (printed on transparency with a laser printer) and is exposed to UV light, in this case, the weak autumn sun.


develop.jpg
It is then washed in developer, which removes the photosensitive ink that was exposed to light. Stubborn photoresist is teased off using a brush.


tank.jpg
The completed image is washed and left to dry, ready for anodising.
The tank is filled with dilute sulphuric acid.
A lead sheet of similar size to the aluminium is cleaned, to expose the surface, and it is placed into the tank as a cathode.
The aluminium plate is then emersed into the tank to create the anode (hence 'anodising'). 4 amps at 12 volts is passed through the acid for 45 minutes, this heats up the acid and gives off hydrogen.


No image.jpg
The aluminium is then removed and washed in distilled water, and then the remaining unexposed photosensitive ink is washed off with acetone. Once the anodised sheet has been washed in acetone, prior to dyeing, no image is visible.


Final.jpg
The aluminium sheet is placed into a tray of dye for 15 minutes. Wherever the metal was exposed to the sulphuric acid, it has become porous and so is more receptive to the dye.
The sheet is then boiled in distilled water for 30 minutes to seal the dye. It is then left to dry, cut to size and sprayed down with compressed air and laquer.
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