Anodising Problems

[tomcannon]

Hi, I have had some aluminium parts water jet cut for a job, and needed them anodised. I decided to try myself as there will be a slow but steady trickle of these parts to do and it is not economical to have them done professionally with minimum orders.

I set up a small rig along with tanks for the pre-cleaning operations etc, the main anodising tank holds in the region of 3 gallons of sulphuric/water mix, approximately 13% acid.

In trials it was going quite well but the aluminium hangers I used (12g aluminium wire) would dissolve over 20mins when used with a decent current of say 10amps at 24v.

To get over this I made a titanium T shape hanger to hang 2 pieces on which did not dissolve of course- but I could not get a good connection between the titanium hanger and the test pieces, as soon as the power was connected and the first lot of bubbles had risen the connection would disappear, I put it down to the poor conductivity of the material as touching the hanger would make small sparks appear between it and the aluminium pieces, current would rise then disappear again.

I then made up a much thicker aluminium hanger (8mm diameter bar welded to 25x6mm flat bar in a T shape), the pieces to be annodised have 8mm holes though 3mm plate, all countersunk (so not much area in contact with hanger) but were a tight fit and pushed onto my new hanger. When powered up though I could not get more than 2-3amps to draw through the set up which dropped to nothing and did very little. When I took it out and popped a piece of 12g wire back in and connected it the current shot back up to 15amps.

I'm obviously overlooking something simple, but I have tired myself over the subject and can't think what. If anyone has any advise it would be greatly appreciated.

The power supply by the way has 12/24v, 20/80a, but I can use resistive wire to lower the current which I did succesfully in testing. The surface area of each workpiece is approx 14"sq.

[doorknob]

I have no direct experience with this, since I am only now gathering the parts and pieces for building a home anodizing line, and probably won't have things set up until summertime. But here are my thoughts anyways...

The parts that I would like to anodize have a single threaded hole of approximately 6mm diameter, along with several smaller holes. My tentative plan is to make hangers using titanium bolts, so that I can thread on the parts rather than using titanium fingers or aluminum wire for the hangers. I haven't investigated it yet, but I hope that the cost of titanium bolts is not so high as to make the plan unaffordable.

I may have missed it in your description - do your parts have any threaded holes that you can use, or can you add a threaded hole in an inconspicuous place, for the purpose of making the electrical connection?

If I don't go ahead with the titanium bolt idea, my fall-back idea is to use aluminum wire hangers, by putting a few threads on the end of each hanger, but that would seem to be a time-consuming approach which would probably require redoing the threads frequently.

[Danodizing]

First make sure you have a good connection. Then do a run of 2 of your 14 sq in parts using constant current. Run at 3 amps for 45 minutes.

[tomcannon]

Thanks, I tried it again tonight with both the titanium hanger and thick aluminium one, once again the titanium one worked for about a minute before the connection fizzled out.

The aluminium one on the other hand worked quite well for a bit, I had to switch the PS (big manual battery charger) onto high to start with and the amps went up to 15, a bit more than I wanted but close enough. I left it for 45mins and the hanger had anodised/corroded itself by probably nearly a mm on all surfaces, the pieces were still hanging there but I guess they had lost connection part way through as I can tell by looking that the anodising is not very thick.

I think you are right and I need to get a better connection on there somehow- but the pieces I am doing are only (approx) 70x90x3mm thick, they have 7 holes in each and all the holes are countersunk fully so not even any square edges in the holes- if that makes sense.

I will have to try with something else with a better chance of a connection- I'm very disappointed by the rubbish conductivity of the titanium though!

p.s. I still dont understand why the bigger the piece in the tank- the lower the current, when I went to disconnect the pieces this afternoon the current was over 20A, when I connected it was 15A- before they lost connection.

[Danodizing]

Part of you connection issue might be pushing that much current through such a small piece of wire. I don't remember off the top of my head how to figure how much current your 12ga wire can handle. Might be worth a shot to use two, one for each part that would double the load your racking can handle.

As far as run time, I'm not familiar with using constant voltage, so I won't be able to help you there.

[doorknob]

It sounds like danodizing is recommending the "low current density" (or LCD) approach to anodizing that I have read about on the Caswell Plating web site. That is the approach that I plan to use as well.

With the LCD method, if I understand it correctly, instead of using a power supply with a constant output voltage and the anodizing current varying during the run, they recommend using a current-regulated power supply (that holds the anodizing current at a constant level while letting the voltage float to whatever level is necessary to provide the desired current), and choosing the current level and anodizing time based on the surface area of the parts and the temperature of the acid bath.

I purchased a hefty, current-regulated power supply for my set-up. It's an HP - Agilent 6268B. For example see:

HP - Agilent 6268B | Used Power Supply (DC) | Used Test Equipment | Refurbished Test Equipment

or

HP AGILENT DC POWER SUPPLY MODEL 6268B 0-40V 0-30A GOOD - eBay (item 260414396975 end time Mar-11-11 19:35:28 PST)

They are usually pretty expensive used, but you can get less expensive ones from other manufacturers (for example, Astron power supplies seem to be popular). I got a great deal on mine (OK, so I'm gloating a bit - I paid $75 for it, but I had to drive 300 miles round trip to get it - yes, I'm crazy).

[Danodizing]

There is a difference between the LCD method and constant current anodizing. LCD uses a much lower sulfuric level and a lower current density, normally in the 6asf range, which has longer run times. A more standard approach to constant current is going to have a high concentration of sulfuric 10% is pretty typical and higher current density, 12-15 asf. LCD is a good way to get started but a much slower process IMO.

Since tomcannon said he thinks he is around 13% the number I suggested were are 15asf

[tomcannon]

Thanks again for the replies- very helpful but I do need to brush up on the subject a bit more so I dont get lost!

I'm in the UK for one, but I'm sure science doesn't change! I should look into a proper PSU of some kind then, what sort of current do you guys recommend with variable voltage type anodising? The parts are actually about 15"sq and I would like to be able to do a few at a time, I'm not going professional with it but I would like to be able to do general stuff as well as these particular bits as I do fabricate and machine parts for part of my living!

Regards,
TC

[Danodizing]

Just starting out I would suggest running at 6asf, but you will need to drop you acid concentration to 7-8%. This will make it easier to maintain temp of the acid bath. Two of your parts at 6asf would need 1.25 amps for 2 hours to build approx. 1 mil.

[Iron-Man]

Hi, I have had some aluminium parts water jet cut for a job, and needed them anodised. I decided to try myself as there will be a slow but steady trickle of these parts to do and it is not economical to have them done professionally with minimum orders.

I set up a small rig along with tanks for the pre-cleaning operations etc, the main anodising tank holds in the region of 3 gallons of sulphuric/water mix, approximately 13% acid.

In trials it was going quite well but the aluminium hangers I used (12g aluminium wire) would dissolve over 20mins when used with a decent current of say 10amps at 24v.

To get over this I made a titanium T shape hanger to hang 2 pieces on which did not dissolve of course- but I could not get a good connection between the titanium hanger and the test pieces, as soon as the power was connected and the first lot of bubbles had risen the connection would disappear, I put it down to the poor conductivity of the material as touching the hanger would make small sparks appear between it and the aluminium pieces, current would rise then disappear again.

I then made up a much thicker aluminium hanger (8mm diameter bar welded to 25x6mm flat bar in a T shape), the pieces to be annodised have 8mm holes though 3mm plate, all countersunk (so not much area in contact with hanger) but were a tight fit and pushed onto my new hanger. When powered up though I could not get more than 2-3amps to draw through the set up which dropped to nothing and did very little. When I took it out and popped a piece of 12g wire back in and connected it the current shot back up to 15amps.

I'm obviously overlooking something simple, but I have tired myself over the subject and can't think what. If anyone has any advise it would be greatly appreciated.

The power supply by the way has 12/24v, 20/80a, but I can use resistive wire to lower the current which I did succesfully in testing. The surface area of each workpiece is approx 14"sq.

You should start here; Caswell Inc. - LCD Aluminum Anodizing Kit
There is a formula for current and size of parts. 10 amps sounds quite high for small parts. Download this document; http://www.caswellplating.com/kits/lcd_ano.pdf it will give you some great information.

I thought about anodizing but decided to go with powder coating, less chemicals etc.

Hope this helps,

Iron-Man

[tomcannon]

Hi again, I haven't had much chance to play with the annodising lately but hoping to again soon, I read through the Caswell pdf and the annodising part wasn't dissimilar to what I was doing.

I may have the acid content slightly too high but not by too much, and 4.5 ASF isn't far off what I was doing. AH- I think I mis-calculated the ASF! I was going by 12 inches square, not 12 square inches, which is 144".....

Could that be why my parts were corroding/annodising so fast the connection became broken and wires burnt though or is it more likely because the acid was slightly too high?

When I was first playing the set up wouldn't seem to actually draw anything unless I cranked the volts and amps up a fair bit, the charger I was using is an old manual type, has 12/24v, and high/low current. On 12v it wouldn't seem to do much at all, 24v was much better but seemingly too much.

Is there a more finesse charger or something that people regularly use- like a laptop type charger or something bigger?

[twocik]

Your ano tank temp sounds too hot, are you running a chiller ?

What's the temp from start to finish ?

If your bath is brand new, might want to run a few more test pieces thru it to build up a little more dissolved aluminum. It's something like 5 grams or so.

Last is your cathode needs to be at least 1:3 ratio (lead or aluminum)...