Strange apperance

[PeterM]

Hello, ive done some anodizing-setups and got good "results" since i managed to get a hard layer, pretty much unscratchable.
BUT, i have some problems with the looks of the part. Im anodizing a small tube with the total area of aprox: 16 square centimeters and along the tubes side theres like stripes going from each end of it in a darker color.

Can anyone please help me out? what am i doing wrong?
I anodize the part at 0.74 Amps for about 40 minutes and use a 19.5% acid solution.

[Fibergeek]

16 sq. cm. = 2.48 sq. in. = .0172 sq. ft.

0.74A / .0172 sq. ft. = 43A per sq. ft.

That's an awfully high current density, it would require:

2.5 Ohms / sq. ft. x 43 A / sq.ft. = 107V to grow the anodize to full potential thickness. You probably don't have that much.

4.5 A / sq. ft. current density for 16 sq. cm. would require 77.4 mA.

Your acid concentration should be about 5% at this current density.

[mcaswell]

Well, the main thing you are doing wrong is NOT following our instructions.
You can download LCD instructions at:

http://www.caswellplating.com/kits/lcd_ano.pdf

We aren't going to give technical support, or offer recommendations on anodizing on this forum, unless you are following this procedure.

Why? Because there is a bunch of nonsense out there in the way of instructions and NO-ONE can give tech support on it all. We certainly aren't even going to try. (Bin thur - dun that)

PLEASE - get on the same page as everyone else here- dump the system you have - and you'll find the rewards are huge!!!

[PeterM]

Thank you for the quick replys. I'll stick to the manual you refer to. I apperently got the wrong information.

[PeterM]

Hello again, i followed your instructions, read the LCD-manual. But i still have the problem.
Im just not getting that nice finish after the anodization-process. I get marks running in the same direction of the tube, on some side more marks than the other side, still theres marks. The marks are in a darker color.

My current setup is: 5% acid, im using a rectifier that gives 77 mA
on a piece thats 16 sq. cm and im anodizing for about 90min.
Ive checked the process and can almost instant see the marks coming, and when i pick up the part it has a pretty hard surface but still a ugly appearance.
Ive tried severals ways of cleaning and prepping the part; ive polished it, used chemicals to remove fats and washed it in detergent and been careful handling it and finishing it off with dest. water.
For connection i use a Aluminium rod which i drilled holes into (and is connected to the rectifier), and then titanium wires that just fits the holes and are fixed with screws from the side. I really dont believe theres a connection problem. My cathode consits of a leadpart. The acid is made by 3:1 dest. water - battery acid.
Can someone please explain why i get this result, im happy with the hardness of the surface, but i also want my parts to look good.
I wish i could send you a picture, but theres no way i can do that atm, hopefully someone recognizes the problem or can give a hint whats causing it.
Ill try demonstrating it with some ascii skillz
__________
|--- _ _ --- |
| ------- __ |
|__--___--_ | << My anodized tube with the marks ALWAYS running the same way as the tubes lengthaxel

Thank you in advance.

[Fibergeek]

I am assuming the marks are not machining or sanding marks left on the tube, anodize will not hide machining marks. Native oxide can be difficult to remove chemically, sanding or beadblasting is the most effective way.

If the tube was formed by DOM (drawn over mandrel) but not then annealed, there will be work hardening present on the surface. These work hardened areas will anodize differently then those areas not work hardened. The solution to this is annealing the tube, to bring the entire tube to the same level of hardness. You may also be able to machine or sand the entire outside surface of the tube to remove the offending hardened areas.

[M_D]

I am thinking along the lines of Fibergeek. I have seen it on extruded 6061 and 2011. Sometimes it shows up as dark lines straight from the anodizing tanks, and other times it doesn't show up until it is dyed, and the darker colors are worse. So, if the lines are perfectly parallel to the length of the tube, I would look at that first. Also, sometimes the effect goes deeper than the surface, so just barely removing any visible (before anodizing) lines may not be enough.

If there are streaks that only closely follow length of the tube, it may be something else.

Do you know the alloy, and what kind of tube it is?

[PeterM]

I get the point, and it seems like your ideas may be the answer to the problem.
The part is DOM (using a pretty old CNC-lathe for the job). Im using a 16 cm diameter aluminium rod, 6012 Alloy. I checked the alloy up with the seller and they said its a good alloy for anodizing. The part is about 15 cm in diameter when finished. Anyway, ive checked the finish on the surface of the part after the lathe-process in microscope, and yes the surface is pretty rugged. So if i get this right, i need to sand the part until i get rid of that "texture" on the part after the lathe-process? A lower speed on the lathe should also be a step in the right direction - since i would result in a better finish?

Thanks for the help, really appreciate it.

[M_D]

The way I understood it was the streaks are running lengthwise on the tube? If the part is turned on a lathe, and there were streaks running around the circumference, I could definately see where an adjustment of the turning process was in order. One thing I would look at is tool sharpness, and using a lubricant or coolant to produce a free cutting action which leaves a good finish and would not tend to work harden the material. Most lathes don't have enough rpm for small diamter aluminum work, so it is doubtful in my mind excessive rpm is contributing to the problem. I turn .75" aluminum at 6,300 rpm, and would often use a higher speed if the lathe was capable of more rpm.

I suspect the DOM tube does have work hardened areas, in which case anealing like Fibergeek suggested may help. If beadblasting were an option for you, it could solve the problem. If that is not an option, it may be benefical to use a heated solution of stipper or lye which will remove aluminum from the surface. You would need to closely monitor the part to ensure it doesn't become pitted.

If your material was originally extruded, and I think it is likely, there could be some steaks of aluminum oxide in that length of stock, which is not unusual. In that case, another piece of material may solve the problem.

I have stripped a few parts with flaws which sound similar, and that removed most if not all of the defects. If you do more parts, and notice flaws, I would reccomend stripping before sealing as that works much faster and more evenly.