Posted by vinnie (68.78.43.5) on March 27, 2009 at 17:17:38:
In Reply to: Problem when anodizing posted by Kevin McArdle on March 25, 2009 at 12:20:18:
: I am currently working on a project to set up my own anodizing line. I am anodizing cylindrical aluminium tubes, 80mm in length and 10 mm outer diameter. I have a problem when I anodize the parts. The anodizing process is removing material from the aluminum tubes instead of building up an oxide layer. I was wondering what the solution would be to this problem.
: The electrolyte I use is 20% sulphuric acid and I use a current of 4.5 amps and 15 volts. Before anodizing I etch the parts in 20% sodium hydroxide and then desmut in phosphoric acid. I have aluminium cathodes and I hang the parts on aluminium welding wire. Also the anodizing tank I use is 100mm x 200mm x 100mm (LxWxH) and i space the cathode/anode as far away as possible.
: All the chemicals I use came from a chemistry lab in my college so they should not contain any impurities.
: Also my parts dont seem to take the dye I am using. After i seal, and the part has dried the dye has just stuck to the outer surface of the part and falls off when rubbed. If anyone can help it would be much appreciated, thanks, Kevin.
Answer a few more questions.
How many do you do at a time?
do you have the positive hooked up to the parts you are anodizing?
Are the parts just hanging from the welding wire or do you have the wire tightly wrapped around the parts?
Are you agitating the solution?
Are the wires you use to hang parts dyeing properly?
If the tank is set up with the proper polarity, positive to parts negative to aluminum anodes, you should anodize eventually. if the part is just hanging on the wire, it is not gonna work, they have to be tight. Aluminum wire is gonna anodize itself since there are no sharp edges and as soon as the wire gets enough coating, it will break the contact to the parts and the part is just gonna dissolve while you think it is anodizing. That is why I ask if you can dye the wire you are using. You also need some aluminum dissolved in your solution. Since you are dissolving the parts you are anodizing, there should be some in there already. If you keep changing the solution, you will need to get some aluminum in the solution. The positioning of the "ground cathodes" is not critical to have them as far as possible. If they are too close they will form hot spots on the parts. All they are doing is making the solution the ground. The area of the cathodes should be three times the size of what you are trying to anodize at a minimum. You can use stainless steel or copper in that little tank, but you have to have a good connection to your negative on whatever rectifier you are using.
4.5 amps should anodize a load of about 1/2 a square foot, however that converts.
Basically, if your wire is anodizing, you just need a better connection to your parts.