Re: Problem when anodizing

Subject:

Comments:
: : : : 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. : : ---- : : I do 1 at a time. : : Yes my parts are wired correctly, positive to the part, negative to the cathodes. : : I loop the ends of the wire around so it acts as a spring against the inside of the aluminium tubes and the fit is very tight. : : I'm not agitating the solution because my tank is quite small. : : The same happens the wires as the parts, they dont take the dye and reduce in size. : : I have tried lowering the concentration of my acid and lowering the amperage because it works out too high for the size of part I am anodizing. The latest experiment i tried was having the power supply at 0.24 amps and 12 volts for 160 minutes (using the "720" rule). My acid concentration was also down to near 10%. Little or no heat was generated from the tank. The parts however did not anodize at all, neither did the wires. The hydrogen bubbles were clearly seen rising off the cathodes so a current was passing through. : : One thing im not certain of is how much aluminium I have dissolved in the electrolyte. At first there was no aluminium dissolved but up to date I have conducted the process 7 to 8 times. : : When you say negative to the cathodes, are you connecting your wire to the positive, sprung thru the part and then contacting the wire with the cathode? Thats what it sounds like you are doing. The negative cathodes are there to allow the sulfuric acid solution to be the ground, thats all. The material is not being dissolved and attached to the part like zinc or copper plating. : You should be able to take a voltmeter, put the positive on your wired part and the negative in the solution. If your wire is touching the negative cathode, the sulfuric acid is just eating away the wire that is electrified just like it will dissolve the part that has no electricity feeding it. : as far as your concentration, the higher percantage of sulfuric you have, the less tendency you will have to burn parts, you will just create more heat. It really is important to agitate your solution since the aluminum is being dissolved and redeposited on the part. : Going to 12 volts and .24 amps seems to be the wrong direction. You want to be between 15 and 18 volts.

Optional Link URL:
Link Title:
Optional Image URL: