Anodizing Problem, part shrinkage.

[kmca_1126]

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. Approximately 0.3mm is taken from the tube wall. I was wondering what the solution to this problem would be.
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.

[acidrain]

I suspect the problem is with the etch and not the actaul ano process.
If your parts are raw and clean, there is no need to etch and desmut. If the parts are polished, that will remove material as well.
Simply degrease and go right to ano.

[kmca_1126]

I only etched the part for 20 seconds and 15 minutes for the desmut. I checked the parts after each step with a vernier calipers and there was almost no difference in the change of the dimensions but it was after the anodizing step that the part reduced in size

[sswee]

A possibility could be reversed polarity. Make sure you have the positive(anode/part) and negative(cathode) correct. It's not common but I have seen some PS reverse polarity if arced. Usually corrects itself by turning the unit off and back on. Check polarity with a digital multi-meter.
SS

[kmca_1126]

Yup I have my electrodes wired up correctly, but even if the polarity was changed, wouldnt that anodize the cathodes? My parts are beings anodized to an extent, the surface is harder than normal as it resists abrasion from aluminium that hasnt being anodized.
Would the fact that the sulphuric acid has little aluminium dissolved in it have anything to do with it?, because i have only conducted this six times with one part at a time.

[spankey666]

isnt 20% sulphuric acid a bit strong ? battery acid i believe is 33% and then is diluted 3:1 with water for the lcd process, i dont know what that makes it but im sure it will be less than 20%. so maybe its desolution occuring
cue replies from otheers that actually know what they're talking about

[kmca_1126]

That means the solution would need to be just over 10% acid. Ill dilute the electrolyte some more and test it out. Thanks for the suggestion.

[kmca_1126]

Ok i diluted my electrolyte some more and still the same happens. The aluminium however actually took some of the dye this time, not much but it does have a tint of colour. I should also note that my parts when anodized are brighter rather than duller.

[smokindragon]

well my guess from what you just stated that the part is now taking on a little color after thinning out acid/water ratio, That I would thin out a bit more.

[spankey666]

you say you are working at 4.5amps. is this constant or are you using a battery charger type power supply ?