Anodizing Problem, part shrinkage.

[spankey666]

to add. are you using proper anodising dyes ?

[coopduc]

I think we need to understand in more detail your set-up, and the parts you are runing. From your description, your tank is extremely small, only about 2 liters. I have never known a tank to be so small. Your part is very small too, and if you are running only one part at a time, there is no way possible you could draw 4.5 amps. One part should draw about 0.3 amp at 15 volts. In order to draw 4.5 amps you would have to run 14 parts at once, then the temp rise in the tank would be extreme, and the bath would be hot to the touch, and vapors would be coming off the surface like steam.

[kmca_1126]

Its a bench power supply. The voltage and amps can be varied. The amps remain at 4.5 throughout the whole process but the voltage begins to drop off over time.

[kmca_1126]

And i had one litre of acid to begin with, but now 1.4 since i diluted it further. The tank does warm up but not by that much even for anodizing for 60 mins.

Also the dyes Im using are fabric dyes but the dying im not as concerned about as the fact that the process is removing material rather than adding to it. Im still waiting to find out the grade of aluminium these parts are to see if that has anything to do with it.

[acidrain]

I didn't do the math, but it sounds like your part size and amps aren't correct.
You'll need to convert everything to inches if you want a true answer, but this it what it sounds like to me:
You have the current density confused with set current. If you are running 4.5 amps per square foot current density (CD), and your part is .5 sq. ft., then your set amps should be 2.25 amps. You need to accurately measure the surface area of the part, and then multiply by your chosen CD. BTW, I prefer 6 amps per sq. ft. At 6 amps CD, the run time will always be 2 hours for a 1 mil build. never try to build over 1 mil... it won't happen.
Also, using pure sulfuric acid has caused problems with other guys... it's best to just use 1:3 battery acid like it suggests in the manual. Using too concentrated of a solution will dissolve the coating as fast as it's grown (dissolution), and give you a soft, chalky coating that won't take dye well.
Heat is a huge factor... you must maintain 68-70 F. If the temp goes over 70F, dissolution will occur. The easiest way to offset that is with a bigger tank.
Agitation is important too... it keeps the heat from building up at the surface of the part. Without agitation, dissolution will occur.

[kmca_1126]

So i'm guessing that the oxide layer dissolves as soon as it forms due to high concentrations and temperatures.
The temperature doesnt seem to exceed 20C/70F. I lowered my concentration of acid to around 15%, but im going to lower it again some more and see if thats the problem.
Ive tried using a lower current on one round and it did the same thing.

[acidrain]

Something is definately wrong here...
Since you didn't convert, I'll do it for you.
At .039 inches per MM, I calculate your part is .05 sq. ft.
.05 X 4.5 CD = .23 amps
Is that what your amps are set at?

[kmca_1126]

Hmm set way higher than that, most times i set the power supply at 4.5 amps because the voltage drops quite a bit if I lower the current. I'll try a lower amperage again next time.

[sswee]

Acidrain is spot on.
I ran the numbers and at a set amperage of 4.5 you are trying to anodize at a CD of 83 amps per square foot. Approximately just under 4X what professional anodizers do in CV.
After conversions, I also got 7.803" squared SA or .054 square feet.
At 4.5 CD gives a set amperage of .244A. If set at 4.5A, it is 18.4X higher than it should be. Something is going to burn.
To loose .006" off of the surface takes some very aggressive action.
Off the top of my head, battery acid is 33% by volume. At a 1:3 ratio, it should be diluted down to 8.25%.
Hope this will get you going the right way.
What is the voltage rating on your PS? Was it 15V?
SS

[kmca_1126]

Yeah the PS was at 4.5 amps and 15 volts for the process but the volts slowly dropped over time. Wasn't much heat coming from the tank so I never suspected that the current was too high. But i'll lower the amperage next time and see what happens.
Also does a concentration as low as 10% for the electrolyte allow for good dying affinity? From my research I have found that higher concentrations form a softer coating but good for dyes, where as lower concentrations form a harder coating but not so good for dyes. Of course im not so sure about the integrity of that claim since my set up is not even working to begin with to see.