Voltage movement

[PeterM]

Hello
I took a long break in trying to learn the secrets of anodizing due to shortage of free time, but now im back with my newbie skills and questions

Today I did a new try.
*16 sq. inch part
*77mA
*Alloy: 60XX
*5% Acid solution
*I used a CC rectifier and amperemeters to log the voltage and ampere values.
*for cathode i used a 50 sq inch lead plate.
*For agitation i used a ceramic rod for fish tanks connected to a small airpump.
*Heating: had none to use
*For connections:
Titaniumwire strongly connected (hammered) into a screw-gizmo i made. To that i connected the cable from + of the rectifier with a scew and bolt.
Since i cant drill holes in my parts i had to use the very stiff titanium wire and bent it and slide it into the part, so that I ended up with 3 connection spots between the wire and the part (also this very very tight - though i know this could be the weak spot also in my run)

Run:
I prepped the part, did waterbreak test and so on. Connected it to the system and turned on the rectifier: My voltage immiedately started to raise...and fast.
After about 5 minutes i was up from 0V to 8V. Then it started raising slower, and after about 7 minutes i had 10.24V. Then..suddenly it started going down. After 25 minutes at 9.34V.
Now my question...shouldnt PAR-value be reached within approx 40 min using LCD method?
And why does my voltage first start to climb like it should, then suddenly so few minutes after start begin to go down?
Is this due to bad connection, or me not using heater to have a specific bath temperature?

Enlighten me metal finishing gurus!
//Peter

[M_D]

Fibergeek or someone else know more than I about PAR, but if I am not mistaken the approximate time to reach it is only predictable with consistent parameters. So you can’t expect for 40 minutes to work for all conditions.

The voltage slope does go up pretty much like you explained it. When speaking of 60xx alloys, it often reaches the peak voltage and can stay close to flat for an extended period of time. If a connection is bad to begin with, or goes bad during the process, the voltage would rise beyond normal. If you anodize too long, the voltage may fall because the anodized layer starts to deteriorate and the part surface becomes a better electrical conductor again.

Your part with 16 square inches is .1111 square feet (16/144=. 1111). Assuming 6 amps current density, you should be anodizing it at .67 amps. I’m not an electronics wiz so perhaps I’m wrong, but isn’t 77 mA equal to .077 amps? If so, that is one problem right there, and that isn’t enough current density to anodize at.

If you used .77 amps instead, that should still give good results, assuming the time and other factors were ok. There is a recent thread explaining the720-rule that might help you to determine the time to anodize, if you are interested.

[PeterM]

Thank you M_D
Im going to try another run today with the changes you suggested. Ill post my results later this day.