Peak voltage question

[mudwhump]

I anodized my first part, and I have a question. First of all here are the details... The part I anodized was a 4.75 X 3.0 square piece of aluminum sheet metal. 4.75 X 3= 14.25 in?. Multiply times 2 for both sides gives 28.50 in? total area. To figure current density of 4.5A/ ft? 28.5/144=.198 .198 X 4.5= 890 mA My power supply is a 24V 10A power supply connected to Fibergeeks VCCS circuit (thanks for the schematic fibergeek, the VCCS works great!). Anodizing bath is 3 gal. distilled water and 1 gal battery acid. Measured solution at 60?f ( I know this is a bit low, but I don't have a heater for the anodizing tank yet). Connected titanium wire to part by drilling a slightly undersized hole and forcing wire through. Cleaned part and it passed water break test. Placed part in anodizing tank, turned on power supply and set to 890 mA. The voltage began to climb, and reached 13.8V after about 3 minutes. After this it began to climb slowly until it reached 15.9V after 30 minutes. At this point it began to slowly drop (about .1V each minute), so I removed from the anodizing tank and rinsed. Dyed part black at 140? and sealed. The part took the dye well.
Here's my question (finally). In the manual it states that the peak voltage needed for 6A/ft? is 15V. I was running my part at 4.5A/ft? and my voltage went to almost 16. Also, PAR was reached quite a bit sooner than anticipated. What could cause this?

[Fibergeek]

Good job Mudwhump. You did everything right. And for once someone provided enough information so I could see what is going on.

The value you reached for PAR is higher than mine. If you're happy with the results that counts the most. I would recommend you get your electrolyte temperature up a bit, and Id be a suspicious about how good you electrical connection is. Both can cause a higher PAR value, and affect the time to PAR.

[potsked]

i notice a re-occurence of 13.8 volts. his descrption mimics my own experience, and im wondering what might be so inherent or fundamental as to cause this. sure the temp sounds low and hey, maybe it's the connection, but i would guess not.

my problem with consistancy was the result of dye temps which ive since rectified. but i have yet to find the cause of this mysterious magical number of 13.8 volts. have you encountered this number in any of your runs?

[Fibergeek]

Potsked,
Mudwhump's basic anodizing curve is right; he went from zero to 13.8V in 3 minutes, but then the voltage continued to climb to 15.9V, which indicated Mudwhump's PAR, and then it went down at a slow rate. The 13.8V value here sounds like a coincidence to me.

Is this what you saw in your experiments? I didn't read it that way, did I misunderstand what you were saying?

[potsked]

i dont follow your logic.

at 13.8 volts with .89 amps would indicate a resistance of 15.5 ohms 3 minutes into the process. considering that par for that amount of sq. footage would be 12.63, i dont see how this shows a correct anodizing curve.

there's also the fact that he reached 15.9 volts after 30 minutes, which indicates 17.87 ohms. this is about 5 ohms too high above par, not to mention an hour early.

if this is a good example of how LCD is supposed to work, then i was way off the entire time.

[Fibergeek]

I'm saying that the curve is right. Ignore the numbers for the moment.

Mudwhump started at zero (when he powered up his supply) the voltage rose within minutes to a higher voltage (the 13.8V) then the voltage went up slowly (to 15.9V) in 30 minutes. At this point it peaked (because it then started to go down) he waited some minutes more to be sure that it was really going down, and then stopped.

This is what I mean by the right curve. The numbers look close to a 6A/sq.ft. current density, not 4.5A/sq.ft, which is why I'm questioning temperature and connection resistance. You're right about the elasped time being too short; to me this means that this coating could have been thicker, assuming that the 4.5A/sq.ft. is right. Mudwhump's calculations are right, but a 25% error in the current measurement (4.5A vs. 6A) sounds like too much to be likely.

Recall that some extra resistance in the connections can make the voltage measurements, hence the resistance calculations, look higher than they really are. I think you experienced this yourself, although neither of us is sure why.

Mudwhump, I hope you don't mind us using your anodization as an example. Sorry we highjacked your thread.

[mudwhump]

Fibergeek, no problem at all... I learn something from all the replies. I think you are right about the connection being the problem. For my next try I'm going to make a small threaded hole in the part (not sheet metal this time) and thread the wire right into it. I'll post my results here.