Problems using 12asf and 4.5asf!

[Chevalier]

I was having problems getting parts to anodize using 12asf, so thinking it could be the fact that the acid was sitting for nearly a year with out being used I decided to try the LCD method. I mixed up 50ml sulphuric acid to 1000ml of distilled water(5% - the same as 1:3 battery acid). I started with a small quantity so I wouldn’t have a ton of waste acid if it didn’t work! I cut a piece of sheet aluminium 1 inch sq. giving a total surface area of 2 inch sq. So using LCD: (2/144)*4.5 = 0.06A and 2.5*(1/(2/144)) = 180 ohms so I needed 0.06Amps and my final voltage should be 10.8V?
I started anodizing and the voltages were as follows:
Time(min): Voltage:
start.......... 0.3
0.5(30sec). 5.5
1.............. 7.5
2.............. 8.0
5.............. 8.2
25............ 9.0
50............ 9.7
60(1 hour) 10
95............ 10.6
120(2 hrs) 11
140.......... 11.2
155.......... 11.4
180(3 hrs) 11.44
195.......... 11.71
215.......... 11.78

The meter on my power supply is anologue so the voltages were measured across the cathode and anode.
The part was drilled and threaded then the wire was forced through the threads, the aligator clip I used has sharp teeth and I forced them into the hanging wire.
I washed the part with soap then cleaned it with Pro-Clean(our equivalent of simple green) and it passed the water break test, then I put it in a 10% sodium hydroxide(lye) bath for 2 min, then in a 10% nitric acid with ammonium bi-flouride bath for 2 min to remove surface impurities, then into the anodizing bath. Seeing the voltages above I thought I had cracked the problem, but unfortunatly not! I took it off after the 215 minute mark thinking there must be something wrong. The red dye didn't take, I got a very very pale pink!(suggesting some anodized layer but not much) Is there something I am missing / any suggestions as to what I should try next? I originally thought the problem was the battery charger so I bought a 0-18V 0-10A lab supply, that didn't make a difference at 12asf so I changed to the LCD method thinking it was the acid, now I have proved that wrong. I figure it is the dye now! it had been sitting for almost a year(although I was also having problems anodizing when I first mixed it).
I also tried another piece doing everything the same just without using the nitric/ammonium bath, the voltage started at 0 but steadly climbed to 18.7V(max on my supply) then after a half hour it was down to 12V and very slowly climbing, its still in the bath now.
Sorry for the long post, I was trying to explain everything... did I miss anything?
Many Thanks.
Robert Harrison

[Fibergeek]

For one thing, you're making things tougher to measure by using a sample that small (2 sq.in.). 1 or 2 milliamps count when you are anodizing in "microscale".

If your sample is 2.0 sq.in. then (2/144) x 4.5 A/sq.ft. = 0.0625A = 62.5mA. Your anodizing current of 0.06A (60mA) is a current density of 4.32A/sq.ft. Your peak voltage calculation of 10.8 V is correct at 4.32A/sq.ft. At 4.5A/sq.ft. you would have expected 11.25V.

Notice that just 2.5mA made a difference of 0.45V.

Since I promised not to talk about the "C word", let it suffice to say that many have trouble with the "C word" until they understand what "tight" means in the DC electrical sense. Erratic voltage swings indicate this problem. 1 year old red dye would certainly be suspect. The pH of the dye changes with time, like a week or maybe less. A year? Who knows.

I don't know how effective the nitric acid / ammonium bifloride would be at removing the native oxide (essential step). I suggest you try using a sample maybe 5X larger to avoid metology issues, and some old fashioned sandpaper to strip the oxide until you get things sorted out.

[Chevalier]

I'll get some new dye tomorrow and give that a shot and let you know how it goes. If I mix the dye to the instructions will that be too weak? I've heard people say they mix half the water/twice the dye to make it stronger. any thoughts on that?
Also, what changes the pH of the dye bath, is it acid that hasn't been properly washed off the part before putting it in the dye or the water evaporating from the bath? then how do you correct the pH levels? I bought a pH tester but it is made to read the pH of soil, I figured it would work, but doesn't seem to!

Thanks for the reply.

[Fibergeek]

Assuming you are using real anodizing dye; and not RIT or something else designed for cloth dyeing, its always best to follow the manufacture's instructions, including the recommended dyeing temperature. You can experiment after you have things working reasonably well. Dyes for cloth work poorly as anodizing dyes, they weren't designed for this.

There are some recent threads here on dyeing, I suggest you read them. pH measurement and adjustment is covered in these threads.