I have also done experimenting with determining surface area this way. There are a couple of problems that make it just too risky. One is the alloy. Different alloys have different current densities. Some alloys like 2024 may take 20 volts to produce a 12 amp current density wile others like 6061 may need only 18 to produce the same 12 amp current density (I'm just pulling numbers out of the air because I don't have my notes with me). Another problem is temperature. Temperature affects current density DRASTICALLY. In my experimenting, I concluded that if you are going to do it this way then it is better to use constant voltage (no more that 8 volts for a stable current reading) and use the current reading for calculations. After that you can switch back over to CC for the anodizing. You would have to take a current reading at the same time every time to be accurate because the current will be rising constantly as the anodize process carries on. Temperature needs to be held within a few degrees (+/- 2 F). If temp can't be held that close then you would also have to know current densities at different temps. You would also have to know the current densities for the particular alloy that you are doing, as they are all different. And finally my numbers for current density won't work for any one else because of differences in ano bath chemistry. You ano bath chemistry is changing slightly all the time from evaporation, rinse water drag in, dissolved alum, etc. so you would have to perform current density test on a regular basis to stay accurate.
It is not impossible to determine surface area this way; it just seems to me that it’s just not practical.
In my opinion there are just too many variables for this to have great success 100% of the time. But, I would love to be informed of any discoveries you may have while pursuing this.
I'm not trying to discourage you; I just thought you should be aware of the obstacles ahead.
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