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.
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