Wow! Pro experimental technique NeoMoses. By all means, continue!
Coating thickness measurements:
MIL-A-8625F describes a method to ascertain anodize coating thickness. Its based on determining the weight of the coating. This requires the use of a scale capable of accurate measurement to a small fraction of a gram.
Another way (what I'm using) is measuring thickness directly using an electronic eddy current gauge. This is cheaper than a precision scale, not to mention easier to do. "Easier" is a relative term; measuring a coating 0.0001" - 0.001" thick is tricky at best. I'm using an Elcometer 345, purchased used on Ebay for $425.00, plus factory calibration "shims" (0.50 and 1.00 mils thick) purchased from Elcometer. New instruments like this go for over $1K.
I have seen that a value for PAR of 2.5 Ohms/sq.ft. holds up well at 6A/sq.ft. current density. It's off at lower and higher current densities, I can't say yet how much off. To me this means that coating resistance isn't the only parameter, the coating density also plays an important part. This means that PAR should have different values based on the actual current density. It will take some time to work this out. Coating density means here, the percentage of pore area vs. solid area over the entire surface of the work. Smaller pores yield higher densities; which show higher values of PAR? I wouldn't swear that it goes in this direction yet. I expect to find some research addressing this if I dig deep enough in the technical literature. I don't want to rely on experiment alone if I don't have to.
As we all know; acid concentration and current density affect pore size, these two parameters seem to dominate, temperature is a second order parameter (weak effect) from what I've seen. There may be others.
We need to have a handle on coating density; in NeoMoses' experiment, we may be seeing the effect of too large of a pore size for deep color dyeing.
NeoMoses, you need a means to measure coating thickness to resolve this, I'll see what I can do.
|