My experience of "chromate conversion" is that all I have seen are in fact
insulating.
In general: all aluminium passivation forms an insulating layer of a thickness.
It may be possible to measure low resistance if the measurement technique
punctures the passivation layer. Even two "flat" surfaces under pressure can
puncture a thin film of passivation and make a pasivated surface appear to be
conductive.
Earthing to passivated panels.
Aluminium panels are often purchased already passivated. If a hole is punched
(or drilled) through this panel, the inside of the hole is conductive. A bolt
and nut fastened through this hole may provide a fortuitous contact with the
side of the hole - seeming to give a conductive contact. This should not be
relied upon.
The other way an electrical contact may be made, is if the head of the bolt,
or the nut, break through the passivation layer. Again, this should not be
relied upon for earthing.
I have two brass weights with a fine-ground finish on the under-side. I will
not say it is "optically flat" but it is very close. these are each connected
to a pair of wires on a four terminal resistance meter. The panel in question
is laid flat and the two weights a lowered carefully onto the surface.This
shows if the surface finish is conductive.
I hope that this helps.
Regards
Tim
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