Alumina (corundum) is too hard relative to any plastic--that is why it's getting stuck. Switch to something else.
Vinegar will not work! If your plastic is not a polyamide or a polyester that can saponify and otherwise hydrolyse, perhaps a concentrated solution of sodium hydroxide can solve your alumina woes?
I use hot, concentrated lye to remove silica, alumina, and aluminasilicates from incoming materials (i.e. picture platinum crucibles and fusion boats that have glass stuck to them).
Steam the parts, and then base digest to form aluminate Al(OH)4-
I'm pretty sure Stepan is right, alpha alumina is acid insoluble (gamma alumina will dissolve in HF/H2
)To answer your original question
: the cheapest way to test to see if it is on the surface would be an optical microscope and looking for the abrasive. Otherwise, get a portable XRF gun and see if it has aluminum in it: http://www.niton.com/?sflang=en