Easy way to find out, get some muriatic acid and some q-tips , dip the q-tip into the acid and use it like a small paintbrush , hit the areas where the Aluminum is stuck at, it will start bubbling and turning black, it may take a couple of applications but all the Aluminum will come off , I have done this on dozens of plated cylinders with good success. Muratic acid washes off with water and you can neutralize it with baking soda.