Erm no. I think its the aluminium oxide. If the wheels are
covered with some sort of protection such as plastics, then im not
sure. If not, it shld be aluminium oxide. The oxide prevents the
aluminium from being corroded, like steel. I got one portable CD
player that has an aluminium covering, it has since blackened alot.
Dont think is dirt leh. Dirt wash already come off. That black
thing must be scrubbed very hard in order for it to be removed.
Dirt isn't so difficult to be cleaned off.
