Anybody know what type of metal aggressive skaters use for sliding
on concrete? What are the properties that make it good for
"grinding".

I'm not planning to intentionally grind the concrete, but I'd like
some material that is durable and will slip on concrete. Conceivably
it can be disposable/replaceable if the replacement is not costly or
time-consuming. It is to be used as a short (about 1 inch) stopgap
between two skate wheels, so rocks and stuff (like a sharp angle
curb) cannot get in between and cause the second wheel to bind.

I'm considering using aluminum and coating it with super glue or
covering it with some material (to be recoated/recovered regularly).

Thanks.