I have a new fire place for a gas fire. What looks like a sandy deposit has appeared on each side of the insert (see photos). I thought it was a deposit from stuff coming down the (newly swept) chimney. But when I try to clean it off, the sandy area just seems to expand. So I am now wondering if the black surface of the insert has been corroded. I’m not sure if that is even possible given the insert is supposed to be cast iron. The surface of the stain is very sandy, and sandy bits come off if you rub it. I would be grateful for any suggestions as to what is going on and how to fix it. Cheers.