Since these rads are made from copper, brass and tin, it's likely oxidation of one of those metals or primer for the black paint. These aren't made from steel that would rust, although rust is the form of oxidation for steel. If you soak the rad in some 50/50 distilled water and white vinegar, it might dissolve (if oxidation) otherwise, it is likely primer or some kind of powder coating for the paint finish.
Edit: Also, for your images you have...if you want them to display on the page as a pic vs. a link...you can add the
and it will display as a thumbnail on your post.