What are you cooking with? Just charcoal, or do you add hardwood? Do you have a picture of it? I'm trying to visualize if you are talking about the bottom of your grill or if you are talking about the little grease trap ashtray under it.
If you leave your charcoal grill out in the elements chances are the rain water is mixing with the ash and creating lye, a caustic chemical that does a great job of cleaning cast iron, but will obliterate aluminum. Is your ashtray aluminum?
If so I would wrap your ashtray in aluminum foil as a protective barrier and replace it when you empty out your ashtray.
If the problem is with an iron grill, you might not have good quality cast iron, in which case rust will wreck havoc with your grill if you don't clean it out and keep it dry (remember cheap hibachi grills?).