With an aluminium car, it is a very good idea to check the sills where the skin wraps around the tubes on a regular basis. AC and Kirkham simply wrap the aluminium around the steel and rivet it - even Astons used to get a thin layer of waxed cotton here (but that could arguably make it worse, I guess), but nothing for 'our' cars. Water, unless pure (which rainwater and road spray is not), is a conductor, so there will be an electrolytic reaction here if it gets damp. Obviously salt will increase this massively. It's well worth ensuring something is in that very fine gap to reduce this effect - either a high quality cavity wax (e.g. Dinitrol High Performance clear), or an anti-corrosive such as ACF-50 (or better, ACF-50 Corrosion Block). ACF-50 needs frequent application as it doesn't last very long, and ideally a spray gun and pit or lift. This area isn't even painted, but gets any spray from the front wheel chucked at it.