I have a slab of aluminum and one part of the slab will be exposed to a very corrosive environment. If I put a sacrificial anode elsewhere on the slab will the section that is exposed still be protected from pit corrosion? I guess I'm unsure because the pit corrosion is a result of a localized reaction and if an anode is added will it prevent the local reaction from taking place.
Generally speaking, there is no protection unless the sacrificial anode is immersed in the electrolyte. It might be better to anodize and seal the aluminum, which should protect it against most acids. There are many aluminum anti-corrosion treatments, but in any case the area under attack would have to be covered. See http://en.wikipedia.org/wiki/Anodizing.