Sacrificial Anodes and Pit Corrosion
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.