A concise Article on Understanding "Galvanic Corrosion" for your review.
Galvanic Corrosion
Noble metals are those that are chemically inert when it comes to oxygen: they don't rust. Most metals have a certain degree of nobility, from graphite and platinum on the "most noble" end of the scale to magnesium and zinc on the "least noble" end of a list of metals called the "Galvanic Series." When two dissimilar metals, like bronze and aluminum, are in close proximity to each other in an electrolyte, like saltwater, a weak wet cell battery is formed. An electrical current flows from the less noble metal, the aluminum boat hull, to the more noble metal, like a stainless steel propeller shaft or an outboard motor's steel lower unit. This flow corrodes the less noble metal.
Metals Less Noble than Aluminum
The lowest two of the bottom three metals on the Galvanic Series are magnesium and zinc. Third from the bottom is aluminum. This means that if you have an aluminum boat in salt water and that boat has an outboard motor, an electrical current will form between the the outboard and the aluminum hull of the boat. The instant that current begins to form, galvanic corrosion begins. Since the steel is more noble than the aluminum boat, the aluminum will begin to corrode.
Read more: The Effects of Saltwater on Aluminum Boats | eHow.com
http://www.ehow.com/info_8568042_effects-saltwater-aluminum-boats.html#ixzz2IVmzhKsT