Copper & Aluminum
Tom,Aluminum (the metal most used in lower units and outdrives) is a less "noble" (acts like a zink) than copper. What happened to you lower unit is that the aluminum in your lower unit was the "zink" that protected the copper based bottom paint. Tin based bottom paints must be used on outdrives and lower units to prevent the kind of corrosion you experienced. Tin is less "noble" than aluminum. Aluminum boats also use tin based paints. A penny dropped into the bilge of an aluminum boat will eventually leave a penny sized hole in the boat! Other posts have the right idea regarding outboards - raise 'em out of the water whenever possible.Here's a table - metals at the top are the most active or least noble:Magnesium and magnesium alloysCB75 aluminium anode alloyZincB605 Aluminium anode alloyGalvanised steel or galvanised wrought ironAluminium 7072 (cladding alloy)Aluminium 5456Aluminium 5086Aluminium 5052Aluminium 3003, 1100, 6061, 356CadmiumAluminium 2117 (rivet alloy)Mild steelWrought IronCast ironNi-Resist13% chromium stainless steel, type 410 (active)50-50 lead tin solder18-8 stainless steel, type 304 (active)18-8 3% Mo stainless steel, type 316 (active)LeadTinMuntz metalManganese bronzeNaval Brass (60% copper-39% zinc)Nickel (active)78% Ni-13,5% Cr-6% Fe (Inconel) (active)Yellow brass (65% copper-35% zinc)Admiralty brassAluminium bronzeRed brass (85% copper-15% zinc)CopperSilicon bronze5% Zn-20% Ni-75% Cu90% Cu-10% Ni70% Cu-30% Ni88% Cu-2% Zn-10% Sn (Composition G-bronze)88% Cu-3% Zn-6,5% Sn-1,5% Pb (Composition M-bronze)Nickel (passive)78% Ni-13,5% Cr-6% Fe (Inconel) (Passive)70% Ni-30% Cu18-8 stainless steel, type 304 (passive)18-8 3% Mo stainless steel, type 316 (passive)Hastelloy CTitaniumGoldPlatinumGraphiteNotice that aluminum is near the top and copper is roughly in the middle, aluminum will "protect" the copper.Rodney KiddC-38 #297, Flying Bear