Why does my outboard or outdrive have corrosion?

There are basically two types of corrosion, the first being stray current and the other galvanic.  Galvanic corrosion occurs when there is an exchange of electrons from dissimilar metals.  Stray current corrosion happens when electrical current flows through water in search of ground.

To help combat these problems, engine manufacturers have installed sacrificial anodes on various components of the engine/outdrive.  The anodes need to be inspected regularly and replaced as needed.

Because outboards and outdrives are made of aluminum and most bottom aint-fouling paints contain copper, you should never use this paint on your outboard or outdrive.  There are special paints made for this application.  Anodes should never be painted.

If your anodes are getting eaten up quickly or the aluminum has excessive corrosion, the first thing to check is to make sure the anodes are properly groundedto the unit, and the unit is grounded back to the battery.  The next step would be to have the water tested at your dock for stray currents.  This is done with a special reference electrode and tester.