What is galvanic corrosion?

Galvanic corrosion occurs when different metals immersed in a conductive electrolyte (salt or fresh water) produce some measurable current flow between eachother.  One metal corrodes preferentially when it is in electrical contact with another. The more corrosion resistant metal (stronger, more “noble”), i.e. the NiBrAl propeller, has the electric current exiting and acting as a cathode, while the less corrosion resistant metal (weaker, less “noble”), i.e. the zinc anode, has electric current entering and acting as the anode.  This flow of current causes the zinc anode to erode while the propeller does not. 

To predict galvanic corrosion, refer to a galvanic chart, where metals are listed from strongest to weakest.  Reactions and thus corrosion can take place between bronze through-hulls, steel screw-heads, steel prop shafts, aluminum propellers, steel and aluminum outdrives and copper-based antifouling paints.