What is the difference between galvanic and stray current corrosion?

Two dissimilar metals immersed in an electrolyte will start to flow electrons from one to the
other via the electrolyte.  The metal giving up electrons is known as the anode and the metal
receiving the electrons is the cathode.  The two metals are a galvanic couple.  In the case of a
vessel in the water, the electrolyte is the salt water.  The common metals found under water on
a vessel are bronze, stainless steel, aluminum, lead keels and sometimes iron or steel.  The
more noble (cathodic) metals, like stainless steel and iron will draw electrons from less noble
(anodic) metals like aluminum or zinc.  The result is the destruction of the anode or galvanic
corrosion.  If the bronze through hulls and seacocks become the anode to the stainless steel
propeller shaft, the through hulls will deteriorate.  This can sink the boat.  If the aluminum
outdrive becomes the anode to the bronze through hulls, goodbye outdrive.  It’s survival of the
fittest.  Zinc and magnesium are the least noble of most marine metals and are commonly
mounted on the bottom of the vessel to become the sacrificial anodes.  Keeping large metal
surfaces under water coated with paints or epoxies also helps.  Typically, a vessel with zinc
anodes will have an underwater current betweens the zincs and more noble metals of less than
0.3 or .4 volts and measured in milliamps.  It’s not much, but remember, it’s working 24/7.
Imagine that your bilge pump float switch starts to leak and 12 volt DC current is introduced to
the salt water standing in your bilge.  It wants to go to ground, so it flows through the salt water
in the bilge to a thru-hull fitting then outside the hull via salt water to the propeller shaft and
finally to the motor and back to the negative terminal of the battery.  This is much more serious
because now instead of a couple hundred millivolts, we’re talking 12.5 to 14 volts.  This is stray
current corrosion, and has been known to destroy a through hull in a matter of hours, causing
boats to sink.  An aluminum outdrive can be eaten to just the internals in a matter of a few
weeks.  Photo below of stray current destruction of lower IO drive from a defective bilge pump.
From the Surveyor's Notebook