Corrosion

The proper term for corrosion that boaters must guard against is Galvanic Corrosion.
For galvanic corrosion to occur, three conditions must be present:
•Dissimilar metals must be present and in contact. Example: a bronze propeller and a stainless steel shaft.
•The metals must be in contact
•The metals must be exposed to an electrolyte such as salt water.

Shaft wipers are small contacts attached to a propeller shaft inside the boat. From these spring loaded contacts, ground wires lead to through bolts securing plate anodes mounted outside the boat, often on the back transom or under the hull. When these contacts fail or an issue arises with the anode, the props are a a very high risk of corrosion.

Corrosion on trim tabs is common. Anodes becoming loose an degrading clean metal to metal contacts allow corrosion to effect the trims. Often its the critical hinges and rivets that are effected. Left untreated the trim and the piston that raises and lowers the trim position are lost.

This video show very serious corrosion effecting a stern drive. The cause was wiring inside the boat being improperly changed around.

Anodes (zincs) that are working properly can at times stop working. Reasons for this include oxide build up between metal to metal contact, being loose and ground wires needing to maintained. When this happens any metal that is supposed to be protected is at a high risk of corrosion.

Propellers are often made of Bronze. Bronze is an Alloy made primarily of Copper, Tin, Zinc and or other metals.
Dezincification is a form of corrosion and weakening of bronze objects in which zinc is dissolved out of the bronze alloy.
Dezincification is the selective loss of zinc from the bronze alloy, leaving behind a porous, copper-rich structure that has little mechanical strength. In this video you will see how simply wrapping up some rope is enough to break large pieces off the propeller blades

A common form of marine corrosion that does not involve two metals in contact with each other is Crevice Corrosion.

This type of corrosion most often effects fasteners (screws) that secure non metal units such as knot meters and transducers to the transoms of boats as well as plastic propellers on electric boats.

In each case, a minute amount of water (electrolyte) is trapped and does not exchange. With time, the water in contact with the metal becomes increasingly acidic until it becomes akin to battery acid.