Corrosion
Electrolysis is a form of corrosion. This is not to be confused with the breakdown of metal by mechanical means such as wear, galling, erosion, or cavitation. Electrolysis is the alteration, decomposition or breakdown of metals or alloys from persistent electrochemical reactions (or by direct chemical attack).
Simplistically Corrosion can be broken down to two possible causes: Galvanic Action or Electrolysis. It is impossible to tell which was responsible after the fact.
Galvanic Action: This is the most common cause of electrolysis in the marine environment. Galvanic corrosion is the interaction of two dissimilar metals connected in an electrolyte (salt-water). In this situation the least noble becomes the anode and corrodes. The more noble is the cathode which in extreme cases can actually be coated with the anode metal. Metals can be connected through a bonding system or directly such as Bronze prop to Stainless shaft. Thus by adding a shaft zinc the zinc deteriorates before the prop.
Electrolysis on the other hand is corrosion caused by stray current. Generally this is much more aggressive causing a lot of damage in a relatively short amount of time.
Typically when Nickel-Aluminium-Bronze (Nibral) corrodes the aluminium leaches out of the alloy leaving irregular shallow pitting in the blade surface. In advanced cases the blade edges become scalloped.
Manganese Bronze on the other hand turns copper red as the zinc leaches out of the alloy. In advanced cases the blade edges actually begin to peal apart much like the leaves of a book. Prior to this the propeller will cease to ring and only thud when tapped with a hammer.
In copper based alloys once corrosion has set in it becomes almost impossible to weld the propeller. The metal has changed sufficiently that attempting to weld the affected areas only results in creating a larger hole.
Corrosion can be aggravated by cavitation. Cavitation is the mechanical breakdown of the propeller material through the implosion of small air bubbles on the surface of the blades as a result of the water “boiling” from low pressure. If the structure of the metal is compromised by corrosion than cavitation erosion occurs much more readily.
The chromium, nickel, and molybdenum content of stainless steel are major contributors to it’s corrosion resistance. Stainless steel is interesting because it is one of the metals that, under certain conditions becomes more noble. The oxygen in the atmosphere and in moving water is sufficient to allow stainless steel to form or repair a tough transparent film of chromium-oxide that renders the metal non-corrodible. When this film is damaged under conditions where there is insufficient oxygen to repair it stainless becomes active and corrodes freely. Typically this corrosion is localized but can be very aggressive. This type of corrosion is often referred to as crevice corrosion.