"The destruction of a metal or alloy by chemical or electrochemical reaction with its environment."
It is fairly difficult to glance at a failed metal boat part and tell immediately what caused that particular failure. Most likely the failure was
caused by a combination of reasons, including wrong choice of alloy by the manufacturer, manufacturing error, such as overheating, contamination or
wrong coating, error in application by the boat builder, water velocity, impurities, or pollution in the electrolyte (sea water), temperature,
vibration, stress, crevice, galvanic or stray current corrosion.
A natural phenomenon is that two different connected metals immersed in sea water, or electrolyte, develop voltage and current.
The metal that is most active electrically (more positive) will deteriorate while protecting the metal that is less positive.
If both of these metals are important to us, we can attach another metal, more positive than the other two, which will deteriorate first and protect the more important metals.
This sacrificial metal will erode, protecting the boat metal that is bonded to it and exposed to the same body of water.
STRAY CURRENT CORROSION (commonly called ELECTROLYSIS):
Stray current corrosion is similar to galvanic corrosion, except that the voltage and current flow is created by an outside electrical source rather than spontaneously
in nature, normally with much more force than galvanic action and can deteriorate our valuable boat metal in a very short period of time.
Turbulent water, high velocity water (especially at bends in cooler tubing), and silt laden water will all cause erosion of metals. Uneven
velocity inside a tube or across metal surfaces of boats in tidal water or drifting will also cause erosion.
Propellers have several special problems, in that they have large uneven surfaces exposed to tidal currents which causes temperature and
electrical differences on different areas of surface, and also differences in voltage due to the different speed between the hub and blades.
Besides balancing the shaft and propellers
, bonding, using shaft straps and an anode system will normally keep
erosion to a
minimum by evening out the voltage over the entire propeller
(click image to enlarge)
Los Angeles Harbor (San Pedro, Wilmington, Long Beach-Shoreline, Alamitos Bay), Huntington Harbor, Newport Beach and Dana Point.