Friday, May 11, 2018

Protecting your boat from galvanic corrosion



  
When boats are neglected in the water for extended durations of time or are completely forgotten, submerged metal parts are subject to two different types of corrosion. They are galvanic corrosion (also known as “natural” corrosion) and electrolytic or stray current corrosion, which is commonly caused by leakage from an interior or exterior electrical flow. In both instances, water is the conductive medium. The unique metal alloys on the boat can be attacked by corrosion relatively quickly depending on which protecting components are already in place. This article concisely explains galvanic corrosion and the precautionary steps that can reduce its effects on outboard motors and sterndrives.

Electrolysis and galvanic corrosion
When two unique metals are in contact with each other and are submerged in a conductive medium (electrolyte), a direct electrical current is generated, much like a battery, producing galvanic corrosion. This current causes ions to be transferred from one metal to the other. Metals are arranged according to a galvanic corrosion graph, with the corrosion potential represented in millivolts (mV). Less useful and very electronegative metals such as zinc, aluminum or magnesium act as anodes and thus deteriorate if stored near lead, bronze or stainless steel which act as cathodes.




Thus, the immersed metal components of a boat stored in the water for a whole season can be changed by the aforementioned electrolysis phenomenon. This is the case, for example, of a sterndrive (aluminum alloy) attached to a stainless steel propeller: the unguarded sterndrive performs as an anode while the stainless steel propeller functions as a cathode. Corrosion starts with paint blistering on the borders and the establishment of a powdery substance on the surface of the sterndrive. In the long run, galvanic corrosion destroys the composition of the unprotected sterndrive.

Preventive measures
To protect from receiving galvanic corrosion, a less valuable metal such as zinc, aluminum or magnesium must be set near interconnected metal parts. The metal, named a conciliatory anode, will dissolve instead of the significant sterndrive parts that should promptly become cathodic.

There are hundreds of anodes available on the market; they come in various sizes and serve different purposes. To accurately protect your boat, know which area to use and where to place the anodes by referring to the motor or boat’s owner’s manual. A boat dealer can also provide information on anodes and will carry replacement kits for sterndrive companies such as Volvo or Mercury. You can also purchase anodes of different sizes and install these on trim tabs, shafts, rudders and any other position to ensure the cathodic protection of your boat.



Anode types and their expected lifespan
The type of sacrificial anode to invest will depend on where you drop anchor. For example, aluminum (Al) anodes can be used in either salt or fresh water. They are light but wear out quickly – particularly in salt water, which is more conductive. Zinc (Zn) anodes are approved for salt or brackish water. Finally, magnesium (Mg) anodes are more expensive and are only efficient in the fresh water.

If you’re planning on docking your boat for an entire season, take this opportunity to replace the anodes. Note that, in some cases, anodes may need to be reinstated more than once a season: for example, if your boat is presented to electric current leakages stemming from shore power hookups, internal leakage or electrical leakage from a boat docked nearby. As a general rule, replace anodes when they are 50% depleted.

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