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Understanding Galvanic Corrosion:
When two dissimilar metals in contact are placed in an electrolyte (an electrically conductive substance, ie seawater or even fresh lake water), electrons begin to flow (just like the plates of a battery), resulting in an electrical current. This action kickstarts Galvanic Corrosion. The metal that loses electrons is called the "anode" and the metal attracting the electrons is called the "cathode". Galvanic corrosion will eventually lead to the complete and utter disintegration of the anode!
Which metal will become the anode, and which will be the cathode?
As you can imagine, in a potential galvanic corrosion situation, the metal that is highly active (ie suspectible to corrosion) will become the anode, while the noble (corrosion resistant) metal becomes the cathode.
Obviously, if you have good quality boat parts, they will be made of corrosion resistant metals (ie, stainless steel) or if they aren't corrosion resistant, they'll be at least coated with a good layer of paint! But as we have learned, virtually all metals will eventually corrode, and paint does get damaged - causing the metal underneath to become vulnerable.
Using Galvanic Corrosion to fight Electrochemical Corrosion!
As mentioned earlier, when your boat's metal is placed underwater, chemical reactions begin to occur between your metal and oxygen dissolved in the water. Even if your metal is corrosion resistant, due to Electrochemical Corrosion, the "rusting process" will eventually happen!
However, by placing a highly active metal nearby your less-active metals, this will cause galvanic corrosion to occur in the more active metal. This sacrificial active metal anode "rusts" preferentially, instead of your precious boat bits (now called the cathode).
In other words, the "rusting process" likes to oxidize whatever metal it can get its hands on. The sacrificial anode is more easily oxidized than the metal that it is protecting.
