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Connected dissimilar metals immersed in an electrolyte generate a voltage and the most active metal deteriorates. This is called Galvanic Corrosion.
One of nature's phenomena is the fact that two different connected metals immersed in an electrolyte (electricity carrying liquid) develop electrical voltage and current. The metal that is most active electrically, will deteriorate while protecting the metal that is less active.
If both metals are important, we can attach another metal more active than each, which will deteriorate and protect them.
The metals used on boats are normally alloys (several metals mixed together.) The higher potential (more active) metals will deteriorate out of the alloy destroying the boat fittings.
To protect boat fittings, attach a more active metal that will sacrifice itself.
It is not reasonable to attach pieces of more active metal to each piece of underwater boat meta. Connect inside the hull all of the boat metal that is immersed (see Bonding) and connect a large piece of immersed more active metal into the system.
The sacrificial metal will erode, protecting the boat metal that is bonded to it and exposed to the same body of water. (The water inside the engine is usually a different body of water. See Engine Zincs).
The Noble Scale shows which metal or alloy is more active. The list below starts with the most active (least noble). Each metal is more active than the metal listed beneath it. This is a partial list. Alloys will have a different position on the list depending on composition.
If you pick any two metals, connect and immerse them in seawater; the metal that is less Noble is more active and will deteriorate while protecting the metal that is more Noble. The further metals are apart on the list, the greater the activity. Magnesium connected to Platinum would deteriorate faster than Magnesium attached to Copper.
In electronics, the more active is called the Anode and the less active the Cathode. A Sacrificial Anode would be the metal attached to the bonding system to protect the boat. Normally, boaters use Zinc as the sacrificial anode, as it is high on the Noble Scale and relatively inexpensive. Magnesium and Aluminum are used extensively in related industries and may be specified for outboard motors. Magnesium voltages are high and can damage Steel and Aluminum. Excessive protection, while using up zincs faster, can also destroy wood, lift paint and create gases.
Shaft Zincs expend faster because of their greater velocity. This material loss does not go to protect the boat metal as the loss is through abrasion rather than electrical output.