Sacrificial Anode or “Zinc”

 A sacrificial anode is an inexpensive mass of metal, traditionally made of zinc, sometimes aluminum, which is electrically connected to the more valuable metal components of a boat, like the propeller or shaft. It protects these parts by “sacrificially” corroding in their place.

 When two different metals are immersed in an ELECTROLYTE (a liquid that can carry electricity) such as salt water, they set up an electrical voltage and a current flows between them, just as in a battery. The most “active” metal (the ANODE or positive) deteriorates by losing positively charged metal ions into the electrolyte (corrosion) and the less active metal (or CATHODE) is protected. This is known as GALVANIC corrosion. If you want to protect both metals (e.g. stainless shaft and bronze propeller) you need to connect a third metal that is more active than the first two. This is where the term “zinc” comes in; it is one of the most active metals and is commonly used as a sacrificial anode.

Order of Activity of Metals:

 Although zinc is the most common material, aluminum and magnesium are more active and can provide additional protection.

 The special aluminum/indium alloy used for sacrificial anodes is better than zinc for a number of reasons:

• It provides a bigger protection voltage (more active).

• It “self cleans” in freshwater. Zinc can form an oxide coating after a few months that insulates it and makes it ineffective.

• It has far more current capacity for the same volume. Thus, an aluminum anode will last 50% longer than the same anode made from zinc.

• It is environmentally friendly. Zinc is a “heavy metal” and causes pollution.

• It is 1½ times lighter than zinc.

Magnesium is too active for most boat applications. If used on a boat with a sterndrive in salt or polluted water, the very low voltage causes the formation of hydrogen bubbles under the paint. This results in the paint being blown off the sterndrive unit in as little as a few hours and then very rapid corrosion of the unit. Magnesium also has a very low capacity – only about 30% compared with zinc.

 Aluminum is the ideal, compromise anode material for all applications. It is safe in all types of water and all types of application and provides longer and better protection than zinc or magnesium.