Zincs and Anodes

Sacrificial Anodes and Galvanic Corrosion

Whenever your boat's hull is submerged in water-salt or fresh-there is the danger that various metal components will be eaten away by galvanic corrosion. This chemical reaction occurs when different metals are touching one another or are electrically joined together by a conductor in an electrically conductive fluid (an electrolyte). The least noble metals are referred to as active and are electrically positive in polarity, while the most noble metals are passive and electrically negative.

Sacrificial anodes can extend the life of your boat's hull, engine, rudder, propeller shaft, engine cooling system, refrigeration condenser and other components by protecting them from the deterioration caused by galvanic corrosion. When dissimilar metals are joined together in an electrolyte, the voltage or potential difference between the two causes formation of a galvanic cell in which one metal (the anode) will corrode while the other (the cathode) remains unharmed. Simply put, because anodes-usually zinc or aluminum-are made of a less noble metal than that of the parts they are designed to protect, they corrode instead of the components.

Galvanic self-corrosion can also occur in a single piece of hardware that's immersed in sea water and not in contact with a more noble metal. This happens when electric current flows internally, near the surface, between the bits of the less noble of an alloy's components, for instance zinc, and a nobler, copper component. Because it is less noble, the zinc can be eaten away, leaving a spongy copper residue. That is why it is so important to use marine-grade fasteners in underwater applications, so they are not destroyed, allowing a hose clamp to fail or your propeller to fall off. The higher the salinity and temperature of the water in which your boat floats, the greater the danger that corrosion will occur.

 

Two excellent resources for more information on galvanic corrosion-what it is and how to combat it-are Nigel Calder's maintenance "bible," the Boatowner's Mechanical and Electrical Manual, and the more specialized book, Your Boat's Electrical System, by Conrad Miller and E.S. Maloney.