TACKLING THE HALO EFFECT

Shielding underwater hardware against corrosion requires a delicate balance of cathodic protection.

- STEVE D'ANTONIO

MONTHLY MAINTENANCE

Halos in bottom paint around underwater hardware is a phenomenon I encounter on a regular basis. And just as often, I'm asked: "What causes this? Is it something to worry about?" The answers lie in an understanding of underwater metals and bonding systems.

Most fiberglass sailing vessels rely on metal alloys for through-hull fittings, struts, rudder gudgeons, hinges and so forth. In most cases, these metals are connected by wires (part of the vessel's bonding system) to a sacrificial anode or anodes, which provide what is commonly referred to as cathodic protection.

From a corrosion perspective, the anode-often called a "zinc," even though it can be zinc, aluminum or magnesium-is less noble, and thus more prone to corrosion, than the metals to which it is connected. As a result, the anode corrodes, or sacrifices itself, in order to protect the other metals.