Understanding Zinc Anodes: Protecting Your Boat from Corrosion

Sacrificial anodes — universally called "zincs" even when they are made of aluminum or magnesium — are the single most important defense against galvanic corrosion on your boat. Without them, the metal components below the waterline would slowly dissolve into the surrounding water, taking your running gear, thru-hulls, and hardware with them.

Here is how it works: when two different metals are submerged in an electrolyte (saltwater is an excellent electrolyte), electrons flow between them. The less noble metal corrodes preferentially — it sacrifices itself to protect the others. Zincs are intentionally made from a metal that is less noble than everything else on your boat, so they corrode first and spare the expensive stuff.

The catch is that they only work when they are actually there. Once a zinc has worn down past about 50%, it has lost most of its effective surface area. Past 70%, it is essentially useless. At that point, corrosion starts attacking the next-least-noble metal — often your shaft, prop, or bronze through-hulls.

Every hull cleaning we perform includes a zinc audit. We measure wear, note condition, and flag any anode that is approaching the replacement threshold. On most vessels, we replace zincs before they reach 40% wear — giving you plenty of margin while avoiding the waste of pulling zincs that still have life in them.

Our divers carry a full inventory of the most common anodes on every dive, so in the vast majority of cases we can replace a worn zinc on the spot. Your boat is never left unprotected while waiting for an order.

If you have never had a formal zinc analysis on your vessel, it is worth considering. We can measure every anode, document the protection status of each metal component, and identify any galvanic issues before they become expensive.