Definition
A sacrificial anode is a block of an electrochemically active alloy fitted to underwater metal or inside an engine's cooling passages, which corrodes preferentially so that the metal it protects does not.
Anodes are consumed deliberately. They are the cheapest component in the corrosion system and they exist so that drive legs, shafts, propellers and heat exchanger internals are not.
Alloy follows the water
Different alloys are specified for full seawater, brackish and fresh water, and an alloy that performs in one performs poorly in another. This matters in Australia more than owners expect, because so many boats live in rivers and estuaries where salinity moves with rainfall. See how to choose the right anode.
Wear rate is a diagnostic
Anodes should waste steadily across a season. Gone in weeks indicates stray current or a bonding fault, and fitting more anodes treats the symptom. Barely touched usually means the anode is not making electrical contact and is protecting nothing.
Pencil anodes
Engines often carry internal anodes in the raw-water passages and heat exchanger. These are easy to forget entirely, and an engine with neglected pencil anodes corrodes internally where nobody looks.