How it works
Connect two metals in an electrolyte and the more active one corrodes preferentially — the mechanism laid out in the galvanic series. A sacrificial anode occupies the active end of the series by design: current flows from anode to structure through the electrolyte, polarizing the protected steel to a potential where its own corrosion effectively stops. Zinc and aluminum alloys serve in seawater and soils; magnesium, the most active, serves where the electrolyte is more resistive, as in fresh water and many soils.
What makes it fail quietly
Three things, all inspectable: consumption — the anode is designed to be eaten, so a wasted anode is a system doing its job right up until it is gone; connection — a corroded or painted-over bond breaks the circuit and protection stops without any visible change; passivation of the anode itself — some anode alloys can film over in the wrong chemistry and stop delivering current. Anode surveys — condition, continuity, and potential measurements on the structure — are what separate a protected asset from one wearing dead jewellery.
With coatings, not instead of them
On tanks, hulls and buried structures, anodes and coatings work as a pair: the coating does the bulk of the work, and the anodes protect the defects and holidays the coating inevitably has. Sizing anodes for bare steel areas is what keeps the pairing economical.