galvanic corrosion
Bolt two different metals together, get them wet, and one of them starts sacrificing itself to save the other — that is galvanic corrosion. It is the everyday version of the two-metal battery: because the two metals sit at different points in the galvanic series, connecting them electrically and putting them in the same water makes one the anode (which corrodes faster than it would alone) and the other the cathode (which corrodes slower, or not at all). One metal's protection is literally paid for by the other's accelerated destruction.
Three ingredients are always required, and removing any one stops it: two metals far enough apart in the galvanic series, an electrical connection between them, and a shared electrolyte (water with dissolved salts) bridging them. The area ratio then decides how vicious it is. Current from the whole cathode is funneled into the anode; if the anode is small and the cathode large, the anode's tiny surface carries a huge current density and is eaten alarmingly fast. This is why a small steel fastener in a big copper or graphite panel is a disaster, while a small copper fastener in a big steel panel is nearly harmless.
The cure follows straight from the three ingredients. Pick metals close together in the series; break the electrical path with insulating washers, sleeves, and gaskets; keep the joint dry or seal out the electrolyte with paint or sealant; and if you must paint, paint the CATHODE (the noble metal), never only the anode — a scratch in paint over the anode concentrates all the attack into one pinhole. The very same effect, deliberately turned around, becomes a defense: a sacrificial zinc anode is galvanic corrosion aimed on purpose at a cheap block so the ship's steel hull survives.
The restored Statue of Liberty is a classic warning: copper skin was riveted to an iron support frame with only a thin shellac barrier between them. Over decades salt-laden sea air soaked the joints, the iron became the anode against the huge noble copper cathode, and the frame corroded and swelled until the skin was pushed apart — a giant galvanic couple.
A large noble cathode plus a small active anode is the worst possible pairing; insulate the two metals or match them.
A common misconception is that touching any two metals is doomed — in dry air or with no continuous electrolyte there is no cell and no galvanic corrosion; it is water plus a large cathode-to-anode area ratio that turns a mismatch into rapid failure.