Stray current corrosion has grown from being a relatively unknown phenomenon 40 years ago to a significant problem today. This is because of the proliferation of electrical hardware and systems on boats, along with the almost universal use of shorepower at all marinas.
Stray currents are usually generated by faulty electrical wiring and equipment both within and without the boat, and act like the applied electric currents described above that cause corrosion to the metal parts (the electrodes) immersed in the surrounding water (the electrolyte). Stray current generation inside the boat is usually caused by leaks to ground from such things as frayed insulation, saltwater, or salt bridges between positive terminations and ground, or current leaks into bilge water. Undersized wiring can create unbalanced potentials in grounding systems, which also can lead to generation of stray currents.
Sources of stray current outside the boat include faulty marina wiring and faulty wiring in your neighbor’s boat. Stray currents can affect your boat even if you aren’t connected to shorepower, in that they can enter your boat via a metal through-hull and exit elsewhere via a metal through-hull if they are bonded together.
In general, stray currents are much stronger than galvanic currents and will therefore cause corrosion to proceed at a much faster rate.
As noted above, if two or more metals are joined as part of the same corroding electrode, the least noble metal will corrode first so those zincs do provide some measure of protection. Keep in mind, though, that strong stray current may cause loss of zincs in a matter of hours instead of the months you would expect from galvanic corrosion. Once the zincs are gone, expect the next metal down the series to follow the zincs.