A bronze prop on a stainless steel shaft will corrode in seawater without a sacrificial anode.
Spring is an ideal time to inspect the sacrificial zincs that protect your boat’s prop, rudder, outdrive and other underwater metals from galvanic corrosion.
When two dissimilar metals touch in an electrolytic solution like seawater, an electrical current flows between them causing the less noble metal to corrode.
You can counteract galvanic corrosion by introducing a less noble metal, usually zinc, into the circuit. The zinc anode will corrode before the other metals, so it makes sense to sacrifice inexpensive zinc anodes to save essential and costly running gear.
Check with the manufacturer to find out what parts of your vessel are protected by zinc anodes. Possible areas include rudders, outdrives, seawater strainers, bronze through-hull fittings and parts of your engine’s raw water cooling system.
Check your zincs frequently, and replace them if you see any corrosion. If less than half the zinc remains after one season, consider replacing it with a larger zinc. It’s cheap insurance.
When replacing zinc anodes, ensure good electrical contact between the anode and the metal it’s protecting. Also, don’t paint zincs as this will diminish their performance. –Van Allen Rice
The USPS Marine Electrical Systems course contains a more thorough discussion of galvanic corrosion and sacrificial anodes. To find a course near you, contact your squadron educational officer or find your local squadron.