Im trying to determine the cause of erosion of the centerboard pivot pin from Arcturus, our 36-foot Soverel keel/centerboard cutter. It is a -inch diameter silicon bronze pin installed new in 2004. I removed the centerboard a few weeks ago and was surprised to see that the pin was heavily corroded. Ive attached a few pictures of the pin. The boat has a fiberglass encapsulated lead keel, with the lead bedded in a resin/sand mixture. The pivot pin fits in a hole drilled through the keel and centerboard trunk (which has encapsulated lead on both sides).
The lead in the keel has the SSB antennae grounded to it, and the mast has a lightning grounding cable attached to it. The DC and AC circuits on the boat are grounded to a separate sintered bronze plate located about 4 feet from the keel. She is fitted with a 50A galvanic isolator on the AC shore power cable.
There is also a zinc installed on the hull, about 5-feet from the keel, that is attached internally to a shaft brush on the stainless steel prop shaft. The shaft is isolated from the engine and carries a MaxProp on the end that the zinc is intended to protect. The bottom paint was eroded around the zinc for about 3 inches, and that area was heavily fouled. The pivot pin is located about 3 inches above the bottom of the keel. The through hulls on the boat are not bonded.
We spoke with Steve DAntonio (stevedmarineconsulting.com), and technical editor Ralph Naranjo, and both agreed that a full diagnosis would require a corrosion analysis, including a survey of the boats anodic protection system using a silver-chloride reference electrode. Although stray current corrosion caused by poor DC connection is a possible cause, both suspected the more likely culprit was galvanic corrosion, which is caused when dissimilar metals of different electrical potential are immersed in saltwater. As DAntonio explained:
If the pin is silicon bronze-and thats hard to confirm as there are many grades of bronze often sold as one or the other-and its in contact with a bushing (likely 316 stainless steel) in the centerboard, the bronze would be anodic to the stainless steel in much the same way a zinc anode is anodic to a propeller or strut. The bronze would, slowly, sacrifice itself to the stainless steel, making this a case of galvanic corrosion.
A stainless bushing is also harder than the bronze pin, so some wear would occur over time as well. Id also recommend a careful inspection of bilge wiring for pumps and float switches. When there is stray-current corrosion, this is usually the source. The pin should be replaced using a known source of silicon bronze or, better yet, monel.
Naranjo echoed the concern that your pin might not be silicone bronze-naval bronze or phosphor bronze-based on the signs of dezincification, the process by which galvanic corrosion dissolves the less noble zinc. He also pointed out that the pin is following a wear pattern typical of centerboard boats with deep draft boards that have a lot of leverage. Based on your description he was also concerned that the pin seems to be excluded from the grounding/bonding. Ideally, all these metals should be tied to the green wire ground system. Finally, he emphasized that centerboard pins should be pulled and inspected at five-year intervals and prior to any major voyage.