Crevice corrosion only occurs with the presence of an electrolyte like salt water. If the chain plates are bedded properly, no salt water will get in. The problem with chainplates is usually at the junction where they pass through the deck or up against the cap rail. Movement of the chain plates breaks the seal and the allows water in but excludes oxygen. I like LifeCaulk or Butyl. Both remain flexible and cope with movement of the chainplates as good as anything. Dolphinite is an organic compound invented sometime around Nelson's victory at the Nile. Works okay on wooden boats but has been surpassed by more modern caulks. For one thing, it dries out over time and then any movement will break the seal. On an FRP boat, it's claim to fame is its easy to work with.