drb1025
Guru
Generally speaking I can see no reason not to fresh water flush. The conductivity of seawater and the effect the salt ion has on metal is undeniable, it's why fresh water engine rooms often look like new after 20 years. Keep this in mind, however, for heat exchangers equipped with zinc anodes, when exposed to fresh water zinc anodes, all of them, develop a coating or scale that essentially puts them to sleep, they stop working. The coating can be removed using a stiff, non-metallic brush or Scotchbtite pad. Therefore, if you flush with fresh water and it remains in the heat exchange, the zinc anodes will not work as well, or at all thereafter, unless cleaned or replaced, which could be counterproductive.
The same is true for hull anodes, vessels that operate or are stored in fresh or brackish water will face the same issue. Zinc anodes are designed for salt water use only, magnesium anodes are designed for fresh water use only,. Aluminum anodes can be used in fresh, salt or brackish water. The only caveat, aluminum anodes tend to be more active, they develop a white froth on their surface that does not impede there effectiveness, however, where pencil anodes are concerned this material can make it difficult to remove them.
How long may a vessel operate in fresh water before a zinc becomes ineffective in salt water? I occasionally go from salt water in Puget Sound and through the locks to fresh water in Lake Washington. I recently hauled out in fresh water and had all the zincs replaced. Did the short trip back to salt water render the new zincs ineffective? Thanks