Galvanic corrosion is the result of creating a short circuited battery composed of different types of metal electrically connected to each other (screwed together, bolted together or in some way having an electrical connections [through the electrolyte doesn't count] and immersed in a conducting fluid, the electrolyte. The farther apart on the galvanic scale (Google it) the metal pieces are, the more active the battery will be and the more rapid the destruction.
Pure water is not a good conductor (or else the power grid would not work when it rains) but adding salt (both chloride or bromide salt will do nicely - it's not so much what salt it is, it is that salt makes the water a conductor) makes an excellent electrolyte allowing a current to flow between the dissimilar metals with the result of the least noble metal being dissolved. If you have all your metals about equal on the galvanic scale the damage will be minimal. If you can tolerate the mechanical weakness, plastic is immune to these effects. The case experienced by Bob Johnson involved a brass fitting. Brass is an mixture of copper and zinc. Immmerse brass in salty water and you have a battery composed of the copper and the zinc in the brass immersed in a conducting electrolyte. The zinc will come out of the brass leaving weak, spongy, copper - a process called de-zincification - destroying the brass fitting. This process in famous for destroying the threads on gate valve stems leaving one with a gate valve stuck open or stuck closed. Bob's iron pipe would have also been attacked in the area of the brass fitting. This happened to me from the well water to the mobile home I lived in while building Bandersnatch's hull. I mixed iron and brass fittings and had a shallow well with lots of salts in the water. The iron fittings were dissolved by galvanic corrosion. If Bob had used a pure copper fitting the copper would be OK but the iron pipe still would have been attacked. The 20" pipe being a big thing, it would have taken a long time to show the results of galvanic corrosion. It would have shown up right around where the copper was screwed in. He should have used iron or plastic fittings. Norm S/V Bandersnatch Lying Julington Creek 30 07.695N 081 38.484W _______________________________________________ Liveaboard mailing list [email protected] To adjust your membership settings over the web http://www.liveaboardnow.org/mailman/listinfo/liveaboard To subscribe send an email to [email protected] To unsubscribe send an email to [email protected] The archives are at http://www.liveaboardnow.org/pipermail/liveaboard/ To search the archives http://www.mail-archive.com/[email protected] The Mailman Users Guide can be found here http://www.gnu.org/software/mailman/mailman-member/index.html
