Surveyor Colin Brown answers a reader's question about copper antifouling. Got a question? Email firstname.lastname@example.org
Q: I was wondering about maybe going down the Coppercoat route for my 32ft steel Wylo II. Is copper antifouling a practical proposition for a steel boat? From the electrolysis point of view, when I get to the berth, will I just find a hole in the water and lots of bubbles? I’m afraid I slept through most of my science lessons at school, so expert guidance would be appreciated!
Colin Brown replies: “Normally I’d say that copper and steel should never be in contact underwater. On the galvanic scale in salt water copper has a potential of -0.4V and mild steel varies between around -0.6V and -0.8V. That’s enough of a difference to form a galvanic cell with steel as the anode. This would lead to corrosion of the steel.
“However, Coppercoat consists of copper powder suspended in resin. Metallic powders do not conduct electricity as discovered in 1890 by Eduard Branley, a French pioneer of radio telegraphy. As such Coppercoat claims that their copper antifouling is electrically inert and will not contribute to galvanic or electrolytic effects and are suitable for all hull materials including aluminium, stern drives and sail drives.
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Here’s PBO’s guide to preparing for, choosing and using antifouling paints
It was October 2015, I was 30 years old, and dreaming of sailing the world some day, perhaps even owning…
“The steel surface should be prepared and primed as required by the primer you use. Normally that would be an epoxy or similar coating.
“That would give an adequate film thickness to cover the steel and keep oxygen and water out. The film thickness is important because even the high spots on the steel have to be covered to prevent oxygen and water reaching the surface.
“Note that none of the above mentions electrolysis. Electrolysis is the splitting (lysis) of a compound using electricity and has nothing to do with corrosion but will also denude metals underwater, particularly in seawater, when electrical current passes between them.”
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