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Cathodic Over-Protection

Posted By Ed Sherman , Monday, August 20, 2018

Back around July 4th I was asked by an old friend to look over three very expensive antique wooden boats that were just beginning to show some strange activity in the bilge areas of the boats. Keep in mind here that these weren’t a collection of derelict wooden junk heaps ready for the scrap pile, but three classic yachts that had undergone multi-million dollar restorations and that were now working in the high end charter business.

The photo above, although not from one of those boats,  illustrates some of the similar activity that was occurring on those boats. Unfortunately this is an all too common scene when I inspect wooden boats and left unattended, this can destroy a boat. What you see here is the early stages of wood delignification, and the process is also taking a more obvious toll on the bronze fasteners you see turning green. When the wood delignifies, all of its inherent strength is lost!

The problem here is cathodic over-protection, which on a wooden boat is really, really easy to achieve quite innocently, which is why I’m writing this. You see the ABYC standard E-2 that addresses cathodic protection identifies appropriate levels of cathodic protection for various hull configurations and materials. Basically the standard calls out negative voltage ranges appropriate for these boats. In the case of wooden boats, the range is -550 mV to -600 mV. therein lies the issue. You see depending upon the anode material you are using, which could be an aluminum alloy, zinc, alloy or in extreme cases in really clean fresh water magnesium anodes, the negative potential achieved by the anodes can easily exceed the potentials these alloys typically achieve. Voltage potential is a value that is achieved by adding or subtracting the amount of anode surface area when compared to the metal area we are attempting to protect. So indeed, we are creating a controlled electro-chemical reaction. The problem with wood is that the value that is considered safe is a mere -50mV. It is almost impossible to maintain that level of protection over the course of a normal boating season in a salt water environment.

So, sure enough, when I tested the three antiques they were all reading in the -900mV range. That amounts to approximately 300 mV overprotected. The only way to reduce that number is to reduce the exposed surface area of the anodes to the seawater on these boats. As I left the dock, the owners had divers down removing anodes and retesting the voltage potentials. I hope we caught it before excessive damage occurred to the wood on these otherwise beautiful boats. In my view, boats in this category should invest in a permanently installed impressed current cathodic protection system engineered to deliver the proper voltage potentials under all conditions as the needs do change if the boat is docked, underway, or the water chemistry changes. Various companies can design and get installed such a system. In the long run, these systems will be much less expensive than sending divers down monthly to service the anodes on your classic wooden yacht.

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