Unless preventive measures are taken, the consequence will be mayhem and eventually murder of the lesser (galvanically speaking) of the two metals. Demise can occur with surprising violence. In plastic sailboats, the most common victim of galvanic corrosion is a bronze propeller on a steel shaft. Rudder fittings, metal struts, and bonded through-hulls are also at risk.
The only way to stop galvanic corrosion from occurring is to make sure all underwater metals are identical or to prevent electrical contact between dissimilar metals, but both of these measures can prove to be impractical. However, we can give corrosion an easy mark, deflecting its violence away from essential underwater components.
Sacrificial anodes, properly installed, are 100 percent effective. But the nature of their task means they have a limited life span, not unlike filters for oil and fuel. If you wait too long to replace depleted zincs, it is your inaction that bears responsibility for bad things happening to your boat’s underwater metal components.
The amount of protection a sacrificial anode provides is directly related to its surface area. The anode surface area required varies with the kind of metal being protected and with the chemical makeup of the water, but you can use one percent of the surface area of the protected metal as a starting point. Check the protected metal frequently. If it shows signs of corrosion despite the zinc, you need more surface area.
|"There is an unfortunate misconception that a sacrificial anode can be mounted anywhere, even hung over the side on a string, and it will still perform its appointed duty."|
Of course, the need to compute anode sizes is far less common than simply replacing depleted zincs with new ones of the same size. You should check all zincs at least annually—more often until you have a sense of their rate of depletion. Replace anodes when they are half depleted. Unless you don’t mind changing anodes underwater, it is generally a wise investment to replace even moderately depleted zincs at every haulout.
There is an unfortunate misconception that a sacrificial anode can be mounted anywhere, even hung over the side on a string, and it will still perform its appointed duty. That is dead wrong! For an anode to provide any protection, it must be in electrical contact with the metal being protected. The conductivity of the water is not adequate. We need low-resistance, metal-to-metal contact—either by mounting the zinc directly to the metal being protected or by connecting the two with a wire. A hanging anode can provide protection if it is connected by a wire to the metal being protected.
If the through-hulls and other underwater fittings of your boat are bonded together electrically, they should also be connected to a sacrificial anode, usually in the form of one or more zinc plates bolted to the hull. The mounting bolts for these anodes should be connected by heavy-gauge electrical cable to the bonding circuit. For underwater components that can be isolated, no good can come from bonding them, but that is the subject for a different article. If your boat is bonded and you allow the protecting anodes to deplete or the electrical connection to deteriorate, the bonded components will begin to corrode.
Zinc plates are also fitted to metal boats to protect the hull. If you own a metal boat, this is a subject in which you should already have expertise. Never paint anodes. They cannot perform their function unless they are exposed. Putting bottom paint or any other coating on a zinc smothers it, rendering it useless.
So back to our whodunit that isn’t, if you discover your prop or through-hulls or rudder gudgeons or heat-exchanger tubes full of holes, YOU know who figuratively pulled the trigger, don’t YOU?
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