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What you should know about
Corrosion
Did you know that your boat could be sinking at the dock while you’re sitting at home watching TV? The elements are working continuously, 24 hours a day, every day – gradually breaking down everything that is exposed to them. For metallic components, this results in what is commonly referred to as “corrosion”. What is corrosion? There are actually several types of corrosion that particularly affect boaters. If you understand the causes, there are things you can do to mitigate the effects. Here are a few of the things you should be aware of:
► Galvanic corrosion results from dissimilar metals being in contact in an electrolytic solution. Water can act as an electrolyte, and salt water is a much more effective electrolyte than fresh water. This is why it’s important to wash your boat down with fresh water after every voyage – not just so it looks shiny, but to remove as much of the salt as possible.
A good example of galvanic corrosion can be seen when a stainless steel fitting is attached directly to an aluminum component, such as a spar. Over time, the aluminum will corrode in the area around the fitting. Technically, it is “giving up ions” to the stainless steel, which is a “more noble” metal. The effect can be reduced by isolating the fitting as much as possible, such as putting a coat of silicon over the aluminum before attaching it.
A better solution is not to put dissimilar metals in direct contact, especially when they are exposed to constant salt water contact. If you’re working on your boat it’s not a bad idea to get a copy of the Galvanic Scale to know which metals are compatible and which are not.
► Stray current corrosion results when two metallic objects in an electrolyte are subjected to different voltages. The current that attempts to flow between the objects causes one object to give up ions to the other object, and corrode. This condition can arise when two components of a boat have different charges, such as when one component is connected into the DC electrical ground (such as the engine block on an inboard engine), and another component is not (such as the rudder). In this case, the rudder will appear to corrode away, sometimes very rapidly.
Stray current corrosion can also occur when two vessels in close proximity (i.e., your dockmates) have different ground potentials. This is usually the result of faulty wiring. It could result in components of your boat, acting as anodes, corroding away while your neighbor is perfectly fine – or vice versa.
One solution to stray current corrosion is to use sacrificial anode made of elements like zinc that are low on the galvanic scale. The anode will corrode away instead of the important (and expensive) components of your boat. That’s why anodes need to be inspected regularly and replaced well before they disappear entirely. Another way of minimizing the effects of stray current corrosion to is to have an effective bonding system. Bonding insures that all metallic components of your boat that are in contact with the water will have a common ground.