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Aluminum doesn't rust, but when you put an aluminum boat in saltwater you may find out that, under the right conditions, it corrodes. Understanding why it may corrode gives you most of the information you need to prevent that corrosion. You can prevent that corrosion with an old and straightforward solution.
Noble metals are those that are chemically inert when it comes to oxygen: they don't rust. Most metals have a certain degree of nobility, from graphite and platinum on the "most noble" end of the scale to magnesium and zinc on the "least noble" end of a list of metals called the "Galvanic Series." When two dissimilar metals, like bronze and aluminum, are in close proximity to each other in an electrolyte, like saltwater, a weak wet cell battery is formed. An electrical current flows from the less noble metal, the aluminum boat hull, to the more noble metal, like a stainless steel propeller shaft or an outboard motor's steel lower unit. This flow corrodes the less noble metal.
Metals Less Noble than Aluminum
The lowest two of the bottom three metals on the Galvanic Series are magnesium and zinc. Third from the bottom is aluminum. This means that if you have an aluminum boat in salt water and that boat has an outboard motor, an electrical current will form between the the outboard and the aluminum hull of the boat. The instant that current begins to form, galvanic corrosion begins. Since the steel is more noble than the aluminum boat, the aluminum will begin to corrode.
Galvanic corrosion in aluminum boats requires two things. First, there must be two dissimilar metals and second they must be close to each other in the saltwater. If your aluminum boat is a rowboat, there's no engine. Instead, the boat is powered by oars made of wood, plastic or even aluminum. Rather than being the anode in a weak battery, corroding as it feeds an electric current to the motor -- the battery's cathode -- the aluminum boat goes merrily on its way no worse for wear, unless it's moored next to a boat with a motor.
A Wall of Cathodic Protection
Since not all aluminum boats used in saltwater are rowboats, it's necessary to impose a wall between the aluminum boat hull and the steel outboard motor. Since you don't want the aluminum boat to act as the anode in this destructive relationship, you must put a wall between your aluminum boat and the motor. A block of magnesium or zinc -- either one, less noble than aluminum -- bolted to the aluminum hull near the motor, becomes a "sacrificial anode," to wither in the aluminum boat's place. You should check the sacrificial anode at least yearly and replace it when it's corroded to half its original size.
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