A process called galvanic corrosion is a common form of deterioration that takes place in aquatic environments. Do not confuse this with electrolysis. They are not the same process. It occurs when two or more different metals make electrical contact with one another in seawater. When this type of connection forms, they have essentially become a battery and some current will flow between the metals. One of the metals in the couple behaves as an anode. This causes it to corrode more quickly than it would if left by itself.
Alternatively, the other metal becomes the cathode and corrodes more slowly than it would alone. Whether either metal in the couple corrodes on its own in seawater is not the issue. The problem is that when there is contact with a different metal, the rates of corrosion will change. Corrosion of the anode will speed up and corrosion of the cathode will slow down or cease altogether. Fortunately, the use of a sacrificial zinc anode helps to alleviate this ever present problem.
This type of corrosion is of special concern especially for the shipping and fishing industries where metals are constantly in contact with salt water. Employing the use of zinc anodes will keep things like steel tanks, rails and ship hulls from rusting. Because the zinc rusts far easier than iron, it corrodes first. To vastly lengthen the life of their metal crab traps, crab fishermen depend on attaching a zinc anode. The crab trap becomes the cathode and stays intact while the anode disintegrates over time. A new zinc anode should be attached after the previous one has rusted away to about half of its original size.
Most boaters are painfully aware that the most common victim of galvanic corrosion is a bronze or aluminum propeller on a steel shaft. The corrosion is counteracted by adding a third metal. As mentioned before, zinc is the metal of choice because it oxidizes more quickly than the other metals. This piece of metal is called a sacrificial anode, because it is "sacrificed" in order to save the other metal parts from rusting away. It's very important that zinc anodes are maintained regularly because if one waits too long, the metal parts they were meant to protect will surely begin to rust and dissolve.
Although making use of zinc anodes is a smart step in protecting metal equipment immersed in sea water, knowing how much zinc to add is even more important. The level of protection offered by the zinc anode is dependent upon its surface area. Other variables include the type of metal that is to be protected as well as the specific chemistry of the water. The anode should be checked frequently to look for any corrosion. If any rust is detected, then zinc with more surface area is needed. If the zinc anode must be replaced in less than a year, then one with more weight should be used instead.
Attaching the anode is another vital step in protecting metal components. Mounting the zinc just anywhere is not going to provide any benefits whatsoever. There must be metal-to-metal contact to allow an electrical connection. The metals can be either in direct physical contact or connected by a wire. This also means that there cannot be paint on any of the metal surfaces or sufficient contact will not be made.
When utilizing zinc anodes, there are crucial steps that cannot be overlooked if the anode is to be effective. Use an anode with enough surface area and weight, attach it so it makes physical contact and be sure there aren't any coatings on either metal that will interfere with the electrical contact.