A process called galvanic corrosion is a common form of rusting that occurs in salt water. 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. In this electrochemical process, they have formed 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 oxidize faster than it would if left by itself. Alternatively, the other metal behaves as 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 between dissimilar metals, 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 take care of this ever present especially for the shipping and fishing industries where metal equipment is always in immersed in salt water. Making use of zinc anodes will prevent like steel tanks, rails and ship hulls from rusting. Because the zinc rusts far easier than iron, it corrodes first. To greatly increase the life of their metal crab traps, crab fishermen depend on attaching a zinc anode. The crab trap becomes the cathode and remains protected while the anode disintegrates over time. A replacement anode should be attached after the previous one has dissolved to about half of its original size.
Most boaters are extremelty cognizant 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 components from corroding. 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 utilizing zinc anodes is a smart step in protecting metal equipment immersed in sea water, knowing the proper amount of zinc to add is even more important. The level of protection provided by the zinc anode is dependent upon its surface area. Other variables include the type of metal that needs protecting as well as the specific chemistry of the water. The anode ought to be inspected regularly to look for any corrosion. If any rust is noticed, 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 will not offer any benefits whatsoever. There must be metal-to-metal contact to allow that electrons will flow. Two options exist: direct physical contact or connected by a wire. This also means that paint cannot cover any of the metal surfaces or sufficient contact will not be made.
Before using a zinc anode, remember 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 make certain that there aren’t any coatings on either metal that will impede the electrical contact.
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