How to Properly Use Zinc Anodes
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 pair acts as an anode. This causes it to oxidize faster than it would if left by itself. Alternatively, the other metal becomes the cathode and corrodes more slowly than it would alone. How each 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. decay of the anode will speed up and decay of the cathode will slow down or cease altogether. Fortunately, the use of a sacrificial zinc anode helps to alleviate this ever present 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 have long practiced attaching a zinc anode. The crab trap becomes the cathode and remains protected 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 extremelty cognizant that the most common victim of galvanic corrosion is a bronze or aluminum propeller on a steel shaft. The corrosion can be avoided 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 making use of zinc anodes is a smart step in protecting metal equipment in contact with sea water, knowing the proper amount of zinc to add is even more important. The amount of protection provided 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 ought to be inspected regularly to look for any corrosion. If any rust is detected, then zinc with a greater 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 protection at all. There must be metal-to-metal contact to allow an electrical connection. Two options exist: 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.
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 mass, attach it so it makes physical contact and be sure there aren’t any coatings on either metal that will impede the electrical contact.
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