Regardless of how familiar you are with the science behind the corrosion of metal, you probably know quite well the end results: rust and loss of material. Zinc anodes are used as part of a protection system that helps to defend metal structures that are buried or submerged against erosion. They are most often used for structures like barges, buried storage containers, docks, harbors, piers, ships, tankers, and underground or underwater pipelines.
Galvanic anodes, or sacrificial anodes, are always made of some type of metal alloy. This may seem counter-intuitive, but the idea behind it is that making the anode out of a metal substance whose voltage is more “active” than that of the structure it protects ensures that the material of the anode will be expended in predilection to that of the structure. To say that the anode has a more “active” voltage means that the anode is more negatively electrochemically charged than the material it is designed to protect. The anode’s greater negative potential in comparison to the underlying material makes it more susceptible to corrosion first, thus leaving the underlying material unscathed for significantly longer than if it were left exposed.
Zinc is just one of the three alloys most often used in making galvanic anodes, along with aluminum and magnesium. Any of the three materials are available in various forms like blocks, extruded ribbon, plates, or rods. Each material also has its own set of benefits and drawbacks. Zinc is considered to be a fairly reliable material and is well-suited for low-resistivity salt water. It is often used for boat and ship hulls, offshore pipelines, propellers and rudders, and inside storage tanks.
Zinc is not, however, suitable for use in high temperatures. At higher temperatures, zinc has a tendency to passivate, or become less negative. If an anode’s negative charge weakens, the current may become disrupted and the anode no longer functional. Zinc also has a comparatively low driving voltage. This means that it generally does not provide a current that is sufficient for holding up in high-resistivity soil. In some cases, though, this circumstantial drawback can turn out to be an advantage. In situations where hydrogen embrittlement (damage to the protected material as a result of hydrogen exposure, which is naturally produced during the corrosion process) is a possibility, lower-voltage zinc anodes may work better to avoid undesirable over-protection.