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A galvanic anode is a component that relies on sacrificial anode theory to protect valuable ferrous metals from corrosion. Anodes achieve this protection by offering a more attractive source of oxidation via an electrical connection to the protected metal. In this way, the galvanic anode “sacrifices” itself to prevent corrosion forming on the metal. These galvanic anodes are generally plates of magnesium, zinc, aluminum or specialist alloys connected to the steel work with a cable and buried in the ground. A galvanic anode lends cost effective, long term and renewable corrosion protection to steel structures in both terrestrial and marine environments.
Corrosion or rust on ferrous metals is the result of an electro-chemical reaction, i.e., oxidization, between moisture and salts in the environment and the metal surface. There are several ways of protecting ferrous metals from this corrosion; sacrificial anodes are among the most cost effective and efficient. This method works by offering a more attractive source of oxidation than that of the material to be protected, thus diverting the corrosion process. A galvanic anode is electrically connected to the protected metal as opposed to galvanizing which is a physical coating. Electrically connected in this case simply means in direct contact either via physical contact or by means of a cable link.
In the case of steel pipelines, the piping is connected at regular intervals with cables to magnesium plates buried in the ground. With its higher negative electrode potential, magnesium draws corrosion causing positive ions away from the steel pipe thus sparing it from the rusting process. The magnesium plate will gradually be consumed in the process but will continue to protect the steel piping until completely depleted. At this point, a new galvanic anode may be installed. The lifespan of a galvanic anode is approximately 20 years, making it a more suitable source of corrosion protection for large scale applications than conventional galvanized coatings.
The galvanic anode is commonly used in applications such as marine engines — where salt water is used for cooling — and steel pylons, boat propellers, steel bridges and pipe lines. Galvanic anode materials differ depending on the specific application and working environment. Steel structures, for instance, will typically be fitted with zinc or magnesium anodes and boats with zinc or aluminum anodes. Specialist applications such as off-shore oil rigs make use of custom formulated alloy compositions for their galvanic anodes.
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