The “protected” metal is the cathode, and remains unoxidized. The more active metal is the sacrificial anode, and is the anode in a galvanic cell. In this method, an easily oxidized and inexpensive metal, often zinc or magnesium (the sacrificial anode), is electrically connected to the metal that must be protected. Another method for protecting metals is cathodic protection. As long as the coating remains, even if scratched, the zinc will oxidize before the iron. Zinc-plated or galvanized iron exploits the fact that zinc is more likely to oxidize than iron. Paint or other coatings will slow corrosion, but they are not effective once scratched. Coating prevents water and oxygen from contacting the metal. One way to prevent, or slow, corrosion is by coating the metal. The rust that forms on iron metal flakes off, exposing fresh metal, which also corrodes. Iron corrodes (forms rust) when exposed to water and oxygen. The thin layer that forms on the surface of the metal prevents oxygen from coming into contact with more of the metal atoms and thus “protects” the remaining metal from further corrosion. Some metals, such as aluminum and copper, produce a protective layer when they corrode in air. Large sums of money are spent each year repairing the effects of, or preventing, corrosion. When the anodes are properly monitored and periodically replaced, the useful lifetime of the iron storage tank can be greatly extended.Ĭorrosion is the degradation of a metal caused by an electrochemical process. The metal being protected serves as the cathode, and so does not oxidize (corrode). The more active metals (lower reduction potential) are called sacrificial anodes because as they get used up as they corrode (oxidize) at the anode. This is also used to protect the metal parts in water heaters. For example, the rusting of underground iron storage tanks and pipes can be prevented or greatly reduced by connecting them to a more active metal such as zinc or magnesium ( ). This is cathodic protection and can be used for metals other than just iron. This suggests that this approach should work with other active metals.Īnother important way to protect metal is to make it the cathode in a galvanic cell. Thus, even if the zinc coating is scratched, the zinc will still oxidize before the iron. Since zinc has a lower reduction potential, it is a more active metal. Zinc is more easily oxidized than iron because zinc has a lower reduction potential. Zinc-plated or galvanized iron uses a different strategy. The chromium tends to collect near the surface, where it forms an oxide layer that protects the iron. For example, stainless steel is mostly iron with a bit of chromium. Other strategies include alloying the iron with other metals. As long as the paint remains intact, the iron is protected from corrosion. The layer of paint prevents the water and oxygen necessary for rust formation from coming into contact with the iron. One way to keep iron from corroding is to keep it painted. The speed of the spontaneous reaction is increased in the presence of electrolytes, such as the sodium chloride used on roads to melt ice and snow or in salt water. Once the paint is scratched on a painted iron surface, corrosion occurs and rust begins to form.
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