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Corals
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A related protective method is used for larger steel structures such as buildings, ships and underwater/underground pipelines. For such large structures, it would not be practical to provide a covering of zinc. What is done is to attach a block of an easily oxidized metal (magnesium is often used) to the structure by a steel cable
Magnesium preferentially oxidizes to form the Mg2+ ion:
Mg(s) = Mg2+(aq) + 2e-
The electrons produced can flow along the steel cable and give the oil rig a small negative charge. This makes it difficult for iron in the structure to oxidize and form positively charged ions as this would add even more electrons to what is an already negatively charged structure and the steel rig remains intact. In time, the block of magnesium is eaten away and has to be replaced. This is much easier and cheaper than carrying out repairs on the structure of the rig.
Modifying metals
The resistance to corrosion of a metal can be changed by providing a surface coating of paint or other metal, but there is another way of changing the properties of the metallic elements that has been known for thousands of years. Bronze is an alloy of copper and tin, and like copper, its major component, it is resistant to corrosion. Bronze is also much harder than is copper and it can be used in engineering. It finds particular use in the manufacture of propellers for ships where corrosion resistance is paramount. Here, there is a further advantage: copper is toxic to molluscs and is effective in inhibiting the growth of barnacles on the propeller which would reduce its operating efficiency .
An alloy is different from a chemical compound which has a fixed ratio (and usually a very simple one) of one element to another. We have seen that the relative proportions of copper and tin in bronze can be varied over a wide range. For any ratio, atoms of the two metals are evenly distributed through the structure and the properties of the overall structure are very dependent on that ratio. The hardness, strength and corrosion resistance of metals can often be ‘tuned’ by carefully adjusting the proportions of elements in the alloy.
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