|Iron is a cheap and strong metal to use as a construction material. It has one basic flaw however, it is very reactive. It will quickly rust, under the right conditions, leaving behind a fine brown powder known as iron oxide(rust). We can prevent iron from rusting by many different methods. The most common way is to paint the metal and prevent oxygen from reacting with it. This method is very expensive as coats of paint need to be applied frequently. If the surface coating of paint is scratched rusting will take place immediately. This is not the preferred method of rust prevention for giant ships that often scrape against rocks, sand and barnacles. Instead, a sacrificial anode is used. A sacrificial anode is a block of metal that is more reactive than iron. The more reactive a metal is the easier it gives away electrons. This reactive block of metal acts as a source of electrons for the iron. When oxygen takes electrons from the iron during the process of rusting iron atoms simply take electrons from the reactive metal.
If we wrap zinc around an iron nail the nail is protected from rusting. Notice that the iron does not have to be completely covered in order to be fully protected. This is a real advantage to using a sacrificial anode.
Notice also the white flakes appearing on the zinc and at the bottom of the test tube. Corrosion is occurring at a great rate but its not iron but rather zinc that is rusting. Simply put, the zinc is sacrificing itself to protect the iron hence the name sacrificial anode.
|The animation on the left shows how electrons flow from zinc to the iron. This process occurs until all the zinc has corroded or the two metals are no longer in contact.
|The hulls of large sea vessels use small zinc blocks placed at regular intervals to prevent rust. Periodically these zinc blocks have to be replaced as they corrode away.
|Oil rigs use blocks of aluminium at the foot of every supporting pillar to prevent corrosion of the iron metal. Aluminium is very reactive metal.
"X" is more reactive than metal "Y".
Both metals are very strong and cheap to produce. A bridge is to be constructed from these two metals. Discuss which metal you will use for the main structure and which will form the sacrificial anode.