Corrosion - Page 6

Making the water more conductive is not the only way that salt can affect corrosion. It is a very powerful in other ways. As explained, when Salt (Sodium Chloride) dissolves it beomes Na+ and Cl- ions in the water. The Cl- ions are very strong little things, and can be very corrosive in their own right.

The animation on the left shows an iron bar corroding, losing iron atoms in the process and developing a hole. The problem with iron is that the iron oxide formed when corrosion takes place simply falls off, exposing new iron metal. This too corrodes and falls off, exposing more iron metal. This process will continue until the iron has completely corroded. You see this to dramatic effect as a hole in the bodywork of a car, where over time the iron has rusted all the way through the panel.

However, other metals do not oxidise in this way. For example, you may think that aluminium does not corrode, as we see unprotected aluminium windows and equipment all around us - and it does not corrode away like iron and steel.
As we saw in experiment 2(1) - aluminium foil in water will not corrode, but when salt is in the water the aluminium foil corrodes rapidly. Why?

The answer lies in the way that Aluminium corrodes. Aluminium corrodes very quickly (actually much more quickly than Iron and Steel). However, the oxide that forms when aluminium atoms oxidise does not fall off like the rust on iron, but clings tightly to the surface of the aluminium. Very quickly, a hard layer of aluminium oxide covers the metal, and protects the aluminium metal beneath. No more aluminium can oxidise, as it has been sealed off from the oxygen and water needed to make the reaction happen.
When salt is added to the water, the powerful Cl- ions attack the aluminium oxide coating, tearing it from the surface and exposing new aluminium metal. As soon as this aluminium metal corrodes into aluminium oxide, it too is stripped from the surface by the Cl- ions. The natural protection that aluminium gets from its oxide coating is lost. This is the reason that aluminium cars and equipment will often corrode badly if they are used in or near the sea.

Copper metal also corrodes in this way, but forms a bright green coating of oxide which protects the copper from further corrosion. This is why the Statue of Libery in New York is still looking as good as the day it was erected in 1886. Had it been made out of Iron it would have rusted away by now (For example the iron and steel Eiffel Tower in Paris, erected in 1889, has undergone major replacement and restoration work to remain standing).

Imagine if we could somehow get steel and iron to behave in this way. If the rust on iron stuck to the surface, instead of simply falling off, we could save a lot of wasted time and effort spent painting and protecting steel. Can this be done ?
Yes it can - by simply mixing some of the metal that has oxide protection (such as copper) into the steel. A mixture of steel and copper is an alloy called "weathering Steel". This is because if it is left outside unprotected, it will still corrode, but the brown rust will cling to the surface of the metal and protect it from further corrosion. This protection is not as strong as pure copper, but will be much better than normal steel. Iron girders used for buildings are often made of weathering steel. This allows them to be put in place for a few months while the building is being erected without corroding a great deal. Once the building is finished and they are sealed off from the rain, the corrosion will stop altogether.

Salt has one other property that makes it promote rusting, it attracts water. When a substance attracts water it is known as Hydroscopic. This means that it will try to get water from anywhere it can.

In experiment 2(2) we saw that a nail with dry salt on its surface will corrode. How can this be - as we know that water is required for corrosion to occur? The answer is that water is present. The salt on the surface of the nail attracts the tiny amounts of water vapour from the surrounding air, and this water then allows corrosion to take place. If you look closely at the photographs in experiemnt 2 you will see that the corrosion happens exactly where the grains of salt are attached to the nail.
The ability of salt to attract water is shown to even greater effect in experiment 2(3). Here the salt can actually pull water through the paint on the nail surface. The nail without salt is completely protected from rust by the paint (the paint is acting in the same way as the hard oxide coatings that we discussed earlier). The nail with salt is not protected, as the salt pulls water through the paint and allows corrosion to take place. It is extremely difficult to remove all traces of salt from metal once it has been corroding in salt water. Special chemicals must be used before the metal is painted in order to make sure that all the chloride (Cl-) has been removed.

Conclusion: salt increases the ability of the water to carry electrons (a current) - and thus speeds up the process of corrosion. The Chloride (Cl-) ions in salt also break down the oxide layer that forms on the surafce of some metals making them corrode when they normally would be protected. Salt also attracts water (even from the air) causing corrosion in areas where water would normally not be present.

If you are trying to prevent corrosion, salt is your worst enemy!