the four experiments we have covered most of the major factors that are involved in corrosion. In this first section we will explore the basics of what happened in each of the experiments, and explain why it happened. Then we will start to explore the chemistry of corrosion and show some real world examples of corrosion and its positive and negative effects. Firstly we should take a look at our experiments and draw some conclusions from the results.
The most obvious result here is that the nail with no water did not corrode at all. We can draw a simple conclusion from this.
Initial conclusion: Water is needed for metals to corrode.
The next difference we observe is that the nail sealed off from the air by a layer of oil does not corrode much. Normally, when water is in contact with the air, air dissolves in the water (If you didn’t know that gases dissolve, think about your own breathing; the oxygen in the air has to dissolve to get into your blood. Dissolved oxygen in water is also how fish “breathe” using their gills).
By applying a coating of oil to the water, we are sealing the water from the air. It is not possible for any oxygen to dissolve in the water. The little oxygen dissolved in the oil covered water soon runs out – and corrosion stops. Oxygen continues to dissolve into the uncovered water, and corrosion continues.
Further conclusion: Oxygen as well as water is needed for metals to corrode. Oxygen alone is not enough; water alone is not enough; both must be present.
When aluminium is exposed to oxygen and water, it does not corrode. Something protects it. However, if we add salt to the water, that protection is lost.
Initial conclusion: Salt accelerates corrosion, and breaks down any protection a metal may have against corrosion.
When a nail is coated in dry salt, it corrodes quite quickly. A non-salty dry nail if left in the same conditions for a long period will corrode a small amount. We know that water is required for corrosion to take place, and as both nails were dried at the start of the experiment, the only source of water for this corrosion is the moisture in the air.
Initial conclusion: Salt attracts moisture from the air, accelerating corrosion when it is present on a metal surface.
When a nail is painted, we are protecting it from both oxygen and water. We would expect there to be no corrosion. However, when a nail is corroded in salt, and painted, it still corrodes. This happens even when the visible signs of salt have been cleaned off the nail. We know that salt attracts moisture – and it must be strong enough to draw moisture through the painted surface.
Initial conclusion: Salt has a very powerful ability to attract water, even through paints and coatings.
Further conclusion: The presence of salt will accelerate corrosion, and make metals corrode in situation where they would normally not corrode. Salt speeds up the corrosion of metals, and once a metal has corroded in salt, it is difficult to clean it from the metal surface.
Metals behave differently when they are joined together during corrosion. Only one of the pair seems to corrode, protecting the other from corrosion. For example, the nail with no attached metal rusts a little bit, the nail attached to aluminium foil does not rust at all (but the aluminium foil does corrode), and the nail attached to the copper wire rusts a great deal, but the copper does not corrode at all. It appears that some metals corrode with more power than others, and when such metals are joined together only the more powerful metal corrodes, and somehow protects the weaker metal.
Initial conclusion (looking at which metal corroded from the pairs we made): Aluminium is a stronger corroder than the iron nail. The iron nail is a stronger corroder than the copper and stronger than the silver solder. The copper is a stronger corroder than the silver solder. Ranked in order (strongest to weakest): Aluminium, Iron, Copper, Silver Solder.
The corrosion of metals is affected by the flow of a current. Metals on the negative side of the current flow do not corrode at all, metals on the positive side of the current corrode.
Initial conclusion: The process of corrosion involves the movement of a current. When a current is forced through metal by a battery, the corrosion can be controlled.