Corrosion - Page 4

We now have another question to answer. If electrons are flowing through the battery to the uncorroded nail on the right, there will be a lot of extra electrons in the uncorroded nail. As we discussed, atoms like to have the correct number of electrons; not more; not less. The nail on the right must get rid of electrons in order to stay neutral - where do these electrons go ?
All of the electrons flowing to the nail on the right come from the corroding nail on the left. Atoms in the nail on the left must be losing their electrons (becoming positively charged in the process). The nail on the left must either get new electrons from somewhere, or throw off these positive atoms that have lost their electrons in order to stay neutral.

A quick look at the picture tells you which of these is happening to the corroding nail. The nail is throwing off iron atoms into the water. As each iron atom is ejected, it leaves behind its electrons which can flow over to the right hand nail. The brown colour that you see is iron oxide (rust), so there must be a reaction taking place in the water around the corroding nail between the ejected iron atoms and oxygen. In order to produce a continuous stream of electrons, the nail ejects more and more iron atoms into the water. This explains one half of the reaction - where the electrons come from. But where do they go at the other end ?

This is a vital part of understanding corrosion, so it is worth dwelling on this point to make sure that you understand. Look at the animation on the left. Two iron atoms leave the nail, and float off, leaving their electrons behind (to make the diagram simple, we show the atoms each leaving 1 electron behind. Actually they leave 2 electrons behind). These electrons flow through the battery as an electrical current to the other nail, where they too must be passed into the water. The nail throwing iron atoms into the water is actually getting smaller by losing atoms - it is "rusting away", the nail throwing electrons into the water loses no iron atoms, so does not corrode at all.

So what happens at the non-corroding nail. Nothing is really visible there, but we know that there must be a reaction that uses up the extra electrons producing something negatively charged. We know from Experiment one that we need water and oxygen for rusting to take place - so it would be a fair assumption that the reaction at the non corroding nail is between oxygen, water, and the extra electrons.
In chemistry, the positive area that frees electrons is called the anode. The negative area that receives electrons is called the cathode. So in this example, the corroding nail is the anode, and the non-corroding nail is the cathode.

Initial Conclusion: Positively charged iron atoms are thrown off the anode (left nail) into the water. The iron atoms are positively charged because they leave their negatively charged electrons behind. The anode (left nail) now has too many electrons, and they flow to the cathode (right nail). Because it now has these new electrons, the cathode (right nail) has too many electrons, and so must eject them into the water.

In this experiment we are using a battery to force the electrons to move in a certain direction, but a battery does not need to be present for iron to rust (or any metal to corrode). Remember - like most metals, iron does not like to exist as pure metal, it wants to return to being iron oxide. Iron atoms will naturally try to wrench free from the iron metal, leave their electrons behind, and become iron oxide. The electrons left behind will flow to another area of the metal and undergo their reaction.

This is the first time we can see that it is possible to protect metal from corrosion by controlling the flow of electrons with electricity. By forcing the electrons to move in a certain direction, we can prevent one piece of metal rusting, and force another to rust instead. This method of protection is call impressed current protection and is often used on ships. To prevent the valuable metal on the ship from rusting, a piece of sacrificial metal is attached to the ship, and a current is forced to flow between the sacrificial metal and the ship itself (through the sea water). The current is set so that electrons flow from the sacrificial metal (the anode) into the ship (the cathode). The sacrificial metal will corrode severely (like the nail on the left), but the flow of electrons into the ship will prevent it rusting (like the nail on the right). When the sacrificial anode has corroded away, it can simply be replaced with a new one - a lot cheaper and safer than replacing corroded panels and components on the ship.