What is Corrosion?
In this article we’re gonna look at corrosion and in particular at why things rust and how we can stop it. Corrosion is a general term that refers to the process by which metals are slowly broken down by reacting with substances in their environment. An example of this is rusting which is where iron reacts with oxygen and water from the environment to form hydrated iron oxide which is more commonly known as rust if we think about what’s happening here and at the atomic level the iron atoms in our original block of metal are each losing three electrons to become iron three-plus ions and at the same time the two oxygen atoms in each oxygen molecule are gaining two electrons so a total of four electrons to become oxygen two minus ions so the iron is being oxidized because it lost electrons while the oxygen is being reduced because it’s gaining electrons and this means that the overall reaction is a redox reaction because both oxidation and reduction are taking place in the same reaction.
The next thing, are the conditions that are required for rusting to take place we said earlier that rusting involves iron reacting with oxygen and water from the environment so both of these have to be present for rusting to take place to see this in practice let’s imagine three test tubes each one with an iron nail in it the first test tube contains water but no oxygen this is because the water has been boiled to remove any of the oxygen that was previously dissolved in it and the layer of oil at the top stops any new oxygen that’s in the air from dissolving into the water. The second test tube contains air and so oxygen and no water and this condition achieved by having the calcium chloride at the the bottom would you remove any water vapor from there and then the final test tube contains both of air; so oxygen and water so if we now lift these test tubes for a while. Only the nail in the last tube would rust because it’s the only one that contains both oxygen and water. One important thing to understand with corrosion is that it’s only the surface of a mess all that corrodes this is because only the surface is exposed to substances in the environment like water and oxygen. When iron rusts through the rust that forms at the surface gradually flakes off revealing a new metal underneath this means that the next layer can also rust and over time that bit will fall away as well so if left long enough the an entire chunk of iron can break down until there’s nothing left in contrast when metals like aluminum corrode are only the surface atoms that are affected this is because when the top layer of aluminum atoms are oxidized to form aluminum oxide they effectively form a protective layer around the piece of aluminum which protects the rest of the metal from the environment the last thing means to look at is how we can prevent iron from rusting. Generally speaking, we can split the methods into two main categories area methods and sacrificial methods the barrier methods work by preventing the oxygen and water from ever touching the iron so that rusting can take place and there are three main types to know one is to paint it which works well for really big pieces of iron like the side of a ship, the second is to oil or grease it which is necessary if the object is moving parts like a bike chain and the third type of barrier protection is electroplating where it is kind of complicated but involves using electrolysis to cover the iron in a thin a layer of another metal which will protect the iron underneath. The other option is called the the sacrificial method involves adding a more reactive metal like aluminum or zinc to the iron this means that if the object gets exposed to oxygen the more reactive metal will be oxidized instead of the iron and so the iron will rust. An example, that uses both of these methods, is galvanizing this involves coating the iron object in a layer of zinc which is effectively the barrier method because it’s forming a protective the coating around the entire nail however, if the zinc coating gets scratched and the underlying irony is exposed the zinc will still react with any oxygen that goes near the nail because the zinc is so much more reactive so it’s an example of the sacrificial method.