Corrosion - Page 8

The effect of sacrifical metals is not always deliberate or desirable. There have been many instances where the designer of a metal object has not fully appreciated the effects of corrosion when metals of different types are in contact.

The Statue of Liberty

Inside the statue of liberty is an iron framework to hold the structure together. Attached to the iron is the copper exterior that we recognise. The copper exterior has corroded and is green in colour. Although this coating slows down the corrosion process, the copper continues to slowly corrode. However - think about what happened when the copper wire was attached to the iron nail?

The designers did try to avoid problems by insulating the ironwork from the copper skin using asbestos, but over the years the asbestos became damp and impregnated with salt - forming the "electrical link" between the two metals. Just like in our example, the interior iron frame of the Statue of Liberty became sacrificial to the copper skin, and there would have been a real danger of the interior iron support rusting away completely. Had this happened the statue would have collapsed.

From your knowledge - what could be done to prevent this ?

If you guessed that a sacrificial anode could be attached to the iron framework you would be right. If we attached a large block of zinc or magnesium to the iron framework, it would be a stronger corroder than both the iron and the copper. The zinc would corrode, protecting the iron AND the copper until there was no zinc left to corrode. You could also use an impressed current to provide protection. Either way, you need to pump electrons into the structure to prevent it from corroding.

Another major problem surfaced in the early days of aluminium, particularly its use in aircraft. Aluminium was favoured because it was very light. However, aluminium is not very strong for use in bolts and screws. Steel is a much stonger fastening and bolting metal. In the early designs of aircraft, steel components were often used.
Just like the aluminium foil attached to the iron nail corroded in our experiments- so the aluminium skin of aircraft corroded because it was sacrificial to the steel components. Eventually the weakened skin would succumb to cracking and other types of fatigue, and corrosion has been cited as a cause of several early aircraft failures. These days airline designers know about these issues, and where different metals are used, it is ensured that they are not in contact with eachother (insulated) so that no electrons can be transferred between them.

Final conclusion: Corrosion is a fascinating subject, and leads to so many other topics it is impossible to cover everything in one go. For example, the effect of two different metals in a solution of water is that electrons (or a current) travels between them. This is how a battery works. A battery is actually a corrosion cell - the electricity supplied is from the corrosion of two different metals inside the battery. This topic should have given you an understanding of the basics of corrosion - although there are many other forms of corrosion. If you would like to know more - please drop us a line and we may write part 2 !