If you were using “cathodic protection,” what would you be trying to protect?

You would be trying to prevent iron from rusting. The rusting of iron is an expensive process. It is estimated that the deterioration of iron due to corrosion costs billions of dollars a year in Canada. The process is simple enough in terms of chemistry. Iron reacts with oxygen from the air to form iron oxide. This is termed an electrochemical reaction because the oxygen actually steals electrons from the iron. Water is required for this reaction to proceed, and the process is faster if the water has substances called electrolytes which can carry an electric current dissolved in it. Salt is a great electrolyte. And our cars are exposed to plenty of it in the winter, which explains why Canadian cars rust so fast. Conversely, airplanes not in use are usually stored in the Arizona desert where there is virtually no humidity. A process known as cathodic protection can be used to prevent rust formation. The iron to be protected is attached to another metal such as zinc or magnesium, which give up electrons to oxygen more readily than does iron. The so- called sacrificial cathode will then corrode and the iron will not. Underground gasoline storage or oil tanks can be protected in this fashion. This is readily demonstrated with a simple experiment. Take two nails, immerse them in salt water but attach a piece of zinc to one of the nails. Watch the difference in corrosion! Rusting can also be prevented by excluding oxygen and moisture. Paint does this quite well. Another possibility is to alloy iron with other metals such as chromium to make stainless steel. In this case chromium reacts with oxygen to form chromium oxide that deposits as a thin impermeable layer on the surface of the metal and protects the iron underneath. Iron can also be coated with a thin layer of another metal which is less prone to oxidation. So called “tin” cans actually are made of iron with a thin coating of tin.

 

Joe Schwarcz

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