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---
  layout: post
  title: If you were using "cathodic protection," what would you be trying to protect?
  author: Joe Schwarcz
  source: McGill Blogs
---
  HYPERLINK
"http://blogs.mcgill.ca/oss/2013/05/06/if-you-were-using-%e2%80%9ccathod
ic-protection%e2%80%9d-what-would-you-be-trying-to-protect/"  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 – May 6th/2013