Metals are utilized widely in our society due to their desirable properties. For example, many buildings and vehicles rely on iron for their structural integrity. Unfortunately, metals are reactive and often encounter conditions that result in corrosion. When this occurs, the chemical reaction can lead to metal fatigue and failure. Corrosion has a great economic effect and presents serious safety concerns. Not surprisingly, corrosion prevention is a major technological problem for our society.

You may be wondering what the process of corrosion has to do with electrochemical cells. The answer may surprise you.

1. Electrochemical Basis of Corrosion

   
   Corrosion is a process that degrades metals electrochemically. In fact, the first step of the corrosion process demonstrates a voltaic cell in action. Let us examine why this is true.

   
   Consider the diagram below, depicting the first step in the corrosion of iron. You can see how iron is oxidized at one location (the anode) and oxygen is reduced at another location (the cathode). This reaction is spontaneous.
   

   
   
   

   
   Both Fe²⁺ ions and OH^- ions are products of this spontaneous redox reaction. These two substances combine to form the low-solubility precipitate, iron(II) hydroxide. Further reactions of the iron(II) hydroxide result in the formation of the familiar brown substance known as "rust".

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2. Corrosion Prevention

   
   Billions of dollars are spent every year to prevent or slow the corrosion of iron. The following are some methods used to prevent the corrosion of iron.
   

   • Mix metals with iron to form a more corrosion-resistant alloy. Alloys have unique sets of properties. Stainless steel is an iron alloy that contains mostly iron, 10% to 30% chromium, a smaller percentage of nickel, and sometimes cobalt.
     
   • Paint exposed iron surfaces. Applying a coating prevents water and oxygen from gaining access to iron and, therefore, prevents oxidation of the iron.
     
   • Plate iron with another metal. Similar to paint, the layer of other metal prevents water and oxygen from gaining access to iron. Chromium and zinc (galvanization) are used commonly for plating iron. You will learn more about electroplating in the next module.
     
   • Apply oils and grease to exposed iron. This method often is used on the moving parts of machinery. Oils and grease are non-polar and, therefore, repel water.
     
   • Use cathodic protection. Cathodic protection forces the iron to become the cathode by means of an impressed current or a sacrificial anode.
     
     1. An impressed current is an electric current that is made to flow towards an iron object.
     2. A sacrificial anode is attached to the iron. A sacrificial anode is a metal that is oxidized more easily than iron. The more active metal is consumed slowly or "sacrificed" at the anode, forcing the iron to become the cathode.