Lecture32 - Chem 162 Lect 32 Spring 2011 Corrosion...

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Chem 162, Lect 32, Spring 2011 Corrosion Corrosion is the oxidation of a metal. Based on the E o chart, which metal, iron or aluminum should corrode more easily? F e Fe 2+ + 2e - + 0.44 volts A l Al 3+ + 3e - +1.66 volts Aluminum is a more active metal (more easily oxidized). Then why is corrosion of iron so common (costs billions of dollars every year), but we never see corrosion of aluminum? Aluminum is very reactive and actually does oxidize. However, it forms a thin-layer protective oxide coating which adheres to the aluminum and protects it from further oxidation. Corrosion of iron is very common. Corrosion is a process which oxidizes the iron to its ions, either Fe 2+ or Fe 3+ . What is the oxidizing agent? The oxidizing agent is oxygen, O 2 ; but not just gaseous O 2 . A shiny dry nail exposed to gaseous O 2 will stay shiny and uncorroded indefinitely. Moisture greatly accelerates the corrosion process. The oxidizing agent is O 2, either oxygen gas dissolved in water or oxygen gas in the air in contact with water. Since corrosion is an oxidation-reduction process, there are two half reactions, and actually two separate areas (cathode and anode)
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Anode: Fe(s) Fe 2+ + 2e - Cathode: 1 2 O 2 + H 2 O + 2e - 2OH - Overall: Fe(s) + 1 2 O 2 + H 2 O Fe(OH) 2 Fe(OH) 2 can get further oxidized to Fe(OH) 3 4Fe(OH) 2 + O 2 + 2H 2 O 4Fe(OH) 3 which can get dehydrated to Fe 2 O 3 , with its familiar red color. 2Fe(OH) 3 Fe 2 O 3 + 3 H 2 O Having ions in the water provides a salt bridge which accelerates the process. Prevention of Corrosion Protective coating such as paint. This protects the metal from moisture or contact with oxygen. Galvanizing:
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Lecture32 - Chem 162 Lect 32 Spring 2011 Corrosion...

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