lecture21sf - Corrosion Corrosion is the oxidation of a...

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Corrosion Corrosion is the oxidation of a metal. Based on the Echart, which metal, iron or aluminum should corrode more easily? Fe 2+ + 2e - → Fe -0.44 volts Al 3+ + 3e - → Al -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 a oxidation-reduction process, there are two half reactions, and actually two separate areas (cathode and anode) where the reaction occurs. Anode: Fe(s) → Fe 2+ + 2e - Cathode: ½ O 2 + H 2 O + 2e - → 2OH - Overall: Fe(s) + ½ O 2 + H 2 O → Fe(OH) 2 Fe(OH) 2 can get further oxidized to Fe(OH) 3 , which can get dehydrated to Fe 2 O 3 , with its familiar red color. Having ions in the water provides a salt bridge which accelerates the process.
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Prevention of Corrosion Protective coating such as paint. This protects the metal from moisture or contact with oxygen. Galvanizing: coating with zinc. Zinc oxidizes more easily than iron, but its oxide forms a protective coating. Alloying with other metals. Stainless steel contains chromium and nickel, which form oxide coatings which protect the iron. Cathodic protection. This method uses a “sacrificial anode” which gets oxidized more easily than the pipe or metal being protected. Magnesium is a sacrificial anode in hot water heaters, or for buried iron pipe. Magnesium is more easily oxidized than iron, so oxidation occurs at the magnesium site (anode), and reduction of water to OH - occurs at the iron site (now the cathode), and the iron is not corroded. Voltaic Cell
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lecture21sf - Corrosion Corrosion is the oxidation of a...

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