1. Protecting Metals from Corrosion

2. a)Natural Protection: Some metals react with substances in the air to form thin natural coatings that adhere tightly to the surface of the metal. i)Aluminum: Aluminum reacts with oxygen (in the air) to form aluminum oxide (Al 2 O 3 ) which forms a thin white coating that sticks to the remaining aluminum. This coating protects the aluminum metal underneath from further corrosion. This aluminum oxide (Al 2 O 3 ) layer is about 5 nm thick whereas aluminum foil (Al) is about 23000 nm thick.

3. Protecting Metals from Corrosion ii)Copper: Copper react with oxygen, water, and carbon dioxide or sulfur dioxide (in the air) to form a thin green coating that sticks to the remaining copper. This coating protects the copper metal underneath from further corrosion. The Statue of Liberty is green! Much of the roofing at the NY State Capitol is green. iii)Zinc: like aluminum, zinc forms a ZnO layer that binds tightly to protect the metal underneath.

4. Protecting Metals from Corrosion b)Man-Made Protection: Metals such as iron and steel (an iron alloy) cannot be protected by their own corrosive products because they flake off and continually expose new metal to corrosion from the elements! i)Paint: A coat of paint covers the metal and prevents moisture and oxygen from reacting with the metal. Many bridges in the USA, such as the Brooklyn bridge, and the San Francisco Bay bridge are in a continual state of being painted in order to protect the metal. ii)Galvanizing: Iron, steel, or even aluminum can be coated with zinc by various methods. The zinc layer becomes a protective ZnO coating. 

5. Protecting Metals from Corrosion iii) Cathodic Protection: Small disks or bars of a more reactive metal (but not too reactive) are attached to the metal being protected. Moisture and oxygen will corrode the more reactive (zinc) metal leaving the protected metal (iron) alone. In this way, the protected metal (iron) is the cathode and the sacrificial metal (zinc) is the anode. A buried iron pipe with zinc disks attached.

6. Moisture and oxygen will corrode the more reactive (Mg) metal leaving the protected metal (steel) alone. In this way, the protected metal (steel) is the cathode and the sacrificial metal (Mg) is the anode. Protecting Metals from Corrosion A steel boat propeller with magnesium disks attached.

7. iv)Metallic Plating: Plating a corrosive-resistant metal such as chromium onto a metal protects it. For years, automobile manufacturers covered iron bumpers with chrome (chromium). Protecting Metals from Corrosion 

8. v)Alloying: The metals to be protected can be alloyed with other metals. Steel (itself an alloy) can be alloyed with chromium to produce stainless steel. vi)Electrolytic Protection: In special cases, a small (-) DC voltage is applied to the metal, making it the cathode so it won’t corrode. Protecting Metals from Corrosion 

9. Ex. (1) Given the reaction for the corrosion of aluminum: 2 Al + 3 O 2 → 2 Al 2 O 3 Which half-reaction correctly represents the oxidation that occurs? (1) Al + 3e - → Al 3+ (3) O 2 + 4e - → 2 O 2- (2) Al → Al 3+ + 3e - (4) O 2 → 2 O 2- + 4e - Ex. (2) Which of the following metals does NOT form its own protective coating against further oxidation? (1) Al(2) Cu(3) Fe(4) Zn

10. Ex. (3) In order to protect steel from corrosion, the process of galvanization is used to coat the steel with what metal? (1) Al(2) Cu(3) Fe(4) Zn Ex. (4) Given the following corrosion reaction for iron: 4 Fe + 3 O 2 → 2 X Which compound is represented by X? (1) FeO(3) Fe 3 O 2 (2) Fe 2 O 3 (4) Fe 3 O 4 Protecting Metals from Corrosion

11. Ex. (5) Because tap water is slightly acidic, water pipes made form iron corrode over time, as shown by the balanced ionic equation: 2 Fe + 6 H + → 2 Fe 3+ + 3 H 2 Explain, in terms of chemical reactivity, why coppers pipes are less likely to corrode than iron pipes? _________________________________________ Protecting Metals from Corrosion As shown by the activity series, copper is less reactive than hydrogen whereas iron is more reactive than hydrogen. Thus iron will corrode in HCl but copper will not corrode in HCl.

12. Ex. (6) Explain, in terms of chemical reactivity, why pieces of nickel metal could protect a lead pipe from corrosion but not protect an iron pipe from corrosion? __________________________________________ Protecting Metals from Corrosion As shown by the activity series, nickel is more reactive than lead whereas nickel is less reactive than iron. Thus nickel will corrode instead of lead, but iron would corrode instead of the nickel.