73 Corrosion
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LEARNING OBJECTIVES
- Define corrosion
- List some of the methods used to prevent or slow corrosion
Corrosion is usually defined as the degradation of metals by a naturally occurring electrochemical process. The formation of rust on iron, tarnish on silver, and the blue-green patina that develops on copper are all examples of corrosion. The total cost of corrosion remediation in the United States is significant, with estimates in excess of half a trillion dollars a year.
CHEMISTRY IN EVERYDAY LIFE
Statue of Liberty: Changing Colors
The Statue of Liberty is a landmark every American recognizes. The Statue of Liberty is easily identified by its height, stance, and unique blue-green color. When this statue was first delivered from France, its appearance was not green. It was brown, the color of its copper “skin.” So how did the Statue of Liberty change colors? The change in appearance was a direct result of corrosion. The copper that is the primary component of the statue slowly underwent oxidation from the air. The oxidation-reduction reactions of copper metal in the environment occur in several steps. Copper metal is oxidized to copper(I) oxide ( [latex]\text{Cu}_2\text{O}[/latex]), which is red, and then to copper(II) oxide, which is black
[latex]\text{2Cu} (s) + \frac{ 1 }{ 2 }\text{O}_2 (g) \rightarrow \text{Cu}_2\text{O} (s) \ \ \ \ \text{(red)}[/latex]
[latex]\text{Cu}_2\text{O} (s) + \frac{1}{2}\text{O}_2 (g) \rightarrow \text{2CuO} (s) \ \ \ \ \text{(black)}[/latex]
Coal, which was often high in sulfur, was burned extensively in the early part of the last century. As a result, atmospheric sulfur trioxide, carbon dioxide, and water all reacted with the CuO
[latex]\text{2CuO} (s) + \text{CO}_2 (g) + \text{H}_2\text{O} (l) \rightarrow \text{Cu}_2\text{CO}_3\text{(OH)}_2 (s) \ \ \ \ \text{(green)}[/latex]
[latex]\text{3CuO} (s) + \text{2CO}_2 (g) + \text{H}_2\text{O} (l) \rightarrow \text{Cu}_2\text{(CO}_3)_2\text{(OH)}_2(s) \ \ \ \ \text{(blue)}[/latex]
[latex]\text{4CuO} (s) + \text{SO}_3 (g) + \text{3H}_2\text{O} (l) \rightarrow \text{Cu}_4\text{SO}_4\text{(OH)}_6 (s) \ \ \ \ \text{(green)}[/latex]
These three compounds are responsible for the characteristic blue-green patina seen on the Statue of Liberty (and other outdoor copper structures). Fortunately, formation of patina creates a protective layer on the copper surface, preventing further corrosion of the underlying copper. The formation of the protective layer is called passivation, a phenomenon discussed further in another chapter of this text.
Perhaps the most familiar example of corrosion is the formation of rust on iron. Iron will rust when it is exposed to oxygen and water. Rust formation involves the creation of a galvanic cell at an iron surface, as illustrated in the figure below. The relevant redox reactions are described by the following equations:
| [latex]\text{anode:}[/latex] | [latex]\text{Fe} (s) \rightarrow \text{Fe}^{2+} (aq) + \text{2e}^-[/latex]