Corrosion

Corrosion

Corrosion occurs when metals deteriorate by chemical processes.

Learning Objectives

Discuss the common causes of corrosion of a metal surface

Key Takeaways

Key Points

  • Corrosion is a two-step process that requires three things: a metallic surface, an electrolyte, and oxygen.
  • During the corrosion process, surface-level metal atoms dissolve into an aqueous solution, leaving the metal with an excess of negative charge. The resultant ions are removed by a suitable electron acceptor.
  • Corrosion can be thought of as metals spontaneously returning to the form of their ores through the process of oxidation.
  • The conductive properties of metal enable the oxidation and reduction steps to take place at separate sites on the metal’s surface.

Key Terms

  • corrosion: Erosion by chemical action, especially oxidation.

When materials deteriorate by chemical processes, the materials are said to corrode. Corrosion is commonly discussed in reference to metals, which corrode electrochemically. This kind of corrosion is a two-step process that requires three things: a metallic surface, an electrolyte, and oxygen. During the corrosion process, a metal atom at the surface dissolves into an aqueous solution, leaving the metal with excess negatively charged ions. These resultant ions are removed by a suitable electron acceptor. Corrosion can be thought of as the spontaneous return of metals to their ores through the process of oxidation.

The conductive properties of metal enable the oxidation and reduction steps that occur during corrosion to take place at separate sites on the metal’s surface. The conductivity allows electrons to flow from the anodic to cathodic regions of the metal.

In a corrosion system, the metal being corroded acts as the anode of a short-circuited electrochemical cell:

[latex]\text{Fe} (\text{s}) \rightarrow \text{Fe}^{2+} (\text{aq}) + 2 \text{e}^-[/latex]

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Corrosion is a nuisance: This photo of the Nandu River Iron Bridge in Hainan, China displays evidence of damage by corrosion.

The free electrons reduce the electron acceptor, resulting in any of the following cathodic steps:

[latex]\text{O}_2 + 2 \text{H}_2\text{O} + 4\text{e}^- \rightarrow 4 \text{OH}^- [/latex]

[latex]\text{H}^+ + \text{e}^- \rightarrow \frac{1}{2} \text{H}_2 (\text{g})[/latex]

[latex]\text{M}^{2+} + 2 \text{e}^- \rightarrow \text{M} (\text{s})[/latex]

In this cathodic step, M is a metal.

How susceptible a particular metal is to corrosion can be determined by its reduction potential. The higher a metal’s reduction potential, the less likely it is to be oxidized.

Corrosion is a common nuisance with real impact. We see its effects in rusted out car frames, the bursting of water mains and the failure of bridges.

Preventing Corrosion

Preventing corrosion reduces both the economic and safety-related damages associated with the process.

Learning Objectives

Discuss the common preventative measures that can be taken against corrosion of a metal surface

Key Takeaways

Key Points

    • Three things are necessary for corrosion to occur: an electrolyte, an exposed metal surface, and an electron acceptor.
    • Corrosion can be prevented by removing one of these conditions.
    • Coating a metal surface with paint or enamel provides a barrier between the metal and the moisture in the environment.
    • The process of coating a metal surface with another metal that is more likely to be oxidized is referred to as sacrificial coating.

 

Key Terms

  • electrolyte: A substance that, in solution or when molten, ionizes and conducts electricity.
  • sacrificial coating: A metal coating that is more likely to be oxidized than the metal it protects.
  • galvanize: To coat with a thin layer of metal by electrochemical means; to electroplate.

Reviewing Corrosion

We have learned that three things are required for the anodic and cathodic steps of corrosion to occur: an electrolyte, an exposed metal surface, and an electron acceptor. It follows, then, that we can prevent corrosion by removing one of these essential conditions. The simplest condition to remove is the exposed metal surface.

Establishing a Physical Barrier

Coating a metal surface with paint or enamel provides a barrier between the metal and the moisture in the environment, thus removing the opportunity for both oxygen and moisture to come in contact with the metal.

Sacrificial Coatings

The process of coating a metal surface with another metal that is more likely to be oxidized is referred to as sacrificial coating. The corrosion-prone iron alloy steel is commonly coated with zinc, a more active metal, in a process known as galvanizing. Corrosion of the sacrificial zinc results in its oxidation; the iron is reduced, which renders it cathodic and inhibits its corrosion.

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A galvanized surface: Protecting iron alloys with a coating of a more active metal through the process of galvanizing prevents the alloys from corroding.

A contrast to the previous scenario can be seen when iron or an iron alloy is plated with a less active metal, such as tin. As long as the tin coating remains intact, corrosion is not possible. If, however, the tin coating becomes degraded, exposing the underlying metal, corrosion will occur. This is because the exposed iron undergoes oxidation and is rendered anodic. The tin accepts electrons from the oxidized iron, and the three criteria for corrosion are met.

Cathodic Protection

Another way to protect against corrosion is to confer a continuous negative electrical charge on a metal. This method is referred to as cathodic protection. Cathodic protection replicates the effects of a sacrificial coating but with a more active metal. The source of negative charge is usually an external direct-current power supply. Cathodic protection is used to protect underground fuel tanks and pipelines, among other things.

Passivation

Passivation is a process through which a thin film of corrosion products builds on a metal surface to serve as a barrier against oxidation. The formation of a passivation layer is affected by environmental pH, temperature, and chemical conditions. The Statue of Liberty, for example, is coated with a blue-green patina caused by several chemical reactions, which serves to protect the copper metal underneath.

Anodization

Anodization is another surface treatment that protects against corrosion. The metal to be protected is bathed in a specific substance, and electrochemical conditions are adjusted such that uniform pores several nanometers wide appear in the metal’s oxide film. These pores allow an oxide film, thicker than a passivation layer, to build up. The resultant protective layer is very hard and very resilient.

Sacrificial Anode Protection

Using the same principle as sacrificial film coating, a sacrificial anode, made of a metal more active than the metal you want to protect, can be used to prevent corrosion on submerged or buried metal structures. The sacrificial anode will corrode before the metal it is protecting does. However, once the sacrificial anode corrodes, it must be replaced; otherwise, the metal it is protecting will begin to corrode as well.

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Cathodic protection prevents corrosion: Galvanic sacrificial anode attached to the hull of a ship; here, the sacrificial anode shows corrosion but the metal it is attached to does not. The anode, a piece of a more electrochemically “active” metal, is attached to the vulnerable metal surface where it is exposed to an electrolyte; the potential of the vulnerable surface is polarized to be more negative until the surface has a uniform potential. At that stage, the driving force for the corrosion reaction with the protected surface is removed. The galvanic anode continues to corrode, consuming the anode material until eventually it must be replaced, but the cathodic material is protected.

Corrosion presents a real threat to the integrity of personal property as well as to that of bridges, roads, and other public infrastructures. Understanding and implementation of strategies that prevent corrosion will decrease both the economic and safety-related damages associated with the process.