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Difficulty: MediumCorrosion of Metals and Prevention Methods

Four identical iron nails are placed into separate test tubes filled with aerated sodium chloride solution. Nail 1 is wrapped with a zinc wire, Nail 2 is wrapped with a copper wire, Nail 3 is completely covered with petroleum jelly, and Nail 4 is left bare. After three days, Nail 2 exhibits significantly more severe rusting than the untreated Nail 4. Which of the following best explains why coupling iron with copper accelerates the corrosion of iron?

  1. Iron is more electropositive than copper, causing iron to serve as the anode and undergo rapid oxidation.Answer
  2. B
    Copper acts as a sacrificial anode by transferring positive ions to iron, increasing rust accumulation.
  3. C
    Copper reacts with water to release hydrogen ions, which directly oxidize the iron surface.
  4. D
    Iron acts as the cathode and undergoes cathodic reduction due to the higher electrical conductivity of copper.

Answer

Iron is more electropositive than copper, causing iron to serve as the anode and undergo rapid oxidation.
When iron is in electrical contact with a less reactive metal like copper in the presence of an electrolyte, a galvanic cell is formed. Because iron has a higher oxidation potential than copper, iron acts as the anode and is oxidized to iron(II) ions much faster than if it were un-coupled. Copper acts as the cathode where atmospheric oxygen is reduced.

Step-by-Step Solution

1
Identify the relative positions of iron and copper in the electrochemical series.
Iron (FeFe) is more electropositive (higher tendency to oxidize) than copper (CuCu).
Metals higher in the reactivity series lose electrons more readily.
2
Determine the galvanic cell configuration formed when iron and copper are in contact in an electrolyte.
Iron becomes the anode (site of oxidation: Fe→Fe2++2e−Fe \rightarrow Fe^{2+} + 2e^-) while copper acts as the cathode (site of reduction: O2+2H2O+4e−→4OH−O_2 + 2H_2O + 4e^- \rightarrow 4OH^-).
Electrons flow from the more reactive metal (anode) to the less reactive metal (cathode).
3
Evaluate the effect of this galvanic coupling on the corrosion rate of iron.
The oxidation of iron is significantly accelerated compared to un-coupled iron.
The presence of a cathodic metal (copper) facilitates the removal of electrons from iron, speeding up rust formation.

Key Concept

Galvanic Corrosion and Electrochemical Series
Estimated Time:1m 0s
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Topics

Corrosion of Metals and Prevention Methods

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