Problem 86 - Chemistry: The Central Science

Question

An iron object is plated with a coating of cobalt to protect against corrosion. Does the cobalt protect iron by cathodic protection? Explain.

Step-by-Step Solution

Answer

Cobalt does not provide cathodic protection to iron objects because it has a lower standard reduction potential (-0.277V) than iron (-0.036V), meaning it is less likely to be reduced and cannot act as an effective cathode. Thus, the cobalt coating will not protect iron objects against corrosion in this manner.

Step 1: Understand the Corrosion process and Cathodic Protection

Corrosion is an electrochemical process where a metal, such as iron, is oxidized to form ions, while other substances, such as oxygen or water, are reduced. This process occurs in the presence of an electrolytic solution. Cathodic protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. This works by providing a more easily reduced substance that won't corrode, which will keep the original metal from corroding.

Step 2: Compare Standard Reduction Potentials

To determine if cobalt protects iron through cathodic protection, we need to compare the standard reduction potentials of cobalt and iron. Standard reduction potentials are measures of the tendency of a chemical species to acquire electrons and be reduced. A higher standard reduction potential implies that a species is more likely to be reduced. The standard reduction potential of iron (Fe) is: Fe³⁺ + 3e⁻ → Fe E° = -0.036V The standard reduction potential of cobalt (Co) is: Co²⁺ + 2e⁻ → Co E° = -0.277V

Step 3: Determine if Cobalt can provide Cathodic Protection

Since Co has a lower reduction potential than Fe, it is less likely to be reduced. However, for cathodic protection to occur, we need cobalt to act as an effective cathode, which means it must be more prone to reduction than iron. In this case, cobalt is not more prone to reduction than iron, which means that cobalt will not provide cathodic protection to iron objects.

Step 4: Explain the Conclusion

Cobalt does not provide cathodic protection to iron objects because it has a lower standard reduction potential than iron. Consequently, cobalt is less likely to be reduced than iron, meaning that it cannot act as an effective cathode to protect the iron. Instead, iron will still be prone to corrosion even with a cobalt coating, so it is not an ideal method for protecting iron against corrosion.