Question

Which of the following metals can offer cathodic protection to iron? Select all the correct choices. \(\mathrm{Mn} \mathrm{Ni} \mathrm{Ph}\)

Short answer

Mn can offer cathodic protection to iron.

Step-by-step solution

Understand Cathodic Protection

Cathodic protection involves using a more reactive metal (anode) to prevent corrosion of a less reactive metal (cathode) such as iron. The more reactive metal should have a higher tendency to lose electrons compared to iron, meaning it has a lower reduction potential.

List the Metals with Their Reduction Potentials

Review the standard reduction potentials for Mn, Ni, Ph, and Fe. The relevant approximate potentials are: Mn: -1.18 V, Ni: -0.23 V, Fe: -0.44 V (for \( ext{Fe}^{2+}\) to \( ext{Fe}\)), and Ph (Lead): -0.13 V.

Compare Reduction Potentials

For effective cathodic protection, the chosen metal must have a more negative reduction potential than iron. The reduction potential of manganese ( Mn: -1.18 V) is more negative compared to iron ( Fe: -0.44 V), meaning Mn is more reactive and can serve as a sacrificial anode. Nickel and lead have less negative potentials than iron and thus cannot.

Key concepts

Reduction Potential

The concept of reduction potential is vital for understanding cathodic protection. In electrochemistry, it refers to the ability of a substance to gain electrons. This is quantified using voltage, where a more negative value indicates a higher tendency to lose electrons, making the element a better candidate for oxidation. In the context of cathodic protection, it's essential to select an anode metal with a more negative reduction potential than the cathode metal (like iron). This ensures the anode will preferentially oxidize and protect the cathode from corrosion. By examining reduction potentials, we determine which metals are more likely to corrode and thereby offer cathodic protection to less reactive metals.

Reactive Metals

Reactive metals are those that readily participate in chemical reactions, primarily oxidation. This reactivity is closely related to their electron configuration; they tend to lose electrons easily. Metals like manganese, with its high reactivity, serve effectively as sacrificial anodes. They sacrifice themselves by corroding in place of the protected metal. In our example, manganese, with a higher activity due to its lower reduction potential (-1.18 V), offers better protection compared to nickel and lead. Understanding the reactivity series helps identify suitable metals for cathodic protection.

Metal Corrosion

Corrosion is the gradual degradation of metals due to environmental reactions, often involving oxygen and moisture. This process can lead to significant structural damage over time. In electrochemical terms, corrosion involves oxidation at the anode, a less favorable reaction. Cathodic protection helps mitigate corrosion by making the protected metal the cathode of an electrochemical cell. This approach shifts corrosion risk to the more reactive anode metal, preserving the integrity and longevity of the structure. Techniques to prevent corrosion include selecting appropriate materials and employing protection systems such as coatings or cathodic protection methods.

Sacrificial Anode

A sacrificial anode is a key component in the cathodic protection strategy. It's a highly reactive metal that effectively "sacrifices" itself, corroding in place of the more important metal structure it's intended to protect. By being connected to the metal needing protection, the sacrificial anode undergoes oxidation, thereby preserving the structural integrity of the less reactive metal, like iron in pipelines. Manganese serves as an excellent example of a sacrificial anode due to its low reduction potential compared to iron, enabling it to protect the iron effectively by corroding first. This process is indispensable in industries that rely on the durability of metal structures exposed to harsh environments.