Question

Which metal, if coated onto iron, would prevent the corrosion of iron? (a) \(\mathrm{Mg}\) (b) \(\mathrm{Cr}\) (c) Cu

Short answer

Magnesium (\text{Mg}) would prevent the corrosion of iron as it is more reactive and will corrode first, protecting the iron.

Step-by-step solution

Understanding Corrosion Prevention

Corrosion prevention in metals can be achieved by coating the metal susceptible to corrosion with a more reactive metal. This more reactive metal can sacrifice itself to protect the underlying metal from corrosion through a process called galvanization.

Assessing Reactivity

To choose the suitable metal for coating iron to prevent its corrosion, compare the reactivity of the options given. The reactivity series shows that magnesium (Mg) is more reactive than iron (Fe), whereas chromium (Cr) and copper (Cu) are less reactive than iron.

Selecting the Correct Metal

Since magnesium is more reactive than iron, it will corrode first, thereby protecting the iron from corrosion. This makes magnesium (Mg) the correct choice for preventing the corrosion of iron.

Key concepts

Galvanization

Galvanization is a potent method employed for corrosion prevention, especially in iron and steel. The process involves applying a protective zinc coating to the metal to shield it from the oxidative effects of its environment. When zinc is applied to the surface of iron or steel, it forms a physical barrier that prevents moisture and oxygen from reaching the underlying metal.

Interestingly, zinc also offers sacrificial protection. If the coating is damaged and the base metal is exposed, zinc is more reactive in the presence of oxygen and moisture. Thus, it corrodes first, protecting the iron or steel from rusting. This sacrificial aspect is what makes galvanization so effective for long-term protection. It is widely used in constructing bridges, automobiles, and household items.

Reactivity Series

The reactivity series is an invaluable tool in predicting how different metals will react, especially when considering corrosion prevention strategies. This series ranks metals from the most reactive to the least reactive based on their tendencies to lose electrons and form positive ions.

Magnesium (Mg), for example, is higher up in the reactivity series compared to iron (Fe), making it more willing to oxidize. When a more reactive metal like magnesium is coated onto iron, it will corrode or oxidize preferentially in what's called a 'sacrificial anode' protection strategy. Understanding the reactivity series allows us to predict and control chemical reactions, such as those involved in corrosion prevention, ensuring the long-term durability of metal structures and components.

Protective Metal Coating

Protective metal coating involves covering a metal with a thin layer of another metal or substance to prevent corrosion. The key to its effectiveness lies in selecting the right coating material based on the working environment of the metal and the reactivity series.

In some cases, other metals like chromium (Cr) are used for coating, not for their reactivity, but because they form a passive layer that protects the metal beneath from further oxidative damage. Although chromium is less reactive than iron, it can be very effective at preventing surface corrosion, providing the metal with a hard, durable, and attractive surface finish. In contrast, copper (Cu) is also less reactive than iron, but it is not used for protection; instead, its use in coating is mainly for electrical conductivity or aesthetic purposes.