Question

The metal that forms a self-protecting film of oxide to prevent corrosion is : (a) Na (b) \(\mathrm{Al}\) (c) Cu (d) Au

Short answer

The metal that forms a self-protecting film of oxide to prevent corrosion is (b) \text{Al}.

Step-by-step solution

- Understanding Corrosion Resistance

Corrosion resistance refers to the ability of a metal to withstand damage caused by oxidation or other chemical reactions. Some metals can form a self-protecting oxide layer that reduces or prevents further corrosion.

- Identifying Metals That Form Protective Oxides

Among the options given, sodium (Na) is highly reactive and does not form a protective oxide layer. Copper (Cu) can form a layer of patina, which is not an oxide and doesn't fully protect it. Gold (Au) is noble and does not corrode but does not form an oxide layer. Aluminum (Al) forms a very thin and strong layer of aluminum oxide that protects it from further oxidation or corrosion.

- Choosing the Correct Answer

Aluminum (Al) is known for forming a self-protecting film of aluminum oxide on its surface when exposed to the atmosphere. This film prevents further corrosion of the metal.

Key concepts

Protective Oxide Layers

One of the remarkable traits of certain metals is their ability to form a protective oxide layer when exposed to oxygen in the environment. This natural phenomenon acts as a shield, preventing ongoing reactions with oxygen and other potentially corrosive elements. The formation of this layer is crucial in the battle against corrosion, a process that degrades metals and can lead to structural failures and safety hazards.

How do these layers actually work? Picture a metal surface, as it reacts with oxygen, a thin film develops—this is the oxide layer. It clings tenaciously to the surface and, crucially, is less reactive than the metal beneath. This means that it resists further reaction with gases or liquids that might continue the corrosion process. It's somewhat similar to applying a coat of paint on wood to protect it from weathering.

Why doesn't every metal form this layer?

Not all metals are capable of forming protective oxides. Some, like sodium (Na), interact with oxygen so readily that the resulting oxide doesn't adhere well, providing no lasting protection. Understanding which metals form protective layers and the conditions under which these layers are stable is a vital part of materials science and plays a significant role in metallurgy and industrial design.

Chemical Reactions in Metallurgy

Metallurgy involves the extraction, refining, alloying, and processing of metals. A core aspect of metallurgy is understanding chemical reactions that metals undergo, particularly those that affect their corrosion resistance. These reactions can be complex and varied, often depending on the surrounding environment and the metal's properties.

For example, in the process of extracting aluminum from its ore, bauxite, an electrolytic reduction reaction takes place in which aluminum oxide is converted to aluminum metal. This same principle of reactivity goes on to play a crucial role in the metal's behavior once it's in use. Knowing how metals react, whether they are likely to form an oxide layer or not, and predicting their durability, are essential skills for metallurgists and engineers.

The delicate balance of metallurgic reactions

Optimizing metallurgical processes often require control of oxygen levels, temperature, and other factors to minimize unwanted reactions and promote beneficial ones, like the formation of protective oxide layers. The study and application of these principles ensure that the right metal is used for the right application, especially in environments where corrosion could be a significant issue.

Properties of Aluminum

Aluminum (Al) is known for its lightweight, strength, and corrosion resistance, making it an indispensable material in multiple industries— from aviation to food packaging. Its resistance to corrosion is due to the thin layer of aluminum oxide that forms almost instantaneously when the pure metal is exposed to air.

This naturally occurring aluminum oxide layer is remarkably impermeable, compact, and sticks to the aluminum substrate with severe tenacity, thus preventing further oxidative damage. It's these properties that give aluminum its impressive durability, especially important for applications where the metal will be exposed to harsh environmental conditions. Importantly, aluminum is also non-toxic and non-sparking, offering additional safety benefits.

Beyond corrosion resistance

Apart from the oxide layer, aluminum's properties like high thermal and electric conductivity, ductility, and the ability to form alloys with other metals broaden its utility. Moreover, when aluminum does react with oxygen to form its protective layer, the reaction is exothermic but self-limiting, which underscores its safety as a construction material.