Which method effectively prevents corrosion by blocking contact with air and moisture?

Question

Examoogle · Accepted Answer

The correct answer is Option D: Applying paint or protective coating. Corrosion is an electrochemical process where metals deteriorate due to oxidation reactions with oxygen and moisture. Applying paint or protective coatings acts as a physical barrier that isolates the metal surface from the surrounding environment, thereby preventing the initiation of the corrosive process.

Step-by-Step Solution & Concepts:
Corrosion is an electrochemical process where metals deteriorate due to oxidation reactions with oxygen and moisture. Applying paint or protective coatings acts as a physical barrier that isolates the metal surface from the surrounding environment, thereby preventing the initiation of the corrosive process.

Working Principle: Corrosion requires the presence of oxygen ($O_2$) and moisture ($H_2O$) to facilitate the transfer of electrons between metal atoms and the oxidant. By applying a non-reactive layer like paint, grease, or enamel, the surface of the metal is sealed, preventing the diffusion of gaseous oxygen and liquid water to the metal substrate. Without this contact, the electrochemical half-cell reactions, specifically the oxidation of the metal ($M \rightarrow M^{n+} + ne^-$), are inhibited.

Key Formulas: $4Fe(s) + 3O_2(g) + 2xH_2O(l) \rightarrow 2Fe_2O_3 \cdot xH_2O(s)$ — The general chemical equation for the rusting of iron

Key Points: Corrosion is a spontaneous electrochemical oxidation process.; Iron corrosion is specifically referred to as rusting.; Protective barriers can be mechanical (paint), chemical (galvanization), or electrical (cathodic protection).; Moisture and air are the primary reactants needed for oxidation.

Advantages: Cost-effective and easy to apply.; Available in various aesthetic colors.; Can be reapplied as needed.

Disadvantages: The barrier is temporary and can chip or peel off.; Requires surface preparation for effective adhesion.

Applications: Iron railings and window grills.; Automobile bodies and car parts.; Structural steel in building construction.

Additional Info: Galvanization is another method where iron is coated with a layer of zinc, which provides both a physical barrier and sacrificial protection.. Option A increases temperature, which often accelerates chemical reactions including corrosion. Option B promotes heat-induced oxidation, and Option C (mixing with water) provides the necessary electrolyte for corrosion to occur faster.

Answer: D is correct — Applying paint or protective coating acts as a physical barrier that prevents oxygen and water from reaching the metal surface, effectively stopping the corrosion reaction.

Exam Tip: Always remember that corrosion requires three components: the metal, oxygen, and an electrolyte (usually water). Removing any one of these stops the process, which is the logic behind techniques like galvanization, alloying, and greasing.

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Option A: Exposing metal to sunlight

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Option B: Heating metal at high temperature

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Option C: Mixing metal with water

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