Problem 11 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Identify the correct statement : [Main Online April 10, 2016] (a) Corrosion of iron can be minimized by forming a contact with another metal with a higher reduction potential (b) Iron corrodes in oxygen free water (c) Corrosion of iron can be minimized by forming an impermeable barrier at its surface (d) Iron corrodes more rapidly in salt water because its electrochemical potential is higher

Step-by-Step Solution

Answer

The correct statement is (c). Corrosion of iron can be minimized by forming an impermeable barrier at its surface.

Step 1: Examine Corrosion Prevention Methods

Corrosion of iron can be minimized by preventing contact between iron and elements that promote oxidation, such as water or air. Forming a contact with a metal with a higher reduction potential does not minimize corrosion; instead, a metal with a *lower* reduction potential should be used to act as a sacrificial anode, such as zinc in galvanization. Thus, option (a) is incorrect.

Step 2: Analyze Corrosion Conditions

Corrosion of iron typically requires oxygen. In an oxygen-free environment, such as deoxygenated water, iron will corrode at a significantly reduced rate, if at all, because one of the main reactants in the oxidation reaction is missing. Therefore, iron does not corrode in oxygen-free water. Thus, option (b) is incorrect.

Step 3: Consider Protective Barriers

To minimize corrosion, creating an impermeable barrier over iron is an effective method. This barrier, such as paint or another protective coating, prevents contact with water and oxygen, reducing corrosion significantly. Thus, option (c) is a correct statement.

Step 4: Assess the Impact of Salt Water

Iron corrodes rapidly in salt water due to the presence of electrolytes, which accelerate the electrochemical reactions necessary for corrosion. The higher concentration of electrolytes increases the conductivity of the water, but it is not due to an increased electrochemical potential of iron. Thus, option (d) is incorrect.

Key Concepts

Electrochemical Reactions

Electrochemical reactions are the driving force behind the corrosion of iron. Corrosion is essentially an electrochemical process where iron undergoes oxidation. This means iron loses electrons and transforms into iron ions. In the rusting process, the electrons lost by iron are accepted by oxygen molecules, which in turn get reduced. The presence of water greatly facilitates this exchange of electrons, completing the electrochemical circuit.
Oxidation occurs on the surface of the iron, while reduction typically happens adjacent to the metal surface where oxygen is available. These reactions create an electrochemical cell, much like a battery, driving the continuous degradation of the iron material. Corrosion can be more aggressively promoted by the presence of salts, as they increase the electrolyte content in water, enhancing ion mobility and speeding up these electrochemical reactions.
Understanding these reactions is crucial for finding methods to disrupt or prevent them, particularly by introducing barriers or alternative reactions that redirect or consume the electrons involved in iron's degradation.

Protective Barriers

Protective barriers are a simple and effective method to prevent iron corrosion. By covering the iron with an impermeable layer, you cut off its exposure to water and oxygen, both of which are required for corrosion to proceed. Common protective barriers include paint, varnish, and metallic coatings like zinc plating, otherwise known as galvanization.
Galvanization is highly effective because zinc not only covers the iron but also acts as a sacrificial anode if the barrier is breached. In such cases, zinc will corrode in preference to the iron, thereby protecting it. This principle is based on the zinc's lower reduction potential, allowing it to sacrifice itself electrochemically to save the iron underneath. By maintaining the barrier's integrity, you can significantly extend the lifespan of iron structures by preventing the electrochemical imbalance that leads to corrosion.

Reduction Potential

The reduction potential is a key concept in understanding how different metals can either contribute to or prevent corrosion. It is essentially a measure of a metal's tendency to gain electrons, or be reduced. Metals with higher reduction potentials are less likely to lose electrons and be oxidized themselves. This property is important in designing anti-corrosion strategies.
In the context of preventing iron corrosion, using metals with lower reduction potential, like zinc, is crucial. When zinc and iron are in electrical contact, zinc, with its lower reduction potential, will oxidize in preference to iron. This process is used in galvanization to protect iron-based structures. Conversely, metals with higher reduction potentials than iron, such as copper, would not be beneficial for protection as they would accelerate the corrosion of iron through galvanic corrosion.
Understanding reduction potentials helps in selecting the right materials for engineering robust, corrosion-resistant structures.

Saltwater Corrosion

Saltwater corrosion is a significant challenge due to the aggressive nature of the electrolytes present in saltwater. These dissolved salts increase the conductivity of water, which means ions can move more easily and conduct electricity better. This results in a faster rate of electrochemical reactions that lead to iron corrosion.
Iron corrodes more rapidly in saltwater because the presence of sodium chloride accelerates the breakdown of the protective oxide layer on iron's surface, exposing it to more oxygen and water. The process is chemically and electrochemically supported by the increased mobility of ions, facilitating both oxidation and reduction reactions of iron corrosion.
This explains why structures such as ships and oil rigs require extensive protective measures, including coatings and cathodic protection, to withstand the harsh conditions of saltwater environments. Such measures aim to reduce the rate of electron exchange in the corrosive cycle, thereby increasing the durability and structural integrity of iron-based materials.