Which one of the following is not true for anodizing process?

1. Corrosion and Methods of Prevention (basic)

Corrosion is the gradual deterioration of a metal surface when it reacts with substances in its environment, such as moisture, oxygen, or acids. From a chemistry perspective, it is an oxidation reaction where the metal loses electrons and transforms into a more stable state like an oxide, hydroxide, or sulphide. While we often focus on the damage it causes to bridges and car bodies, it is essentially nature's way of returning refined metals to their original ore-like state. Science, Class X, Chemical Reactions and Equations, p.13

Different metals react differently to the atmosphere, leading to distinct visual results:

• Iron: Develops a reddish-brown flaky substance called rust (hydrated iron oxide) when exposed to moist air for long periods. Science, Class X, Metals and Non-metals, p.53
• Silver: Turns black because it reacts with sulphur in the air to form a coating of silver sulphide (Ag₂S).
• Copper: Reacts with moist carbon dioxide to lose its shiny brown surface and gain a green coat of basic copper carbonate. Science-Class VII, The World of Metals and Non-metals, p.50

To prevent this economic and structural loss, we use various protection methods. A highly effective industrial technique for metals like aluminium is Anodizing. Unlike painting, which adds a layer on top, anodizing uses electrolysis to thicken the metal's natural oxide layer. During this process, a clean aluminium article is made the anode (the positive electrode) in an electrolytic cell containing dilute sulphuric acid (H₂SO₄). When current passes through, oxygen gas is evolved at the anode. This oxygen reacts immediately with the aluminium surface to create a hard, durable, and corrosion-resistant layer of aluminium oxide (Al₂O₃). Science, Class X, Metals and Non-metals, p.42

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.13; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.53; Science-Class VII, NCERT (Revised ed 2025), The World of Metals and Non-metals, p.50; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.42

2. Fundamentals of Electrolysis: Anode and Cathode (basic)

At its core, electrolysis is a process that uses direct electric current (DC) to drive a chemical reaction that wouldn't happen on its own. Imagine it as a way of using "electrical force" to pull molecules apart. This happens in an electrolytic cell, which consists of two solid conductors called electrodes dipped into a liquid called an electrolyte. The electrolyte is a substance (like a salt or acid solution) that contains free-moving ions, allowing it to conduct electricity Science, Class VIII, NCERT (Revised ed 2025), Electricity: Magnetic and Heating Effects, p.55.

The two electrodes have very specific roles based on how they are connected to the power source. Because opposites attract, the charged ions in the liquid migrate toward the electrode with the opposite charge. In an electrolytic cell:

This process is the only way to extract highly reactive metals like Sodium (Na), Magnesium (Mg), and Aluminium (Al) from their ores. Because these metals have a stronger affinity for oxygen than carbon does, they cannot be "reduced" by heating with carbon. Instead, we use electrolytic reduction. For instance, when we electrolyze molten sodium chloride (NaCl), the pure metal sodium is deposited at the cathode, while chlorine gas is liberated at the anode Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.52. Similarly, in the Chlor-alkali process used for making bleach and soaps, chlorine gas is evolved at the anode and hydrogen gas at the cathode Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.30.

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.52; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.30; Science, Class VIII, NCERT (Revised ed 2025), Electricity: Magnetic and Heating Effects, p.55

3. Properties of Aluminium: The Passive Oxide Layer (intermediate)

4. Strategic Materials: Alloys in Aviation and Space (intermediate)

In the high-stakes world of aviation and aerospace, materials must be more than just strong; they must be strategic. This means they need a high strength-to-weight ratio and the ability to withstand extreme environmental stress. Aluminium is the cornerstone of this industry because of its elasticity, conductivity, and light weight. As noted in Geography of India, Industries, p.38, it is vital for manufacturing aeroplanes and defence accessories. However, pure metals often lack the specific resilience needed for supersonic speeds or space vacuums, which is why we turn to alloying and surface treatments.

Alloying is the art of mixing metals to enhance their properties. For instance, adding Nickel to iron makes it tougher and more ductile (ideal for armour plating), while Chromium allows iron to resist rusting by forming stainless steel (Certificate Physical and Human Geography, Manufacturing Industry, p.284). In aviation, we take this a step further with Anodizing. This is an electrochemical process that artificially thickens the natural oxide layer on metals like aluminium. By making the aluminium article the anode in an electrolytic cell, we use electricity to evolve oxygen gas at its surface. This oxygen reacts immediately with the metal to form a thick, integrated layer of Aluminium Oxide (Al₂O₃), which is incredibly hard and corrosion-resistant.

During this electrolysis, while oxygen is doing its work at the anode, hydrogen gas is typically evolved at the cathode. This process is fundamental to the aerospace industry because it ensures that the structural components of an aircraft do not degrade when exposed to moisture or chemicals at high altitudes. It transforms a common metal into a durable, high-performance material capable of enduring the rigours of flight (Science, Metals and Non-metals, p.56).

Sources: Geography of India, Industries, p.38-39; Certificate Physical and Human Geography, Manufacturing Industry, p.284; Science, Metals and Non-metals, p.56

5. The Anodizing Process: Mechanism and Science (exam-level)

Nature has a clever way of protecting certain metals. When aluminium is exposed to air, it spontaneously forms a very thin, invisible layer of aluminium oxide (Al₂O₃). This layer acts as a "skin" that prevents further oxygen from reaching the metal underneath, effectively stopping corrosion in its tracks Science, Class X (NCERT 2025 ed.), Chapter 3, p. 42. However, for industrial uses—like in aircraft or high-end cookware—this natural layer is too thin and fragile. Anodizing is the controlled electrochemical process we use to intentionally thicken this protective layer, making the metal significantly more durable and resistant to wear.

To understand the science, we look at the setup of an electrolytic cell. In this process, a clean aluminium article is made the anode (the positive electrode) and is immersed in an electrolyte, typically dilute sulphuric acid (H₂SO₄). When an electric current is passed through the solution, a specific chemical reaction occurs at each electrode. It is crucial to remember that in this setup, oxygen gas is evolved at the anode. This freshly released oxygen doesn't just bubble away; it reacts immediately with the surface of the aluminium anode to create a uniform, robust oxide coating.

One of the most interesting properties of this newly formed oxide layer is its porous structure. Before the layer is "sealed," these tiny pores can absorb dyes, allowing manufacturers to give aluminium products vibrant, metallic colors that won't chip or peel off, because the color is actually trapped inside the oxide layer itself Science, Class X (NCERT 2025 ed.), Chapter 3, p. 42. This combination of strength, lightweight nature (compared to steel), and aesthetic versatility is why anodized aluminium is the gold standard in the aircraft industry and modern architecture.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.42

6. Solving the Original PYQ (exam-level)

Now that you have mastered the basics of electrolysis and oxidation, this question serves as a perfect application of those principles. In your previous lessons, you learned that oxidation occurs at the anode. The term anodizing itself is a huge clue; it implies that the metal article is being treated as the anode in an electrochemical cell. As the current passes through the acid electrolyte, oxygen gas is released specifically at the anode, where it reacts with the metal surface to create a thickened, protective oxide layer. This is the core mechanism explained in Science, class X (NCERT 2025 ed.).

To solve this, you must look for the conceptual inconsistency in the statements. While Option (A) and Option (B) correctly describe the purpose (corrosion resistance) and the materials (Al, Ti, Mg) involved, Option (C) contains a technical trap. It correctly identifies the aluminium as the anode but falsely claims oxygen is evolved at the cathode. In reality, hydrogen gas is evolved at the cathode, while oxygen is evolved at the anode to facilitate oxidation. This subtle swap of gas locations is a classic UPSC trap designed to test whether you understand the specific directional flow of ions and gases during electrolysis.

Finally, Option (D) is a factual application of the process. Because anodizing creates a surface that is both lightweight and highly durable, it is a staple in the aircraft industry where weight-to-strength ratios are critical. Therefore, since the question asks for the statement that is not true, Option (C) is the correct answer because it misidentifies the site of oxygen evolution. Always remember: in anodizing, the action happens at the anode!