Which one of the following is not true for anodizing process?
Clean aluminium article is the anode and oxygen gas is evolved at the cathode.
The question asks us to identify the statement that is not true for the anodizing process. Let's analyze each option to determine the incorrect statement regarding this surface finishing technique.
Understanding the Anodizing Process
Anodizing is an electrochemical process that converts the metal surface into a durable, corrosion-resistant anodic oxide finish. It involves immersing the metal part (like aluminium) in an electrolyte bath and passing an electric current through it. The metal part serves as the anode (the positive electrode).
Analyzing the Options for Anodizing
Let's evaluate each statement:
Option 1: Corrosion Resistance Improvement
Statement: It makes aluminium corrosion resistant.
Analysis: This is a primary benefit of the anodizing process. The oxide layer formed enhances the metal's resistance to corrosion and wear. Therefore, this statement is true.
Option 2: Applicable Metals
Statement: Metals like aluminium, titanium and magnesium can be anodized.
Analysis: While most common in aluminium, the anodizing process can indeed be applied to other metals like titanium and magnesium to improve their surface properties. Therefore, this statement is generally considered true.
Option 3: Electrode Roles and Gas Evolution
Statement: Clean aluminium article is the anode and oxygen gas is evolved at the cathode.
Analysis: This statement contains two parts. Let's examine each:
- "Clean aluminium article is the anode": In the anodizing process, the metal being treated, such as aluminium, is connected to the positive terminal of the power supply, making it the anode. This part of the statement is correct.
- "oxygen gas is evolved at the cathode": At the cathode (the negative electrode), reduction occurs. In aqueous electrolytes commonly used for anodizing (like sulfuric acid), water is reduced, leading to the evolution of hydrogen gas (\(H_2\)). The reaction is typically: \(2H_2O + 2e^- \rightarrow H_2(g) + 2OH^-\) Conversely, oxygen gas (\(O_2\)) is typically evolved at the anode (the aluminium surface) due to the oxidation of water or anions from the electrolyte. The reaction can be represented as: \(2H_2O \rightarrow O_2(g) + 4H^+ + 4e^-\) Therefore, the claim that oxygen gas evolves at the cathode is incorrect.
Since one part of the statement is incorrect, the entire statement is not true.
Option 4: Use in Aircraft Industry
Statement: It is used in the aircraft industry.
Analysis: The enhanced durability, corrosion resistance, and aesthetic qualities provided by anodizing make it highly valuable in the aircraft industry for components made of aluminium alloys. Therefore, this statement is true.
Conclusion
Based on the analysis, the statement that inaccurately describes the anodizing process is the one concerning the location of oxygen gas evolution. In anodizing, the aluminium article functions as the anode, and it is at the anode that oxygen gas is primarily evolved, while hydrogen gas is evolved at the cathode.
Therefore, the statement "Clean aluminium article is the anode and oxygen gas is evolved at the cathode" is the one that is not true for the anodizing process.
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