Question

Etchant used for steels is

• A. mixture of sodium thiocyanate and ammonium nitrate
• B. mixture of nitric acid and ethyl alcohol
• C. sodium picrate solution
• D. mixture of ammonia and ferric chloride

Hint:

For steel metallography, remember these two key etchants:

• Nital (Nitric acid + Alcohol): The most common, general-purpose etchant.

• Picral (Picric acid + Alcohol): Also common, gives better resolution of fine pearlite.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
In metallography, an etchant is a corrosive chemical solution used to reveal the microstructure of a polished metal sample. Different phases and grain boundaries corrode at different rates, creating topographical contrast that can be seen with a microscope. The question asks for a common etchant for steels.

Step 2: Detailed Explanation:
Let's analyze the options:

• mixture of sodium thiocyanate and ammonium nitrate: This is not a standard etchant for steels.

• mixture of nitric acid and ethyl alcohol: This is the composition of

Nital, one of the most widely used etchants for carbon steels, low-alloy steels, and cast irons. Typically, it's a solution of 1-5% concentrated nitric acid in ethanol or methanol. It effectively reveals the boundaries between ferrite and cementite in pearlite, as well as grain boundaries.

• sodium picrate solution: This is also an etchant for steels, but it is used for a specific purpose: to selectively darken or tint the cementite (\(Fe_3C\)) phase, leaving ferrite bright. While it is used for steels, Nital is the more general-purpose and common etchant.

• mixture of ammonia and ferric chloride: This etchant is more commonly used for copper alloys like brass and bronze.
Given the options, the mixture of nitric acid and ethyl alcohol (Nital) is the most common and universally recognized etchant for general-purpose microstructural analysis of steels.

Step 3: Final Answer:
The most common etchant used for revealing the microstructure of steels is Nital, which is a mixture of nitric acid and ethyl alcohol.