Question:
Which of the following elements, when present in iron, enhances its corrosion resistance?
Which of the following elements, when present in iron, enhances its corrosion resistance?
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Look for the element that forms a thin, self healing passive oxide film on the metal surface, the same mechanism used in stainless steel.
Updated On: Jul 28, 2026
- H
- Cr
- S
- C
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The Correct Option is B
Solution and Explanation
Step 1: Recall what corrosion resistance in iron depends on.
Iron corrodes because its natural surface oxide (rust, mainly hydrated iron oxide) is porous and does not protect the metal underneath. An alloying element raises corrosion resistance only if it forms a stable, adherent, self healing oxide film on the surface.
Step 2: Check option (A), hydrogen.
Hydrogen dissolves in iron and steel during processes like pickling or welding. It does not form any protective oxide. Instead it causes hydrogen embrittlement, which makes the metal crack under stress. It plays no role in raising corrosion resistance.
Step 3: Check option (B), chromium.
When the chromium content in iron goes above about 10.5 percent, it reacts with oxygen to form a thin, continuous layer of chromium oxide \(Cr_2O_3\) on the surface.
This layer is called a passive film. It sticks tightly to the metal and heals itself if scratched, so it blocks further attack by oxygen and moisture. This is exactly the mechanism used in stainless steels.
Step 4: Check option (C), sulfur.
Sulfur is normally an unwanted impurity in iron. It tends to form iron sulfide (FeS) at grain boundaries, which causes hot shortness (cracking during hot working) and can also act as a site where pitting corrosion starts. Sulfur lowers corrosion resistance rather than improving it.
Step 5: Check option (D), carbon.
Carbon is the basic element that turns iron into steel, but higher carbon content forms carbides such as \(Fe_3C\) at grain boundaries. In alloy steels these carbides can pull chromium out of the surrounding matrix (sensitization), which generally makes the microstructure more prone to corrosion, not less.
Step 6: Final answer.
Only chromium builds the passive oxide film that protects iron from corrosion.
\[ \boxed{\text{Cr}} \]
Iron corrodes because its natural surface oxide (rust, mainly hydrated iron oxide) is porous and does not protect the metal underneath. An alloying element raises corrosion resistance only if it forms a stable, adherent, self healing oxide film on the surface.
Step 2: Check option (A), hydrogen.
Hydrogen dissolves in iron and steel during processes like pickling or welding. It does not form any protective oxide. Instead it causes hydrogen embrittlement, which makes the metal crack under stress. It plays no role in raising corrosion resistance.
Step 3: Check option (B), chromium.
When the chromium content in iron goes above about 10.5 percent, it reacts with oxygen to form a thin, continuous layer of chromium oxide \(Cr_2O_3\) on the surface.
This layer is called a passive film. It sticks tightly to the metal and heals itself if scratched, so it blocks further attack by oxygen and moisture. This is exactly the mechanism used in stainless steels.
Step 4: Check option (C), sulfur.
Sulfur is normally an unwanted impurity in iron. It tends to form iron sulfide (FeS) at grain boundaries, which causes hot shortness (cracking during hot working) and can also act as a site where pitting corrosion starts. Sulfur lowers corrosion resistance rather than improving it.
Step 5: Check option (D), carbon.
Carbon is the basic element that turns iron into steel, but higher carbon content forms carbides such as \(Fe_3C\) at grain boundaries. In alloy steels these carbides can pull chromium out of the surrounding matrix (sensitization), which generally makes the microstructure more prone to corrosion, not less.
Step 6: Final answer.
Only chromium builds the passive oxide film that protects iron from corrosion.
\[ \boxed{\text{Cr}} \]
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