Question

In order to increase the corrosion resistance of stainless steel, the following element is added

• A. Nickel
• B. Molybdenum
• C. Carbon
• D. Chromium

Hint:

Chromium is the core element that makes stainless steel "stainless"! Always remember: Minimum 10.5% Chromium $\rightarrow$ Formation of a passive $\text{Cr}_2\text{O}_3$ surface oxide layer $\rightarrow$ Complete protection against corrosion and rust.

Answer 1

The Correct Option is D

Concept: Stainless steel is an iron-based alloy known for its exceptional corrosion resistance. This protective capability is achieved by adding specific alloying elements that alter the surface chemistry of the metal. The primary element responsible for this transformation is Chromium (Cr). To classify a steel alloy as "stainless," it must contain a minimum of approximately 10.5% to 12% chromium by weight. When exposed to oxygen in the atmosphere or water, chromium reacts preferentially to form an exceedingly thin, continuous, adherent, and chemically unreactive (passive) layer of chromium oxide ($\text{Cr}_2\text{O}_3$) on the surface of the steel. This invisible film acts as a highly effective physical barrier that seals the underlying iron matrix, stopping oxygen and moisture from causing oxidation (rusting). If the surface is scratched or damaged, the passive layer self-heals instantly in the presence of oxygen.

Step 1: Examine the metallurgical role of each listed option.
Let's analyze the typical engineering purpose of each alloying element in steel design:
• Nickel (Ni): Added to stabilize the austenite (FCC) crystal structure down to room temperature, which enhances ductility, toughness, and formability, while also providing moderate resistance to acid environments.
• Molybdenum (Mo): Added in smaller quantities (typically 2-4%) to improve resistance to localized pitting and crevice corrosion in chloride-rich environments (such as seawater). However, it is used as a secondary addition alongside chromium, not as the primary defining element.
• Carbon (C): Added primarily to increase hardness, tensile strength, and wear resistance through carbide formation and martensitic transformation. Higher carbon content can actually reduce corrosion resistance by forming chromium carbides, which depletes chromium at the grain boundaries (a problem known as sensitization).
• Chromium (Cr): The essential primary alloying element required to establish baseline corrosion resistance and passivity in stainless steels.

Step 2: Select the primary element based on the question context.
Since Chromium is the fundamental element required to form the passive protective oxide film that defines stainless steel, it is the correct choice. This matches Option (D).