Stainless steels — essential alloying addition For manufacturing stainless steel, carbon steel is alloyed primarily with which element to achieve corrosion resistance?

Introduction / Context:
Stainless steels rely on a protective passive film to resist corrosion. Selecting the correct alloying element(s) is essential for achieving this passive behavior in aggressive environments.

Given Data / Assumptions:

• Corrosion resistance in stainless steels emerges when chromium content is above a threshold (about 10.5%).
• Nickel, molybdenum, and others tailor phase balance and localized corrosion resistance but are not the fundamental passivators.

Concept / Approach:
Chromium forms a stable, self-healing chromium oxide film on the steel surface in the presence of oxygen and moisture. This passive film drastically slows corrosion rates. Nickel stabilizes the austenitic structure and improves toughness; molybdenum enhances pitting resistance; tungsten is rarely used for general stainless grades.

Step-by-Step Solution:
Identify the passivating element: chromium → Cr2O3 passive layer.Confirm threshold: ≥ 10.5% Cr defines stainless behavior.Therefore, the essential alloying addition is chromium.

Verification / Alternative check:
All standard stainless families (ferritic, martensitic, austenitic, duplex) include chromium above the threshold; compositions in codes and datasheets confirm this.

Why Other Options Are Wrong:
Nickel and molybdenum are beneficial but not strictly essential for passivity; tungsten is uncommon; “none” contradicts the definition of stainless steel.

Common Pitfalls:
Assuming nickel creates corrosion resistance; overlooking low-nickel ferritic/martensitic stainless steels that are still stainless due to chromium.

Final Answer:
Chromium