Stainless steels — identify the element chiefly responsible for corrosion resistance Which alloying element is primarily responsible for imparting passivation and corrosion resistance to stainless steels?

Introduction / Context:
Stainless steels are defined by their ability to form a thin, adherent, self-healing oxide film that protects the underlying metal from corrosion. The alloying chemistry that enables this passivation must be understood to choose the correct grade and to diagnose failures.

Given Data / Assumptions:

• Typical stainless steels have chromium above about 10.5–12% by mass.
• Other elements (Ni, Mo, N, Si, Mn) modify mechanical and corrosion behavior but do not substitute for the passivating element.

Concept / Approach:
Chromium is the essential element. It forms a continuous chromium oxide film (Cr2O3) that passivates the steel. Nickel stabilizes the austenitic phase and improves formability and certain corrosion modes but does not by itself create stainless behavior. Molybdenum improves pitting and crevice corrosion resistance in chlorides. Carbon must be controlled to avoid sensitization (chromium carbide precipitation) that undermines passivity.

Step-by-Step Solution:

Verification / Alternative check:
Standards define “stainless” at a minimum chromium content; corrosion testing shows sharp deterioration when effective chromium is tied up in carbides.

Why Other Options Are Wrong:

• Silicon and manganese: deoxidisers and strength modifiers; limited effect on passivation.
• Carbon: must be minimized; excess harms corrosion resistance.
• Nickel: important austenite stabilizer but not the main passivator.

Common Pitfalls:
Assuming nickel is what makes steel stainless; overlooking sensitization near 500–800°C.

Final Answer: