Alloying for corrosion resistance in steels — role of Ni and Cr State whether the statement is correct: “Corrosion resistance of steel is increased by adding nickel and chromium.”

Introduction / Context:
Stainless and corrosion-resistant steels rely on specific alloying elements to form protective surface films and to stabilize phases that resist attack. Nickel and chromium are the two most important elements in many corrosion-resistant steel grades.

Given Data / Assumptions:

• General atmospheric and aqueous corrosion scenarios.
• Alloying additions at levels typical of stainless and weather-resistant steels.

Concept / Approach:
Chromium is essential for passivation: above about 10.5–12% Cr, a thin, adherent chromium-oxide film forms that self-heals in oxidizing environments, creating “stainless” behavior. Nickel stabilizes the austenitic phase and improves resistance in many environments, enhancing toughness and resistance to certain acids and chloride stress corrosion cracking (in appropriate compositions). Together, Ni and Cr significantly increase corrosion resistance beyond that of plain carbon steel.

Step-by-Step Solution:
Recognize chromium’s passivation role → protective Cr2O3 film.Recognize nickel’s role → austenite stabilization and improved corrosion performance in many media.Therefore, the statement that Ni and Cr increase corrosion resistance is correct.

Verification / Alternative check:
Common stainless families (austenitic 300-series, ferritic 400-series) are based on Cr (and often Ni). Data sheets show large improvements in corrosion rate compared with unalloyed steels.

Why Other Options Are Wrong:

• “Only chromium” or “only nickel” neglect how many grades rely on both.
• “True only above 20% Cr” is unnecessarily restrictive; many grades with ~12–18% Cr already show strong corrosion resistance.
• “Disagree” contradicts well-established metallurgy.

Common Pitfalls:
Assuming nickel alone creates stainless behavior. Chromium-driven passivation is the cornerstone; nickel enhances and broadens resistance in many environments.

Final Answer:
Agree