How much chromium is required for stainless steel?

To qualify as stainless steel, an alloy must contain at least 10.5% chromium by mass. This minimum chromium level is essential for forming a protective passive layer of chromium oxide, which prevents rust and corrosion on the steel’s surface.

Chromium Content Range by Type

• Minimum Requirement: 10.5% chromium is the baseline for stainless classification. Below this threshold, the passive oxide layer is not stable enough to resist oxidation effectively.
• Common Range: Most commercial stainless steels contain 10–30% chromium, depending on grade and intended use.
• Austenitic Steels (e.g., 304, 316): 16–24% chromium, providing high corrosion resistance and good formability.
• Ferritic Steels: 12–30% chromium, with low carbon content and good oxidation resistance.
• Martensitic Steels: 11–18% chromium, allowing them to be hardened while maintaining moderate corrosion resistance.

Functional Role of Chromium

Chromium serves as the primary passivating element in stainless steel. It reacts with oxygen to form a thin, adherent, and self-repairing chromium oxide film (Cr₂O₃), which acts as a barrier against further oxidation and corrosion. Increasing chromium content enhances protection against high temperatures and aggressive chemical environments.

For example,

• 10.5% Cr offers oxidation resistance up to about 700 °C,
• 16% Cr protects up to nearly 1200 °C,
• 18% Cr (as in 304 steel) achieves balanced corrosion and heat resistance.