309/309S vs. 310/310S at High Temperatures

Oxidation Resistance of 309/309S vs. 310/310S at High Temperatures

Oxidation and Stainless Steel

Under most conditions, metal alloys react with their environment. The most common reaction is oxidation, where metals combine with oxygen to form oxides.

In stainless steel, chromium helps resist oxidation. A thin, stable chromium oxide layer (Cr₂O₃), called chromia, is formed on the surface. This layer grows slowly and adheres well to the metal, preventing faster-forming oxides from developing.

	Type 309 (UNSS30900)	Type 309S (UNS S30908)	Type 309H (UNS S30909)	Type 309Si (DIN 1.4828)	Type 310 (UNS S31000)	Type 310S (UNS S31008)	Type 310H (UNS S31009)	Type 310Si (DIN 1.4841)
C	0.2	0.08	0.04 min 0.10 max	0.2	0.25	0.08	0.04 min 0.10 max	0.2
N	—	—	—	0.11	—	—	—	0.11
Mn	2	2	2	2	2	2	2	2
P	0.045	0.045	0.045	0.045	0.045	0.045	0.045	0.045
S	0.03	0.03	0.03	0.015	0.03	0.03	0.03	0.015
SI	0.75	0.75	0.75	1.50 min 2.50 max	1.5	1.5	0.75	1.50 min 2.50 max
Cr	22.00 min 24.00 max	22.00 min 24.00 max	22.00 min 24.00 max	19.00 min 21.00 max	24.00 min 26.00 max	24.00 min 26.00 max	24.00 min 26.00 max	24.00 min 26.00 max
Ni	12.00 min 15.00 max	12.00 min 15.00 max	12.00 min 15.00 max	11.00min 13.00 max	19.00 min 22.00 max	19.00 min 22.00 max	19.00 min 22.00 max	19.00 min 22.00 max
Fe	Balance	Balance	Balance	Balance	Balance	Balance	Balance	Balance

How Protection Works

Once a thick and stable chromium oxide layer is formed, it blocks further oxidation. As the layer grows thicker, oxidation slows down. This is because oxygen and metal ions must travel farther through the oxide layer.

This behavior is called protective scale formation. It is similar to the passivation process seen in corrosion at lower temperatures.

Alloy Composition and Oxidation Resistance

The chromium content in stainless steel plays a key role in high-temperature oxidation resistance.

Alloys with at least 20% chromium are usually considered heat-resistant.
Replacing iron with nickel can also improve high-temperature performance.

309/309S vs. 310/310S

Both ATI 309 and ATI 310 are highly alloyed austenitic stainless steels. Because of their composition, they offer excellent resistance to oxidation, especially at elevated temperatures.

How Oxidation Is Measured

The resistance to oxidation is often measured by tracking weight gain after exposure to high heat.

When a metal oxidizes, it absorbs oxygen and becomes heavier.
More weight gain usually means more oxidation and lower resistance.
Less weight gain suggests better protection and higher resistance.

Summary

In summary:

309/309S and 310/310S form protective chromium oxide layers.
Higher chromium and nickel levels improve oxidation resistance.
310/310S, having more alloying elements, typically perform better in extreme heat.
Weight change testing is used to compare their oxidation performance over time.