How does the oxidation resistance of duplex stainless steel pipes perform in high-temperature

The oxidation resistance of duplex stainless steel tubes under high-temperature and high-pressure conditions is closely related to their alloy composition (especially the contents of chromium, nickel, molybdenum, etc.), the temperature range of use, and the environmental medium. Generally, it shows good oxidation resistance at medium temperatures (≤350℃), but has obvious limitations at high temperatures (>400℃). The specific analysis is as follows: 

I. Core Mechanism of Antioxidant Properties: Formation and Stability of Oxide Film

The antioxidant property of duplex stainless steel (such as 2205, 2507, etc.) relies on the formation of a dense chromium oxide (Cr₂O₃) passivation film on the surface: The chromium element (typically accounting for 21%-25%) combines with oxygen at high temperatures to form a continuous and stable oxide film, which can prevent oxygen from further diffusing into the base material, thereby inhibiting oxidative corrosion.

In addition, some models (such as 2507) contain a small amount of silicon (≤1%), and silicon can combine with oxygen to form SiO₂, which, together with Cr₂O₃, strengthens the density of the oxide film and further enhances the antioxidant capacity. 

II. Oxidation Resistance Performance under High Temperature and High Pressure

1. Medium-high pressure environment (≤350℃): Excellent oxidation resistance

In the medium-high pressure environment (such as chemical high-pressure heat exchangers, heat exchange pipelines in oil and gas extraction, with pressures up to 10-30 MPa) below 350℃, the oxide film stability of duplex stainless steel is good. Specifically, it is manifested as:

Low oxidation rate: The oxide film is not easily damaged by high-pressure steam or media containing trace oxygen. The annual oxidation loss is usually less than 0.1mm, which is much lower than that of ordinary carbon steel or low-alloy steel pipes.

Resistance to medium-coordinated corrosion: If there are small amounts of corrosive media such as chloride ions, sulfides, etc. (such as high-temperature high-pressure brine) in the environment, the oxide film can still maintain integrity, avoiding oxidation film peeling due to corrosion (this performance is superior to some austenitic stainless steels, such as 304, because 304 has a lower chromium content, and the oxide film is prone to damage in the presence of chlorine-containing media).

2. Medium-high-temperature high-pressure environment (350-450℃): Oxidation resistance decreases, usage conditions need to be controlled

When the temperature exceeds 350℃, the oxidation behavior of duplex stainless steel begins to change:

Decrease in oxide film stability: High temperature accelerates the diffusion of chromium, which may lead to local thickening of the oxide film or the formation of pores. Especially under high pressure, if the medium contains steam or water vapor, it may cause "steam oxidation", resulting in a decrease in the bonding force between the oxide film and the substrate, and local peeling.

The influence of alloy phase: 350-450℃ is the sensitive temperature range for the precipitation of σ phase (brittle intermetallic compound) in duplex stainless steel. The formation of σ phase consumes chromium in the matrix, leading to a local reduction in chromium content, weakening the repair ability of the oxide film, and indirectly exacerbating oxidation corrosion.

Therefore, in this temperature range, the oxidation resistance of duplex stainless steel can still meet short-term (such as equipment start-up and shutdown) usage requirements, but long-term (>1000 hours) usage will lead to an increase in oxidation loss, and regular detection of the oxide film condition is required to control risks.

3. High-temperature high-pressure environment (>450℃): Oxidation resistance significantly deteriorates, not recommended for use

When the temperature exceeds 450℃ (especially above 500℃), the oxidation resistance of duplex stainless steel will significantly decline:

Oxide film failure: At high temperatures, the diffusion rate of chromium accelerates sharply, and the oxide film gradually transforms from the dense Cr₂O₃ to a loose Fe-Cr composite oxide (such as FeCr₂O₄), unable to prevent oxygen from invading, and the oxidation rate increases exponentially (annual oxidation loss can reach 0.5-1mm, far exceeding the equipment's allowable range).

Matrix deterioration: High temperature causes the balance between austenite and ferrite in duplex stainless steel to be disrupted, with ferrite decomposition and the precipitation of carbonitrides in austenite, further reducing the material's mechanical properties and oxidation-coordinated ability, and even causing "high-temperature embrittlement + oxidation corrosion" combined failure. 

III. Indirect Effects of High Pressure on Oxidation Performance

High pressure (above 10 MPa) itself does not directly damage the oxide film, but it can intensify oxidation through the following ways:

Increased medium concentration: Under high pressure, the partial pressure of oxygen, water vapor, or corrosive gases (such as CO₂, H₂S) in the environment increases, and the reaction rate with the metal surface accelerates, which may cause the oxide film to be "broken through" in local areas (such as welds, stress concentration points), leading to local oxidation.

Stress synergy: The continuous stress brought by high pressure may cause the oxide film to develop microcracks due to "stress fatigue", and the crack-free areas lose protection, becoming weak points for oxidation and corrosion, especially during temperature fluctuations (such as equipment start-stop), where thermal stress and mechanical stress combine, increasing the risk of crack propagation. 

IV. Differences in Oxidation Resistance of Different Types of Duplex Stainless Steel

The oxidation resistance of duplex stainless steel increases with the increase in chromium content. Typical comparisons of the models are as follows:

Image 1 

V. Comparison with Other High-Temperature Resistant Materials

Compared with high-chromium-nickel austenitic stainless steel (such as 310S), the high-temperature oxidation resistance of duplex stainless steel is significantly weaker:

310S contains 25% Cr and 20% Ni. It can still form a stable Cr₂O₃-NiO composite oxide film below 800℃, and its oxidation resistance is much better than that of duplex stainless steel;

The advantage of duplex stainless steel lies in the medium-temperature high-pressure + corrosive medium (such as chloride-containing) environment, while 310S is more suitable for pure high-temperature (＞600℃) and non-strongly corrosive environments (such as boiler superheaters). 

Summary

The duplex stainless steel pipe exhibits excellent oxidation resistance in medium-temperature and high-pressure conditions (≤350℃, pressure ≤30MPa) and in environments containing corrosive media (such as chloride ions, sulfides). It can meet the demanding requirements in the fields of chemical engineering and oil and gas. However, in high-temperature and high-pressure conditions (＞450℃), its oxidation resistance significantly deteriorates. In such cases, 310S and other high-temperature-resistant austenitic stainless steel must be used instead. In practical applications, the usage temperature must be strictly controlled (long-term not exceeding 350℃), and the oxygen and water vapor content in the environmental medium should be monitored to prevent equipment failure due to oxidation corrosion.