Overview

Oxidation is a problem for all alloys. It consumes the elements inside the alloy and destroys the internal structure of the alloy. Eventually, the alloy will suffer from degradation or even destruction due to oxidation.

Therefore, studying the oxidation laws of alloys is of great significance for the correct use of alloys, improvement and development of new alloys.

In this article, we will describe the principle of oxidation in detail and explain the main anti-oxidation methods of superalloys. This will make you more aware of the importance of material selection.

What is Oxidation

Oxidation of alloys refers to the process of chemical reactions between alloys and oxidizing substances to form oxides. In a high temperature environment, the oxidation reaction will accelerate sharply. This is destructive to the alloy.

There are narrow and broad definitions of metal oxidation. Oxidation in a narrow sense refers to the reaction process of metals and oxidizing substances to form metal oxides. Oxidation in a broad sense refers to the process by which metal atoms lose electrons. Among them, the oxidation reaction in the narrow sense is the most basic and common, because oxygen is the most common gas in nature and industrial environments.

Superalloys contain many alloying elements. Among them are active elements that are easily oxidized (such as aluminum, chromium, silicon, etc.), and refractory metals (such as niobium, tungsten, molybdenum, tantalum, etc.). In high temperature oxidizing environments, alloying elements form oxides. This reduces the effective area of the workpiece and increases the stress on the alloy. At the same time, oxidation occurs inside the metal. This aids in the generation and propagation of cracks. Finally, the creep strength and fatigue strength of the alloy are affected.

The Principle of Oxidation Resistance of Superalloys

There are three classifications of superalloys. They are based on iron, nickel and cobalt. These alloys are called iron-based superalloys, nickel-based superalloys and cobalt-based superalloys. Pure iron, pure nickel and pure cobalt are susceptible to oxidation at high temperatures. And the oxides they generate on the metal surface cannot become protective oxide films, so they cannot effectively prevent the metal from further oxidation.

There are antioxidant elements that react with oxygen to produce a dense oxide film. This oxide film covers the surface of the metal. It separates the metal from oxygen and thus prevents further oxidation of the metal. Among them, chromium is the most common antioxidant element. Therefore, in order to improve the high temperature strength and to ensure that the alloy has good oxidation resistance, iron-based alloys are actually iron-nickel-chromium alloys, cobalt-based alloys are mostly cobalt-nickel-chromium alloys, and nickel-based alloys are actually nickel-chromium alloys or nickel-chromium-cobalt alloys. Then further alloying is done on this basis.

Antioxidant Elements in Superalloys

Chromium

Chromium is the most important antioxidant element. When chromium is added to superalloys, it oxidizes to form Cr2O3, which forms an oxide film to protect the alloy. Chromium is usually added to superalloys at 15% or more to ensure the formation of a continuous and dense Cr2O3 oxide layer.

Aluminum

Aluminum is also the most important antioxidant element. When the aluminum content is 0% ~ 6%, NiO is formed on the outer layer of the metal surface, while Al2O3 oxide is generated on the inner layer. When the aluminum content increases to 6% ~ 17%, Al2O3 is formed on the metal surface at the beginning of oxidation, but then the oxide gradually changes to NiO due to insufficient aluminum content. When the aluminum content is greater than 17%, stable Al2O3 can be formed on the surface, and the required aluminum content decreases as the temperature increases.

Silicon

The addition of 0.5% to 1.3% silicon to the alloy allows the formation of SiO2 in the inner layer of the alloy surface, which improves the cyclic oxidation resistance and to some extent has the same effect as aluminum. Incoloy 330 is a good example. It has a certain amount of silicon added to achieve better oxidation resistance.

Selection of Antioxidant Elements

According to the results of many years of research and production use, the formation of Cr2O3 oxide film or Al2O3 oxide film on the surface of superalloys is effective and feasible for protection against further oxidation of superalloys. Below 900°C, Cr2O3 should be preferred as the main oxide film. Above 1000°C, Al2O3 should be preferred as the primary oxide film.

In the dispersion strengthened alloy Incoloy MA956, a large amount of aluminum ensures the oxidation resistance of the alloy above 1000°C.

The Comprehensive Effect of Antioxidant Elements

When the third element is added on the basis of adding the antioxidant element, the formation of the oxide film can be promoted. For example, superalloys often contain both chromium and aluminum. Due to the synergistic effect of the two, the antioxidant improvement is very obvious.

When the superalloy contains 10% chromium, stable Al2O3 can be formed as long as the aluminum content reaches 5% which is much lower than the requirement that the aluminum content in the alloy must be greater than 17%. Obviously, chromium plays a very important role as a third element.

Elements that Reduce the Oxidation Resistance of Alloys

In nickel-based alloys and cobalt-based alloys, refractory metals (such as tungsten, molybdenum, niobium, tantalum, etc.) both improve corrosion resistance and reduce corrosion resistance. Overall, these elements still reduce the corrosion resistance of the alloy. Among them, tungsten has the most harmful effects.

Conclusion

Oxidation refers to the phenomenon in which the metal elements in the alloy react with oxygen, resulting in a decrease in the performance of the alloy.

The oxides formed by the reaction of some metal elements with oxygen can prevent further oxidation. These elements are called antioxidant elements. They are mainly chromium, aluminum, silicon and so on.

We produce a wide range of superalloys with very good oxidation resistance. If you have needs, you can contact us.