Oxidation is the reaction of metal and oxygen. If the oxide formed is continuous and effective in separating the alloy from the atmosphere, the oxide is protective. However, if the oxide fails to act as a separator, problems with corrosion occur.
When an alloy is heated in air, an oxide layer forms on its surface. This layer can consist of oxides of iron, chromium and other elements, depending on the composition of the material.
Up to a certain temperature, known as the scaling temperature or maximum service temperature, the oxide layer is continuous and dense, and protects the underlying material from high-temperature corrosion attack. Above the scaling temperature the layer tends to crack and will, therefore, lose its protective capacity. The material will then corrode rapidly. Rapid changes in temperature will accelerate the corrosion attack.
Increased contents of chromium (Cr), silicon (Si) and aluminium (Al) increase the resistance to oxidation. The addition of rare earth metals and reactive elements such as titanium (Ti), zirconium (Zr) and yttrium (Y) has a highly positive effect on oxidation resistance.
Material exposed to pure steam will also undergo oxidation because oxygen is present in the form of H2O. As a rule, steam produces faster oxidation than air. Oxide layers formed in steam are more iron-rich and porous, and therefore give inferior protection. Air with high water content will similarly cause more rapid oxidation.
The resistance to oxidation in steam increases with the chromium (Cr) content of the steel. Silicon (Si) and aluminium (Al) also have positive effects.
The following grades have very high or excellent resistance to oxidation:
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