Chromium in stainless steel can provide corrosion resistance
Chromium (Cr) is particularly effective in oxidizing environments such as those containing nitric acid. After added chromium in stainless steel, an oxide with a chemical ratio of iron dioxide will be formed on its surface. Since the affinity of chromium and oxygen is much higher than that of iron, the presence of chromium increases the stability of this oxide. Steel with a chromium content of more than 10.5% is considered to be stainless in the atmosphere. In more aggressive environments, a higher ratio is required.
Chromium is also a ferrite forming element.
Fe-Cr alloys containing more than 12% Cr are all ferrite alloys. However, increasing the chromium content in Fe-Cr-C or Fe-Cr-Ni-C alloys will promote the formation and residual of ferrite in martensitic stainless steel, austenitic stainless steel and duplex stainless steel. In ferritic alloy steel, chromium is the main alloying element that stabilizes the ferrite structure.
The corrosion resistance of stainless steel depends on Cr, but because Cr is one of the components of steel, the protection methods are different.
When the addition amount of chromium reaches 10.5%, the atmospheric corrosion resistance of steel is significantly increased, but when the chromium content is higher, although the corrosion resistance can still be improved, it is not obvious. The reason is that when chromium is used to alloy steel, the type of surface oxide is changed to a surface oxide similar to that formed on pure chromium metal. This tightly adhered chromium-rich oxide protects the surface and prevents further oxidation. This oxide layer is extremely thin, and the natural luster of the steel surface can be seen through it, giving the stainless steel a unique surface. Moreover, if the surface layer is damaged, the exposed steel surface will react with the atmosphere to repair itself and re-form this oxide "passivation film" to continue to play a protective role.
Therefore, all stainless steels have a common characteristic, that is, the chromium content is above 10.5%.
Posted by stainless steel pipe and fittings supplier, KAYSUNS.