What are the roles of chromium in stainless steel?

Chromium is a ferrite forming element, which can significantly improve the passivation ability of iron. Chromium plays a fundamental role in the corrosion resistance of stainless steel. In the definition of stainless steel, it is clearly required that the chromium content is ≥ 10.5%, which is the main element of the corrosion resistance of stainless steel. The higher the chromium content in the stainless steel, the better the corrosion resistance. This is because stainless steel can form a stable surface protective film based on Cr2O3 in the oxidizing medium, that is, passivation occurs, and chromium oxide Cr2O3 is enriched in the film. This chromium-rich oxide film has good stability in many media, thereby improving the corrosion resistance of steel. At the same time, chromium can effectively increase the electrode potential of the solid solution (ferrite, martensite or austenite), so that the electrode potential of the original pure iron (low carbon steel) changes from negative to positive, thereby preventing steel corrosion.

In addition, chromium plays an important role in the mechanical and technological properties of stainless steel. Chromium can improve the hardenability of stainless steel and has been widely used in low alloy structures. This effect of chromium reduces the rate of transformation from austenite to ferrite and carbides, and makes the isothermal transformation diagram of austenite significantly shift to the right, thereby reducing the critical cooling rate of quenching of stainless steel, resulting in the hardenability of stainless steel increase. Some martensitic stainless steels can obtain a martensitic structure by air quenching.
Chromium can improve the oxidation resistance of stainless steel. As the chromium content increases, the oxidation resistance of stainless steel will increase significantly. In martensitic chromium stainless steel, its oxygen resistance is 4-9 times higher than that of ordinary steel. The temperature of martensitic chromium stainless steel without skin is 700~850'C. The tendency of carbides formed in chromium steel is greater than that of manganese. Less than tungsten. Chromium can improve the high temperature strength of stainless steel and the wear resistance of high carbon steel.