Generally, the content of chromium must reach 13 ~ 25% (by weight) to change the oxidation of steel surface.

If we insert a freshly cut stainless steel plate into water, a layer of chromium oxide will be formed on the surface in less than one second, which can slow down the oxidation rate of steel by at least 10,000 times. At this rate, it will take at least one million years to corrode a steel plate with a thickness of one centimeter.

★ But stainless steel doesn't really rust. If you look at it under a microscope, there are actually many small rust spots on its surface. ;

Look carefully again, these spots all contain sulfur, and the existence of sulfur is inevitable in the process of steelmaking.

=> These stainless steels are in good condition when exposed to air, but they will soon rust once put into water. At this time, a small amount of molybdenum can be added. For example, the steel nails that are driven into the bone during fracture all contain molybdenum, and the price of this stainless steel is much higher.

Trace manganese sulfide will make stainless steel start to rust,

But how did they start?

-& gt； Recent studies have found that it is not that these sulfur atoms turn into sulfuric acid and other things in water that cause corrosion, but that in the process of manufacturing and cooling stainless steel, chromium in steel moves to some places, partially replacing the position of manganese in manganese sulfide, and then the chromium content nearby becomes low.

When the chromium content is lower than 13%, stainless steel will rust.