Stainless steel refers to steel that does not rust in weak corrosive media such as air, water, steam, etc. and is resistant to corrosion in strong corrosive media such as acids, alkalis, and salt solutions. Some of the steels that do not rust in weakly corrosive media do not necessarily resist corrosion in strong corrosive media, while steels that are resistant to corrosion in corrosive media generally have good rust properties.

Stainless steel is made by adding a certain amount of chromium element to the carbon steel. Why is it not rust and corrosion resistant? The main reason is that steel contains chromium. Due to the influence of chromium, a strong and dense oxide film is formed on the surface of the steel under the action of corrosive medium, which is called "passivation film". This film is extremely thin and transparent, almost invisible to the naked eye, and still sees a shiny metallic surface. This film isolates the metal from the external medium and prevents the metal from being further corroded. It also has the ability to repair itself. If it is destroyed, the chromium in the steel will re-generate the passivation film with the oxygen in the medium and continue to protect. effect.

Carbon steel is not the same as this. Under the action of corrosive medium, the surface of carbon steel will quickly form a loose layer of iron oxide, which is often called rust. It can't prevent the metal from being isolated from the medium. The external oxygen atoms continue to spread inward and the steel will continue to rust. Corrosion, so that it is completely destroyed.

Stainless steel can not rust and corrosion, what is the minimum chromium content required? The US and European standards stipulate that it is not less than 10.5% (mass fraction, the same after), and the Japanese industrial standard stipulates that it is generally not less than about 11%. It is generally considered to be not less than 12%. In fact, the corrosion resistance of stainless steel, the content of chromium content, depends on the type of corrosion medium, concentration, temperature, pressure, flow rate, and other alloying elements other than chromium in steel.

In order to make stainless steel have good corrosion resistance and good mechanical and physical properties, according to different requirements, in addition to adding a higher content of alloying element chromium, the steel is also matched with nickel, molybdenum, manganese, nitrogen, etc. Other alloying elements. In this way, not only can the chemical composition of the passivation film be changed, its corrosion resistance in harsh media can be enhanced, but also the steel can obtain sufficient strength, plasticity and toughness, as well as good process properties such as weldability and processing. Sex and so on.

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