Stainless steel does not rust, the reason is that it contains metal chromium. Chromium is the 24th element in the periodic table with the symbol Cr. Stainless steel must not only contain chromium, but also must contain a certain amount of chromium in order to achieve anti-rust effect.
The first aspect of chromium's antiseptic effect is passivation. One theory is that under the action of the oxidizing medium, the stainless steel surface forms a layer of anti-corrosion chromium-rich oxide film, which can prevent the oxidizing medium from penetrating through the metal surface. In addition, once this layer of oxide film is damaged, it can recover by itself. The second theory of passivation believes that because the adsorbed oxygen atoms are acted on by the electrons of the metal, the battery is coupled and can fight against electrochemical corrosion, thereby improving the corrosion resistance. A third theory of passivation states that chromium robs one of the outermost electrons of the iron atom, passivating stainless steel.
The second aspect of chromium corrosion protection is that chromium increases the electrode potential of the iron-based solid solution. When chromium reaches a certain value, this increase can be mutated, that is, when it reaches one-eighth, two-eighth, three-eighth... When atomic weight is applied, its corrosion is greatly weakened. The chromium content reaches one-eighth, that is, 12.5%, it can resist the corrosion of atmosphere, water vapor and dilute nitric acid. If you want to resist the erosion of concentrated acid, you need to reach the second mutation value of two-eighths, that is, 25%. Therefore, stainless steel to contain chromium, is only a qualitative statement. And containing chromium 12 5%, is quantitative, less than this amount is not called stainless steel.
Industrial applications of stainless steel, chromium content between 12% and 30%, usually can resist atmospheric corrosion of 13% chromium steel called stainless steel, chromium containing more than 17% steel called acid steel, collectively referred to as stainless acid steel. In addition, there are heat-resistant non-peeling steel and electric heating alloy that can resist high temperature oxidation.
All metals react with oxygen in the atmosphere to form an oxide film on the surface. Unfortunately, the iron oxide formed on ordinary carbon steel continues to oxidize, allowing the rust to expand and eventually form holes. The surface of carbon steel can be secured by electroplating with paint or oxidation-resistant metals (e.g., zinc, nickel and chromium), but, as is known, this protection is only a thin film. If the protective layer is destroyed, the following steel will begin to rust resistant to air, steam, water and other weak corrosive media and acid, alkali, salt and other chemical etching media corrosion of steel. Also known as stainless acid resistant steel. In practical applications, the steel that is resistant to the corrosion of weak corrosive media is often called stainless steel, and the steel that is resistant to the corrosion of chemical media is called acid-resistant steel. Due to the difference in the chemical composition of the two, the former is not necessarily resistant to chemical medium corrosion, while the latter is generally stainless. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel. Chromium is the basic element to make stainless steel corrosion resistance, when the chromium content in the steel reaches about 1.2%, chromium and the oxygen in the corrosive medium, the formation of a thin oxide film on the surface of the steel (self-passivation film), can prevent further corrosion of the steel matrix. In addition to chromium, commonly used alloying elements are nickel, molybdenum, titanium, niobium, copper, nitrogen, etc., to meet the requirements of various uses on the structure and properties of stainless steel.