In order to improve the corrosion resistance of steel, usually chromium or an increase in the proportion of alloy elements can promote passivation, adding Ni, Mo, Mn, Cu, Nb, Ti, W, Co, etc., these elements not only improve the resistance of steel corrosion, while changing the internal structure of the steel as well as physical and mechanical properties. Typically, people chromium greater than 12% or greater than 8% nickel alloy steel called stainless steel precision molds processing. This steel or having a corrosion resistance in certain corrosive media in the atmosphere, and has a high strength at elevated temperatures (> 450 ℃). Chromium amounted to 16% to 18% of steel known as acid or acid-resistant stainless steel, customarily known as stainless steel. When the steel contains chromium amounted to more than 12%, in contact with the oxidizing medium, since the electrochemical reaction, the surface quickly form a layer of chromium-rich passive film to protect the metal from corrosion inside; but in a non-oxidizing corrosion medium, is still difficult to form a firm passivation film.