Corrosion resistant alloy

• Overview

  Corrosion resistant alloys are metal materials that are resistant to erosion by medium.

  Corrosion resistant alloys usually have one of three conditions

  
  

  1 Metals with high thermodynamic stability.

  Generally, the standard electrode potential can be used to judge the stability of the positive value is higher. The less stable. Precious metals with good corrosion resistance, such as Pt, Au, Ag, Cu, etc. belong to this category.

   

  2 Easily passivated metals.

  Many metals can form dense oxide film with protective effect in oxidizing medium, this phenomenon is called passivation. Ti, Zr, Ta, Nb, Cr, Al and so on are the most easily passivated metals.

   

  3 A metal with a surface capable of forming an insoluble and well-protected corrosion product film.

  This condition occurs only when the metals are exposed to certain corrosive media, such as Pb and Al in H2SO4 solution, Fe in H3PO4 solution, Mo in hydrochloric acid solution, and Zn in the atmosphere.

• Classification

  Corrosion resistant alloy is mainly divided into iron base alloy (namely stainless steel), nickel base corrosion resistant alloy, active metals.

  
  

  1 Fe base CRA

  Corrosion resistant stainless steel mainly refers to the ordinary atmospheric or seawater corrosion resistance of 300 series stainless steellike304, 316 L, 317L, etc. Strong corrosion resistance of austenitic stainless steellike904L, 254SMo, duplex steel 2205,2507, etc., corrosion resistant alloy containing Cu like alloy 20, etc.

  
  

  2 Ni base CRA

  It is mainly Hastelloy alloy and Ni-Cu alloy, etc. Because the metal Ni itself is face-centered cubic structure, its crystallographic stability makes it able to accommodate more alloying elements, such as Cr, Mo, etc., than Fe, so as to achieve the ability to resist various environments. At the same time nickel itself has a certain ability to resist corrosion, especially the stress corrosion caused by chloride ions. In the strong reducing corrosion environment, complex mixed acid environment, containing halogen ion solution, Hastelloy as the representative of nickel corrosion resistant alloy has absolute advantages over iron based stainless steel

   

  3 Active metals

  Thetypical representatives of good corrosion resistanceare Ti, Zr, Ta, etc. The most typical representative is Ti, titanium has a wide range of applications, mainly used in some stainless steel can not adapt to the corrosion environment. Titanium corrosion resistance principle: in the oxidizing atmosphere, the formation of dense oxide film to provide protection; Therefore, it can not be used in corrosive environment with strong reducibility or high sealing. At the same time, the application temperature of titanium materials is generally less than 300 degrees Celsius. In particular, none of the active metals can be used in fluorinated environments.

• Selection of Ni base CRA

  1 Pure nickel

  Its typical representative is nickel 200 (UNS N02200). Nickel 200 has excellent corrosion resistance to a wide range of reducing acids and salts, but is not suitable for strongly oxidizing conditions such as nitric acid. The most notable characteristic of pure nickel is its resistance to corrosion by strong alkali, especially when the strong alkali is in the molten state. Although excellent corrosion resistance to dry halogen environments, nickel is not suitable for environments below the dew point of water. A low carbon nickel 201 (UNS N02201) is preferred for use at temperatures above 6000°F.

   

  2 Ni-Cu alloys

  The typical example is Alloy 400. The corrosion resistance of nickel-copper alloy 400 is the same as that of nickel. It is best used under reducing conditions and can be damaged by oxidizing atmosphere. Alloy 400 has excellent corrosion resistance to halogen acids and halogen compounds, especially to hydrofluoric acid and high temperature gases rich in fluorine or hydrogen fluoride. The alloy is widely used to treat sulfuric acid solutions, sea water and salt water. For applications requiring higher strength requirements, such as valve and pump parts, it is often manufactured in alloy K-500 (N05500), a precipitation-hardened variant of alloy 400.

   

  3 Ni-Cr-Fe alloys

  (1)Alloy 600

  The addition of chromium in nickel matrix enhances the adaptability of alloy 600 in oxidizing environment. Its corrosion resistance to inorganic acids is only medium, but its corrosion resistance to organic acids is very good. Therefore, it is widely used in the processing of fatty acids, and it is also widely used in the processing and production of strong alkaline chemicals. Alloy 600 is also considered an excellent material for high-temperature applications requiring both heat and corrosion resistance. The excellent performance of the alloy in high temperature halogen environment makes it the best material choice for organic chloride processing. In other high temperature degradation processes, Alloy 600 has been proved to have excellent oxidation, carburizing and nitriding properties.

   

  (2) Alloy 690

  Alloy 690 has the highest chromium content of all nickel alloys and is suitable for manufacturing pressure equipment. It is highly resistant to oxidation media. It can be used effectively in high temperature concentrated sulfuric, nitric and hydrofluoric nitrate mixed acids and oxidizing salt environments. The high chromium content also improves its vulcanization resistance in high temperature vulcanization environments.

   

  (3) Alloy 825

  Alloy 825 is sometimes included in the Super austenitic stainless steel series because it contains about 30% iron. This alloy, like Alloy 20, was developed primarily for sulfur and phosphorous applications. Although alloy 825 has a strong resistance to hydrochloric acid corrosion, it is susceptible to chloride pitting and crevice corrosion, especially in non-flowing, non-ventilated solutions. The high iron content of alloy 825 makes it less resistant to strong alkalis and halogens than alloys with higher nickel content.

   

  4 Ni-Cr-Mo alloys

  The addition of molybdenum toNi-Cr alloys enhances the corrosion resistance to oxidizing and reducing inorganic acids and salts. The addition of nickel makes the alloy have the corrosion of pore and crevice caused by milk water chloride.

  (1) Alloy 625

  Alloy 625 is a kind of material with excellent fatigue resistance. Alloy 625LCF is a modified steel type of 625. It has the properties required by bellows, and the difference is that it has excellent low cycle fatigue and thermal fatigue resistance. Like alloy 600, Alloy 625 can be effectively used as a heat and corrosion resistant material. Its high temperature strength and combined resistance to halide, oxidation and carburizing have made Alloy 625 widely used in chemical and petrochemical processing equipment, which are often exposed to extremely high temperatures.

   

  (2) C alloys

  For the chemical industry in special corrosive environments, alloy C-276 is considered an excellent alloy with exceptional resistance to various acids, acid salts and other corrosive substances encountered in chemical processing. Alloy C-276 is suitable for harsh environments such as chlorides and hypochlorites with moisture. Because the alloy C-276 contains molybdenum, this alloy is more resistant to pitting and crevice corrosion caused by chloride.

  In the selection of materials with better metallurgical properties and corrosion resistance than alloy C-276, several alloys are available for reference, such as C-22, 622, 59, 686, and C-2000. In all alloys with approximately equal molybdenum content, chromium content is much higher than that of alloy C-276. Some alloys also contain W and Cu. These minor alloying elements have complicated effects on metallurgical properties and corrosion resistance.

   

  5 Ni-Cr-Fe-Mo alloys

  Usually referred to as "G" alloys

  The corrosion resistance of alloy G-3 exceeds that of alloys 400, 600, and 825 in many applications. This alloy is particularly resistant to sulfuric acid and impurious phosphoric acid corrosion and can withstand the erosion of both reducing and oxidizing environmental conditions. The recently developed alloy G-30 has better welding performance and overall improved corrosion resistance, especially to the heat affected zone of the weld.

   

  6 Ni-Mo alloys

  Usually referred to as "B" alloy

  Alloy B-2 has outstanding resistance to sulfuric acid, phosphoric acid and hydrochloric acid under reduction conditions. It is particularly suitable for equipment processing hydrochloric acid at any concentration and up to the boiling point temperature. Oxidizing compounds have an adverse effect on the corrosion resistance of this alloy, of particular note strong oxidants such as iron and copper salts may cause inclusion. Recently recommended alloys B-3 and B-4 have better properties than B-2, mainly because unwanted microstructure will be minimized.