Corrosion resistance characteristics and application of common special metal materials
With the rapid development of China’s economy, especially the acceleration of the technical transformation and upgrading of the chemical industry, the application of special metal materials in China is more and more extensive, especially in the petroleum, chemical and other related industries, which also put forward higher and higher requirements for the materials of production equipment, thus driving the relevant research institutes and processing enterprises in China (such as Hefei General Institute, Baoji non-ferrous metal Institute) The development and research of the processing plant, as well as the improvement of the capability of the relevant equipment manufacturers for the manufacturing technology and equipment of special metal materials and equipment (such as the application of explosive composite technology), can not be separated from the promotion of the national government (China National titanium office, Sinopec localization Office), so that the localization process is accelerating. At the same time, some international suppliers of special materials (Krupp, MCC, etc.) vigorously promoted the application of special materials, which to varying degrees promoted the development and application of special metal materials in China.
Titanium and titanium alloy
The production of titanium alloy in China is basically in step with that in foreign countries, but its popularization and application are backward, especially for civil use. At the same time, due to the disordered competition of foreign smuggling titanium materials and some equipment processing enterprises in recent years, some enterprises without production capacity and some small and medium-sized township enterprises adopt inferior materials or substitute inferior ones to a certain extent, which has also disrupted the titanium equipment market, making the equipment manufacturers talk about “titanium” color change, so this situation also hinders the development of China’s titanium equipment industry The obstruction should be noticed by the relevant management department, and it should also be the forerunner of other special materials under development.
Common titanium brand (Chinese material standard)
Corrosion resistance of titanium
Titanium is a metal with strong passivation tendency. It can quickly form a stable protective film in air and in oxidizing or neutral aqueous solution. Even if the film is damaged for some reasons, it can recover automatically. Therefore, titanium has excellent corrosion resistance in oxidizing and neutral media.
Because of the great passivation property of titanium, in many cases, when it contacts with dissimilar metals, it does not accelerate the corrosion, but may accelerate the corrosion of dissimilar metals. If Pb, Sn, Cu or Monel alloy contact with titanium to form electric couple, the corrosion of these materials will be accelerated, while titanium will not be affected. However, in hydrochloric acid, when titanium is in contact with low carbon steel, new hydrogen is produced on the surface of titanium, which destroys the titanium oxide film, not only causes hydrogen embrittlement of titanium, but also accelerates the corrosion of titanium, which may be due to the high activity of titanium to hydrogen.
The iron content in titanium has an effect on the corrosion resistance of some media. The reason for the increase of iron is not only the reason of raw materials, but also the penetration of contaminated iron into the weld bead during welding, which increases the local iron content in the weld bead. At this time, the corrosion is uneven. When using iron parts to support titanium equipment, iron contamination on the iron titanium contact surface is almost inevitable to accelerate corrosion in the iron contamination area, especially in the presence of hydrogen. When the titanium oxide film on the contaminated surface is damaged mechanically, hydrogen penetrates into the metal. According to the temperature, pressure and other conditions, hydrogen diffuses correspondingly, which makes titanium produce different degrees of hydrogen embrittlement. Therefore, it is necessary to avoid surface iron contamination when titanium is used in medium temperature, medium pressure and hydrogen containing systems.
In general, titanium does not suffer from pitting.
Titanium also has corrosion-resistant fatigue stability.
Titanium has good resistance to crevice corrosion, especially ti-0.3mo-0.8ni and ti-0.2pd alloy. Therefore, ti-0.3mo-0.8ni and ti-0.2pd alloy are widely used in sealing surface materials of container equipment to solve the crevice corrosion problem of sealing surface of equipment.
Application of titanium
Due to its excellent corrosion resistance, titanium is widely used in petroleum, chemical industry, salt making, pharmaceutical, metallurgy, electronics, aviation, aerospace, marine and other related fields.
Titanium has excellent corrosion resistance to most salt solutions. For example, titanium is more corrosion resistant than high chromium nickel steel in chloride solution without pitting. However, the corrosion rate is higher in AlCl3, which is related to the production of concentrated hydrochloric acid after hydrolysis of AlCl3. Titanium also has good stability to hot sodium chlorite and hypochlorite of various concentrations. Therefore, titanium is widely used in vacuum salt making and bleaching powder refining industries.
Titanium has good corrosion resistance to most alkaline solutions. Titanium is relatively stable in sodium hydroxide and potassium hydroxide solution with concentration less than 50%. If the alkali solution contains chloride ion or chloride, its corrosion resistance is even higher than that of nickel and zirconium. But when the temperature and concentration increase, the corrosion will increase. At present, chlor alkali industry is the largest application field of civil titanium materials in China.
Titanium is not corrosion-resistant in the dry chlorine gas, but also has the risk of ignition. However, it has high stability in the wet chlorine gas, which is more stable than zirconium, Hastelloy C and Monel alloy, even in the medium of sulfuric acid, hydrochloric acid and chloride in the saturated chlorine gas. Therefore, titanium material is the preferred material for the key equipment of titanium dioxide production by sulfuric acid method.
Due to the good corrosion resistance of titanium in hydrocarbons, even when it contains acid and chloride impurities. Therefore, titanium is also widely used in organic chemical industry, such as PTA (purified terephthalic acid), PVA (vinylon), etc.
Titanium has excellent corrosion resistance in seawater, so it is also widely used in offshore oil drilling platform, desalination and other marine fields.
Nickel and nickel base alloys
Production status of nickel and nickel base alloy in China
Pure nickel for industrial use in China can be produced by itself, but some nickel base alloys are mainly imported.
Types of nickel and nickel base alloys
Corrosion resistance of nickel and nickel base alloy
Nickel tends to be blunt. Under normal temperature, the surface of nickel is covered with an oxide film, which makes it corrosion resistant in water and many salt aqueous solutions.
Nickel is quite stable at room temperature in non oxidizing dilute acids, such as < 15% hydrochloric acid, < 17% sulfuric acid and many organic acids. However, the corrosion rate of nickel increased significantly with the increase of oxidants (FeCl2, CuCl2, HgCl2, AgNO3 and hypochlorite) and aeration.
Nickel is completely stable in all alkali solutions, whether high temperature or molten alkali, which is the outstanding characteristic of nickel.
Monel alloy is more resistant to corrosion than nickel in reducing medium and copper in oxidizing medium. It is more resistant to corrosion than nickel and copper in phosphoric acid, sulfuric acid, hydrochloric acid, salt solution and organic acid.
In any concentration of hydrofluoric acid, Monel alloy is very corrosion-resistant when oxygen enters into it. However, when there are aeration and oxidants in the solution, or there are harmful impurities such as iron salt and copper salt in the solution, its hydrofluoric acid resistance is reduced. In addition to platinum and silver, it is one of the best materials that can resist hydrofluoric acid corrosion.
It is very corrosion resistant in caustic solution, but when the concentration of sodium hydroxide is very high, the corrosion resistance of Monel alloy is worse than that of nickel, but it is still more alkali resistant than other metal materials.
Monel alloy is easy to produce stress corrosion cracking. It is better to use Monel alloy after annealing at 530-650 ℃.
Hastelloy B (B-2, B-3) and Hastelloy C-276 are commonly used. They have high corrosion resistance in non oxidizing inorganic and organic acids, such as dilute sulfuric acid resistant to 70 ℃, hydrochloric acid, phosphoric acid, acetic acid and formic acid resistant to all concentrations, especially concentrated hydrochloric acid.
Hastelloy alloy is stable in caustic and alkaline solution, and completely stable in organic medium, seawater and fresh water.
White copper (B10, B30)
Cupronickel is a copper nickel alloy. Cupronickel can be produced in China, mainly by Luotong.
The corrosion resistance of white copper is similar to that of pure copper. But for hydrofluoric acid with concentration less than 70%, it is corrosion-resistant without oxygen and below boiling point. In organic acids, the corrosion rate of white copper is small, but in alkaline solutions and organic compounds.
B30 (70-30 copper nickel alloy) can be used to replace pure nickel in caustic soda or diaphragm electrolytic caustic soda to make membrane evaporator, especially in falling film part. It can not only improve service life, but also save 70% nickel. B10 (91-9 Cu Ni alloy) can also replace pure nickel to make evaporation tube, evaporation chamber and other equipment of rising film evaporator.
White copper has high corrosion resistance in seawater, so B10 and B30 white copper are often used in heat exchanger cooled by seawater.
Commonly used zirconium and zirconium alloy brands are: non nuclear zirconium R60702, r60703, r60704, r60705, r60706.
Although there is no specification for zirconium and zirconium alloy containers in China, zirconium materials for nuclear and non nuclear use can be produced
Zirconium has better corrosion resistance than stainless steel, nickel base alloy and titanium. Its mechanical and technological properties are also suitable for making vessels and heat exchangers. However, due to its high price, it was seldom used in the past. But with the development of China’s chemical industry, more and more zirconium materials are used in many equipment with strong corrosion, which greatly improves the service life and reliability of the equipment and achieves better economic benefits. At present, from the production of zirconium material to the design, manufacture and inspection technology of equipment, it has become increasingly mature, which provides the basis for the wide application of zirconium containers.
Tantalum (TA1, TA2, tanb3, tanb20)
Tantalum has high chemical stability, and it has strong resistance to chemical corrosion and atmospheric corrosion under 150 ℃, even in the polluted industrial atmosphere.
Tantalum is resistant to any concentration of hydrochloric acid and nitric acid at boiling temperature, and the mixture of fuming nitric acid and fuming sulfuric acid from room temperature to 150 ℃. Tantalum is stable to other acids except hydrofluoric acid, smoky sulfur trioxide, high temperature concentrated sulfuric acid and concentrated phosphoric acid.
Tantalum is more stable than gold or platinum in the acidic and alkaline media below 200 ℃.
Tantalum has poor corrosion resistance in concentrated alkali solution. It is not resistant to potassium iodide and solutions containing fluorine ions.
Tantalum corrosion is uniform and overall corrosion, insensitive to notch, and does not occur corrosion fatigue, corrosion cracking and other local types of corrosion. By using this property of tantalum, cladding and lining materials can be made.
Other special metal materials
- Low grade duplex stainless steel (type 2304)
- Standard duplex stainless steel (type 2205)
- Super duplex stainless steel (type 2507)
For Ferritic Austenitic duplex stainless steel, it has the characteristics of both ferritic steel and austenitic steel. The existence of austenite reduces the brittleness of high chromium ferritic steel, prevents the tendency of grain growth, and improves the toughness and weldability of ferritic steel. The existence of ferrite improves the yield strength of Cr Ni austenitic steel, and makes the steel have the characteristics of stress corrosion resistance and low tendency of hot cracking during welding. This kind of steel contains high Cr, Ni, Cu, Mo and other corrosion-resistant alloy elements. Although the dual phase structure is easy to cause micro cell corrosion, if the alloy element content reaches a certain value, the two phases are likely to be passivated in the medium, and there is no dual phase selective corrosion, and its resistance to uniform corrosion and pitting corrosion is good.
Today, duplex stainless steel has been used in a variety of applications, not only in chemical, petrochemical and pharmaceutical applications, but also in pulp and paper, food and beverage, architecture, buildings and structures.
However, the most important applications of duplex stainless steel are reactors and other industrial equipment in chemical industry, chemical fertilizer, petrochemical industry, electric power and pulp and paper industry. In most applications, duplex stainless steel is considered as a cost-effective alternative material, which fills the gap between ordinary austenitic steel such as 316L and higher alloy.
Although it is generally believed that biphasic alloys are used because of their corrosion resistance to chemical products, the most important thing is in hot water solution media where austenitic stainless steels do not have sufficient resistance to pitting and stress corrosion cracking.
AL-6XN alloy is a super austenitic stainless steel discovered by Allegheny Ludlum company in the United States. It has higher resistance to pitting corrosion, crevice corrosion and pressure crevice corrosion than the standard 300 series alloy, and lower cost than the traditional nickel base alloy.
In stainless steel, Cr, Mo, Ni and C have corrosion resistance to different media respectively. Cr is the representative of corrosion resistance in natural and oxidation environment. The increase of Cr, Mo and Ni content increases the pitting resistance, nickel provides austenite structure, and nickel molybdenum increases the pressure crevice corrosion ability to chloride ion and the corrosion resistance to environment.
AL-6XN alloy with high Ni (24%) – Mo (6.3%) has good resistance to pressure crevice corrosion. Molybdenum has the ability to resist chloride ion pitting, nickel further enhances the ability to resist pitting, and can provide higher strength than 300 austenitic stainless steel, so it is often used in the thinner parts of the equipment. In AL-6XN, high content of chromium, molybdenum and nickel also provide corrosion resistance of stainless steel during forming and welding.
High chromium, molybdenum, nickel and nitrogen make AL-6XN have better resistance to chloride ion pitting and crevice corrosion, which makes AL-6XN applied in many environments, such as food, seawater or other chemical environment.
Although special metal materials have their own good corrosion resistance, but also relatively high price, which is one of the reasons why some of them can not be widely promoted, but metal composite technology promotes the application of these special metal materials on the other hand.
Metal composite material is a new metal material composed of a, B, C and other metal or alloy components, which is compounded by different processing processes. The interface forms metal bond set combination, and has the same or better performance of the original monomer metal material. It is neither a nor B (or C). It not only optimizes the material design, but also embodies the principle of rational use of materials. It is one of the development directions of material science and engineering.
The composite methods include explosive composite, explosive rolling composite and rolling composite. Nowadays, explosive composite is mostly used in China.
Composite materials include: composite plate (two layers, three layers), composite rod, composite tube.
- The reasonable combination and proportion of the properties of the composite and base materials;
- Determine the thickness ratio of the two materials as required;
- Save precious and rare metals and reduce equipment cost;
- Reduce the design thickness of the structure or increase the service stress of the structure.
At present, China has relevant national standards for composite materials, such as GB8547-87 titanium steel composite plate, GB8546-87 titanium stainless steel composite plate, JB4733-94 explosive stainless steel composite plate for pressure vessels, etc.
In conclusion, due to the good corrosion resistance and machining performance of special metal materials, it can greatly meet the corrosion resistance demand of the equipment produced by the manufacturer and improve the corrosion resistance level of the equipment. In recent years, the promotion and application in China has achieved certain results, but with the rapid development of China’s economy, especially the gradual formation of the global economic integration pattern and China’s accession In the WTO, there is a huge space for the development of China’s special metal materials (including entering the international market), but it is necessary for the relevant national industry management departments to formulate necessary standards and relevant policies and norms to promote the development of the whole industry.
Source: China Special Alloy Pipe Fittings Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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