Corrosion resistance characteristics and application knowledge summary of commonly used special metal materials
With the rapid development of China’s economy, especially the acceleration of technological transformation and upgrading in the chemical industry, the application of special metal materials in China has become more and more extensive, especially in the petroleum, chemical and other related industries. The materials for production equipment have also been proposed more and more. Higher anti-corrosion requirements have led to the development and research of relevant Chinese research institutes and processing companies, and related equipment manufacturers’ ability to manufacture technology and equipment for special metal materials. The improvement (such as the application of explosive composite technology) during this period is also inseparable from the promotion of the Chinese government (China Titanium Office, Sinopec Localization Office), which has accelerated the localization process. At the same time, the vigorous publicity of some international special material suppliers (Krupp of Germany, American Metallurgical Union, etc.) has also promoted the application of special materials. These factors have promoted the development and application of special metal materials in China to varying degrees.
Titanium and titanium alloys
The production of titanium alloys in China is basically synchronized with that of foreign countries, but its promotion and application lag behind, especially for civilian use. At the same time, due to the disorderly competition of smuggled titanium materials abroad and some equipment processing enterprises in recent years, some enterprises without production capacity and some small and medium-sized township enterprises have used inferior materials or shoddy materials have also disrupted the titanium equipment market to a certain extent. Make equipment manufacturers talk about “titanium” color change, so this situation also plays a certain role in hindering the development of China’s titanium equipment industry, which must attract the attention of relevant management departments, and it should also serve as a warning for other special materials under development .
Commonly used titanium grades
- TA1 (Grade2): Industrial Pure Titanium
- TA2 (Grade3): Industrial Pure Titanium
- TA9 (Grade11): Ti-0.2Pd
- TA10 (Grade12): Ti-0.3Mo-0.8Ni
- TC4 (AB-1): Ti-6Al-4V
The corrosion resistance of titanium
Titanium is a metal with a strong tendency to passivation. It can quickly form a stable oxidizing protective film in the air and in an oxidizing or neutral aqueous solution. Even if the film is damaged for some reason, it can quickly and automatically recover. Therefore, titanium has excellent corrosion resistance in oxidizing and neutral media.
Due to the great passivation performance of titanium, in many cases, when it comes in contact with dissimilar metals, it does not accelerate the corrosion, but may accelerate the corrosion of dissimilar metals. For example, in a low-concentration non-oxidizing acid, if Pb, Sn, Cu or Monel alloy is contacted with titanium to form a galvanic couple, these materials will corrode faster, and titanium will not be affected. In hydrochloric acid, when titanium is in contact with low-carbon steel, new hydrogen is generated on the surface of titanium, which destroys the titanium oxide film, which not only causes hydrogen embrittlement of titanium, but also accelerates the corrosion of titanium. This may be due to the high degree of hydrogen on titanium. Caused by activity.
The iron content in titanium has an impact on the corrosion resistance of some media. The reason for the increase in iron is that in addition to the raw materials, the contaminated iron penetrates into the weld bead during welding, which increases the local iron content in the weld bead. Time corrosion has a non-uniform nature. When using iron to support titanium equipment, iron contamination on the iron-titanium contact surface is almost unavoidable and accelerated in the iron-contaminated area, especially in the presence of hydrogen. When the titanium oxide film on the contaminated surface is mechanically damaged, hydrogen will penetrate into the metal. According to conditions such as temperature and pressure, hydrogen will diffuse correspondingly, which causes hydrogen embrittlement of titanium to varying degrees. Therefore, titanium should be used in medium temperature and medium pressure and hydrogen-containing systems to avoid surface iron contamination.
Under normal circumstances, titanium does not undergo pitting corrosion.
Titanium also has corrosion fatigue stability.
Titanium has better crevice corrosion resistance, especially Ti-0.3Mo-0.8Ni and Ti-0.2Pd alloys. Therefore, Ti-0.3Mo-0.8Ni and Ti-0.2Pd alloys are widely used as sealing surface materials for container equipment to solve Crevice corrosion on the sealing surface of the equipment.
Application of Titanium
Due to the excellent corrosion resistance of titanium materials, titanium materials are widely used in petroleum, chemical, salt, 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 solutions and has no pitting corrosion. However, the corrosion rate is higher in aluminum trichloride, which is related to the production of concentrated hydrochloric acid after the hydrolysis of aluminum trichloride. Titanium also has good stability to hot sodium chlorite and various concentrations of hypochlorite. Therefore, titanium materials are widely used in vacuum salt making and bleaching powder industries.
Titanium has good corrosion resistance to most alkaline solutions. Titanium is relatively stable in sodium hydroxide and potassium hydroxide solutions with a concentration of less than 50%. If the alkali solution contains chloride ions or chlorides, its corrosion resistance even exceeds that of nickel and zirconium. However, as the temperature and concentration increase, corrosion will increase. At present, the chlor-alkali industry is the largest application of titanium for civil use in China.
Titanium is not resistant to corrosion in dry chlorine and has a fire hazard. However, it has high stability in humid chlorine, exceeding zirconium, Hastelloy C and Monel, and even in sulfuric acid, hydrochloric acid and sulfuric acid saturated with chlorine. It is also stable in media such as chloride, so titanium is the material of choice for key equipment in the production of titanium dioxide by sulfuric acid.
Because titanium has good corrosion resistance in hydrocarbons, it is also good even when it contains acid and chloride impurities. Therefore, titanium materials are also widely used in organic chemicals, such as PTA (purified terephthalic acid), PVA (vinylon) and so on.
Titanium materials have excellent corrosion resistance in seawater, so titanium materials are also widely used in marine fields such as offshore oil drilling platforms and seawater desalination.
Nickel and nickel-based alloys
China’s production status of nickel and nickel-based alloys
China’s industrial pure nickel can be produced by itself, but some nickel-based alloys mainly rely on imports.
Commonly used nickel and nickel-based alloy models are: pure nickel N6; Monel 400; Hastelloy B, Hastelloy B-2; Hastelloy C-276, etc.
Corrosion resistance of nickel and nickel-based alloys
Nickel has a greater tendency to turn into a passive state. At normal temperatures, the surface of nickel is covered with an oxide film, which makes it resistant to corrosion in water and many salt solutions.
Nickel in non-oxidizing dilute acids, such as <15% hydrochloric acid, <17% sulfuric acid and many organic acids, is quite stable at room temperature. However, when oxidants (FeCl2, CuCl2, HgCl2, AgNO3 and hypochlorousite) and ventilation are added, the corrosion rate of nickel increases significantly.
Nickel is completely stable in all alkali solutions, whether it is high temperature or molten alkali, which is the outstanding characteristic of nickel.
Monel is more corrosion-resistant than nickel in reducing media, and more corrosion-resistant than copper in oxidizing media, and more corrosion-resistant than nickel and copper in phosphoric acid, sulfuric acid, hydrochloric acid, salt solutions and organic acids.
In any concentration of hydrofluoric acid, Monel is very resistant to corrosion when oxygen enters a little. But when there is aeration and oxidant in the solution, or when there are harmful impurities such as iron salt and copper salt in the solution, its hydrofluoric acid resistance will decrease. In addition to platinum and silver among metal materials, it is one of the best materials resistant to hydrofluoric acid corrosion.
It is very resistant to corrosion in caustic alkali solution, but when the concentration of sodium hydroxide is 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 prone to stress corrosion cracking. It is best to use it after annealing at 530-650℃ to eliminate stress.
Hastelloy alloys commonly used are Hastelloy B (B-2, B-3) and Hastelloy C-276, which have high corrosion resistance in non-oxidizing inorganic and organic acids, such as resistance to dilute at 70°C. Sulfuric acid, resistant to all concentrations of hydrochloric acid, phosphoric acid, acetic acid and formic acid, especially resistant to concentrated hydrochloric acid.
Hastelloy is stable in caustic alkali and alkaline solutions, and completely stable in organic media, sea water and fresh water.
Cupronickel (B10, B30)
Cupronickel is a copper-nickel alloy, and China can satisfy the production of cupronickel.
The corrosion resistance of cupronickel is basically similar to that of pure copper, and severe corrosion occurs in inorganic acids, especially nitric acid. But for hydrofluoric acid with a concentration of <70%, it is corrosion resistant in the absence of oxygen and below the boiling point. Copper-nickel alloy has little corrosion in organic acids, and has a low corrosion rate in alkaline solutions and organic compounds.
In caustic soda, or in diaphragm electrolytic caustic soda, B30 (70-30 copper-nickel alloy can be used to replace pure nickel to make film evaporator equipment, especially the falling film part. It can not only increase the service life, but also It saves 70% of nickel. B10 (91-9 copper-nickel alloy) can also replace pure nickel to manufacture the evaporation tube, evaporation chamber and other equipment of the rising film evaporator.
Cupronickel has high corrosion resistance in seawater, so B10 and B30 cupronickel is often used for heat exchangers cooled with seawater.
Zirconium
Commonly used zirconium and zirconium alloy grades are: non-nuclear zirconium R60702, R60703, R60704, R60705, R60706.
Although China does not have specifications for zirconium and zirconium alloy containers, it has been able to produce zirconium materials for nuclear and non-nuclear use.
Zirconium has better corrosion resistance than stainless steel, nickel-based alloys and titanium, and its mechanical properties and process properties are also very suitable for manufacturing containers and heat exchangers. But because of its high price, it was used less in the past. However, with the development of China’s chemical industry, many highly corrosive equipment is increasingly using zirconium materials, which greatly improves equipment life and reliability, and achieves better economic benefits. At present, from the production of zirconium materials to the design, manufacturing and inspection technology of equipment, it has become mature day by day, providing a foundation for the wide application of zirconium containers.
Tantalum
- Ta1, Ta2, TaNb3, TaNB20
Tantalum has high chemical stability, and has strong resistance to chemical corrosion and atmospheric corrosion below 150°C. It is also corrosion resistant even in polluted industrial atmospheres.
Tantalum is resistant to hydrochloric acid and nitric acid of any concentration at boiling temperature, and resistant to mixed acids composed of fuming nitric acid and fuming sulfuric acid at room temperature to 150°C. Except for hydrofluoric acid, fuming sulfur trioxide and high-temperature concentrated sulfuric acid and phosphoric acid, tantalum is stable to other acids.
Tantalum has high stability in acidic and alkaline media below 200°C, even higher than gold and platinum.
Tantalum has poor corrosion resistance in concentrated lye. Not resistant to potassium iodide and solutions containing fluoride ions.
The corrosion of tantalum is uniform overall corrosion, not sensitive to the incision, and no local types of corrosion such as corrosion fatigue and corrosion cracking occur. Using this characteristic of tantalum, it can be used as a coating and lining material.
Other special metal materials
Duplex steel
- 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 presence 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 presence of ferrite improves the yield strength of Cr-Ni austenitic steel, and at the same time makes the steel have the characteristics of resistance to stress corrosion and low tendency of hot cracking during welding. This type of steel contains high corrosion-resistant alloying elements such as Cr, Ni, Cu and Mo. Although the dual-phase structure is easy to cause corrosion of the microbattery, if the alloy element content reaches a certain value, the two phases may 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 various applications, not only in chemical, petrochemical and pharmaceutical applications, but also in pulp and paper, food and beverage, construction, buildings , duplex stainless steel pipe and structures.
But the most important applications of duplex stainless steel are reactors and other industrial equipment in the chemical, fertilizer, petrochemical, power and pulp and paper industries. In most applications, duplex stainless steel is considered a cost-effective alternative material, filling the gap between ordinary austenitic steels such as 316L and higher alloys.
Although it is generally believed that dual-phase alloys are used because of their corrosion resistance to chemical products, the most important thing is in the hot aqueous medium where austenitic stainless steel does not have sufficient resistance to pitting corrosion and stress corrosion fracture.
AL-6XN
AL-6XN alloy is a super-austenitic stainless steel discovered by Allegheny Ludlum in the United States. It has higher resistance to pitting, crevice corrosion and pressure crevice corrosion than the standard 300 series alloys. It is more resistant to corrosion than traditional nickel-based alloys. The alloy cost is low.
In stainless steel, Cr, Mo, Ni, and C have corrosion resistance to different media. Cr is the representative of corrosion resistance in natural and oxidizing environments. The increase in the content of Cr, Mo, Ni increases the resistance to pitting corrosion, nickel provides an austenite structure, and nickel molybdenum increases the pressure crevice corrosion resistance and resistance to chloride ions. Reduce the corrosion resistance of the environment.
High nickel (24%)-molybdenum (6.3%) alloy AL-6XN has better resistance to pressure crevice corrosion. Molybdenum has the ability to resist chloride ion pitting corrosion. Nickel further enhances the ability to resist pitting corrosion, and can provide higher strength than 300 austenitic stainless steel, so it is often used in thinner parts of equipment. In AL-6XN, the higher content of chromium, molybdenum and nickel also provides corrosion resistance during forming and welding of stainless steel.
High chromium, molybdenum, nickel and nitrogen make AL-6XN have better resistance to chloride ion pitting corrosion and crevice corrosion, which makes AL-6XN used in many environments, such as food, seawater or other chemical environments.
Metal composite
Although special metal materials have their own good corrosion resistance, they are also relatively expensive. This is one of the reasons why some of them cannot be promoted on a large scale. However, metal composite technology promotes these special metal materials from another aspect. Applications.
Metal composite material is a new metal material compounded by several metal or alloy components such as a, b, c, etc. through different processing techniques. Each interface forms a metal bond set and has the same or better performance as the original single metal material. . It is neither a nor b (or c). It integrates the advantages of the constituent elements, and overcomes the shortcomings of the single-group elements. It not only optimizes the material design, but also reflects the principle of rational use of materials. It is one of the current development directions of materials science and engineering.
The compounding methods are: explosive compounding, explosive-rolling compounding, and rolling compounding. Nowadays, most of China adopts explosive compounding method.
The types of composite materials include: composite boards (two-layer, three-layer), composite rods, and composite pipes.
Advantage:
- Reasonable combination and ratio of the properties of the composite material and the base material;
- Determine the thickness ratio of the two materials as needed;
- Save precious and rare metals and reduce equipment cost;
- Reduce the design thickness of the structure or increase the use stress of the structure.
At present, China has relevant Chinese 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 steel plate for pressure vessels”, etc.
In summary, due to the good corrosion resistance and machining performance of special metal materials, it can greatly meet the corrosion resistance requirements of manufacturers’ production equipment and improve the corrosion resistance level of equipment. In recent years, the promotion and application in China has achieved certain results.
Author: China Iron and Steel Research Institute
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.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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