Corrosion resistance characteristics and applications of commonly used special metal materials
1. Titanium and titanium alloys
The production of titanium alloys in China is basically in sync with foreign countries, but its promotion and application lags behind, especially for civilian use. At the same time, due to the disorderly competition among foreign smuggled titanium materials and some equipment processing companies in recent years, some companies without production capacity and some small and medium-sized township enterprises have used inferior materials or shoddy goods, which has also disrupted the titanium equipment market to a certain extent. It makes equipment manufacturers talk about "titanium" discoloration. Therefore, this situation also plays a certain role in hindering the development of China's titanium equipment industry. It must attract the attention of relevant management departments and should also serve as a warning for other special materials that are being developed. .
Commonly used titanium grades (with national material standards)
1. Corrosion resistance characteristics of titanium
Titanium is a metal with a strong tendency to passivate. It can quickly form a stable oxidative protective film in the air and in oxidizing or neutral aqueous solutions. 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 in contact with dissimilar metals, it does not accelerate corrosion, but may accelerate the corrosion of dissimilar metals. For example, in low-concentration non-oxidizing acids, if Pb, Sn, Cu or Monel alloy are contacted with titanium to form a galvanic couple, the corrosion of these materials will be accelerated, while titanium will not be affected. In hydrochloric acid, when titanium comes into contact with low carbon steel, new hydrogen is generated on the titanium surface, which destroys the titanium oxide film, which not only causes hydrogen embrittlement of titanium, but also accelerates titanium corrosion. This may be because titanium is highly resistant to hydrogen. due to activity.
The iron content in titanium has an impact on the corrosion resistance in some media. In addition to the raw materials, the reason for the increase in iron is often that contaminated iron penetrates into the weld bead during welding, causing the local iron content in the weld bead to increase. This Corrosion has a non-uniform nature. When iron parts are used to support titanium equipment, iron contamination on the iron-titanium contact surface is almost inevitable. Corrosion is 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 penetrates into the metal. Depending on conditions such as temperature and pressure, hydrogen diffuses accordingly, which causes varying degrees of hydrogen embrittlement in titanium. Therefore, when titanium is used in medium temperature and medium pressure and hydrogen-containing systems, surface iron contamination must be avoided.
Under normal circumstances, titanium does not suffer from pitting corrosion.
Titanium also offers corrosion fatigue stability.
Titanium has good 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 the problem of Crevice corrosion problem on equipment sealing surface.
2. Application of titanium materials
Due to its excellent corrosion resistance, titanium materials are widely used in petroleum, chemical industry, salt production, pharmaceuticals, 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 aluminum trichloride is hydrolyzed. Titanium also has good stability to hot sodium chlorite and various concentrations of hypochlorite. Therefore, titanium materials are widely used in vacuum salt production 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 concentrations less than 50%. If the alkaline solution contains chloride ions or chlorides, its corrosion resistance even exceeds that of nickel and zirconium. However, as temperature and concentration increase, corrosion will increase. The chlor-alkali industry is now the largest field of domestic civil titanium applications.
Titanium is not corrosion-resistant in dry chlorine and is a fire risk, but it has high stability in moist chlorine, exceeding zirconium, Hastelloy C and Monel, and even in sulfuric acid, hydrochloric acid and saturated chlorine. It is also stable in media such as chloride, so titanium is the first choice material for key equipment in the production of titanium dioxide by the sulfuric acid method.
Because titanium has good corrosion resistance in hydrocarbons, it is also good even when it contains acids and chloride impurities. Therefore, titanium materials are also widely used in organic chemicals, such as PTA (purified terephthalic acid), PVA (vinylon), etc.
Titanium has excellent corrosion resistance in seawater, so titanium is also widely used in marine fields such as offshore oil drilling platforms and seawater desalination.
2. Nickel and nickel-based alloys
1. Domestic production status of nickel and nickel-based alloys
Domestic industrial pure nickel can be produced by itself, but some nickel-based alloys mainly rely on imports.
Types of nickel and nickel-based alloys (some have national material standards)
Commonly used nickel and nickel-based alloy models include: pure nickel N6; Monel 400; Hastelloy B, Hastelloy B-2; Hastelloy C-276, etc.
2. Corrosion resistance of nickel and nickel-based alloys
Nickel has a greater tendency to pass into a passive state. At normal temperatures, 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, when adding oxidants (FeCl2, CuCl2, HgCl2, AgNO3 and hypochlorite) and ventilation, the corrosion rate of nickel increases significantly.
Nickel is completely stable in all alkaline solutions, whether high temperature or molten alkali. This is the outstanding characteristic of nickel.
Monel alloy is more corrosion-resistant than nickel in reducing media, and more corrosion-resistant than copper in oxidizing media. It is 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 alloy is very resistant to corrosion when oxygen does not enter much. However, when there is aeration and oxidants in the solution, or when there are harmful impurities such as iron salts and copper salts in the solution, its resistance to hydrofluoric acid decreases. Among metal materials, apart from platinum and silver, it is one of the best materials resistant to hydrofluoric acid corrosion.
It is very corrosion-resistant in caustic alkali solutions, but when the concentration of sodium hydroxide is very high, although the corrosion resistance of Monel alloy is worse than that of nickel, it is still more alkali-resistant than other metal materials.
Monel alloy is prone to stress corrosion cracking and is best used after annealing at 530-650°C to eliminate stress.
The commonly used Hastelloy alloys are Hastelloy B (B-2, B-3) and Hastelloy C-276. They have high corrosion resistance in non-oxidizing inorganic acids and organic acids, such as resistance to 70°C dilute Sulfuric acid, resistant to all concentrations of hydrochloric acid, phosphoric acid, acetic acid and formic acid, especially hot concentrated hydrochloric acid.
Hastelloy is stable in caustic and alkaline solutions and completely stable in organic media, seawater and fresh water.
Three white copper (B10, B30)
Cupronickel is a copper-nickel alloy. Cupronickel can be produced domestically and is mainly produced by Luoyang Copper.
The corrosion resistance of white copper is basically similar to that of pure copper. Severe corrosion will occur in inorganic acids, especially nitric acid. However, hydrofluoric acid with a concentration of <70% is corrosion-resistant in the absence of oxygen and below the boiling point. White copper does not corrode greatly in organic acids, and the corrosion rate is very small in alkaline solutions and organic compounds.
In the caustic soda process or in the diaphragm electrolytic caustic soda, B30 (70-30 copper-nickel alloy can be used to replace pure nickel to manufacture film evaporator equipment, especially the falling film part. It can not only improve the service life, but also Save 70% of nickel. B10 (91-9 copper-nickel alloy) can also replace pure nickel to manufacture evaporation tubes, evaporation chambers and other equipment of rising film evaporators.
White copper has high corrosion resistance in seawater, so heat exchangers cooled by seawater often use B10 and B30 white copper.
Four zirconium materials
Commonly used zirconium and zirconium alloy grades include: non-nuclear zirconium R60702, R60703, R60704, R60705, and 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. Its mechanical properties and process properties are also very suitable for manufacturing containers and heat exchangers. However, due to its high price, it was rarely used in the past. However, with the development of the domestic chemical industry, many highly corrosive equipment are increasingly using zirconium materials, which greatly improves equipment life and reliability and achieves better economic benefits. At present, the technology from the production of zirconium materials to the design, manufacturing and inspection of equipment has become increasingly mature, providing a foundation for the wide application of zirconium containers.
5. Tantalum materials (Ta1, Ta2, TaNb3, TaNB20)
Tantalum has high chemical stability and is highly resistant to chemical corrosion and atmospheric corrosion below 150°C. It is corrosion-resistant even in polluted industrial atmosphere.
Tantalum is resistant to hydrochloric acid and nitric acid of any concentration at boiling temperature, and to a mixed acid composed of fuming nitric acid and fuming sulfuric acid from room temperature to 150°C. Except for hydrofluoric acid, fuming sulfur trioxide, and high-temperature concentrated sulfuric acid and concentrated 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 alkali solutions. Not resistant to potassium iodide and solutions containing fluoride ions.
The corrosion of tantalum is uniform and comprehensive corrosion, insensitive to cuts, and does not cause localized types of corrosion such as corrosion fatigue and corrosion cracking. This characteristic of tantalum can be used as coating and lining materials.
6. Other special metal materials
1. 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 resistant to stress corrosion and has a small tendency to hot crack during welding. This type of steel contains high levels of corrosion-resistant alloy elements such as Cr, Ni, Cu, and Mo. Although the dual-phase structure can easily cause microbattery corrosion, if the alloy element content reaches a certain value, both phases may be passivated in the medium, and dual-phase selective corrosion will not occur. It has good resistance to uniform corrosion and pitting corrosion. .
Today, duplex stainless steels are used in a variety of applications, not only in chemical, petrochemical and pharmaceutical applications, but also in pulp and paper, food and beverage, and construction, buildings and structures.
But the most important applications of duplex stainless steel are in reactors and other industrial equipment in the chemical, fertilizer, petrochemical, power and pulp and paper industries. In most applications, duplex stainless steels are considered a cost-effective alternative material, filling the gap between common austenitic steels such as 316L and higher alloys.
Although it is generally believed that duplex alloys are used because of their resistance to corrosion by chemical products, this is most important in hot water solution media where austenitic stainless steels do not have sufficient resistance to pitting corrosion and stress corrosion cracking.
2. AL-6XN
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 to chloride ions than the standard 300 series alloy, and is more resistant to corrosion than traditional nickel-based alloys. Alloy cost is low.
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 oxidizing environments. The increase in the content of Cr, Mo, and Ni increases the resistance to pitting corrosion. Nickel provides an austenite structure. Nickel and molybdenum increase the pressure crevice corrosion ability and resistance to chloride ions. Reduce the corrosion resistance of the environment.
High nickel (24%)-molybdenum (6.3%) alloy AL-6XN has good resistance to pressure crevice corrosion. Molybdenum has the ability to resist chloride ion pitting corrosion. Nickel further enhances the resistance to pitting corrosion and can provide higher strength than 300 austenitic stainless steel, so it is often used in thinner parts of equipment. The higher levels of chromium, molybdenum and nickel in AL-6XN also provide corrosion resistance when forming and welding the stainless steel.
High chromium, molybdenum, nickel and nitrogen make AL-6XN have good 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.
7. Metal composite materials
Although special metal materials have their own good corrosion resistance, they are also relatively expensive, which is one of the reasons why some of them cannot be promoted on a large scale. However, metal composite technology has promoted these special metal materials on the other hand. Applications.
Metal composite materials are new metal materials that are composed of several metal or alloy components such as a, b, and c through different processing techniques. Each interface forms a set of metal bonds and has the same or better performance as the original single metal material. . It is neither a nor b (or c). It combines the advantages of constituent components and overcomes the performance deficiencies of single components. It not only optimizes material design, but also embodies the principle of rational use of materials. It is one of the current development directions of materials science and engineering.
Compounding methods include: explosion compounding, explosion-rolling compounding, and rolling compounding. Nowadays, most of the domestic methods use explosion compounding.
Composite material varieties include: composite panels (two-layer, three-layer), composite rods, and composite pipes.
advantage:
Reasonable combination and ratio of properties of cladding materials and base materials;
Determine the thickness ratio of the two materials as needed;
Save precious and rare metals and reduce equipment costs;
Reduce the structural design thickness or increase the structural service stress.
At present, the country 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 Steel Plate for Pressure Vessels", etc.
To sum up, because special metal materials have good corrosion resistance and machining performance, they can greatly meet the corrosion resistance needs of manufacturers' production equipment and improve the corrosion resistance level of equipment. In recent years, their promotion and application in China have achieved certain results. However, with the rapid development of China's economy, especially the gradual formation of the global economic integration pattern and China's accession to the WTO, there is huge room for the development of domestic special metal materials (including entering the international market), but it requires relevant national industry management departments. Develop necessary standards and related policies and regulations to promote the development of the entire industry.
