Corrosion characteristics and application promotion of commonly used special metal materials

With the rapid development of the domestic economy, especially the accelerated technological transformation and upgrading of the chemical industry, special metal materials are more and more widely used in the country, especially in the petroleum, chemical and other related industries, the production of equipment materials also put forward more and more high anticorrosion requirements, which led to the domestic research institutes and processing enterprises (such as the Northwest Nonferrous Metals Research Institute and its industrialization company, Western Materials, etc.) to this Development and research, as well as related equipment manufacturers for special metal materials equipment manufacturing technology and equipment to improve capacity (such as the application of explosive composite technology), during this period cannot be separated from the promotion of the national government, so that the process of localization continues to accelerate. At the same time, some international special materials suppliers (Germany Krupp, the United States, such as Metallurgical Alliance) of the vigorous publicity also promote the application of special materials pace; these factors are in different degrees to promote the development and application of domestic special metal materials.

1. Titanium and titanium alloy

China’s production of titanium alloys with the basic synchronization of foreign countries, but its promotion and application must catch up, especially for civilian use. At the same time, due to foreign smuggling of titanium and some equipment processing enterprises in recent years of disorderly competition, some still need the production capacity of enterprises. Some small and medium-sized townships and villages use poor quality or substandard materials and also, to a certain extent, disturb the titanium equipment market so that equipment manufacturers talk about the ‘titanium’ color, so this situation is also in China’s titanium equipment. This situation also plays a certain role in hindering the development of China’s titanium equipment industry, must attract the attention of the relevant management departments, and should also be the development of other special materials of the previous lesson.
1). Titanium corrosion resistance characteristics
Titanium is a metal with a strong tendency to passivation in the air; oxidizing or neutral aqueous solution can quickly generate a layer of stable oxidizing protective film, even if the film is damaged for some reason, but it also can be quickly restored automatically. Therefore, titanium has excellent corrosion resistance in oxidizing and neutral media.
Because of titanium’s great passivation properties, in many cases with dissimilar metals in contact, it does not accelerate corrosion but may accelerate the corrosion of dissimilar metals. Such as, in the low concentration of non-oxidizing acid, if the Pb, Sn, Cu, or Monel alloy and titanium are in contact with the formation of galvanic coupling, these materials’ corrosion accelerated, while titanium is not affected. And in hydrochloric acid, titanium, and mild steel contact, due to the titanium surface produces newborn hydrogen, destroying the titanium oxide film not only causes titanium hydrogen embrittlement but also accelerates the corrosion of titanium, which may be due to the titanium has a high degree of activity on hydrogen.
Iron content in titanium has an impact on the corrosion resistance of certain media; the reason for the increase in iron in addition to the raw material often welding stained iron penetration into the welding channel, so that the welding channel in the localized iron content increases, when the corrosion has an uneven nature. Using iron parts to support titanium equipment and iron staining on the iron titanium contact surface is almost inevitable in the iron-stained area corrosion acceleration, especially in hydrogen. When mechanical damage occurs to the titanium oxide film on the stained surface, hydrogen penetrates the metal, and depending on the temperature, pressure, and other conditions, hydrogen diffusion occurs accordingly, which makes titanium produce varying degrees of hydrogen embrittlement. Therefore, titanium should be used at moderate temperatures and pressure and in hydrogen-containing systems to avoid surface iron contamination.
In general, titanium will not occur pore corrosion.
The titanium also has corrosion fatigue stability.
Titanium has better crevice corrosion resistance, especially Ti-0.3Mo-0.8Ni and Ti-0.2Pd alloys, so 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 of the sealing surface of the equipment.
2). Application of titanium
Due to the excellent corrosion resistance of titanium, titanium is widely used in petroleum, chemical, salt production, pharmaceutical, metallurgy, electronics, aviation, aerospace, marine, and other related fields.
Titanium has excellent corrosion resistance for most salt solutions, such as titanium in chloride solution, high chromium-nickel steel corrosion resistance, and no pore corrosion phenomenon. However, the corrosion rate is higher in aluminum trichloride, which is related to the hydrolysis of aluminum trichloride to produce concentrated hydrochloric acid. Titanium also has good stability to hot sodium chlorite and various hypochlorite concentrations. Therefore, titanium is widely used in vacuum salt-making and bleaching essence industries.
Titanium has good corrosion resistance to most alkali solutions. Titanium is stable in sodium hydroxide and potassium hydroxide solutions with less than 50% concentrations. If the alkali solution contains chloride ions or chlorides, its corrosion resistance exceeds that of nickel and zirconium. However, the corrosion will increase with increasing temperature and concentration. Nowadays, the chlor-alkali industry is the largest field of domestic civil titanium application.
Titanium is not corrosion-resistant in dry chlorine gas. At the same time, there is a risk of fire, but in the wet chlorine gas has high stability, exceeding zirconium, Hastelloy C, and Monel alloy, and even in saturated chlorine, sulfuric acid, hydrochloric acid and chlorides and other media are also stable, so titanium is the sulfuric acid method of titanium dioxide production of the key equipment of choice for the material.
As titanium material in the hydrocarbon corrosion resistance is very good, even in the presence of acid and chloride impurities is also very good. Therefore, titanium is also widely used in organic chemicals, such as PTA (fine terephthalic acid), PVA (vinylon), etc.
Titanium has excellent corrosion resistance in seawater, so titanium is also widely used in offshore oil rigs, desalination, and other marine fields.
Titanium material processing technology is becoming more and more mature; western materials is an internationally renowned titanium material processing enterprise with extraordinary technology research and development capabilities and including melting, ingot casting, forging, rolling, processing, and other technical means; titanium products including pipe, plate, wire, composite plate, pipe fittings, flanges, forgings, pressure vessels, and another complete industrial chain, is the most powerful technology-based company in China. Its subsidiary Yaang Pipe Industry Co., Limited. completed the first localization of titanium equipment in the 80s, solved problems such as welding, non-destructive testing, structural design, and forming process, and maintained a sustained leading position.

2. Nickel and nickel-based alloys

1). Nickel and nickel-based alloys in the domestic production situation
The domestic industrial pure nickel can be completely produced by ourselves, but some nickel-based alloys mainly depend on imports. Commonly used nickel and nickel-based alloy models are pure nickel N6; Monel400; Hastelloy B, Hastelloy B-2; Hastelloy C-276.
2). Corrosion resistance of nickel and nickel-based alloys
Nickel has a greater tendency to turn into a passive state. At general temperature, the surface of nickel is covered with an oxide film, which makes it corrosion-resistant in water and many saline aqueous solutions.
Nickel is fairly 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 increases significantly with the addition of oxidizing agents (FeCl₂, CuCl₂, HgCl₂, AgNO₃, and hypochlorite) and with aeration.
The outstanding property of nickel is that it is completely stable in all hot and molten alkali solutions.
Monel alloys are more resistant to corrosion than nickel in reducing media and more resistant to corrosion than copper in oxidizing media, and are more resistant to corrosion than nickel and copper in phosphoric acid, sulphuric acid, hydrochloric acid, salt solutions, and organic acids.
Any hydrofluoric acid concentration in the oxygen is little when the monel alloy is very corrosion-resistant. However, when the solution is aerated and oxidizing agents or the presence of iron salts, copper salts, and other harmful impurities in the solution, it is resistant to hydrofluoric acid performance decline. In addition to platinum and silver in metal materials, it is among the best materials to resist hydrofluoric acid corrosion.
It is very resistant to corrosion in caustic solutions. Still, when the concentration of sodium hydroxide is very high, the corrosion resistance of Monel alloys, although worse than nickel, is more resistant to alkali than other metal materials.
Monel alloys are prone to stress corrosion cracking; annealing at 530-650 ℃ is best to eliminate stress after use.
Hastelloy commonly used Hastelloy B (B-2, B-3) and Hastelloy C-276; they have high corrosion resistance in non-oxidizing inorganic and organic acids, such as resistance to dilute sulfuric acid at 70 ℃, resistant to all concentrations of hydrochloric acid, phosphoric acid, acetic acid, and anthranilic acid, especially resistant to hot concentrated hydrochloric acid.
Hastelloy is stable in caustic and alkaline solutions and is completely stable in organic media, seawater, and freshwater.

3. Zirconium

Commonly used zirconium and zirconium alloy grades are non-nuclear zirconium R60702, R60703, R60704, R60705, and R60706.
Although China does not have a specification for zirconium and zirconium alloy containers, it has been able to produce nuclear and non-nuclear zirconium.
Zirconium has better corrosion resistance than stainless steel, nickel-based alloys, and titanium; mechanical properties and process performance are also very suitable for manufacturing vessels and heat exchangers. However, due to the high price, it was used less in the past. However, with the development of the domestic chemical industry, strong corrosion equipment is increasingly using zirconium, greatly improving equipment life and reliability to achieve better economic benefits. From the production of zirconium to the design of equipment, manufacturing and inspection technology has also become increasingly mature, providing a basis for the wide application of zirconium containers.
Currently, the zirconium processed material produced by Western Materials has occupied 80%+ of the domestic market share, of which a large part is realized in Yaang Pipe Industry Co., Limited. It is reported that the company’s manufacturing technology strength is at the domestic leading level.

4. Tantalum material

Ta1, Ta2, TaNb3, TaNB20

Tantalum has high chemical stability and resistance to chemical corrosion, and atmospheric corrosion below 150 ℃ is very strong; even in a polluted industrial atmosphere is also corrosion-resistant.
Tantalum is resistant to hydrochloric acid and nitric acid at any concentration at boiling temperature and to mixed acids consisting of fuming nitric acid and fuming sulfuric acid at room temperature up to 150℃. Tantalum is stable to all acids except hydrofluoric acid, fuming sulfur trioxide, and concentrated sulfuric and phosphoric acids at high temperatures.
Tantalum has high stability in acidic and alkaline media below 200℃, even higher than gold and platinum.
Tantalum has poor corrosion resistance in concentrated alkaline solutions. It is not resistant to potassium iodide and solutions containing fluoride ions.
Tantalum corrosion is uniform overall corrosion, insensitive to cuts, does not occur corrosion fatigue and corrosion rupture, and other local types of corrosion. Using this characteristic of tantalum, it can be used as cladding and lining material.

5. White copper (B10, B30)

White copper is a copper-nickel alloy; white copper can meet domestic production, mainly by Luo Copper.
The corrosion resistance of white copper is similar to that of pure copper, and serious corrosion occurs in inorganic acids, especially in nitric acid. But for the concentration of <70% of hydrofluoric acid, in the absence of oxygen and below the boiling point of the case is corrosion-resistant. White copper corrodes little in organic acids, and the corrosion rate is small in alkaline solutions and organic compounds.
In caustic soda, or the diaphragm electrolysis caustic soda, can be used B30 (70-30 copper-nickel alloy to replace pure nickel manufacturing membrane evaporator equipment, especially the descending part of the membrane, which not only improves the service life but also saves 70% of the nickel. B10 (91-9 copper-nickel alloy) can also be used instead of pure nickel to make the ascending membrane evaporator tubes, evaporation chamber, and other equipment.
White copper in seawater has high corrosion resistance, so the heat exchanger cooled with seawater often uses B10, B30 copper.

6. Other special metal materials

1). Duplex steel

• Low-grade duplex stainless steel (2304 type)
• Standard duplex stainless steel (2205 type)
• Super duplex stainless steel (Type 2507)

For ferritic-austenitic duplex stainless steels, it combines the properties of both ferritic and austenitic steels. Austenite reduces the brittleness of high-chromium ferritic steels, prevents grain growth, and improves the toughness and weldability of ferritic steels. The presence of ferrite improves the yield strength of Cr-Ni austenitic steels and, at the same time, makes the steel resistant to stress corrosion and welding with a small tendency to thermal cracking and other characteristics. These steels contain high Cr, Ni, Cu, Mo, and other corrosion-resistant alloying elements. Although the duplex organization can easily cause microcell corrosion, there is no selective corrosion if the alloying element content reaches a certain value so that the two phases will likely passivate in the medium. Its resistance to uniform corrosion and pore corrosion performance is good.
Today, duplex stainless steel has been used in various applications, not only in chemical, petrochemical, and pharmaceutical applications, but also for pulp and paper, food and beverage, construction, buildings, and structures.
However, the most important applications for 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 steels are considered cost-effective alternatives, filling the gap between common austenitic steels such as 316L and higher alloys.
Although it is generally accepted that duplex alloys are used because of their resistance to the corrosive properties of chemical products, the most important are in hot aqueous media where austenitic stainless steels do not have sufficient resistance to pore corrosion and stress corrosion fracture.
2). AL-6XN
AL-6XN alloy is super austenitic stainless steel discovered by Allegheny Ludlum in the U.S. It has a higher resistance to pitting, crevice corrosion, and pressure crevice corrosion than the standard 300 series alloys against chloride ions. It is less costly than traditional corrosion-resistant nickel-based alloys.
In stainless steel, Cr, Mo, Ni, and C, respectively, corrosion resistance to different media. Cr is in the natural and oxidizing environment and is corrosion resistance representative; Cr, Mo, and Ni content growth increased pitting corrosion resistance, nickel provides austenitic structure, nickel, and molybdenum increased pressure crevice corrosion resistance to chloride ions and corrosion resistance to reduce the environment.
The high nickel (24%)-molybdenum (6.3%) alloy AL-6XN has good resistance to pressure crevice corrosion. Molybdenum provides resistance to chloride pitting, and nickel further enhances pitting resistance and provides higher strength than 300 austenitic stainless steel, so it is often used in thinner equipment sections. Higher chromium, molybdenum, and nickel levels in AL-6XN also provide corrosion resistance in forming and welding stainless steel.
The high chromium, molybdenum, nickel, and nitrogen give AL-6XN better resistance to chloride pitting, and crevice corrosion, which allows AL-6XN to be used in many environments such as food, seawater, or other chemical environments.

7. Metal composites

Although the special metal materials have their own better corrosion resistance, but also have relatively high prices, which is some of them cannot be widely promoted one of the reasons. Still metal composite technology, on the other hand, promotes the application of these special metal materials.
Metal composite materials are made of a, b, c, and so on, several metal or alloy group elements, by different processing technology composite of new metal materials from all walks of life to form a set of metal bonding combined with the original monomer metal materials with the same or better performance. It is neither a nor b (or c). It is a combination of the advantages of the constituent elements so that the performance of a single group of elements to overcome the shortcomings of the group, which optimizes the material design but also reflects the principle of rational use of materials, is one of the current directions of development of materials science and engineering.
Composite methods are explosion composite, explosion-rolling composite, rolling composite, and now most of the domestic use of explosion composite.
Composite material varieties include composite plate (two-layer, three-layer), composite rod, and composite tube.
Advantages:

• Reasonable combination and proportion of the properties of the compound material and the base material;
• According to the need to determine the thickness ratio of the two materials;
• Saving precious scarce metals and reducing the cost of equipment;
• Reduce the thickness of the structural design or improve the structure by using stress.

At present, the country has composite materials related to national standards, such as GB8547-87 “titanium – steel composite plate”, GB8546-87 “titanium – stainless steel composite plate”, JB4733-94 “pressure vessels with exploded stainless steel composite steel plate,” and so on.