Selection and Application Guide: corrosion resistant nickel alloy
Nickel alloy has excellent corrosion resistance, strength, toughness, metallurgical stability, processability and weldability. Many nickel alloys also have excellent heat resistance and are ideal for applications requiring high temperature strength and chemical resistance at high temperatures.
The forging and rolling nickel base alloy (defined as the alloy containing more than 45% nickel) introduced here includes various forging and rolling corrosion-resistant alloys commonly used in chemical plants.
The chemical composition and UNS number of these alloys are listed in Table 1. For the convenience of readers, some commonly used trade names are listed beside.
Table.1 nominal chemical composition of corrosion resistant nickel alloy
Properties of alloy
Nickel alloy is more expensive than stainless steel. However, economic comparisons based on initial costs rather than life cycle costs can be misleading. For example, the price of Ni Cr Mo alloy is about 5 times of that of 18Cr-8Ni stainless steel and 2 times of that of super austenitic stainless steel. However, due to the excellent corrosion resistance of nickel alloy, the increase of initial cost can often be compensated by the prolongation of equipment life, the reduction of maintenance cost and the saving of long-term cost caused by very few downtime.
The physical properties of nickel alloy are very similar to those of 300 series austenitic stainless steel. The thermal expansion rate of nickel base alloy and carbon steel is about the same, but obviously lower than that of 300 series stainless steel.
Although the thermal conductivity of pure nickel is higher than that of carbon steel, the thermal conductivity of most nickel alloys is obviously lower, even lower than that of austenitic stainless steel in some cases.
In addition to pure nickel, the strength of nickel alloy used in chemical processing industry is much higher than that of 300 series stainless steel. Nickel alloys also have excellent ductility and toughness (see Table 2 for room temperature mechanical properties). The maximum allowable stress of most alloys used in chemical equipment is shown in Section VIII of ASME Boiler and pressure vessel code.
Nickel alloy is full austenite microstructure. Almost all nickel alloys used in the chemical industry are solution strengthened. The increase of their strength comes from the addition of effective hardening elements such as molybdenum and tungsten, rather than the formation of carbides. Like austenitic stainless steel, solid solution nickel alloy can not be strengthened by heat treatment, but only by cold working.
Another kind of nickel base alloy can be strengthened by precipitation hardening heat treatment. Most of these alloys are specially used for ultra-high strength applications, such as those used in deep oil and gas production and ultra-high pressure process.
In addition to the application of Precipitation Hardening Nickel base alloy in valves and rotating mechanical parts, its application in chemical equipment is limited. These alloys include heat-resistant Superalloys for gas turbines, combustion chambers, and aerospace applications.
Table.2 mechanical properties of corrosion resistant nickel alloy at room temperature
Compared with the traditional stainless steel and super austenitic iron-based alloy, the ability of nickel alloy to resist all kinds of acid, alkali and salt corrosion is improved by one grade. One of the outstanding advantages of nickel alloy is its excellent corrosion resistance in the aqueous solution containing halide ions. In this respect, nickel alloy is far superior to austenitic stainless steel, which is easily corroded by chloride and fluoride solutions.
The excellent corrosion resistance of nickel alloy is not only low metal loss, but also better resistance to local corrosion, especially pitting / crevice corrosion, intergranular corrosion and stress corrosion cracking. Compared with uniform corrosion, these forms of local corrosion are the main cause of corrosion failure in the chemical industry.
The excellent corrosion resistance of nickel alloys is due in part to the inherent low reactivity of nickel (relative to iron), as shown by its inert oxidation potential in the electromotive force (EMF) sequence. Similar to stainless steel, chromium containing nickel alloy has passivation ability (i.e. it spontaneously forms an ultra-thin and tough surface oxide, which plays an effective role in preventing corrosion).
Another advantage of nickel over iron is that nickel can hold a large proportion of alloy elements without forming brittle phase. Chromium, molybdenum and copper are usually added to the alloy to improve corrosion resistance. The role of alloy elements will be briefly introduced below.
See Table 3 for comparison of corrosion resistance of nickel alloy in common medium conditions of chemical plant. These general guidelines are not intended for specific purposes and are only used as a starting point for the selection process.
Table.3 Corrosion resistance of nickel alloy
Most nickel alloy welding uses SMAW, GTAW and GMAW. The ductility of nickel alloy weldments is very good, and their low thermal expansion characteristics reduce the residual stress and bending deformation. Post weld heat treatment is only required for precipitation hardening type. See Table 4 for technical conditions of nickel alloy electrode and filler metal issued by American Welding Society (AWS).
The welding process of nickel alloy is very similar to that of austenitic stainless steel. However, in order to obtain full penetration weld, it may be necessary to improve the joint shape and welding technology. Nickel alloy does not allow the presence of contaminants more than steel, which can cause weld embrittlement.
The high ductility, low thermal expansion and the ability to allow dilution of various metal elements of nickel rich welding materials make them widely used in welding of dissimilar metals. It includes not only the welding of nickel base alloy and iron base alloy, but also the welding of stainless steel and carbon steel and alloy steel. Similarly, nickel alloy can be overlaid on the surface of carbon steel without the risk of cracking.
Table.4 technical conditions of nickel alloy electrode and filler metal
Types of nickel alloys
There are many kinds of nickel alloy products available on the market, including plate, sheet, strip, pipe, joint and forging. ASTM Specifications for some products are listed in Table 5. Some nickel alloys also have cast products, which usually have different characteristics from their forged and rolled products.
Nickel alloy is generally classified according to its main alloy composition. The following is a brief introduction of nickel alloy widely used in chemical equipment:
- Pure nickel: pure nickel (alloy 200) has very good corrosion resistance to various reducing acids and salts, but it is not suitable for strong oxidizing media such as nitric acid. The most important characteristic of pure nickel is its unparalleled ability to resist caustic corrosion, even molten caustic corrosion. Although the corrosion resistance of pure nickel is outstanding in the dry halogen medium, its corrosion resistance is insufficient below the water dew point. For applications above 600 ° F, a derivative grade of alloy 200, nickel 201 (UNS uo2201) with low carbon content, is preferred.
- Like nickel, the corrosion resistance of nickel copper alloy 400 is the best in reductive medium, while the corrosion resistance is not good in gas filled and oxidizing chemicals. Alloy 400 has very good corrosion resistance to hydrofluoric acid and halide, especially to hydrofluoric acid and high temperature gas containing fluorine or hydrogen fluoride. This alloy is widely used in the treatment of sulfuric acid solution, seawater and brine. For applications requiring high strength, such as valve and pump components, alloy K-500 (no5500) is often used, which is a precipitation hardening derivative brand of alloy 400.
- Ni Cr Fe alloy 600: the addition of chromium in the nickel matrix increases the applicability of alloy 600 in oxidizing environment. Although alloy 600 has medium corrosion resistance to inorganic acids, its corrosion resistance to organic acids is very good, so it is widely used in fatty acid processing. Alloy 600 is also widely used in the production, storage and transportation of hydroxides and alkaline chemicals. Alloy 600 is also an excellent material for high temperature applications with both heat and corrosion resistance requirements. The excellent performance of the alloy in high temperature halogen environment makes it an ideal material for organic chlorination process. Alloy 600 also shows excellent anti-oxidation, anti carburizing and anti nitriding properties.
- Ni Cr Mo alloy 625: the addition of Mo to Ni Cr alloy increases the corrosion resistance of the material to oxidizing and reducing inorganic acids and salts. Molybdenum makes the alloy resistant to chloride ion pitting and crevice corrosion. Alloy 625 is a kind of high-strength material with excellent fatigue resistance. Alloy 625lcf is a derivative brand of alloy 625. It is specially used for bellows and has excellent low cycle fatigue resistance and heat-resistant fatigue performance. As with alloy 600, alloy 625 can be used as both corrosion-resistant and heat-resistant materials. Alloy 625 has excellent high temperature strength, halogen corrosion resistance, oxidation resistance and carburizing resistance, which makes it an ideal material for chemical and petrochemical equipment operating in harsh high temperature environment.
- Nickel Chromium Alloy 690: Alloy 690 has the highest chromium content in the nickel alloy used in the manufacture of pressure equipment, and has a strong corrosion resistance to oxidizing medium. It can be effectively used in hot concentrated sulfuric acid, nitric acid and nitric / hydrofluoric acid mixed acid and oxidizing salt medium. High chromium content also improves the corrosion resistance of the material in high temperature vulcanization environment.
- Ni Cr Fe alloy 825: Alloy 825 is sometimes included in the super austenitic stainless steel series because it contains nearly 30% iron. It performs well in sulfuric acid and phosphoric acid media, similar to alloy 20, and its main purpose is to be used in sulfuric acid and phosphoric acid media. Although the corrosion resistance of alloy 825 to hydrochloric acid is acceptable, it is prone to chloride ion pitting and crevice corrosion, especially in non flowing and non aerated solutions. Alloy 825 has high iron content, so its corrosion resistance to alkali and halogen is lower than that of alloy 825 with high nickel content.
- Nickel chromium iron molybdenum “g” alloy: the corrosion resistance of alloy G-3 exceeds alloy 400, alloy 600 and alloy 825 in many applications. The alloy is especially resistant to the corrosion of sulfuric acid and impure phosphoric acid, and can withstand the conditions of reducibility and oxidation. Later developed alloy G-30 has better welding performance and overall improvement of corrosion resistance, especially the corrosion resistance of weld heat affected zone.
- Nickel chromium molybdenum “C” alloy: Alloy C-276 is an excellent alloy material used in the chemical industry to cope with extremely corrosive medium conditions (beyond the scope of stainless steel), which has outstanding corrosion resistance in various acids, acid salts and other corrosive chemicals. Alloy C-276 performs well in harsh environments such as wet chlorine and hypochlorite. Due to the high content of molybdenum in alloy C-276, it has good corrosion resistance to pitting and crevice corrosion caused by chloride ion. The process of searching for materials with better metallurgical and corrosion resistance than alloy C-276 has promoted the development and commercialization of several patented “C” series alloys, which are alloy C-22, 622, 59, 686 and C-2000. The molybdenum content of these alloys is about the same, while the chromium content is much higher than that of alloy C-276. Some brands also contain tungsten or copper. The influence of these minor alloy elements on metallurgical properties and corrosion resistance is complex, which will not be discussed in this paper.
- Nickel molybdenum “B” alloy: Alloy B-2 has outstanding corrosion resistance to sulfuric acid, phosphoric acid and hydrochloric acid with reducibility. It is especially suitable for hydrochloric acid equipment with all concentration ranges and temperatures up to boiling point. Oxidizing chemicals have a negative effect on the corrosion resistance of the alloy, especially the strong oxidants such as iron and copper ions which are impurities in the solution. Later developed alloys B-3 and B-4 have better properties than alloy B-2. One of the advantages of these new grades is to minimize the formation of poor microstructure (which may cause embrittlement) during processing.
Table.5 ASTM Specifications for some product forms
Development of nickel alloy
Like iron and copper, nickel has been used in alloys since the beginning of civilization. But compared with steel, brass and bronze, nickel alloy is the successor of chemical industry.
The first nickel alloy of commercial importance is alloy 400, which was developed and marketed by international nickel company (later called inco alloy company) in 1905. Its trademark is Monel. The next important milestone is the advent of nickel molybdenum alloy B and nickel chromium molybdenum tungsten alloy C around 1930. Their inventors are Haynes Stellite (now Haynes International), two of which are registered trademarks of Hastelloy. The next important stage of nickel base alloy development comes from inco company. Nickel chromium ferroalloy 600 was developed in 1931 and nickel iron chromium alloy was developed in 1949, respectively named Inconel and Incoloy.
Inco and Haynes used the popularity and reputation of the original trademarks to launch about 50 kinds of corrosion-resistant and heat-resistant alloys of Monel, Inconel, Incoloy and Hastelloy series.
Germany’s Krupp VDM company is a later well-known nickel base alloy developer and producer. Its trademarks are nickel, nimofer and nicorros.
The three world leading companies are also continuing to develop new nickel and improved alloys of interest to the chemical industry. Their proprietary alloys mentioned herein are listed in table a.
All the early patented alloys have long lost their patent protection, and now many other manufacturers in the world can produce these alloys, either their own trade names or general alloy names that meet the standards or technical specifications. As with drugs, there are still arguments about the equivalence between generic products and patented products.
The commonly used standards such as ASTM and din may have less restrictive requirements than the standards set by famous alloy manufacturers. The internal standards of manufacturers control the chemical composition and metallurgical parameters of alloys more strictly, aiming to ensure higher material properties.
Table.A commodity name of alloy
Source: China Alloy Steel Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (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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