Guide for selection of corrosion and heat resistant nickel base alloys
Why choose a nickel-based alloy?
Nickel-based alloys are both heat and corrosion resistant, making them ideal materials for high temperature corrosive environments. Nickel alloys are useful for chemical equipment exposed to high temperatures (usually defined as temperatures above about 1000 ° F) in corrosive processes.
In many cases, high-strength, chemically resistant nickel alloys are the best (if not the only) materials for harsh environments (beyond the corrosion resistance of austenitic and super-austenitic stainless steels). Although more expensive than iron-based alloys, the excellent performance of heat-resistant and corrosion-resistant nickel alloys makes them the most economical choice for long-term use.
Composition of nickel-based alloy
The chemical composition of the heat-resistant and corrosion-resistant nickel-based alloys (defined here as alloys with a nickel content of more than 45%) widely used in the chemical processing industry is shown in Table 1. Alloys are identified by common grades and UNS codes. Although early patents on alloys have expired, they still use their original trade names. For example, the universal alloys 600, 601, and 625 are commonly associated with the INCONEL trademark held by Inco. Similarly, Alloy X is a well-known alloy in the Hastelloy trademark of Haynes International. The newly developed alloys are still proprietary materials that can only be purchased from their developers, as shown in the notes in Table 1.
Table.1 Nominal chemical composition of wrought heat-resistant nickel-based alloy,%
Note: * is the maximum value.
- 1. Haynes rhenium alloys 214TM, 230, 242TM (products of Haynes International Corporation).
- 2. Nicrofer (R) alloy 45TM, Nicrofer 6025HT-alloy 602CA (Krupp VDM).
The alloys introduced are forging and rolling materials, and the strength is mainly increased by solid solution strengthening treatment. It does not include precipitation-hardened nickel-based alloys or oxide dispersion strengthened (ODS) alloys produced by powder metallurgy. These superalloys, which are mainly used in gas turbines and aerospace equipment, are rarely used in chemical equipment. The corresponding casting grades of the alloys listed in Table 1 are also outside the scope of this article.
Physical and mechanical properties
The physical properties of nickel alloy are similar to those of 300 series Cr Ni stainless steel. There may be significant differences in thermal conductivity and thermal expansion of different alloys, which should be considered in equipment design.
As a whole, the mechanical properties of nickel base alloy are excellent, both in strength and plasticity. The minimum strength and plasticity index at room temperature are listed in Table 2. Except for alloy 601, the minimum yield strength is much higher than 30ksi, which is the yield strength of common austenitic stainless steel.
The strength of nickel based materials is higher than that of iron-based materials, and it increases with the increase of temperature, as shown in Table 3. It is worth noting that at 1500 ° f (816 ° C), the nickel base alloy maintains 40% – 75% of its room temperature yield strength, while stainless steel only maintains 20% – 35% of the room temperature yield strength.
The superiority of nickel alloy extends to creep fracture, as shown in Table 4. When stainless steel is at or above 2000 ° f (1093 ° C), its effective strength is basically lost, while nickel alloy can still play an effective role as a medium stress component. For example, for alloys 600, 601, 214, 230, and 333, the breaking strength at 1000 hours at 2000 ° f (1093 ° C) is about 1.0ksi, for alloys 617 and 602ca, the breaking strength at this condition is about 1.4ksi.
The ASME Boiler and pressure vessel code includes alloy allowable stress values in addition to alloys 214, 242, and 45tm.
Table.2 Minimum mechanical properties at room temperature
Table.3 0.2% of typical high temperature yield strength, KSI
Table.4 Typical creep rupture strength, KSI
Another important feature considered in the selection of alloys for high temperature applications is metallurgical stability, also known as thermal stability, which refers to the ability of materials to resist the formation of brittle microstructure phases or aging (i.e. long-term exposure to high temperature) precipitates. The so-called “aging brittleness” mainly shows the decrease of plasticity and toughness, which may also lead to the decrease of corrosion resistance.
Although some alloys, such as alloy 600 and 601, do not actually undergo aging embrittlement, most of them suffer from different degrees of damage. Alloy 625 is one of them. When it is exposed to temperatures from 1200 ° f (649 °C) to 1400 ° f (760 °C), its plasticity and impact strength decrease significantly. At higher temperatures, these properties are partially restored due to the re dissolution of brittle precipitates. It is unusual for equipment to fail due to reduced plasticity and toughness, which is due to the very high initial properties of the non aged nickel alloy.
The most common form of corrosion in high temperature chemical processing environment is gas phase corrosion, especially oxidation, sulfuration and halogenation (chlorination and fluorination). In the severe high temperature environment, other forms of performance degradation are mainly carburizing, nitriding and hydrogen corrosion. Because there is no metal loss and surface pit, these corrosion forms are not traditional corrosion, but mainly show the damage of metallurgical mechanical properties, the most common is embrittlement.
See Table 5 for the effect of alloy composition on slowing down or intensifying high temperature chemical corrosion of nickel base alloy.
The effects of chromium, molybdenum, cobalt, tungsten, silicon and aluminum can be both beneficial and harmful, depending on the specific exposure conditions, especially temperature and reducing or oxidizing atmosphere.
In fact, several degradation modes of material properties may occur simultaneously. For example, many industrial environments contain both oxygen and chlorine, and metals suffer from very lethal corrosion when exposed to chlorine oxidation. Liquid phase such as molten salt, molten ash or molten metal can also cause abnormally serious corrosion. These corrosive substances are rarely encountered in the chemical processing. This paper will not discuss them.
Table.5 Effect of alloy elements on high temperature corrosion of nickel base alloy
Forms of corrosion
The common types of high temperature chemical corrosion are briefly described below. Due to the strong ability of hydrogen corrosion resistance of nickel alloy, hydrogen corrosion is omitted.
- (1) Oxidation is the most common form of corrosion at high temperature. It is characterized by the formation of metal oxidation corrosion products. These so-called oxide scales are usually very dense and have strong adhesion, so they can slow down further corrosion. However, under very bad conditions, oxide skin can be penetrated or peeled off. Chromium is the most important element to make the alloy have oxidation resistance. As in the case of stainless steel, the addition of a small amount of aluminum, silicon and rare earth elements can further improve the stability and adhesion of oxides, especially under thermal cycling conditions. The stable oxide skin not only slows down the further oxidation, but also acts as an effective screen wall to resist other types of corrosion.
- (2) Sulfide produces oxide skin rich in metal sulfide. The reductive curing environment is usually more corrosive than the oxidizing curing atmosphere. Nickel base alloy is more likely to be vulcanized than stainless steel because of its low melting point. As with oxidation, the addition of chromium can significantly improve the vulcanization resistance.
- (3) Chlorinated stainless steel will corrode rapidly when exposed to high temperature chlorine and chlorine compounds. Due to the instability of ferric chloride and oxychloride, serious chlorination may occur without obvious oxide scale formation. Nickel base alloy is much more resistant to chloride than iron alloy, and is an ideal material for chlorine or chloride environment.
- (4) Carburizing in high carbon active atmosphere, carbon often diffuses into metal matrix and forms metal carbide. This form of corrosion is called carburizing, which will cause serious damage to mechanical properties, especially plastic and impact strength. Nickel base alloys show good resistance to carburization, because unlike iron, nickel is not a strong carbide forming element.
- (5) Nitriding refers to the diffusion of nitrogen into the metal lattice to form metal nitrides. In the chemical industry, this phenomenon is mainly encountered in the high temperature ammonia atmosphere. As with carburizing, the damage appears as embrittlement rather than metal loss. Nickel does not form nitrides, so nickel rich alloys have excellent nitriding resistance.
- (6) Internal corrosion carburizing and nitriding are not the only high-temperature degradation mode of materials characterized by internal damage. In fact, all high-temperature corrosion is driven by diffusion, and the corrosion under the surface is mainly along the grain boundary. This is suitable for oxidation, vulcanization, especially halogenation. In many cases, internal corrosion penetrates deeper into the metal than the surface metal loss. Therefore, the evaluation of high temperature corrosion should not only be based on thickness or metal loss, but also on metallographic examination.
Nickel base alloys can be welded by common welding processes, including SMAW, GTAW and GMAW. Generally, nickel alloy weldments have good plasticity, and their lower thermal expansion characteristics are beneficial to reduce residual stress and deformation. Post weld heat treatment is generally not required for solution strengthening alloy.
American Welding Society (AWS) standards A5.11 and A5.14 give the applicable welding materials of nickel base alloy, as shown in Table 6. Although filler metals matching alloys 601, 214, 230, 242, and 602ca are not included in AWS standards A5.11 and A5.14, they are commercially available. For weldments exposed to corrosive high temperature environment, it is better to select matching filler metal, because it can ensure that the weld metal and base metal have the same corrosion resistance. Nickel rich welding materials are also widely used for welding between nickel base and iron base alloys and for surfacing on iron base materials.
Table.6 Technical conditions of AWS welding materials
- 1. These welding materials are not completely matched with the base metal, so the high-temperature corrosion resistance of the weld metal and the base metal may be different.
- 2. Matching filler metal can be purchased, but it is not included in AWS technical specification.
The welding process of nickel base alloy is similar to that of stainless steel. However, due to the poor fluidity of high nickel weld pool, in order to obtain the full penetration weld, the design of welding joint and welding technology should be modified properly. Nickel base alloy is more sensitive to weld embrittlement caused by pollution than iron base alloy. The welding area should be protected carefully to prevent contamination by foreign substances.
Forging and rolling heat-resistant and corrosion-resistant nickel base alloy products have various shapes, including medium plate / sheet, seamless pipe and welded pipe products. Table 7 lists the ASTM standards for some products. Most of the alloys listed in Table 1 are also capable of producing castings, but the chemical composition of the casting products has changed slightly.
Table.7 ASTM standards for some products
Properties of alloy
This paper briefly summarizes the main characteristics of each alloy, which affect the applicability of the material in the high temperature corrosive environment of the chemical plant.
- Alloy 600 has excellent anti-oxidation, anti chlorination, anti carburization and anti nitriding ability, but its anti vulcanization ability is poor. Alloy 600 is widely used in high temperature chlorine/hydrogen chloride environment and ammonia environment.
- Alloy 601 has excellent resistance to carburization and periodic oxidation. It has medium strength, but its thermal stability is very good. Alloy 601 is widely used in polluted combustion environment.
- Alloy 617 integrates excellent high temperature strength, thermal stability, oxidation resistance and carburizing resistance, and is suitable for the production of nitric acid and petrochemical products.
- Alloy 625 has both high strength and good overall corrosion resistance, including corrosion resistance in aqueous medium. It has excellent fatigue fracture resistance but moderate thermal stability. Alloy 625 is widely used in chemical / petrochemical equipment.
- Alloy X has outstanding strength, processability, oxidation resistance, carburizing resistance and nitriding resistance. It is a good alloy used in corrosive combustion environment as stressed parts.
- Alloy 214 has good oxidation resistance (up to 2200 ° f (1204 ℃)), anti chlorination, anti carburization and anti nitriding ability, medium thermal stability, processability and welding performance. It is an alloy suitable for extreme corrosion environment, with limited product form and quantity.
- Alloy 230 has the best balance of strength, thermal stability, fatigue fracture resistance, oxidation resistance and processability. It is suitable for high-strength parts in harsh combustion environment.
- Alloy 242 has the best resistance to fluoride and fluoride corrosion. It has very high strength and good thermal stability. Alloy 242 is not suitable for temperatures above 1500 ° f (816 ° C). It is often used in the production of fluoropolymers.
- Alloy 333 has excellent oxidation resistance, carburization resistance, vulcanization resistance and mechanical properties. It can be used in all kinds of chemical / petrochemical equipment.
- Alloy 45tm is the best choice for the application requiring the comprehensive ability of anti chlorination/oxidation/vulcanization/carburization. This alloy is suitable for incineration and gasification processes.
- Alloy 602ca has outstanding resistance to periodic oxidation (up to 2200 ° f (1204 ℃)) and carburization, as well as good resistance to oxidation / sulfurization gas. It has high creep strength at very high temperature.
The price of the alloy mentioned in this paper is about two to five times higher than that of 310 stainless steel. Because there is little difference between nickel base alloy and stainless steel in the cost of processing and manufacturing, the price difference is greatly reduced from the perspective of installation equipment cost.
Another important consideration in economic analysis is higher performance of high-performance materials, lower maintenance costs and longer service life. From the perspective of life cycle cost analysis, nickel base alloy is often proved to be the most economical choice.
Note: heat resistant alloy and corrosion resistant alloy.
Alloy manufacturers and drafters of technical standards think it is convenient to divide alloy into heat-resistant alloy or corrosion-resistant alloy. This kind of random classification mainly focuses on whether the chemical composition and microstructure of the alloy is the best corrosion resistance in aqueous medium or the best performance at high temperature. It can be further subdivided according to whether the chemical corrosion resistance of the heat-resistant alloy is mainly used or the best strength and metallurgical stability are used in the high temperature corrosive environment. In fact, there are many overlaps between heat-resistant and corrosion-resistant alloys. Some multipurpose nickel base alloys can be used effectively in many occasions.
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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