Selection and Application of Nickel-Based Alloy Tubes
The wrought nickel-based alloy tubes introduced here (defined as alloys containing more than 45% nickel) include various types of wrought corrosion-resistant alloys commonly used in chemical plants.
The chemical compositions and UNS numbers of these alloys are listed in Table 1. For the convenience of readers, some common trade names are also listed next to them.
1 Properties of Alloys
Nickel alloys are more expensive than stainless steel. However, economic comparisons based on initial cost rather than life cycle cost can be misleading. For example, the price of Ni-Cr-Mo alloy is about 5 times that of 18Cr-8Ni stainless steel and 2 times that of austenitic stainless steel. However, due to the superior corrosion resistance of nickel alloys, the increase in initial cost can often be compensated by the long-term cost savings brought about by extended equipment life, reduced maintenance costs and less downtime.
The physical properties of nickel alloys are very similar to those of 300 series austenitic stainless steel. The thermal expansion rate of nickel-based alloys is roughly equal to that of carbon steel, and significantly lower than that of 300 series stainless steel.
Although pure nickel has a higher thermal conductivity than carbon steel, most nickel alloys have significantly lower thermal conductivity, in some cases even lower than that of austenitic stainless steel.
Except for pure nickel, nickel alloys used in the chemical processing industry are much stronger than 300 series stainless steel. Nickel alloys also have very good ductility and toughness (see Table 2 for room temperature mechanical properties). The allowable stresses for most alloys used in chemical equipment are given in Section VIII of the ASME Boiler and Pressure Vessel Code.
Nickel alloys are fully austenitic microstructures. Almost all nickel alloys used in the chemical industry are solid solution strengthened. Their increased 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 alloys cannot be strengthened by heat treatment, but can only be strengthened by cold working.
Another large class of nickel-based alloys can be strengthened by precipitation hardening heat treatment. Most of these alloys are specialized for high-strength applications, such as those used in deep oil and gas production and high-pressure processes.
Precipitation hardened nickel-based alloys have limited application in chemical equipment, except for valves and rotating machinery parts. This class of alloys includes heat-resistant alloys used in gas turbines, combustors, and aerospace applications.
2 Corrosion resistance
Nickel alloys are a step up from conventional stainless steels and austenitic iron-based alloys in their ability to resist corrosion by a variety of acids, bases, and salts. One of the outstanding advantages of nickel alloys is their excellent corrosion resistance in aqueous solutions containing halide ions. In this regard, nickel alloys are far superior to austenitic stainless steels, which are susceptible to corrosion by chloride and fluoride ion solutions.
This excellent corrosion resistance behavior of nickel alloys is manifested not only in lower metal loss, but also in better resistance to localized corrosion, especially pitting/crevice corrosion, intergranular corrosion, and stress corrosion cracking. Compared with uniform corrosion, these forms of localized corrosion are the main causes of corrosion failure in the chemical industry.
The excellent corrosion resistance of nickel alloys is partly due to the inherently lower reactivity of nickel (relative to iron), as shown by its more inert oxidation potential in the electromotive force (EMF) series. Like stainless steel, nickel alloys containing chromium have the ability to passivate (i.e., spontaneously form a thin, tough surface oxide that acts as an effective corrosion inhibitor).
Another advantage of nickel over iron is that nickel can accommodate large proportions of alloying elements without forming brittle phases. Alloying additions that improve corrosion resistance are usually chromium, molybdenum, and copper. The role of alloying elements is briefly described below.
A comparison of the corrosion resistance of nickel alloys under common media conditions in chemical plants is shown in Table 3. These general guidelines are not intended for specific purposes and are only a starting point for the material selection process.
Most nickel alloys are welded using manual arc welding (SMAW), gas tungsten arc welding (GTAW), and gas metal arc welding (GMAW). Nickel alloy welds are very ductile, and their low thermal expansion characteristics reduce residual stresses and bending deformation. Only precipitation hardening grades require post-weld heat treatment.
The welding process for nickel alloys is very similar to that for austenitic stainless steels. However, since the molten pool of nickel-rich welds is more viscous and has poorer penetration, it may be necessary to improve the joint shape and welding technology to obtain a fully penetrated weld. Nickel alloys are less tolerant of contaminants than steel materials, which can cause weld brittleness.
The high ductility, low thermal expansion rate and ability to allow dilution of various metal elements of nickel-rich welding materials make them widely used in the welding of dissimilar metals. This includes not only the welding of nickel-based alloys with iron-based alloys, but also the welding of stainless steel with carbon steel and alloy steel. Similarly, nickel alloys can be welded to the surface of carbon steel without the risk of cracking.
