Nickel and the nickel-base alloys constitute a family of alloys with increasing importance in many industrial applications because they can be corrosion resistant in a wide variety of service environments that range from sub-zero to elevated temperatures. Some types have an almost unsurpassed corrosion resistance in certain media, but nickel alloys are usually more expensive than, for example, iron-base or copper-base alloys or than plastic materials of construction. Aside from its application as an alloying element, Nickel as such is used as a material of construction, as a plating on a steel substrate or another less noble metal, or as a cladding on steel. The plating may be deposited either by electrochemical techniques or as an electroless plating deposited by a chemical reduction process.

Nickel is found as a constituent in most meteorites and often serves as one of the criteria for distinguishing a meteorite from other minerals. Iron meteorites, or siderites, may contain iron alloyed with from 5 to nearly 20% nickel. The USA 5-cent coin (whose nickname is "nickel") contains just 25% nickel. Nickel is a silvery white metal that takes on a high polish. It is hard, malleable, ductile, somewhat ferromagnetic, and a fair conductor of heat and electricity.

Non-chromium-containing

Nickel-Molybdenum Alloys: The element molybdenum possesses exceptional resistance to sulfuric, phosphoric, and particularly hydrochloric acids. Recognition of this resistance led to an alloy development program which indicated that about 25% molybdenum alloyed with nickel yielded an alloy resistant to hydrochloric acid.

Alloy B (UNS N10001): The first commercial Ni-Mo alloy with high Mo-content was Alloy B (e.g., Hastelloy B). It is an almost chromium-free (1% Cr) nickel alloy (61% Ni) with about 30% Mo, developed to resist hydrochloric acid up to the atmospheric boiling point. It is available in wrought and cast forms. Work hardening presents some fabrication difficulties, and machining is somewhat more difficult than, for example, for Type 316 stainless steel. Conventional welding methods can be used. This alloy is now almost obsolete because in some severe environments, such as boiling hydrochloric acid, the heat-affected zones of welds were subject to intergranular corrosion. It was found that Mo-rich precipitates formed in the grain boundaries of UNS N10001 which locally depleted the alloy of molybdenum. It is been replaced by Alloy B-2.

Alloy B-2 (UNS N10665): The chemistry of the original alloy has been modified to achieve freedom from intergranular corrosion. A low-carbon variant has been developed and the modified alloy is Alloy B-2 (UNS N10665, or "Hastelloy B-2"/ TM). Alloy B-2 is gradually replacing Alloy B in most applications. There is no cast counterpart to UNS N10665. However, ASTM A-494 Grade N12M-1 and N12M-2 cast nickel-molybdenum alloys are normally employed for cast components.

Nickel-Copper Alloys

• Alloy 400 (UNS N04400): Nickel and copper form solid-solution, single-phase alloys over the entire range of concentrations. Nickel ore from the Sudbury district of Ontario contained about two-thirds nickel and one-third copper. Thus, an alloy of this composition could be directly smelted from this ore. The resulting alloy was called "MONEL" and is now widely known by this name and also as "Alloy 400" and UNS N04400.
  
  
• Monel K-500 (UNS N05500): By the addition of small amounts of aluminum (2.7%) and titanium (0.6%) an age-hardenable (precipitation-hardening) nickel alloy UNS N05500 (MONEL K-500) is obtained with higher mechanical strength than UNS N04400. It has increased hardness and abrasion resistance and is non-magnetic. Its corrosion resistance is comparable to Alloy 400, but it is, for example, preferred for pump shafts, impellers, bolts, valve trim and spindles, doctor blades and scrapers, etc.
  
  
• Alloy 505: Alloy 505 is a nickel-copper cast alloy with about 66% Ni, 30% Cu, and 4% Si. It has very good non-galling characteristics. It is excellent for bearings or bushings. High strength can be developed by heat treatment.
  
  
• Monel R-405: This alloy is similar to Alloy 400 but has controlled sulfur added for improving machining characteristics. It is not widely used.

Chromium-containing
Nickel-Chromium Alloys: The addition of about 10% or more chromium to nickel yields alloys that are considerably more resistant to oxidizing conditions than nickel itself. The prototype alloy in this category of nickel-chromium materials is Alloy 600 (UNS N06600, "Inconel 600" /TM). It contains about 76% Ni and 15% Cr.

Annealed UNS N06600 is a single-phase alloy with an austenitic structure; but by the addition of small amounts of aluminum, titanium and niobium (columbium) an age-hardenable alloy (UNS N07750, "Inconel X-750" /TM) with mechanical properties superior to Alloy 600 may be made.

Ni-Cr-Mo Alloys: The prototype alloys of this group was "Hastelloy C" [/TM] (UNS N10002, or Alloy C). Alloy C is a nickel alloy with about 15% Cr and 15% Mo (plus about 60% Ni, 5% W, and 5% Fe), which was developed for enhanced resistance to both oxidizing environments and strongly reducing acids. It was so corrosion resistant that it set a standard to which other metals and alloys were often compared. Alloy C can be produced in wrought and cast form. Fabrication, machining and welding present no unusual problems, except that it tends to work-harden.

Inert-gas-shielded or metal-arc-welding processes are usually recommended. The original Hastelloy C is now almost obsolete, and is replaced by a number of variants : the composition of the alloy has seen several modifications that have increased metallurgical stability and resistance to localized corrosion in the heat affected zones of welds. In addition, other nickel-chromium-molybdenum alloys have been developed with excellent corrosion resistance.

Nickel-Iron-Chromium Alloys: The basic alloy in this group is Alloy 800 ("Incoloy 800"/TM ; UNS N08800). With its nominal composition 21Cr - 33Ni - 47Fe it is in fact a superstainless steel. It differs from Type 310 stainless steel (25Cr-20Ni) primarily because of its higher Ni content. Strictly speaking, it is neither an iron-based nor a nickel-based alloy (neither element is present at more than 50% concentration), so the N designation in the UNS system is arbitrary. Alloy 800 is strong, and is resistant to oxidation and carburization at elevated temperatures. It resists sulfur attack, internal oxidation, scaling, and corrosion in a wide variety of atmospheres.

Ni-Fe-Cr-Mo Alloys: The group of Ni-Fe-Cr-Mo alloys contains both a series of so-called "medium alloys" (Alloys 20, Alloy 825) and a number of highly alloyed materials (Hastelloys F and G, Incoloy 925 /TM). These alloys not only have Mo (and eventually Cu) added, but also have higher Cr and Ni content than the common nickel-iron-chromium alloys (e.g. Alloy 800).