Nickel and nickel-base alloys are vitally important to modern industry
because of their ability to withstand a wide variety of severe operating
conditions involving corrosive environments, high temperatures, high
stresses, and combinations of these factors.
There are several reasons for these capabilities. Pure nickel is ductile and
tough because it possesses a face-centered cube crystal structure up to its
melting point. Nickel has good resistance to corrosion in the normal
atmosphere, in natural freshwaters and in deaerated nonoxidizing acids, and
it has excellent resistance to corrosion by caustic alkalis.
Therefore, nickel offers very useful corrosion resistance itself and
provides an excellent base for developing specialized alloys. Intermetallic
phases can be formed between nickel and some of its alloying element: this
enables the formulation of very high strength alloys for both low- and
Copper. Additions of copper provide improvement in the resistance of
nickel to nonoxidizing acids. In particular alloys containing 30 to 49 % Cu
offer useful resistance to nonaerated sulfuric acid (H2SO4) and offer
excellent resistance to all concentrations of nonaerated hydrofluoric acid
(HF). Additions of 2 to 3% Cu to nickel-chromium-molybdenum-iron alloys have
also been found to improve resistance to hydro-chloric acid (HCl), H2SO4 and
phosphoric acid (H3PO4).
Chromium additions impart improved resistance to oxidizing media such as
nitric (HNO3) and chromic (H2CrO4) acids. Chromium also improves resistance
to high-temperature oxidation and to attack by hot sulfur-bearing gases.
Iron is typically used in nickel-base alloys to reduce costs, not to
promote corrosion resistance. However, iron does provide nickel with improved
resistance to H2SO4 in concentrations above 50%.
Molybdenum in nickel substantially improves resistance to
nonoxidizing acids. Commercial alloys containing up to 28% Mo have been
developed for service in nonoxidizing solutions of HCl, H3PO4 and HF as well
as in H2SO4 in concentrations below 60%. Molybdenum also significantly
improves the pitting and crevice corrosion resistance of nickel base alloys.
Silicon is typically present only in minor amounts in most nickel-base
alloys as a residual element from deoxidation practices or as an intentional
addition to promote high-temperature oxidation resistance. In alloys
containing significant amounts of iron, cobalt, molybdenum, tungsten or
other refractory elements, the level of silicon must be carefully controlled
because it can stabilize carbides and harmful intermetallic phases.
Cobalt. The corrosion resistance of cobalt is similar to that of
nickel in most of environments. Because of this and because of its higher
costs and lower availability, cobalt is not generally used as a primary
alloying element in materials designed for aqueous corrosion resistance. On
the other hand, cobalt imparts unique strengthening characteristics to
alloys designed for high-temperature service.
Niobium and Tantalum. In corrosion resistant alloys, both niobium and
tantalum were originally added as stabilizing elements to tie up carbon and
prevent intergranular corrosion attack due to grain-boundary carbide
Aluminium and titanium are often used in minor amounts in corrosion
resistant alloys for the purpose of deoxidation or to tie up carbon and/or
nitrogen, respectively. When added together, these elements enable the
formulation of age-hardenable high-strength alloys for low- and elevated
Carbon and Carbides. There is evidence that nickel forms a carbide of
the formula Ni3C at elevated temperatures, but it is unstable and decomposes
into a mixture of nickel and graphite at low temperatures. Because this phase
mixture tends to have low ductility, low-carbon forms of nickel are usually
preferred in corrosion-resistant applications.
Nickel and its alloys, like the stainless steels, offer a wide range of
corrosion resistance. However, nickel can accommodate larger amounts of
alloying elements - mainly chromium, molybdenum, and tungsten - in solid
solution than iron. Therefore, nickel-base alloys in general can be used in
more severe environments than the stainless steels. In fact, because nickel
is used to stabilize the austenite phase of some of the highly alloyed
stainless steels, the boundary between these and nickel-base alloys is rather
The nickel-base alloys range in composition from commercially pure nickel to
complex alloys containing many alloying elements. A distinction is usually
made between those alloys that are primarily used for high-temperature
strength, commonly referred to as superalloys, and those that are primarily
used for corrosion resistance.
Nickel-base alloys are frequently used because of their improved resistance
to environmental embrittlement over steels and stainless steels. However,
nickel-base alloys can exhibit environmental embrittlement under the combined
action of tensile stresses (either residual or applied) and specific
environmental conditions. In the most severe cases, cracking or failure may
result after an incubation period in which no apparent damage has occurred.
These incubation periods may be of the order of minutes, days, months or
The embrittlement of nickel-base alloys by the combined action of tensile
stress and a suitable environment is thought to occur by two phenomena:
hydrogen embrittlement and Stress Corrosion Cracking (SCC).
No inference is made as to mechanisms of embrittlement or to what extent
hydrogen is involved in SCC. Phenomenologically, hydrogen embrittlement is
distinguished from SCC in this section by the influence of two parameters
(environmental temperature and anodic/cathodic polarization) on the
susceptibility of alloys to embrittlement. Increasing the temperature from
ambient generally results in increasing susceptibility to SCC and decreasing
susceptibility to hydrogen embrittlement. Cathodic polarization often results
in increasing hydrogen embrittlement and decreasing SCC susceptibility.
The nickel-base alloys are generally used to combat SCC where austenitic
stainless steels have failed because of SCC. However, two events have
recently occurred that require increased knowledge of the SCC resistance of
nickel-base alloys. First, a large number of alloys have been developed and
included in the market: this has resulted in an almost continuous change in
performance (alloy content) between stainless steels and the numerous
nickel-base alloys. Second, the nickel-base alloys have been historically
considered to be immune to SCC in all but a few environments, but the
increased requirements for current processes have extended the use of
materials to temperatures at which the SCC of nickel-base alloys must be
Stress-corrosion cracking of nickel-base alloys has been found to occur in
three types of environments: high-temperature halogen-ionic solutions,
high-temperature waters, and high-temperature alkaline environments. In
addition, SCC has been detected in liquid metals, near-ambient-temperature
polythionic acid solutions, and environments containing acids and hydrogen
Hydrogen-embrittlement of nickel-base alloys is exemplified by three forms:
brittle (usually intergranular) delayed fracture, a loss in reduction of
area while often retaining a microvoid coalescent fracture, or a reduction
in properties such as fatigue strength. Although cleavage-type cracks have
been reported in nickel-base alloys they are not the predominant mode of
Nickel-base alloys are used for corrosion resistance or for combined
corrosion resistance and high temperature strength in a wide range of
commercial applications. These various applications may demand resistance to
aqueous corrosion mechanisms, such as general corrosion, localized attack,
and SCC, or resistance to elevated temperature oxidation, sulfidation and
carburization. Many nickel-base alloys have been developed to resist these
and other forms of attack. The alloys often find application in areas
outside the specific industry or process for which they were designed.
Caustic Soda. The chemical-processing industry involves a great
variety of corrosive environments. Thus, a variety of nickel-alloys are used
in this industry.
Water. Nickel and nickel-base alloys generally have very good
resistance to corrosion in distilled water and freshwater. Typical corrosion
rates for Nickel 200 (commercially pure nickel) in a distilled water storage
tank at ambient temperature and domestic hot water service are
<0,0025mm/yr and <0,005mm/yr respectively. Nickel-copper alloys such as
400 and R-405 also have very low corrosion rates and are used in freshwaters
systems for valve seats and other fittings.
Atmospheres. Nickel and nickel-base alloys have very good resistance
to atmospheric corrosion. Corrosion rates are typically less than 0,0025
mm/yr, with varying degrees of surface discoloration depending on the alloy.
Corrosion of alloy 400 is negligible in all types of atmospheres, although a
thin gray-green patina will develop. In sulfurous atmospheres, a brown
patina may be produced.
Nickel alloys are used in pulp and paper mills generally where conditions
are the most corrosive. Alloys 600 and 800 have been utilized for over 25
years for digester liquor heater tubing because their high nickel content
provides excellent resistance to chloride SCC. In the disposal of organic
wastes in unevaporated black liquor, alloy 600 has been used for the reactor
vessel, transfer lines and piping.