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Corrosion of Nickel-Base Alloys

Abstract:Nickel and nickel-base alloys are vitally important to modern industry because of theirability to withstand a wide variety of severe operating conditions involving corrosiveenvironments, 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 st ructure 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...

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, hightemperatures, 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 high-temperature service.

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 precipitation.

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 temperature service.

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 diffuse.

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

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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 years.

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. F irst, 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 considered.

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 sulfide (H2S).

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

fracture.

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