Corrosion of Nickel-Base Alloys

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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...

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 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 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. 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 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 <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.

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