Corrosion of Nickel-Base Alloys

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Introduction to Nickel-Base Alloys

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. The exceptional performance of these materials stems from several fundamental characteristics that make them indispensable in critical applications.

Pure nickel exhibits remarkable ductility and toughness because it possesses a face-centered cubic crystal structure that remains stable up to its melting point. This structural stability provides nickel with good resistance to corrosion in normal atmospheric conditions, natural freshwaters, and deaerated nonoxidizing acids. Additionally, nickel demonstrates excellent resistance to corrosion by caustic alkalis, making it particularly valuable in chemical processing applications.

The inherent properties of nickel provide an excellent foundation for developing specialized alloys through strategic alloying additions. Intermetallic phases can form between nickel and various alloying elements, enabling the formulation of very high strength alloys suitable for both low- and high-temperature service conditions.

Alloying Elements and Their Effects on Corrosion Resistance

The performance characteristics of nickel-base alloys are significantly enhanced through careful selection and control of alloying elements. Each element contributes specific properties that expand the operational capabilities of the base material.

Copper Additions

Copper additions provide substantial improvement in nickel's resistance to nonoxidizing acids. Alloys containing 30 to 49% copper offer useful resistance to nonaerated sulfuric acid (H₂SO₄) and demonstrate excellent resistance to all concentrations of nonaerated hydrofluoric acid (HF). Furthermore, additions of 2 to 3% copper to nickel-chromium-molybdenum-iron alloys have been found to improve resistance to hydrochloric acid (HCl), H₂SO₄, and phosphoric acid (H₃PO₄).

Chromium Enhancement

Chromium additions impart improved resistance to oxidizing media such as nitric acid (HNO₃) and chromic acid (H₂CrO₄). Beyond aqueous corrosion resistance, chromium also improves resistance to high-temperature oxidation and provides protection against attack by hot sulfur-bearing gases, making it essential for elevated temperature applications.

Iron and Molybdenum Contributions

Iron is typically incorporated into nickel-base alloys primarily to reduce costs rather than to promote corrosion resistance. However, iron does provide nickel with improved resistance to H₂SO₄ in concentrations above 50%. Molybdenum substantially improves resistance to nonoxidizing acids, with commercial alloys containing up to 28% molybdenum 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.

Other Important Elements

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, silicon levels must be carefully controlled because it can stabilize carbides and harmful intermetallic phases.

Cobalt exhibits corrosion resistance similar to nickel in most environments. Due to its higher cost and lower availability, cobalt is not generally used as a primary alloying element in materials designed for aqueous corrosion resistance. However, cobalt imparts unique strengthening characteristics to alloys designed for high-temperature service.

Niobium and tantalum serve as stabilizing elements in corrosion-resistant alloys, originally added to tie up carbon and prevent intergranular corrosion attack due to grain-boundary carbide precipitation. Aluminum and titanium are often used in minor amounts for deoxidation purposes or to tie up carbon and nitrogen, respectively. When added together, these elements enable the formulation of age-hardenable high-strength alloys for low- and elevated temperature service.

Environmental Embrittlement and Stress Corrosion Cracking

While nickel-base alloys are frequently selected for their improved resistance to environmental embrittlement compared to steels and stainless steels, they can exhibit environmental embrittlement under the combined action of tensile stresses 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 range from minutes to years.

The embrittlement of nickel-base alloys occurs through two primary phenomena: hydrogen embrittlement and stress corrosion cracking (SCC). These mechanisms are distinguished by their response to environmental temperature and anodic/cathodic polarization. Increasing temperature from ambient conditions generally results in increasing susceptibility to SCC and decreasing susceptibility to hydrogen embrittlement. Cathodic polarization often increases hydrogen embrittlement while decreasing SCC susceptibility.

Stress Corrosion Cracking Environments

Stress corrosion cracking of nickel-base alloys has been identified in three primary types of environments: high-temperature halogen-ionic solutions, high-temperature waters, and high-temperature alkaline environments. Additionally, SCC has been detected in liquid metals, near-ambient-temperature polythionic acid solutions, and environments containing acids and hydrogen sulfide (H₂S).

Hydrogen Embrittlement Characteristics

Hydrogen embrittlement of nickel-base alloys manifests in 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.

Industrial Applications and Performance

Nickel-base alloys find extensive use in corrosion resistance applications or for combined corrosion resistance and high-temperature strength across a wide range of commercial applications. These 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.

Chemical Processing Applications

The chemical-processing industry involves a great variety of corrosive environments, necessitating the use of various nickel alloys. In caustic soda production, nickel-base alloys provide essential corrosion resistance in highly alkaline conditions where conventional materials would fail rapidly.

Water Systems Performance

Nickel-base alloys demonstrate excellent resistance to corrosion in distilled water and freshwater systems. Typical corrosion rates for Nickel 200 (commercially pure nickel) in distilled water storage tanks at ambient temperature are less than 0.0025 mm/yr, while domestic hot water service applications see rates below 0.005 mm/yr. Nickel-copper alloys such as 400 and R-405 also exhibit very low corrosion rates and are widely used in freshwater systems for valve seats and other critical fittings.

Atmospheric Corrosion Resistance

Nickel-base alloys exhibit excellent resistance to atmospheric corrosion across various environmental conditions. Corrosion rates are typically less than 0.0025 mm/yr, with varying degrees of surface discoloration depending on the specific alloy composition. Alloy 400 shows negligible corrosion in all types of atmospheres, although a thin gray-green patina may develop over time. In sulfurous atmospheres, a brown patina may form, but this does not significantly impact the underlying corrosion resistance.

Pulp and Paper Industry Applications

Nickel alloys are strategically employed in pulp and paper mills where conditions are most corrosive. Alloys 600 and 800 have been successfully utilized for over 25 years in digester liquor heater tubing because their high nickel content provides excellent resistance to chloride stress corrosion cracking. In organic waste disposal applications involving unevaporated black liquor, alloy 600 has been effectively used for reactor vessels, transfer lines, and piping systems.

Conclusion

Nickel-base alloys represent a critical class of materials that bridge the gap between conventional stainless steels and the most demanding industrial applications. Their ability to accommodate larger amounts of alloying elements in solid solution enables superior performance in severe environments while maintaining excellent mechanical properties. Understanding the specific effects of alloying elements, potential embrittlement mechanisms, and appropriate application guidelines ensures optimal performance and longevity in service. As industrial processes become increasingly demanding, nickel-base alloys continue to evolve to meet new challenges while maintaining their fundamental advantages in corrosion resistance and high-temperature performance.