Effects of Alloying Elements in Nickel-Base Alloys
Different elements in an alloy can significantly change the mechanical properties, corrosion resistance, and microstructure of a metal. While chromium, nickel, molybdenum, and iron may be the primary alloying elements, other elements such as tungsten, carbon, aluminum, titanium, copper, and sulfur can also have significant effects. Understanding these elements and their positive and negative effects on alloys can help determine the uses for certain alloys.
Nickel (Ni)
Improves high temperature strength, resistance to oxidation, nitriding, carburization, and halogenation. It also provides metallurgical stability. Additions of this element improve the alloy's resistance to reducing acids and alkalines, as well as resistance to stress corrosion cracking.
Chromium (Cr)
Alloying with chromium improves the alloy's resistance to high temperature oxidation and sulfidation, as well as resistance to general oxidizing environments. Such oxidizing media include nitric acid and chromic acid. Additions are typically between 15% and 30%, but can be as high as 50%.
Molybdenum (Mo)
Mo additions significantly improve the alloy's resistance to non-oxidizing acids, such as hydrochloric acid (HCl), phosphoric acid (H3PO4), and hydrofluoric acid (HF). Molybdenum has also been shown to improve the alloy's resistance to sulfuric acid (H2SO4) at concentrations below 60%. Molybdenum improves the alloy's resistance to pitting and crevice corrosion and imparts high-temperature strength to the alloy.
Iron (Fe)
This element reduces alloy cost, improves the alloy's resistance to high-temperature carburization, and controls thermal expansion.
Tungsten (W)
This element, like Mo, improves the alloy's resistance to reducing acids and localized corrosion, and enhances the alloy's strength and weldability.
Carbon (C)
Degrades the alloy's corrosion resistance, but improves its strength at elevated temperatures.
Aluminum (Al)
The addition of aluminum promotes the formation of a tightly adherent aluminum oxide scale at elevated temperatures that resists oxidation, carburization, and chloride attack. In combination with titanium, aluminum also promotes age hardening in some alloys.
Titanium (Ti)
As mentioned above, titanium promotes age hardening, and, due to the formation of chromium carbides after heat treatment, it also combines with carbon to reduce susceptibility to intergranular corrosion.
Copper (Cu)
Improves resistance to reducing acids. Alloys containing 30% to 40% copper have excellent resistance to all concentrations of non-aerated hydrofluoric acid (HF). If copper is added to nickel-chromium-molybdenum-iron alloys, its resistance to hydrochloric acid, phosphoric acid, and certain concentrations of sulfuric acid can be improved.
Cobalt (Co)
Cobalt imparts unique strengthening properties to high-temperature alloys. Cobalt also improves the resistance of nickel alloys to carburization and sulfidation. This is because Co increases the solubility of C in Ni-base alloys and cobalt sulfide has a higher melting point than nickel sulfide.
