Nickel retains an austenitic, face-centered-cubic (fcc) crystal structure up to its melting point, providing freedom from ductile-to-brittle transitions and minimizing the fabrication problems that can be encountered with other metals. In the electrochemical series, nickel is more noble than iron but more active than copper. Thus, in reducing environments, nickel is more corrosion resistant than iron, but not as resistant as copper. Alloying with chromium provides resistance to oxidation thus providing a broad spectrum of alloys for optimum corrosion resistance in both reducing and oxidizing environments. Nickel-based alloys have a higher tolerance for alloying elements in solid solution than stainless steels and other iron-based alloys but maintain good metallurgical stability. These factors have prompted development of nickel-based alloys with multiple alloying additions to provide resistance to a wide variety of corrosive environments.
Nickel – Provides metallurgical stability, improves thermal stability and weldability, improves resistance to reducing acids and caustics, and increases resistance to stress corrosion cracking particularly in chlorides and caustics.
Chromium – Improves resistance to oxidizing corrosives and to high-temperature oxidation and sulfidation, and enhances resistance to pitting and crevice corrosion.
Molybdenum – Improves resistance to reducing acids, and to pitting and crevice corrosion in aqueous chloride containing environments. It contributes to increased high-temperature strength.
Iron – Improves resistance to high-temperature carburizing environments, reduces alloy costs, and controls thermal expansion.
Copper – Improves resistance to reducing acids (particularly non-aerated sulfuric and hydrofluoric) and to salts. Copper additions to nickel-chromium-molybdenumiron alloys provide improved resistance to hydrochloric, phosphoric and sulfuric acids.
Aluminum – Improves resistance to oxidation at elevated temperatures and promotes age hardening.
