The Nickel retains its austenitic cubic structure until it reaches the melting point, which provides freedom for ductile-brittle transition and greatly reduces manufacturing problems caused by the presence of other metals. Nickel is more inert than iron and more active than copper, so the order of corrosion resistance in reducing environment is Copper > Nickel > Iron. The addition of chromium on the basis of nickel provides the alloy with antioxidant properties, which can produce a wide range of alloys with the best resistance to both reducing and oxidizing environments.
For a certain nickel-based alloy, there are a variety of changes in the specific environment such as concentration, temperature, ventilation, liquid (gas) flow velocity, impurities, abrasion, cycle process conditions, etc., which will produce a variety of corrosion problems. These questions can be answered in nickel and other alloying elements.
Nickel-based alloys can hold more alloying elements in the solid solution state while maintaining good metallurgical stability compared with stainless steel and other Fe-based alloys. These factors allow the addition of a wide variety of alloying elements, making nickel-based alloys widely used in a wide variety of corrosive environments. The common elements in Nickel-based alloys are:
Nickel provides metallurgical stability, improves thermal stability and weldability, increases corrosion resistance to reducing acids and caustic soda, and in particular increases stress corrosion cracking resistance in chloride and caustic soda environments.
Chromium improves the properties of oxidation resistance, high-temperature oxidation resistance, vulcanization resistance, high pitting resistance and gap corrosion resistance.
Molybdenum improves the corrosion resistance of reducing acid, the resistance to pitting corrosion, gap corrosion and high-temperature strength under the environment of chloride aqueous solution.
Iron improves resistance to high temperature carburizing environment, reduces alloy cost and controls thermal expansion.
Copper improves corrosion resistance to reducing acids (especially sulfuric acid and hydrofluoric acid, which are used in airless applications) and salts. The addition of copper to nickel-chromium-molybdenum-ferroalloys helps to improve corrosion resistance to hydrofluoric acid, phosphoric acid and sulfuric acid.
Aluminum improves high-temperature oxidation resistance and aging hardening.
The combination of titanium and carbon reduces the intergranular corrosion caused by chromium carbide precipitation during heat treatment and improves the aging strengthening.
The combination of Niobium and carbon reduces intercrystalline corrosion caused by chromium carbide precipitation during heat treatment and improves pitting corrosion resistance, clearance corrosion performance and high-temperature strength.
Tungsten improves the resistance to reducing acid and local corrosion as well as its strength and weldability.
Nitrogen improves metallurgical stability, pitting corrosion resistance, gap corrosion resistance and strength.
Cobalt provides enhanced high temperature strength, carbonation resistance, and vulcanization resistance.
Many of these alloying elements can be combined with nickel over a wide range of compositions to form single-phase solid solutions, ensuring excellent corrosion resistance of the alloys under many corrosive conditions. The alloy also has good mechanical properties under complete annealing without worrying about harmful metallurgical changes during manufacturing or hot working. Many high Nickel alloys can be strengthened by solid solution hardening, carbide precipitation, precipitation (aging) hardening and dispersion hardening.