Knowledge related to nickel based corrosion-resistant alloys

 　　Nickel based corrosion-resistant alloys are alloys based on nickel as the matrix, containing more than 50% nickel, possessing certain comprehensive properties such as high temperature strength, and being able to resist oxidation or water prone medium corrosion. They are called nickel based corrosion-resistant alloys. Correspondingly, corrosion-resistant alloys with nickel content greater than 30% and nickel plus iron content greater than 50% are commonly referred to as iron nickel based corrosion-resistant alloys. The Ni Cu alloy (Monel alloy Ni 70 Cu30) produced in the United States in 1905 was the earliest nickel based corrosion-resistant alloy. In 1914, the United States began producing Ni-Cr-Mo-Cu corrosion-resistant alloys (Illium R), and in 1920, Germany began producing Ni-Cr-Mo corrosion-resistant alloys containing about 15% Cr and about 7% Mo. In the 1970s, there were nearly 50 corrosion-resistant alloy grades produced in various countries.

　　Nickel based corrosion-resistant alloys often have austenitic structures. Under the conditions of solid solution and aging treatment, there are also intermetallic phases and metal carbonitrides present on the austenitic matrix and grain boundaries of the alloy. Various corrosion-resistant alloys are classified according to their composition and characteristics as follows: Ni-Cu alloy has better corrosion resistance than nickel in reducing media, and better corrosion resistance than copper in oxidizing media. It is the best material for high-temperature resistance to fluorine gas, hydrogen fluoride, and hydrofluoric acid in the absence of oxygen and oxidants (see Metal Corrosion). Ni Cr alloys are mainly used under oxidizing medium conditions. It is resistant to high-temperature oxidation and corrosion by gases such as sulfur and vanadium, and its corrosion resistance increases with the increase of chromium content. This type of alloy also has good resistance to hydroxide corrosion (such as NaOH, KOH) and stress corrosion.

　　Ni Mo alloys are mainly used under corrosive conditions in reducing media. It is the best alloy to resist hydrochloric acid corrosion, but its corrosion resistance significantly decreases in the presence of oxygen and oxidants. Ni Cr Mo (W) alloy combines the properties of both Ni Cr alloy and Ni Mo alloy mentioned above. Mainly used under conditions of oxidation-reduction mixed media. This type of alloy exhibits good corrosion resistance in high-temperature fluorinated hydrogen gas, hydrochloric acid and hydrofluoric acid solutions containing oxygen and oxidants, and in wet chlorine gas at room temperature.

　　Ni Cr Mo Cu alloy has the ability to resist both nitric acid and sulfuric acid corrosion, and also has good corrosion resistance in some oxidation-reduction mixed acids.



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