Introduction of Hastelloys of the “B”, “G” and “C” Families and Their Corrosion Resistance Performance

Overview

Hastelloys are currently divided into three families of B, C, G. These alloys are mainly used for strong corrosive applications where iron-based Cr-Ni, Cr-Ni-Mo stainless steel and non-metallic materials cannot. In order to improve the corrosion resistance, as well as cold & hot workability, Hastelloy has undergone three major improvements.
The development processes are as follows:

• • Hastelloys of the B Family: B → B-2(00Ni70Mo28) → B-3
• • Hastelloys of the C Family: C → C-276(00Cr16Mo16W4) → C-4(00Cr16Mo16) → C-22(00Cr22Mo13W3) → C-2000(00Cr20Mo16)
• • Hastelloys of the G Family: G → G-3(00Cr22Ni48Mo7Cu) → G-30(00Cr30Ni48Mo7Cu)

Currently the most widely used second-generation materials are N10665 (B-2), N10276 (C-276), N06022 (C-22), N06455 (C-4) and N06985 (G-3).

Ni-Mo alloy (Hastelloys of the B Family)

The chemical compositions of Hastelloys of the B family are listed as below in Table 1. Hastelloy B, developed as early as the 1920s, is an alloy type of the Ni-Mo alloy family. Due to its high carbon content, its heated welds are sensitive to the corrosion of non-oxidizing acids, such as acetic acid, formic acid, hydrogen chloride. In the 1960s, Hastelloy B-2 was developed with the application of argon acid decarburization technology. Although this alloy can resist corrosion in the heat affected zone of the welds, it is difficult to manufacture. The study of the composition of the alloy B-2 prompted the development of Hastelloy B-3. As the unfavorable intermetallic phases in the internal structure of the precedent alloys are eliminated or reduced, the corrosion resistance and manufacturing properties of alloy B-3 are effectively improved.

The applications of alloys B-2, B-3 have been extended to address corrosion problems in HCl solutions and humidified HCl gases over a temperature range of 70-100 °C and of all concentrations. These alloys exhibit very good corrosion resistance to sulfuric acid of concentration less than 60% at room temperatures and at high temperature as to heating to the boiling point. The only drawback of this alloy is that it is less resistant to oxidizing media because it does not contain chromium. Alloy B-2 has been successfully used in the organic vulcanization of acetic acid, pharmaceuticals, ethylbenzene, styrene, and cumene.

Ni-Cr-Fe-Mo-Cu alloy (Hastelloys of the G Family)

The chemical compositions of Hastelloys of the B family are listed as below in Table 2.

Hastelloy G, developed in the 1960s, is a type of Ni-Cr-Fe alloy, strengthened with molybdenum and copper elements to improve the corrosion resistance to sulfuric acid and phosphoric acid. The alloy features good corrosion resistance in the welds, capable of solving the corrosion problem caused by oxidizing, reducing medium and mixed acid, fluorine silicate, sulfate, concentrated nitric acid, fuel gas in coal-fired power plant and hydrofluoric acid. With more molybdenum and nickel contained within than alloy 825, Hastelloy G features remarkably enhanced resistance to local corrosion and can effectively resist the intergranular corrosion of chloride. However, the alloy G has ceased to be in use and has been replaced by alloy G-3.

Hastelloy G-3 is an upgraded version of alloy G with corrosion resistance similar to that of Alloy G, but with better corrosion resistance to the weld’ heat affected zone (HAZ) and also better weldability. Because the alloy G-3 contains low carbon, its carbide deposition process is slow; meanwhile the high molybdenum contained within gives it stronger resistance to local corrosion. At present, alloy G-3 has replaced the alloy G almost in all industrial applications, and the alloy 825 in many applications since these applications require good local corrosion resistance.

Hastelloy G-30 is a modified version of Hastelloy G-3 with the addition of more chromium but reduced molybdenum. Alloy G-30 exhibits good corrosion resistance in commercial concentrated acids and other complex mixed acids such as nitric acid/hydrochloric acid, nitric acid/hydrofluoric acid. Besides, it also features good corrosion resistance to sulfuric acid. Hastelloy G-30 is typically applied in these fields like phosphoric acid, mixed acid, nuclear fuel reprocessing, immersion operating equipment, petrochemical, agrochemical production (fertilizers, pesticides, insecticides, herbicides) and mining industries.

Ni-Cr-Mo alloy (Hastelloys of the C Family)

Ni-Cr-Mo alloys, developed in the 1930s, are originally Hastelloys of the C family. These alloys are new materials that optimize the Ni-Cr alloy to form an antioxidant medium, and the Ni-mo alloy to form an excellent anti-reductive medium. The combination of these two types results in a general corrosion resistant material Ni-Cr-Mo alloy family, which chemical compositions are shown in table 3. These materials are very corrosion resistant in many harsh corrosive environments, especially in the chemical industry and some other industries. These alloys exhibit good resistance to pitting corrosion and crevice corrosion in low PH value, high chloride and oxidizing environments. Ni-Cr-Mo alloys can completely avoid stress corrosion cracking. These features altogether enable these alloys to be used in industrial equipment for many years, despite their limitations. Today, Hastelloy C, initially used in the 1930s, has been largely eliminated except for the use of certain casting materials. The latter half of 20th century witnesses great development of the corrosion-resistant Hastelloys, such as alloy C-276 in the 1960s, alloy C-4 in the 1970s, alloy C-22 and 622 in the 1980s, alloy C-59, 686, C-2000 and MAT21 in the 1990s. The improved corrosion resistance performance of these alloys not only overcomes the limitations of the C alloy, but also extends the applications of the materials to the chemical industry with more demanding material requirements.

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