745 NiMo Alloys B Family Alloys

B-4/N10629 Bal 28 3 1.2 0.005 B-10/N10624 Bal 24 6 8.0 0.005

Alloy B, the original alloy in the Ni-Mo family, developed in the 1920s, suffered from heat affected zone (HAZ) corrosion in nonoxidizing acids (i.e., acetic, formic, and hydrochloric) due to its higher carbon content. In the decade of the 1960s, improved argon-oxygen decarburization (AOD) melting technology led to development of alloy B-2. This alloy solved the HAZ corrosion problem but suffered from poor fabricability. Recent developments of controlled chemistry alloy B-2, alloy B-3, and Nimofer 6629—Alloy B-4, UNS N10629—solved both these problems by eliminating/reducing the formation of detrimental intermetallic phases with further improvement in corrosion resistance behavior. Greater details on fundamental behavior and understanding of Ni-Mo alloy systems are available in the open literature [10, 11]. Alloy B-2, B-3, and B-4 are recommended for service in handling all concentrations of HCl in the temperature range of 70-100°C handling of wet HCl gas. It has excellent resistance to pure H2SO4 up to boiling point in concentrations below 60%. One weakness of the alloy is its lack of chromium and hence its very poor corrosion resistance in the presence of oxidizing species. Alloy B-2 has been successfully used in the production of acetic acid, pharmaceuticals, alkylation of ethyl benzene, styrene, cumene, organic sulfonation reactions, melamine, herbicides, and many other products. Alloy B-4, the improved version of alloy B-2, is being tested and has already found applications in production of resins encountering hydrochloric acid due to the presence of aluminum chloride in the temperature range of 120-150°C. In one chemical company in Spain, alloy B-4 was tested and specified for use in production of pesticides, where severe corrosive conditions exist due to presence of hydrochloric acid. The "C" family alloys were totally inadequate under these conditions. Alloy B-4 has solved both the fabricability problems, encountered with alloy B-2 and the susceptibility to stress corrosion cracking in many corrosive environments.

A very recent development in the Ni-Mo family has been the introduction of alloy B-10 (Nimofer 6224). One of the major weaknesses of the B, B-2, B-3, and B-4 alloys was their inability to handle the presence of oxidizing species in the corrosive media. Unacceptable and very high corrosion rates resulted. Under such conditions, the C family alloys with their higher chromium contents, such as alloy C-276 or alloy 59, could easily handle the oxidizing species but lacked sufficient molybdenum to counteract against the highly acidic hydrochloric or sulfuric acid reducing conditions. Alloy B-10 was an intermediate alloy between the C and B family, where the molybdenum level was significantly higher than the C family but somewhat lower than the B family. Also, the chromium and iron levels were increased to 8 and 6%, respectively, to counteract against the presence of the oxidizing corrosive species, which may be present in the environment. This alloy has already found successful uses in very specific crevice corrosion conditions caused in waste incinerators. Many other field tests are under way with this alloy [12].

7.4.6 Ni-Cr-Fe Alloys

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