Molybdenum nickel-based alloys

It is used in certain nickel-based alloys, such as the "Hastelloys(R)" which are heat-resistant and corrosion-resistant to chemical solutions. Molybdenum oxidizes at elevated temperatures. The metal has found recent application as electrodes for electrically heated glass furnaces and foreheaths. The metal is also used in nuclear energy applications and for missile and aircraft parts. Molybdenum is valuable as a catalyst in the refining of petroleum. It has found applications as a filament material in electronic and electrical applications. Molybdenum is an... 

Plain Carbon and Low Alloy Steels. For the purposes herein plain carbon and low alloy steels include those containing up to 10% chromium and 1.5% molybdenum, plus small amounts of other alloying elements. These steels are generally cheaper and easier to fabricate than the more highly alloyed steels, and are the most widely used class of alloys within their serviceable temperature range. Figure 7 shows relaxation strengths of these steels and some nickel-base alloys at elevated temperatures (34). 

Silver-palladium-manganese brazes possess excellent creep characteristics and have been developed for high-temperature applications involving the use of cobalt or nickel-based alloys, heat-resistant steels, molybdenum and tungsten. Their liquidus temperatures lie in the range 1 100-1 250°C. 

Replacing some of the nickel with iron produces a family of alltws with intermediate corrosion resistance between stainless steels and the Ni-Cr-Mo alloys. Alloys such as Incoloy 825 and Hastelloy G-3 and G-30 are in this family. Incoloy 825 has 40 percent Ni, 21 percent Cr, 3 percent Mo, and 2.25 percent Cu. Hastelloy G-3 contains 44 percent Ni, 22 percent Cr, 6.5 percent Mo, and 0.05 percent C maximum. These alloys have extensive applications in sulfuric acid systems. Because of their increased nickel and molybdenum contents they are more tolerant of chloride-ion contamination than are standard stainless steels. The nickel content decreases the risk of stress-corrosion cracking molybdenum improves resistance to crevice corrosion and pitting. Many of the nickel-based alloys are proprietary and are coverecf by the following specifications ... 

Marked susceptibility to grain disintegration can also be observed in the highly corrosion-resistant nickel-based alloys, such as the chromium- and molybdenum-containing type NiMo 16 Cr (material no. 2.4812) (Figure 20.31), which is, however, no longer in use. 

Nominal compositions of some commercial nickel-base alloys containing copper, molybdenum, or chromium are given in Table 23.3. The Ni-Cu alloys are readily rolled and fabricated, whereas the Ni-Cr alloys are less readily, and the Ni-Mo-Fe and Ni-Mo-Cr alloys are difficult to work or fabricate. 

Nickel-based alloys, which form the bulk of alloys produced, are basically nickel-chrome alloys with a face-centered cubic solid-solution matrix containing carbides and the coherent intermetallic precipitate y-NijlAfTi). This latter precipitate provides most of the alloy strengthening and results in useful operating temperatures up to 90% of the start of melting. Further additions of aluminum, titanium, niobium, and tantalum are made to combine with nickel in the y phase, and additions of molybdenum, tungsten, and chromium strengthen the solid solution matrix. 

Nickel-copper and nickel-chromium-molybdenum alloys are the nickel-base alloys that are t5fpically used in seawater. The nickel-copper alloys have good corrosion resistance in high velocity seawater, but do exhibit localized corrosion in quiescent seawater [79]. Alloy 625, a nickel-chromium-molybdenum alloy, is susceptible to crevice corrosion in both quiescent and flow conditions [97-700]. Other nickel-chromium-molybdenum alloys, such as Alloys C-276, C-22, 59 and 686 have increased seawater crevice corrosion resistance as compared to Alloy 625 [97,98],... 

Molybdenum, and sometimes tungsten, are also added to Ni-Cr-Fe allo3rs for improved localized corrosion (pitting and crevice corrosion) resistance. An example is alloy C-276 (UNS N10276), which contains about 57Ni-15.5Cr-15.5Mo-4W-5Fe and is one of the most pit-resistant nickel-base alloys available. 

New nickel-base alloys such as alloy G-30 (N06030), alloy 59 (N06059), and alloy 686 (N06686) have been developed with higher levels of chromium or molybdenum, or both, for increased resistance to localized or general corrosion in severe environments. These materials are being included in test procedures such as ASTM G 28. 

From a corrosion standpoint, the roles of various alIo)dng elements in the cobalt-base alloys parallel those seen in the nickel-base alloys. Chromium, molybdenum, and tungsten, for example, are highly soluble in both atomic forms of cobalt. Chromium is added to most of the commercially important alloys, and provides passivity over a wide range of potentials. Molybdenum and tungsten enhance resistance to corrosion within the active regime. 

At higher elevations, there is fume and carry-over with the flue gases that cause deposits to form on the superheater, generating bank, and economizer tubes. Deposits may be molten depending on the concentrations of chloride and potassium. Superheater temperatures are usually between 300 C at the steam inlet and 500°C at the outlet. Although there are a variety of materials used in superheaters, ranging from chromium-molybdenum steels to stainless steels and nickel-base alloys, changing materials offers httle improvement when superheater deposits are molten. 

Among the nickel materials, the nickel-based alloys alloyed with chromium and molybdenum are to a great extent resistant to local corrosion in seawater, even at higher temperatures. They are used, even though the higher-alloyed stainless steels do not meet the requirements. 

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