The Nickel-Molybdenum Alloys

The composition of this alloy (54% nickel, 15% molybdenum, 15% chromium, 5% tungsten and 5% iron) is less susceptible to intergranular corrosion at welds. The presence of chromium in this alloy gives it better resistance to oxidizing conditions than the nickel/molybdenum alloy, particularly for durability in wet chlorine and concentrated hypochlorite solutions, and has many applications in chlorination processes. In cases in which hydrochloric and sulfuric acid solutions contain oxidizing agents such as ferric and cupric ions, it is better to use the nickel/molybdenum/ chromium alloy than the nickel/molybdenum alloy. 

In the environmental series, nickel is nobler than iron but more active than copper. Reducing environments, such as dilute sulfuric acid, find nickel more corrosion resistant than iron but not as resistant as copper or nickel-copper alloys. The nickel molybdenum alloys are more corrosion resistant to a reducing environment than nickel or nickel-copper alloys [16]. Nickel-based superalloys are extremely prone to weld cracking. 

By alloying nickel with both molybdenum and chromium, an alloy is obtained resistant to oxidizing media imparted by alloyed chromium, as well as to reducing media imparted by molybdenum. One such alloy, which also contains a few percent iron and tungsten (AUoy C), is immune to pitting and crevice corrosion in seawater (10-year exposure) and does not tarnish appreciably when exposed to marine atmospheres. Alloys of this kind, however, despite improved resistance to Cl, corrode more rapidly in hydrochloric acid than do the nickel-molybdenum alloys that do not contain chromium. 

Steel is the most common constructional material, and is used wherever corrosion rates are acceptable and product contamination by iron pick-up is not important. For processes at low or high pH, where iron pick-up must be avoided or where corrosive species such as dissolved gases are present, stainless steels are often employed. Stainless steels suffer various forms of corrosion, as described in Section 53.5.2. As the corrosivity of the environment increases, the more alloyed grades of stainless steel can be selected. At temperatures in excess of 60°C, in the presence of chloride ions, stress corrosion cracking presents the most serious threat to austenitic stainless steels. Duplex stainless steels, ferritic stainless steels and nickel alloys are very resistant to this form of attack. For more corrosive environments, titanium and ultimately nickel-molybdenum alloys are used. 

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 ... 

Powder Formation. Metallic powders can be formed by any number of techniques, including the reduction of corresponding oxides and salts, the thermal dissociation of metal compounds, electrolysis, atomization, gas-phase synthesis or decomposition, or mechanical attrition. The atomization method is the one most commonly used, because it can produce powders from alloys as well as from pure metals. In the atomization process, a molten metal is forced through an orifice and the stream is broken up with a jet of water or gas. The molten metal forms droplets to minimize the surface area, which solidify very rapidly. Currently, iron-nickel-molybdenum alloys, stainless steels, tool steels, nickel alloys, titanium alloys, and aluminum alloys, as well as many pure metals, are manufactured by atomization processes. 

K. Tachibana and M.B. Ives, Selective Dissolution Measurements to Determine the Nature of Films on Nickel-Molybdenum Alloys, Passivity of Metals, The Electrochemical Society, 1978, p 878-897... 

Figure 8.15 Nickel-molybdenum alloy deposition, high concentration of NiS04 (1.0 mol dm ) and low concentration of Na2Mo04 (0.005 mol dm ), 0.7 mol sodium citrate, pH = 7.5, dependence on the rotation rate for 400, 1000, and 2000 rpm (Podlaha and Landolt). " (Reproduced with permission from J. Electrochem. Soc. 143, 885 (1996), 1996, The Electrochemical Society.)...

Because the binary nickel-molybdenum alloys have poor physical properties (low ductility, poor workability), other elements, for example, iron, are added to form ternary or multicomponent alloys. These are also difficult to work, but they mark an improvement over the binary alloys. Resistance of such alloys to hydrochloric and sulfuric acids is better than that of nickel, but it is not improved with respect to oxidizing media (e.g., HNO3). Since the Ni-Mo-Fe alloys have active corrosion potentials and do not, therefore, establish passive-active cells, they do not pit in the strong acid media to which they are usually exposed in practice. 

Because of the lack of chromium. Alloy B is not resistant to oxidizing conditions (for example, HNO3) or to oxidizing metal chlorides (such as FeCh) [18]. Contamination of nonoxidizing acids with oxidizing ions (such as Fe " or Cu " ) causes a large increase in corrosion rate [19]. Nickel-molybdenum alloys must not be used where oxidizing conditions exist. 

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],... 

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. 

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. 

The chromium-free nickel-molybdenum alloys, for example NiMo28 (DIN-Mat. No. 2.4617, HasteDoy B-2), show only minimal amounts of surface corrosion in seawater, but are nonetheless practically unsuitable for use in seawater because of their sensitivity to pitting and crevice corrosion. 

Nickel-copper alloys do not follow the scheme shown in Table 4.2. They use M as the first letter (examples are M35-1 and M25.5). Nickel-molybdenum alloys use the letter N as the beginning letter, such as N7Mn and N12MV. 

This is a chromium-nickel-molybdenum alloy, with its composition shown in Table 8.4. It has excellent resistance to chloride pitting and stress corrosion cracking environments. It finds use in the chemical processing and utility industries. 

N7M is the cast equivalent of Hastelloy B2. This nickel-molybdenum alloy has excellent corrosion resistance in all concentrations and temperatures of hydrochloric acid. If ferric or cupric ions are present, however, severe attack will occur. It is also good for sulfuric, acetic, and phosphoric acids. ... 

Sulfate Reducing Bacteria SRBs have been implicated in the corrosion of cast iron and steel, ferritic stainless steels, 300 series stainless steels and other highly alloyed stainless steels, copper nickel alloys, and high nickel molybdenum alloys. They are almost always present at corrosion sites because they are in soils, surface water streams and waterside deposits in general. The key s5unptom that usually indicates their involvement in the corrosion process of ferrous alloys is localized corrosion filled with black sulfide corrosion products. 

Nickel-molybdenum alloys are known by the name Hastelloy, a trademark of the Cabot Corp. These alloys have very good resistance to concentrated oxidizing agents. 

In order to reach a better understanding of the role of molybdenum in the presence of sulfur, studies were undertaken on simple systems, i.e., binary and ternary single-crystal alloys on which the surface concentrations of sulfur and of Mo could be precisely measured by radiochemical ( S) and spectroscopic (XPS) techniques. The experiments performed on nickel-molybdenum alloys [Ni-2Mo and Ni-6Mo(at %)] provided the first direct evidence of a specific surface interaction between molybdenum and adsorbed sulfur leading to the removal of sulfur from the surface and thereby the attenuation or the disappearance of the detrimental effects of adsorbed sulfur [34,35],... 

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... 

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