Chromium and Molybdenum

The physical and mechanical properties of steel depend on its microstmcture, that is, the nature, distribution, and amounts of its metaHographic constituents as distinct from its chemical composition. The amount and distribution of iron and iron carbide determine most of the properties, although most plain carbon steels also contain manganese, siUcon, phosphoms, sulfur, oxygen, and traces of nitrogen, hydrogen, and other chemical elements such as aluminum and copper. These elements may modify, to a certain extent, the main effects of iron and iron carbide, but the influence of iron carbide always predominates. This is tme even of medium alloy steels, which may contain considerable amounts of nickel, chromium, and molybdenum. 

The first iron—nickel martensitic alloys contained ca 0.01% carbon, 20 or 25% nickel, and 1.5—2.5% aluminum and titanium. Later an 18% nickel steel containing cobalt, molybdenum, and titanium was developed, and still more recentiy a senes of 12% nickel steels containing chromium and molybdenum came on the market. 

The two corrosion-resistant alloys presented ia Table 5 rely on chromium and molybdenum for their corrosion resistance. The corrosion properties of IJ1 timet are also enhanced by tungsten. Both alloys are available ia a variety of wrought product forms plates, sheets, bars, tubes, etc. They are also available ia the form of welding (qv) consumables for joining purposes. 

Carbon steels heated for prolonged periods at temperatures above 455°C (8.50°F) may be subject to the segregation of carbon, which is transformed into graphite. When this occurs, the structural strength of the steel will be affected. Killed steels or low-alloy steels of chromium and molybdenum or chromium and nickel should be considered for elevated-temperature seivices. 

For hot wall vessels, the increased strength may be such that the use of chromium and molybdenum alloy steels will be cheaper. Also, these steels may be required to prevent hydrogen attack and to reduce oxidation and sulfidation. 

The fact that the pH values of the pure metals were lower than the theoretical values was attributed to the formation of hydroxy-chloro complexes of the metal and to the high chloride ion concentration in the pit, and the results highlight the very pronounced decrease in pH that can occur in an occluded cell, particularly when the alloy contains high concentrations of chromium and molybdenum. They also showed that migration of chloride ions into the solution in the pit can result in a 7-12-fold increase in concentration, and that the potential in the pit is in the active region. 

Amorphous Fe-3Cr-13P-7C alloys containing 2 at% molybdenum, tungsten or other metallic elements are passivated by anodic polarisation in 1 N HCl at ambient temperature". Chromium addition is also effective in improving the corrosion resistance of amorphous cobalt-metalloid and nickel-metalloid alloys (Fig. 3.67). The combined addition of chromium and molybdenum is further effective. Some amorphous Fe-Cr-Mo-metalloid alloys passivate spontaneously even in 12 N HCl at 60° C. Critical concentrations of chromium and molybdenum necessary for spontaneous passivation of amorphous Fe-Cr-Mo-13P-7C and Fe-Cr-Mo-18C alloys in hydrochloric acids of various concentrations and different temperatures are shown in Fig. 3.68 ... 

Fig. 3.68 Critical concentrations of chromium and molybdenum necessary for spontaneous passivation of amorphous Fe-Cr-Mo-13P-7C and Fe-Cr-Mo-18C alloys in hydrochloric acid of various concentrations and temperatures ...

The possibility of being involved in olefin metathesis is one of the most important properties of Fischer carbene complexes. [2+2] Cycloaddition between the electron-rich alkene 11 and the carbene complex 12 leads to the intermediate metallacyclobutane 13, which undergoes [2+2] cycloreversion to give a new carbene complex 15 and a new alkene 14 [19]. The (methoxy)phenylcar-benetungsten complex is less reactive in this mode than the corresponding chromium and molybdenum analogs (Scheme 3). 

Stellite (alloy of cobalt, tungsten, chromium, and molybdenum) used for surgical instruments. 

A specialty group of alloys has been created with chromium and molybdenum significantly above the standard stainless grades. Many of these grades are proprietary and contain chromium of 20 percent and molybdenum of 6 to 7 percent, and most also contain nitrogen to... 

Hydrogenation processes usually require standards and materials that may not be warranted in other operations of the petroleum industry. At certain combinations of elevated temperature and hydrogen partial pressure, both chemical and metallurgical changes occur in carbon steel, which in advanced stages can render it unsuitable for safe operation. Alloy steels containing chromium and molybdenum can be used under such conditions. 

A remarkable reaction, discovered by McGuire and Hegedus in 1982 [292], is the photochemical conversion of heteroatom-substituted chromium and molybdenum carbene complexes into intermediates with ketene-like character (Figure 2.21). This reaction has been reviewed by Hegedus [203]. 

The existence of quadruple bonds was first recognized in 1964289 and since then a great many compounds of rhenium, technetium, chromium and molybdenum have been shown to contain them. Both bridged and unbridged compounds are known with a large variety of ligands. The bonding has been described in detail.289... 

The reaction of WCU with K2C8H8 in THF gives a reasonably air-stable material that was shown to be [W2(C8Hg)3]. The W—W bond length of 2.375 A, although long, is consistent with the presence of a quadruple bond.329 However, it is recognized that this compound (and the related chromium and molybdenum) is electronically different from other quadruply bonded species. 

Group 6 In order of increasing atomic number, these are chromium, molybdenum, and tungsten. Chromium and molybdenum have one eleeiron in their outer shells tungsten has two electrons in its outer shell. Although chromium and molybdenum also have other valences, all of the dements in this group have a 6+ valence in common. 

Identify the element with the larger atomic radius in each of the following pairs (a) cobalt and manganese (b) copper and zinc (c) chromium and molybdenum. 

Nickel, either as a Raney catalyst or in the form of nickel boride, is also effective in the reduction of the C=0 bond. An increase in the catalytic activity can be brought about by metal promoters (chromium and molybdenum). Copper chromite may also be used. 

XPS data have been also reported in the past [33] for chromium and molybdenum corrolates and discussed with respect to analogous porphyrinates. 

The large differences in metal-centered redox potentials between chromium and molybdenum have been attributed to the existence of a greater interaction between the Cr dxy orbital and the a and/or k orbitals of the corrole ligand. Such interaction would in fact reduce d electron density at the metal. Another factor considered to cause the potential difference has been a strong covalent interaction between chromium and N-donor ligands. 

Work Item F determination of lead, cadmium, chromium and molybdenum by ETAAS after pressure digestion. 

Some examples of polynuclear polymeric metal arene complexes, obtained by cryosynthetic reactions from tin, titanium, vanadium, chromium, and molybdenum, are represented [202,542,543]. However, the preparative possibilities of such syntheses are not yet clear [201,202]. 

Polymers of 1,4-dienes are obtained in the presence of titanium, and also with Co, Ni, and Rh, where allyl complexes can be isolated. 1,2-Polybutadiene can be produced in the presence of Pd, which is not generally regarded as a Ziegler catalyst. Chromium and molybdenum systems have also been used. Whereas structural isomerism is controlled by the metal in the catalyst center, the geometric isomerism is determined by the ligands and counterions. 

The mass spectra of binuclear cyclopentadienylmetal carbonyl derivatives of chromium and molybdenum provide useful indications of the relative strengths of the metal-metal bonds in various compound types. The mass spectrum of [C5H5Cr(CO)3]2 (S. M = Cr) exhibits no bimetallic ions but only monometallic ions such as CsHsC CO) (n = 3,... 

Exposure of a tetrahydrofuran solution of W(CO)6 and the 1,2-B9C2H,, 2 ion to ultraviolet radiation produced immediate carbon monoxide evolution. Ultimately the air-sensitive (1,2-B9C2H,, )W(CO)32 ion was obtained as the tetramethylammonium salt (18). The corresponding chromium and molybdenum complexes have been obtained in the same manner (17, 18). These dianions undergo nucleophilic reactions characteristic of the analogous 77-C5H5Mo(CO)3 ion... 

Chromium- and molybdenum-based catalysts yield predominantly 1,2-poly-butadiene. Several soluble chromium compounds such as Cr(Acac)3 give apparently hydrocarbon-soluble catalysts when activated with AIR3. Also, soluble catalysts are formed from soluble molybdenum compounds such as Mo(Acac)3 or Mo02(Acac)2 on reaction with AlEt3 in the presence of ethyl acetate or sulphides. ESR studies of Mo(OR)xCl4 x-based catalysts showed the Mo(III) species to be present in the catalysts. It should be mentioned, however, that catalysts based on group 6 metals have not been extensively studied [7],... 

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