Molybdenum dissolution rates

The initial dissolution rate of molybdenum discs in liquid aluminium is seen from equation (5.17) to be 1.97xlO 7 ms"1. After 300 s, it drops to 1.77 x 10 ms. Assume that in the 0-300 s time range the dissolution rate... 

A second important group of alloying elements beside the passivity promoters are the dissolution moderators or blockers. These elements slow down the anodic dissolution rate. A prominent example is Mo in iron. Molybdenum does not form stable passive films but increases the stability of alloys significantly. Indirect conclusions that Mo hinders the surface mobility of Fe were confirmed during the last years with scanning probe techniques. Marcus and also Strehblow [135,136] confirmed in their surface science approach that the mobility of Fe is slowed down significantly. This increases the activation energy for the ion-transfer into the electrolyte and thus stabilizes the alloy. 

Vanadium, niobium, tantalum and molybdenum and their alloys have demonstrated good corrosion resistance in liquid Pb-Li [8]. These materials have dissolution rates in the range 0.001 to 0.004 g/m /day at 645 C [8], which is far superior to the performance of Fe-based alloys. However, the cost associated with many of these materials would make their application impractical for a full-scale reactor. 

Baraboshkin AN, Esina N.O., Talanova M.l.(1988) The effect of oxychemical impurities on the growth and dissolution rates of monocrystalline molybdenum facets, Vysokochistye Veschestva 24, 206-207... 

Fe-Cr or Fe-Cr-Ni alloy with and without molybdenum. Figure 3-20 shows an example of such an observation (Ole-fjord and Elfstrom, 1982). To the question of how a small content of molybdenum (a few at.%) can slow down the dissolution rate of the whole surface, the answer (not proved directly, however) is that molybdenum is located preferential-... 

Anodic protection of high alloy steels (chromium-nickel and chromium-nickel-molybdenum) is possible in all sulfuric acid concentrations and necessary especially at elevated temperatures (up to approx. 120 °C). For example, the protection of steel containing 18% Cr, 10% Ni, and 2% Mo in sulfuric acid of 20-60% concentration (at 47 °C) causes a decrease of the corrosion rate of over 1000 times. For a given acid concentration, the dissolution rate during anodic protection depends on the composition of the steel and the temperature of the solution. 

Edwards e/a/. carried out controlled potential, slow strain-rate tests on Zimaloy (a cobalt-chromium-molybdenum implant alloy) in Ringer s solution at 37°C and showed that hydrogen absorption may degrade the mechanical properties of the alloy. Potentials were controlled so that the tensile sample was either cathodic or anodic with respect to the metal s free corrosion potential. Hydrogen was generated on the sample surface when the specimen was cathodic, and dissolution of the sample was encouraged when the sample was anodic. The results of these controlled potential tests showed no susceptibility of this alloy to SCC at anodic potentials. 

The alloying elements molybdenum and copper do not, by themselves, enhance passivity of nickel in acid solutions, but instead ennoble the metal. This means that, in practice, these alloying elements confer benefit in precisely those circumstances where chromium does not, viz. hydrogen-evolving acidic solutions, by reducing the rate of anodic dissolution. In more oxidising media the anodic activity increases, and, since binary Ni-Mo and Ni-Cu alloys do not passivate in acidic solutions, they are generally unsuitable in such media. 

Organic hydroperoxides have also been used for the oxidation of sulphoxides to sulphones. The reaction in neutral solution occurs at a reasonable rate in the presence of transition metal ion catalysts such as vanadium, molybdenum and titanium - , but does not occur in aqueous media . The usual reaction conditions involve dissolution of the sulphoxide in alcohols, ethers or benzene followed by dropwise addition of the hydroperoxide at temperatures of 50-80 °C. By this method dimethyl sulphoxide and methyl phenyl sulphoxide have been oxidized to the corresponding sulphone in greater than 90% yields . A similar method for the oxidation of sulphoxides has been patented . Unsaturated sulphoxides are oxidized to the sulphone without affecting the carbon-carbon double bonds. A further patent has also been obtained for the reaction of dimethyl sulphoxide with an organic hydroperoxide as shown in equation (19). 

Effecting deposition-precipitation by decreasing the pH level is interesting with metal ions present in the stable state in aqueous solution as anions [35]. With silica no interaction is observed, which has led to the development of the electrochemical reduction procedure. To apply metal ions, such as, molybdenum or vanadium, on alumina, a homogeneous decrease in pH level is interesting. The pH level has been decreased by injection of nitric acid or perchloric acid and electro-chemically. However, the rate of crystallization of the hydrated oxides of vanadium(V) and molybdenum(VI) was observed to be fairly low. To prevent dissolution of the alumina supports the pH cannot be decreased to levels below about 3, at which the crystallization of the hydrated metal oxides does not proceed rapidly. 

The corrosion resistance of some metals ultimately depends on the presence of a thin oxide film formed by the reaction of the metal with the environment. This is the case of titanium, tantalum, zirconium, molybdenum, aluminium, cobalt, chromium, etc. Table 9.23 gives the physicochemical properties of the oxides formed on some metals. A low oxide solubility is important to guarantee a low rate of corrosion, since any loss in oxide thickness, due to chemical dissolution, will tend to be balanced by oxidation of the metallic substrate. The oxides should also possess low ionic conductivity. 

Molybdenum reduces the rate of anodic dissolution in the active state (i.e. in the absence of a passive film on the surface). This conclusion is reached by comparing the current-potential curves of the same... 

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