Chromate compounds

Around 1800, the attack of chromite [53293-42-8] ore by lime and alkaU carbonate oxidation was developed as an economic process for the production of chromate compounds, which were primarily used for the manufacture of pigments (qv). Other commercially developed uses were the development of mordant dyeing using chromates in 1820, chrome tanning in 1828 (2), and chromium plating in 1926 (3) (see Dyes and dye intermediates Electroplating Leather). In 1824, the first chromyl compounds were synthesized followed by the discovery of chromous compounds 20 years later. Organochromium compounds were produced in 1919, and chromium carbonyl was made in 1927 (1,2). 

Most of the CL6 found in nature is a result of domestic and industrial emissions (Steven et al. 1976). Interaction of +6 chromic oxide, dichromate, or chromate compounds with organic compounds can result in reduction to the comparatively less toxic trivalent form (Taylor and Parr 1978). 

For the oxidation of valencene to nootkatone, strong oxidation agents like oxygen in the presence of metal salts, peroxides or chromate compounds are required. There are also several patents on the bioconversion of valencene to nootkatone which results in natural nootkatone (Scheme 13.9) [10,11]. 

Utility systems for equipment and space heating and cooling frequently use heavy metal corrosion inhibitors in their heat transfer fluids. Chromate compounds are among the best corrosion inhibitors available. Nonchromate inhibitors that have proved to be feasible substitutes include polyphosphates, organophosphates, zinc, molybdates, and aromatic azoles. Some of these compounds have their own environmental impacts, however. Azoles, for instance, can be quite dangerous to human health. 

Chromate compounds have been considered by many to be the best inhibitors available. They are typically composed of mixtures of sodium bichromate and chromic acid, and art through passivation of the metal surfaces. Passivation involves formation of a tough metal oxide layer or other film on the surfaces. Chromate concentrations of 200 to 1000 ppm in the cooling water are generally required, although for environments where bimetallic influences exist, chromate levels must be much higher. For instance, when steel and copper surfaces are present in the system, chromate levels often exceed 2000 ppm (BETZ 1982, pp. 207, 212). 

Chromate compounds are insoluble, except when they contain ions from the first bulleted point. 

As formed, carbon black is a fluffy powder possessing low density. The densification process involves the removal of occluded air by agitation and followed by dry or wet process pelletization. In both the dry and wet pelletization process, nearly spherical pellets or beads will form that are typically composed of >99% carbon black and trace impurities such as sulfur. Thus, carbon black is sold as a low density powder or as a pelleted form in pigmenting and other end uses. The choice of a fluffy or pelleted carbon black for dispersion in a given system depends upon the dispersion and handling equipment and end use. For example, pelleted carbon blacks are used most frequently in production of black masterbatch carbon black powders are typically used to tint chromatic compounds. 

Chromium chromium metal chromite (mineral form) chromic acid hexavalent (chromate) compounds carbamate, phosphate, and triacetate forms lung, nasal passages 2.78... 

All cases of accidental chromate poisoning in cattle have resulted from the exposure of animals to chromate compounds associated with oil-field activities. Chromates are used as a corrosion inhibitor between the pipe and casing and are often added to drilling fluids (in the form of chromeUgnosulfQnate) to improve thermal stability. One recorded case involved 20 mature cows and their 8-month-old calves. 

The effects of chromium compounds have been studied since World War II. While chromic acid mist was known to produce septal perforations in the nose, chrome holes of the skin, and chronic dermatitis, occupationally caused lung malignancies from certain chromate compounds were a newly discovered epidemiological finding. Exposure to vinyl chloride produced angiosarcoma (cancer of the liver) in workers exposed to the chemical. This wasn t discovered until an epidemiological survey revealed workers in plants that had vinyl chloride in the process had abnormally high incidences of liver cancer. 

Hanmura et al. [122, 123] found two simple chromium cations Cr(II)OH and Cr(III)0 (models If and 2f) that could dimerize ethylene into 1-butene without using any organometallic cocatalyst, and proposed that they could be treated as simple homogenous cluster models for the Phillips catalyst. Baker and Carrick [111] reported ethylene polymerization over BC (model 3f), a hexavalent chromate compound bearing two triphenylsilyl ligands, at elevated temperatures (>130°C)... 

Carbonates (CO ), phosphates (PO ), chromates Compounds containing alkali metal ions and the... 

Aubriet, F, Maunit, B., Muller, J.F. (2000) Studies on alkali and alkaline earth chromates by time-of-flight laser microprobe mass spectrometry and Fourier transform ion cyclotron resonance mass spectrometry part I differentiation of chromate compounds. International Journal of Mass Spectrometry, 198,189-211. 

The most important are oxides, sulfides, selenides, and chromate compounds of metals, mainly Zn, Ti, Cd, Cr, Ni, Co, Pb, Mo. Carbon black is also often included into this group. They have high thermostability and good dispersibility in PR... 

Figure 3.13 Effect of diethylaluminum ethoxide to triethylaluminum ratio on polymer Melt Index (12.16) for the Bis(triphenylsUyl) chromate catalyst supported on silica. The aluminum alkyl mixture is added after the deposition of the chromate compound onto the silica. Reprinted from [17] with permission from John WUey and Sons.

Rare earth compounds as replacements of chromate compounds in polymer coatings... 

For many decades, chromate compounds have been successfully used as anticorrosive inhibitors in the surface treatment of aluminium and others alloys. The use of chromates is however restricted worldwide, as they are considered highly toxic and carcinogenic [1], This has stimulated research aimed at the development of effective and environmentally acceptable alternatives to chromates. To obtain the beneficial properties of chromatation, two approaches can be utilised passive [2] and active [3,4] corrosion protection. Passive protection is normally provided by a barrier film that prevents contact between the corrosive species and the metal surface and therefore hinders a corrosion process. However, when a defect is formed in the barrier layer, the coating cannot stop corrosion in that place. The second approach is active corrosion protection, which employs inhibitive species that can decrease corrosion activity. An important point is that both strategies must be used together to protect the metallic substrate adequately. 

Conversion coatings are adherent surface layers of low-solubility oxide, phosphate, or chromate compounds produced by the reaction of suitable reagents with the metallic surface. These coatings affect the appearance, electrochemical potential, electrical resistivity, surface hardness, absorption, and other surface properties of the material. They differ from anodic coatings in that conversion coatings are formed by a chemical oxidation-reduction reaction at the surface of... 

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