Chromate, hexavalent

Acid treatments The principal acid processes were developed in the USA under the name Alodine, and are marketed in the UK as Alocrom and under other names. The original solutions were based on acid solutions containing phosphate, chromate and fluoride ions. Immersion for up to 5 min in the cold or warm solution leads to the deposition of a greenish film containing the phosphates of chromium and aluminium, and possibly some hexavalent chromate. The more recent Alocrom 1 200 process uses an acid solution containing chromate, fluoride and nitrate. Room-temperature immersion for 15 s to 3 min deposits golden-brown coatings which contain chromate as a major constituent. 

For metallic iron and nickel electrodes, the transpassive dissolution causes no change in the valence of metal ions during anodic transfer of metal ions across the film/solution interface (non-oxidative dissolution). However, there are some metals in which transpassive dissolution proceeds by an oxidative mode of film dissolution (Sefer to Sec. 9.2.). For example, in the case of chromium electrodes, on whidi the passive film is trivalent chromium oxide (CrgOj), the transpassive dissolution proceeds via soluble hexavalent chromate ions. This process can be... 

Hexavalent chromate [Cr(VI)] is still used within the industry to meet critical high corrosion control and other metal surface finishing requirements. Cr(VI) is toxic and its control generates a hazardous, costly waste. 

Contact dermatitis can occur from exposure to cement and has been considered an occupational hazard for construction workers (Turk and Rietschel 1993 Roto et al. 1996 Zachariae et al. 1996). The problem develops because chromium(VI)-con-taining residues from blast furnaces are incorporated into cement. Some countries add ferrous sulfate to cement to reduce the amount of soluble hexavalent chromate in the product (Turk and Rietschel 1993 Roto etal. 1996 Zachariae etal. 1996). In the UK, chromium and chromates accounted for 8.1% of the allergic contact dermatitis cases (Meyer et al. 2000). In Finland, chromium causes 5.6% of ACD, with tanners, cast concrete workers, leader goods workers, and metal plating/coating workers being at greatest risk (Kanerva et al. 2000). In Denmark, the chromium(VI) content of 35% of... 

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

Scheme 1 Plausible structures of surface-stabilized hexavalent chromate species Cr(VI)0 c,surf the silica surface of the Phillips Cr/silica catalyst (n > 1)...

Formaldehyde is a by-product of the redox reaction between ethylene and hexavalent chromate species, resulting in the formation of divalent chromium species. Subsequently, the Cr(ll) species coordinated with formaldehyde might act as the active precursor at lower temperatures to produce the new short olefins with both odd and even numbers of carbon atom. The experimental evidence obtained from the early stage of ethylene polymerization cannot be rationalized... 

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)... 

In the absence of organometallic cocatalyst, the hexavalent chromate species on the Phillips catalyst is first reduced to a lower valence state by ethylene monomers. Experimentally, we found that the exposure of ethylene to Phillips catalyst during the induction period at RT led to the reduction of Cr(VI)0 c surf precursors to Cr(II) 0 c surf species with the simultaneous formation of formaldehyde and unsaturated hydrocarbon species, such as propylene and butene. The proposed reaction mechanisms during the induction period are shown in Scheme 15 [79]. 

There were three kinds of Cr(II) sites generated after the reduction of hexavalent chromate species by ethylene monomers. 4g represented the naked cluster model for the Cr(II) site of the Phillips catalyst, providing more room for ethylene coordination to the Cr center. The calculations showed that the initiation reactions between the Cr(ll)0 c surf species and ethylene molecules may occur after the desorption of one or two formaldehyde molecules (on 4g-l or 4g-2). For 4g-2, two formaldehyde molecules were adsorbed on the Cr(ll) center from the opposite side above the chromasiloxane ring, with formation of two Cr-O bonds of 2.131 A. 

As an important industrial catalyst, the Ti-modified Phillips catalyst is widely used in ethylene polymerization. Recently, the mechanism of ethylene polymerizatiOTi by the Ti-modified Phillips catalyst has been studied theoretically and experimentally [71]. In the DPT calculations, six mononuclear chromium cluster models including three hexavalent chromate sites (16g, 17g, and 18g) and the corresponding divalent chromium sites (4g, 12g, and 15g) were employed to mimic various Ti-modification environments on the surface of the Phillips catalyst, as shown in Fig. 26. Among these cluster models, 16g/4g represented Cr(VI)/Cr(II) sites without Ti-modification. In 17g/12g and 18g/15g, the Ti/Cr atomic ratio was set to 1 1 and 2 1, respectively. 

Most types of cement contain small quantities of chromates. These chromates passivate the zinc surface, which is then not attacked by fresh concrete. If the cement contains less chromate than will give at least 20 ppm in the final concrete mix, the hot dip galvanized bars can be dipped in a chromate solution or chromates can be added on the basis of twice the minimum required. Avoid handling the chromates directly, for this can cause dermatitis hexavalent chromate is a toxic substance. 

In Denmark, follow-up of patients with chromate allergy after the introduction of ferrous sulphate in Danish cement to reduce hexavalent chromate concentration in cement continued to show poor prognosis. Only 30% of workers who remained on the job had total clearance of their dermatitis (Avnstorp 1989). 

Trivalent chromate is not considered toxic, but hexavalent chromate has considerable toxic effects. In sufficient concentrations, it (1) causes cancer, particularly lung cancer (Bidstrup 1983), (2) causes respiratory symptoms of bronchitis (Langard 1983), (3) affects the immune system [Snyder et al. (1986) found a lower level of interleukin 6 produced by pokeweed nitrogen-stimulated mononuclear cells isolated from patients exposed to chromate in the soil], and (4) causes irritant dermatitis and chrome ulcers of the skin and mucous membranes. 

Dermatitis occurs more commonly with hexavalent than trivalent chromate. Trivalent chromate binds very readily to protein and, thus, penetrates the skin poorly little trivalent chromate gets past the stratum corneum, whereas hexavalent chromate penetrates easily and deeply into the dermis and is then transformed to trivalent chromate, whereupon it readily forms the hapten with the protein and is processed as an allergen. Polak (1983) demonstrated that it is difficult to sensitize guinea pigs with trivalent chromate but, once sensitized, they react on patch testing in the same way as those sensitized to hexavalent chromate. 

The potential source of water-soluble hexavalent chromate [Cr(VI)] in cement is the trivalent [Cr(III)] compound (Cr203) in the raw materials from which it is produced. Cr(III) compounds are oxidised to Cr(VI) compounds (CrO ) when heated in the kilns to temperatures of approximately 1400 °C. The content of chromium in cement is determined by the presence of chromium compounds in the raw materials, by the kiln lining and by chromium steel abrasion during the... 

Di- and polyisocyanates Diethylene ycol diaminopropyl ether Hexavalent chromate Melamineformaldehyde resins [9003-08-1]... 

A few reports of contact allergy to non-amine hardeners have been published in the case of dicy-andiamide (Senff et al. 1988), dodecenyl succinic anhydride (Goransson 1977), methylhexahydrophthalic anhydride (Kanerva et al. 1997), hexavalent chromate, an accelerator additive (Handley and Burrows 1994 Bruze et al. 1996) and polysulfides (Bruze et al. 1996). 

Guin JD, Work WJ (1995) Plastics Epoxy resins. In Guin JD (ed) Practical contact dermatitis. A handbook for the practitioner. McGraw-Hill, Inc., USA, pp 433-446 Handley J, Burrows D (1994) Dermatitis from hexavalent chromate in the accelerator of an epoxy sealant (PR1422) used in the aircraft industry. Contact Dermatitis 30 193-196 Hansson C (1994) Determination of monomers in epoxy resin hardened at elevated temperature. Contact Dermatitis 31 333-334... 

Black rubber can sensitize hydraulic engineers. Assembly workers build the frame and both rivet and seal. Set-up men, riveters, retouching specialists, and engine fitter-mechanics may come into contact with sealants that may contain phenol-formaldehyde resin (Beck 1989), epoxy base (Handley and Burrows 1994), hexavalent chromate in hardeners for epoxy-based sealants (Handley and Burrows 1994) and polysulfides in two-part sealant bases (Wilkinson and Beck 1993 Bruze et al. 1996). Thiocol, a synthetic rubber based on polysulfide polymers, is used by aircraft fitters to dip screws and rivets into before use, and a fingertip dermatitis localized to the thumb and first two fingers of the dominant hand was seen in five of six fitters with positive patch-test reactions to the polysulfide-polymer-containing system (Wilkinson and Beck 1993). 

Handley J, Burrows D (1994) Dermatitis from hexavalent chromate in the accelerator of an epoxy sealant (PR1422) used in the aircraft industry. Contact Dermatitis 30 193-196... 

The chromate pretreatment layer, which is also called the chromate conversion coating (CCC) varies in thickness depending on the chemistry of the process and the application method used. The CCC layer is, however, usually not thicker than a few microns, which in coating weight is somewhere between 5 and 25 mg/m, expressed as Cr [19], This CCC layer improves the adhesion between the metal and the primer, it aids in the protection of scratches and defects and it also protects cut edges of the metal to some extent [20]. The hexavalent chromate in the CCC layer is known for its low solubility and the self-healing effect, which means lliat it only leaches out on demand when the base metal has been scratched [21]. 

According to this equation the hexavalent chromate ion is reduced to trivalent chromium oxide thereby oxidizing the metal to its oxide. A surface layer forms that contains both Cr203 and MO. In reahty, the corrosion reactions that take place during chromatizing do not follow such a simple stoichiometry because mixed oxides... 

Hexavalent chromium Cr 0.1 Platings, especially for surface passivation of environmentally unstable metals, pigments, and plastics colorant as lead chromate hexavalent chromate finish for plated metal enclosures, fasteners, clips, and screws... 

Cook R, Elliott J, Sapp S, (2007) Electrochemical and computational screening of nontoxic organic replacements for hexavalent chromate corrosion inhibitors , in The 2007 Tri-Service Corrosion Conference (Denver, CO, December 3-7,2007). 

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