Trivalent Chromate

The surface finishing process where hexavalent chromium ion would be issued (used) is mostly for chromate conversion treatment. Chromate is a chemical conversion coating process. This means that the surface finishing is by chemical reactions between chemical agents and materials. The chromate conversion treatment uses a chromate bath composed of hexavalent chromium ions. Currently trivalent chromate and some topcoats are used. 

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 incidence of chromate allergy in footwear dermatitis varies in different studies. Trivalent chromate is used hence, the sensitization potential is low. Nevertheless, chromate allergy should always be considered in shoe dermatitis and possibly even as a factor in hand dermatitis due to wearing leather gloves. Chromate is used in leather for other purposes than tanning, e.g. water-repellent trivalent chromium stearate chloride or stain-repellent trivalent chromium and fluorinated carboxylic acids, or as a dye. In those carrying out tanning, the sensitization risk is increased... 

A Trivalent Chromate Process (TCP) that complies with the above regulations and is free from hexavalent chromium is offered in the market and is used in some applications (Alpha Metal Finishing). 

Masking by oxidation or reduction of a metal ion to a state which does not react with EDTA is occasionally of value. For example, Fe(III) (log K- y 24.23) in acidic media may be reduced to Fe(II) (log K-yyy = 14.33) by ascorbic acid in this state iron does not interfere in the titration of some trivalent and tetravalent ions in strong acidic medium (pH 0 to 2). Similarly, Hg(II) can be reduced to the metal. In favorable conditions, Cr(III) may be oxidized by alkaline peroxide to chromate which does not complex with EDTA. 

The compositions of the conversion baths are proprietary and vary greatly. They may contain either hexavalent or trivalent chromium (179,180), but baths containing both Cr(III) and Cr(VI) are rare. The mechanism of film formation for hexavalent baths has been studied (181,182), and it appears that the strength of the acid and its identity, as well as time and temperature, influences the film s thickness and its final properties, eg, color. The newly prepared film is a very soft, easily damaged gel, but when allowed to age, the film slowly hardens, assumes a hydrophobic character and becomes resistant to abrasion. The film s stmcture can be described as a cross-linked Cr(III) polymer, that uses anion species to link chromium centers. These anions may be hydroxide, chromate, fluoride, and/or others, depending on the composition of the bath (183). 

In all circumstances it is important to ensure that the inhibitor is chemically compatible with the liquid to which it is added. Chromates, for example, cannot be used in glycol antifreeze solutions since oxidation of glycol by chromate will reduce this to the trivalent state which has no inhibitive properties. 

Complex oxide film is formed in a basic solution, whereas the films described earlier are formed in an acidic solution. Complex oxide conversion coating reactions do not contain either hexavalent or trivalent chromium ions. However, the sealing rinse contains much greater quantities of hexavalent and trivalent chromium ions than do the sealing rinses associated with phosphate conversion coatings and chromate conversion coatings. 

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 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... 

Since the reduction of chromate(VI) to metal is a six-electron process, it would seem logical to plate the metal from an electrolyte based on a lower oxidation state of chromium. A good deal of work has been done on the development of trivalent chromium plating,8 but it is only recently that a commercial process has been available in the United Kingdom. 

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