Chromium/chromates

Chromate conversion coatings are thin, noncrystalline, adherent surface layers of low solubiHty phosphoms and/or chromium compounds produced by the reaction of suitable reagents with the metal surface (2,3). The two classes of chromate coatings are chromium phosphates (green chromates) and chromium chromates (gold chromates). 

Chromium Chromate. Chromium chromate treatment baths are acidic and made up from sources of hexavalent chromium and complex fluoride, fluorosiHcate, fluorozirconate, fluorotitanate, and siHcofluorides. Optional additional components added to accelerate coating rate are free fluoride, ferricyanide, and other metal salts such as barium nitrate. Conversion coating on aluminum precedes by the following reactions (2,3,17) ... 

Operation nd Control. Control of chromium chromate conversion coating baths is accompHshed by controlling chromium concentration and pH. The quaHty of the conversion coating is sensitive to aluminum accumulations in the coating bath as well as to rinse water purity. Sulfate contamination is a particular problem. 

In order that a chromate film may be deposited, the passivity which develops in a solution of chromate anions alone must be broken down in solution in a controlled way. This is achieved by adding other anions, e.g. sulphate, nitrate, chloride, fluoride, as activators which attack the metal, or by electrolysis. When attack occurs, some metal is dissolved, the resulting hydrogen reduces some of the chromate ion, and a slightly soluble golden-brown or black chromium chromate (CtjOs CrOs xHjO) is formed. 

Several immersion treatments using solutions containing chromates have been developed for aluminium. It is not always clear to what extent the films formed can properly be called chromate films, i.e. films containing a substantial amount of a slightly soluble chromium chromate, but even if the film consists largely of aluminium oxide or hydroxide or other salt with chromate physically absorbed, it will still provide a reservoir of soluble chromate at the metal surface. Treatments fall into two classes alkaline and acid. The latter are of more recent development. 

The chromate film deposited by the Cronak process on zinc consists largely of a hydrated chromium chromate and contains some 10% by weight... 

Chromium(II) chloride, 6 528t, 531, 564t Chromium(III) chloride, 6 532 physical properties, 6 528t Chromium(IV) chloride, 6 535 Chromium(III) chloride hexahydrate, physical properties, 6 528t Chromium chromate coatings, 76 219—220 Chromium complexes, 9 399 Chromium compounds, 6 526-571 analytical methods, 6 547-548 economic aspects, 6 543-546 environmental concerns, 6 550—551 health and safety factors, 6 548-550 hydrolysis, equilibrium, and complex formation constants, 6 530t manufacture, 6 538-543... 

Chromium chromate, potassium dichromate, chromium trioxide... 

SYNS CHROMIC acid, CHR0MIUM(3+) salt (3 2) CHROMIUM CHROMATE (MAK)... 

The readiness with which chromium chromate is converted by ammonia, alkali carbonate, or hydroxide, or by boiling water, into chromium hydroxide and chromate is considered to be evidence of its salt-like constitution. 

Chromium Chromate or chromium dioxide has already been described (see p. 39). ... 

Chromite, FeCr204, is the principal ore of chromium. Chromates and dichromates are obtained from it by fusion with sodium or potassium carbonate with access of air the iron is oxidized from the ferrous to the ferric condition and the chro-... 

A number of oxides, with compositions intermediate between those of chromic oxide, Cr203, and chromic anhydride, CrOs, has been reported. These oxides include CrsOg, CrgOs, CrgOg, CrgOig, CrgOig, and CrgOig. They are sometimes represented as chromium chromates. Their individuality is not well established. 

The brown colour of the unstable intermediate product points to the formation of chromium chromate. The hydrogen dioxide oxidizes the chr-omium chromate forming chromic acid and water. All this characterizes a catalytic reaction. Attempts by E. H. Eiesenfeld, and E. Spitalsky to work out a detailed explanation of the catalyzed reaction have not been successful. When a large excess of hydrogen dioxide is present, the rate of decomposition is approximately proportional to the cone, of the chromic acid. If Ci denotes the cone, in mols of K2Cr04 per litre, and ki the velocity constant,... 

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