Dichromate, chromate coatings

A process for zinc alloy consists of immersing in a sodium dichromate solution with other additives, at room temperature, followed by rinsing and drying to produce a dull yellow zinc chromate coating. 

Another example of where ultrasound influences the discharge rate of chromium is in the chromium electroplating of steel plates. When cold-rolled steel plate is elecUo-lytically chromated from a silent aqueous solution containing chromates and dichromates, a chrome coating of 13 mg m is obtained after 1 s, whilst in the presence of ultrasound a coating of 45 mg m is obtained [25] in the same time. The product is also accompanied with an increase in the brightness of the metal. Similar results have been found by other workers [26]. 

Chromium(IV) oxide, Cr02, is used as a ferromagnetic coating for chrome magnetic tapes. Sodium chromate, Na2Cr04(s), is used to prepare many other Cr compounds. 1 n acid solution, Cr04 forms the dichromate ion, Cr207 in both ions, the oxidation number of Cr is -1-6. 

The efficiency of the electrolytic oxidation is limited to some extent by the reduction of chlorate at the cathode. To check this reduction as far as possible, a little chromate or dichromate is added to the bath. This is partially reduced at the cathode to tripositive chromium, which forms a coating of a chromic chromate around the cathode and thus inhibits the cathodic reduction of chlorate or hypochlorite. 

Cr(VI) speciation in coating baths is an important variable in CCC formation. Cr(VI) in solution can rapidly speciate via hydrolysis and condensation reactions to chromate, Cr04, dichromate, Cr207 , or bichromate HCr04 [117]. The predominant species depends on Cr(VI) concentration and pH as shown in Fig. 15, and not on the form of the salt or... 

Emulsion systems for anticorrosive coatings are usually copolymers of vinylidene chloride, vinyl chloride, and an alkyl acrylate or methacrylate. Relatively small amormts of surfactant and colloid are used in order to minimize water sensitivity in the dried coating. Flash rusting inhibitors such as ammonium chromate/dichromate or alkali metal nitrate/nitrites may also be added. 

Aqueous deposition methods are used to apply conversion coatings for decorative and protective reasons. These conversion coatings are formed by immersing the substrate metal in an aqueous solution of chromic acid, chromate or dichromate salts. For the deposition to occur activating ions such as sulfate, nitrate, chloride, or fluoride must be added. Then as hydrogen is generated by the attack of the activating ions some of the chromium ions are reduced to form a hydrated chromium chromate which is deposited on the substrate surface. 

Chrome baths always contain a source of hexavalent chromium ion (e.g., chromate, dichromate, or chromic acid) and an acid to produce a low pH which usually is in the range of 0-3. A source of fluoride ions is also usually present. These fluoride ions will attack the original (natural) aluminum oxide film, exposing the base metal substrate to the bath solution. Fluoride also prevents the aluminum ions (which are released by the dissolution of the oxide layer) from precipitating by forming complex ions. The fluoride concenfration is critical. If the concentration is too low, a conversion layer will not form because of the failure of the fluoride to attack the natural oxide layer, while too high a concentfa-tion results in poor adherence of the coating due to reaction of the fluoride with the aluminum metal substrate. 

The classical zinc-corrosion inhibitor has been mercuric or mercurous chloride, which forms an amalgam with the zinc. Cadmium and lead, which reside in the zinc alloy, also provide zinc anode corrosion protection. Other materials like potassium chromate or dichromate, used successfully in the past, form oxide films on the zinc and protect via passivation. Surface-active organic compounds, which coat the zinc, usually from solution, improve the wetting characteristic of the surface unifying the potential. Inhibitors are usually introduced into the cell via the electrolyte or as part of the coating on the paper separator. Zinc cans could be pretreated however, this is ordinarily not practical. 

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