Chromium Phosphates

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 Phosphate. Chromium phosphate treatment baths are strongly acidic and comprise sources of hexavalent chromium, phosphate, and fluoride ions. Conversion coating on aluminum precedes by the foUowing reactions (24) ... 

Chromium phosphate coatings can be deposited with very low (0.05—0.15 g/m ) weights to give colodess coatings for appHcations such as a paint base or very heavy (2.0—5.0 g/m ) weights for decorative appHcations. 

Operation and Control. Control of a chromium phosphate conversion coating bath requires monitoring chromium and aluminum concentrations, active fluoride level, and temperature. Coating weight is very sensitive to active, ie, uncomplexed, fluoride. An innovative electrochemical method using a siHcon electrode (25) is employed for measuring active fluoride. A special precaution in chromium phosphate bath operation is the... 

Product Utilization. The principal appHcation for chromium phosphate coatings is as a paint base for painted aluminum extmsions and aluminum beverage can stock. In these appHcations, extremely demanding performance criteria are met by the chromium phosphate conversion coatings. As an example, the Architectural Aluminum Manufacturer s Association Voluntary Specification 605.2-92 requires humidity and salt spray testing for 3000 hours and allows only minimal incidence of paint failure after testing (26). 

The reactions which take place when the mixed etch primer is applied to a metal are complex. Part of the phosphoric acid reacts with the zinc tetroxychromate pigment to form chromic acid, zinc phospliates and zinc chromates of lower basicity. The phosphoric acid also attacks the metal surface and forms on it a thin chromate-sealed phosphate film. Chromic acid is reduced by the alcohols in the presence of phosphoric acid to form chromium phosphate and aldehydes. It is believed that part of the chromium phosphate then reacts with the resin to form an insoluble complex. Excess zinc tetroxy chromate, and perhaps some more soluble less basic zinc chromes, remain to function as normal chromate pigments, i.e. to impart chromate to water penetrating the film during exposure. Although the primer film is hard... 

The most important phosphate-containing pigments are zinc phosphate [5.67]-[5.69], [5.71] basic zinc phosphate [5.70], [5.71] chromium phosphate [5.72] aluminum triphosphate [5.73] barium phosphate [5.53] aluminum zinc phosphate [5.68] and zinc/iron phosphate [5.71], Phosphate ions form protective coatings of basic iron(III) phosphate on an iron surface. Their composition is described in [5.64]. 

Table 40. Composition and properties of zinc and chromium phosphate pigments...

Chromium phosphate [16453-74-0], CrP04-3 H20, Mr 201.0, is produced from chromium(III) salts and alkali phosphates. The physical and chemical properties are listed in Table 40 important areas of use are given in Table 39, [5.72],... 

Chromium phosphate has a low solubility. It is therefore nearly always used in combination with other anticorrosive pigments. It is an extremely good long-term inhibitor, but is less effective during the initial phase of corrosion protection. 

Chrome green proper consists of chromium sesqui-oxide, Guignet green being more or less hydrated oxide. Other analogous products, such as Arnaudon s, Plessy s and Schnitzer s greens, consist essentially of basic chromium phosphates. 

Chromium Phosphides have been prepared from the elements or by the action of phosphorus vapour on potassium dichromate, or from carbon and chromium phosphate. These compounds have the appearance of grey powders, which are hardly attacked by dilute acids but are decomposed by alkalies. 

Recently, there has been reference in the literature has to further phosphate-based pigments, such as barium phosphate, manganese phosphate, lead phosphate, and chromium phosphate. Utilization of such products has been limited, due to economical aspects [5.71]. 

Chromium may also be precipitated as chromium phosphate, CrP04 (see p. 87). 

Copper-chromium-boron (CCB) and copper-chromium-phosphate CCP... 

The tin-free steel is made corrosion-resistant by a very thin coating of chromium phosphate, chromium or chromium oxide, or aluminium. Steel use for packaging purposes makes up around 5 percent of the total world steel consumption. Tin and chrome are in rather short supply tin makes up 0.4 percent, and chromium 0.01 percent of the Earth s crust, but because amounts used are so small, supply does not appear likely to be a problem in the near future, ... 

Arnaudon s Green Chromic phosphate Chromium monophosphate Chromium orthophosphate Chromium phosphate EINECS 232-141-0 Phosphoric acid, chromium(3+) salt (1 1) Plessy s green. Chromic phosphate, used as a pigment. 

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