Zinc chromate pigment

Some less soluble hexavalent chromium compounds (lead chromate and zinc chromate pigments calcium chromate) are carcinogenic in rats, producing tumors at the sites of administration by several routes. Lead chromate also produces renal carcinomas after intramuscular administration in rats. ... 

Davies JM Lung cancer mortality among workers making lead chromate and zinc chromate pigments at three English factories. Brjf Ind Med Mi2y.l58-169, 1984... 

Basic zinc potassium chromate 4 ZnO K20 4 Cr03 3 H20 [37300-23-5] basic zinc chromate zinc chromate pigment lemon yellow C.I. Pigment Yellow 36 lemon yellow triclinic flakes 2.5-5... 

Molybdenum-based anticorrosive pigments offer a nontoxic alternative to the zinc chromate pigments [5.120], They all have a neutral color (white) but the pure compounds are very expensive. To produce economically competitive pigments molybdate and phosphate pigments are combined, or molybdate compounds are applied to inorganic fillers (e.g., zinc oxide, alkaline-earth carbonates, or talc) [5.75], [5.121]—[5.123]. 

ISO 1249 - Zinc chromate pigments - Basic zinc potassium chromate pigments and zinc tetrahydroxychromate pigments - specifies the requirements, and corresponding test methods for zinc chromate pigments suitable for use in paints and/or corrosion-inhibiting coatings [5.59]. 

Standard Classification for Dry Pigmentary Titanium Dioxide Products Standard Specification for Zinc Yellow (Zinc Chromate) Pigments Standard Test Methods for Sampling and Testing of Flaked Aluminum Powders and Pastes... 

Some pigments exert an inhibitive effect on the corrosion of metal. The mechanisms are complex and not always fully understood. Inhibitive pigments include red lead, zinc phosphate and zinc chromate. 

Etch primers are widely used. They are mostly based on polyvinyl butyral and contain chromates and phosphoric acid. They are said to act both as primers and as etching solutions because it is believed that the chromates and phosphoric acid form an inorganic him, which provides adhesion, while oxidised polyvinyl butyral provides an organic him. For direct application to new galvanised steel, the best known primers are based on calcium orthoplumbate pigment and metallic lead, but these are now less used for environmental reasons. Zinc-dust paints and zinc-phosphate pigmented paints are also used, but the trend is to use pretreatments to assure good adhesion. 

Red lead, zinc chromate, calcium plumbate and zinc dust were for many years of special importance as pigments for metal primers. When dispersed in raw or lightly-treated linseed oil, the first three possess the ability to inhibit the corrosion of mild steel and will function very well on wire-brushed rusted surfaces. In other media the tolerance towards rusted surfaces decreases with decreasing quantities of available oil, but performance on clean steel will usually be maintained and often improved. 

It is usual to define primers in terms of the principal inhibiting pigment e.g. zinc phosphate, zinc dust or zinc chromate, and the topcoats in terms of the binder, e.g. alkyd, chlorinated rubber, etc. This practice can be confusing, however, and lead to the selection of incompatible coatings. 

Zinc chromate and zinc tetroxychromate have also been used successfully in anticorrosive paints. Both pigments function by releasing chromate ions which passivate the steel surface. In common with lead pigments, those... 

Inhibitive Pigment a pigment which retards or prevents the corrosion of metals by chemical and/or electrochemical means, as opposed to a purely barrier action. Red lead and zinc chromate are examples of inhibitive pigments as opposed to red iron oxide which has little or no inhibitive action. 

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 lARC has concluded that there is sufficient evidence in humans for the carcinogenicity of chromium(Vl) compounds as encountered in the chromate production, chromate pigment production, and chromate plating industries. In experimental animals there is sufficient evidence for the carcinogenicity of calcium chromate, zinc chromates, strontium chromate, and lead chromate. ... 

After this primer is applied vinyl chloride-vinyl acetate copolymer is added in a series of thin films. The total thickness is usually 5 mils. Pigments like iron oxide, lead, or zinc chromate prevent corrosion of the metal substrate in acid environments and may also be included in the coating. The final coated metal has good resistance to water and many chemicals with about a ten-year lifetime. 

Chromate pigments chrome green molybdate red chrome orange chrome yellow, zinc yellow, alkaline earth chromates ... 

The anticorrosive pigments zinc chromate, zinc tetraoxychromate, and strontium chromate are described in Section 5.2.6... 

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