Chromate pigments

Lead chromate Pigment Yellow 34 formula xPbCrO -yPbSO  

Chrome yellow pigments are bright yellow with high chroma values. They are predominantly lead chromates, but some variants have a mixed crystal phase with lead sulfate. Important chrome yellow pigments are  

Primrose chrome This is a pale, greenish-yellow pigment with a metastable orthorhombic crystal structure containing a solid solution of lead chromate and lead sulfate. The crystals are normally stabilized by additives. Due to a less-stable crystal form, they have poorer fastness properties compared to other monoclinic chrome pigments. 

Lemon chrome This is a greenish-yellow pigment (slightly redder than primrose) with a monoclinic crystal structure, which is more stable compared to other crystal forms. The undertone is dependent on the lead sulfate content - the more sulfate, the greener the shade. 

Middle chrome This is a reddish-yellow pigment precipitated as pure lead chromate in the monoclinic crystal structure. 

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Bismuth V n d te. The use of lead chromate pigments has been slowly phased out of many appHcations. Thus a search for more environmentally acceptable relatively inexpensive yeHow pigments having exceHent coloristic properties has been ongoing. 

The basic compositions of the most common commercially available lead chromate pigments are given ia Table 2 (44). 

The principal producers of lead chromate pigments ia North America are Cookson, DCC (owned by Kikuchi), Engelhard, and Wayne Pigment. In Europe, lead chromates are produced by Ciba ia Holland, CapeUe ia Erance, and Heubach GmGH ia Germany. 

Owing to the low dielectric constant of organic vehicles, these pigments can ionise only after water has permeated the film, consequently their efficiency is associated with the nature of the vehicle in which they are dispersed, a point which is sometimes overlooked when comparing the relative merits of chromate pigments. 

Safety No year goes by without some widely used chemical being declared suspect on toxicity grounds. The paint industry has responded rapidly to eliminate toxic chemicals from coatings or to show how they can be used safely in an industrial environment. Examples are the elimination of specific ether-alcohol solvents and the introduction of air-fed hoods for spraying isocyanates. Of particular interest in corrosion prevention is the current pressure to eliminate chromate pigments. Currently there are no equally effective alternatives and the emphasis has had to be on safe usage. The search for replacements continues. 

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 18 substances are shown in Table 1. Five of these substances are now (June 2012) included in Annex XIV (Authorisation list) [29]. These substances are the lead-chromate pigments, pigment yellow 34 and pigment red 104, and the phthalates DEHP, DBP and BBP, which all are used as components/additives in some printing inks. As evident from Table 1, cobalt(II) salts and trichloroethylene appear on the Annex XIV candidate list [28], and they are also used as additives in some printing inks. ECHA has recommended in its third recommendation [30] to put these two substances on the Authorisation list [29]. Besides these seven substances of very high concern (SVHC substances) appearing on either the Authorisation list or its candidate list, 11 other substances that meet one or more of the criteria for the candidate list are shown in Table 1. These substances are also... 

Under certain conditions, dry mixtures of lead chromate pigments with the azo-dyes l-(2. 4 -dimtrobenzcncazo)-2-hydroxynaphthalene (dinitroaniline orange) or l-(4 -chloro-2 -nitrobenzcncazo)-2-hydroxynaphthalene (chlorinated para-red) may lead to violent explosions dining mixing/blending operations. 

Lead arsenates, 74 797 Lead azide, 70 727-729 74 792 Lead babbitt, 3 52, 52t arsenic addition to, 3 271 Lead-base babbitts, 24 797 Lead-based paint, 74 765 Lead-based paint remediation, 18 75 Lead-based pigments, 79 386. See also Lead chromate pigments Lead benzoate, 74 794 Lead bisilicate, 74 796 Lead blast furnace, 74 737 Lead borate, 74 797 Lead bromide, 74 785 Lead bullion, 74 742, 745... 

Lead chromate pigments, 19 405, 407-408 Lead Chromate Yellow, pigment for plastics, 7 370t... 

The presence of lead and hexavalent chromium in these products is of chronic hazard concern and the EU has classified these pigments as harmful substances. Lead is soluble at stomach-acid concentrations and can accumulate in the organism. The results of a high lead intake include inactivation of enzymes and disturbances in the synthesis of haemoglobin. Hexavalent chromium compounds are considered to be carcinogenic. For these reasons the usage of lead chromate pigments has declined considerably in recent years. 

Lead chromate pigments are not sufficiently fast to high sulfur dioxide concentrations. 

Lead chromate pigments are effected by materials containing sulfuric acid, such as soot or moist dead leaves, to which automotive finishes are frequently exposed. 

Chromates. Epidemiological studies from around the world have consistently shown excess risks for lung cancer in workers involved in chromate and chromate pigment production. The epidemiological studies do not clearly implicate specific compounds but do implicate chromium(VT) compounds." (A recent report also implicated insoluble chromium(HI) as a cause of lung cancer in... 

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. ... 

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. ... 

Davies JM Long term mortality study of chromate pigment workers who suffered lead poisoning. Br jf Ind Med 41(2) 170-178, 1984... 

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... 

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