Chromate and Molybdate Pigments

By far the most important products are chrome yellow and molybdate red. The chrome yellow pigments are Pb(Cr,S)04 mixed phases or pure PbCr04 and are produced by precipitation from the aqueous phase. The different hues from green-tinged yellow to bright and mid-yellow to golden yellow can be controlled by the crystal modification (orthorhombic or monoclinic) or by varying the lead sulfate content. 

Despite the adoption of appropriate safety measures during manufacture, such as closed units and dust removal plants, ensuring emission values for chromate and lead below their respective MAK values (0.1 mg/m as Cr03 and 0.1 mg/m as Pb), chromate pigments are being increasingly substituted by other pigments. 

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Weisberg, H. E. Chromate and Molybdate Pigments, Paint and Varnish Production, 1968. 

Lead Chromates and Molybdates. The lead chromates appear in several shades of yellow. The primrose and lemon are solid solutions of lead sulfate in the chromate and have the stable monoclinic stmcture. The medium shade contains no sulfate. Chrome orange is a compound with lead oxide (PbCrO TbO). Molybdate orange is a combination of lead chromate and sulfate with molybdate (PbMoO ). These pigments have the advantages of... 

Inhibitive Primers. The traditional primers are red lead and linseed oil, alkyd resins and zinc potassium chromate, proprietary epoxy, alkyd, urethane and latex systems with modified phosphate, borate and molybdate pigments. Chromate-based pigments, although in use in aircraft coatings, automotive primers and coil coating primers, are subject to environmental concern on the toxicity of hexavalent chromium which is likely to result in their replacement by innocuous products. 

Most of the pigments used in conventional paints and coatings are also suitable for waterborne systems. Some exceptions are lead chromates and molybdates, manganese lakes (Pigment Red 48 and 52), some perylenes (Pigment Red 223), benzim-idazolone yellow (Pigment Yellow 151), and isoindolines (Pigment Yellow 13a) which all have limited alkali resistance. 

Minium, Pb304, of orange color, is widely used in alkyd paints that serve as primary paints on steel for corrosion protection. In a similar way as chromates and molybdates, minium promotes the passivation of the steel surface, although the reaction mechanisms involved are not all known in detail. Unfortunately, pigments containing lead or chromate are toxic and they will have to be replaced with other compounds that are less harmful to the environment. Molybdates or organic molecules such as the tannins can fulfill this task, but their protective power is still inferior. For certain applications zinc rich paints, provide an interesting alternative. They contain... 

Molybdenum orange is a solid solution of lead molybdate, lead chromate and often lead sulfate. It is used as a pigment in paints, plastics and printing inks. The pigment is used for red hues in automobile and appliance paints. 

The most important chromate pigments include the lead chromate (chrome yellow) and lead molybdate pigments (molybdate orange and molybdate red) whose colors range from light lemon yellow to reds with a blue hue. Chrome yellow,... 

Lead chromates, lead molybdates, chrome greens, and fast chrome greens are supplied as pigment powders, low-dust or dust-free preparations, or as pastes. For standard specifications, see Table 1. 

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]. 

Unlike chromate ions in chromate pigments, the Mo04 ions in molybdate pigments are not chemically reduced in most coatings. Hence, they are ineffective for cathodic protection. Their protective action is assumed to be due to activity in the anodic region, similar to that of phosphate ions. As with the protective phosphate films, molybdate films are very resistant to chloride and sulfate [5.124], The duration... 

Molybdenum salts used as catalysts include cobalt molybdate for hydrogen treatment of petroleum stocks for desulfurization, and phospho-molybdates to promote oxidation. Compounds used for dyes are sodium, potassium, and ammonium molybdates. With basic dyes, phosphomolyb-dic acid is employed. The pigment known as molybdenum orange is a mixed crystal of lead chromate and lead molybdate. Sodium molybdate, or molybdic oxide, is added to fertilizers as a beneficial trace element. Zinc and calcium molybdate serve as inhibitory pigments in protective coatings arid paint for metals subjected to a corrosive atmosphere. Compounds used to produce better adherence of enamels are molybdenum trioxide and ammonium, sodium, calcium, barium, and lead molybdates. 

From a regulatory prespective, it is the trivalent character of its chromium content that distinguishes chrome titanium yellow from the hexavalent character of the chromium in lead chromate and lead molybdate pigments. 

As stated previously throughout this chapter, the pigment types most adversely impacted by this series of federal and state regulations are cadmium, mercury-cadmium, lead chromate, and lead molybdate colorants. Generally, these regulations make a distinction between hexavalent chromium, which is preceived as carcinogenic, and trivalent chromium. It is this distinction that allows the con-... 

The colorants affected by this legislation include the lead chromate and lead molybdate yellows and oranges, cadmium pigments, and mercury cadmium pigments (which were little used by the time this legislation began to take effect). 

Molybdates are used in a variety of industries. Sodium molybdate is used for the synthesis of pigments such as molybdate chrome orange, which is a homogeneous mixture of lead molybdate, lead chromate and lead sulphate. This use is likely to decline because of concerns about health hazards associated with lead, but phosphomolybdates and phosphotungstomolybdates are used to complex certain dyestuffs to produce pigments. A few of the best-known of these are Malachite Green, Rhodamine Band Methyl Violet, also used as indicators in analytical chemistry. 

SYNS CHROMIC ACID, LEAD and MOLYBDENUM SALT CHROMIC ACID LEAD SALT with LEAD MOLYBDATE C.I. PIGMENT RED 104 LEAD CHROMATE, SULPHATE and MOLYBDATE MOLYBDENUM-LEAD CHROMATE MOLYBDENUM ORANGE... 

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