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Composition of the Anticorrosion Coating 43 Use of these pigments has been limited because of their expense. Zinc phosphate versions of the molybdate pigments have been introduced in order to lower costs and improve both adhesion to steel and film flexibility [23,47,96–98]. The molybdate pigment family includes: • Basic zinc molybdate • Basic calcium zinc molybdate • Basic zinc molybdate/phosphate • Basic calcium zinc molybdate/zinc phosphate In general, tests of these pigments as corrosion inhibitors in paint formulations have returned mixed results on steel. Workers in the field tend to refer somewhat wistfully to the possibilities of improving the performance of molybdates through combination with other pigments, in the hope of obtaining a synergistic effect. A serious drawback is that, in several studies, molybdates appeared to cause coating embrittlement, perhaps due to premature binder aging [99–102]. Although molybdate pigments are considered nontoxic [103], they are not completely harmless. When cutting or welding molybdate-pigmented coatings, fumes of very low toxicity are produced. With proper ventilation, these fumes are not likely to prove hazardous [101]. The possible toxicity is about 10% to 20% that of chromium compounds [103,104]. 2.3.7.4 Silicates Silicate pigments consist of soluble metallic salts of borosilicates and phosphosilicates. The metals used in silicate pigments are barium, calcium, strontium, and zinc; silicates containing barium can be assumed to pose toxicity problems. The silicate pigments include: • Calcium borosilicates, which are available in several grades, with varying B 2O 3 content (not suitable for immersion or semi-immersion service or water-based resins [23]) • Calcium barium phosphosilicate • Calcium strontium phosphosilicate • Calcium strontium zinc phosphosilicate, which is the most versatile phosphosilicate inhibitor in terms of binder compatibility [23] The silicate pigments can inhibit corrosion in two ways: through their alkalinity and, in oleoresinous binders, by forming metal soaps with certain components of the vehicle. Which process predominates is not entirely clear, perhaps because the efficacy of the pigments is not entirely clear. When Heyes and Mayne examined calcium phosphosilicate and calcium borosilicate pigments in drying oils, they found a mechanism similar to that of red lead: the pigment and the oil binder react to form metal soaps, which degrade and yield products with soluble, inhibitive anions [105]. Van Ooij and Groot found that calcium borosilicate worked well in a polyester binder, but not in an epoxy or polyurethane [106]. This hints that the alkalinity © 2006 by Taylor & Francis Group, LLC
44 Corrosion Control Through Organic Coatings generated within the binder cannot be very high, otherwise the polyester — being much more vulnerable to saponification — would have shown much worse results than either the epoxy or the polyurethane. Metal soaps, of course, would not be formed with either an epoxy or polyurethane. However, the possibility of metal soaps cannot be absolutely ruled out for a polyester without knowing exactly what is meant by this unfortunately broad term. The state of Massachusetts had a less-positive experience with the same pigment, although possibly a different grade of it. In the 1980s, the state of Massachusetts repairpainted a number of bridges with calcium borosilicate pigment in a conventional oleoresinous binder — a vehicle that would presumably form metal soaps. Spot blasting was performed prior to coating. The calcium borosilicate system was judged less forgiving of poor surface preparation than is LBP, and attaining the minimum film build was found to be critical. Massachusetts eventually stopped using this pigment because of the high costs of improved surface preparation and inspection of film build [0]. Another silicate, calcium barium phosphosilicate, has been tested in conjunction with six other pigments on cold-rolled steel in an epoxy-polyamide binder [0, 0]. After nine months’ atmospheric exposure in a marine environment (Biarritz, France), the samples with calcium barium phosphosilicate pigment — and those with barium metaborate — gave worse results than either the aluminum triphosphate or ionexchanged calcium silicate pigments. (These in turn were significantly outperformed by a modified zinc phosphate as well as by zinc chromate pigment.) 2.3.8 BARRIER PIGMENTS 2.3.8.1 Mechanism and General Information Barrier coatings protect steel against corrosion by reducing the permeability of liquids and gases through a paint film. How much the permeability of water and oxygen can be reduced depends on many factors, including: • Thickness of the film • Structure of the film (polymer type used as binder) • Degree of binder crosslinking • Pigment volume concentrations • Type and particle shape of pigments and fillers Pigments used for barrier coatings are diametrically opposed to the active pigments used in other anticorrosion coatings in one respect: in barrier coatings, they must be inert and completely insoluble in water. Commonly used barrier pigments can be broken into two groups: • Mineral–based materials, such as mica, MIO, and glass flakes • Metallic flakes of aluminium, zinc, stainless steel, nickel, and cupronickel In the second group, care must be taken to avoid possible electrochemical interactions between the metallic pigments and the metal substrate [109]. © 2006 by Taylor & Francis Group, LLC
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© 2006 by Taylor & Francis Group,
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Corrosion of Ceramic and Composite
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Published in 2006 by CRC Press Tayl
Page 7 and 8: Preface This book has been written
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Abrasive Blasting and Heavy-Metal C
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6 Weathering and Aging of Paint Thi
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7 Corrosion Testing — Background
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