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Composition of the Anticorrosion Coating 33 reaches an equilibrium value of 20 nm. The film prevents the outward diffusion of iron. Phosphate ions do not appear to directly contribute to the oxide film formation but rather act to complete or maintain it by plugging discontinuities with anion precipitates of Fe(III) ions. Romagnoli has noted that Pryor used soluble phosphates rather than the generally insoluble phosphates used in coatings, so care should be taken in extrapolating these results [37]. Other studies have found both oxyhydroxides and iron phosphates incorporated in the protective film [59]. • Inhibitive Aqueous Extracts Formed with Certain Oleoresinous Binders Inhibitive aqueous extracts form with certain oleoresinous binders. Components of the binder, such as carboxylic and hydroxyl groups, form complexes with either the zinc phosphate or the intermediate compounds formed when the zinc phosphate becomes hydrated and dissociates. These complexes can then react with corrosion products to form a tightlyadhering, inhibitive layer on the substrate [21,39,43–46,52]. • Polarization of the Substrate Clay and Cox [60] have suggested that nearly insoluble basic salts are formed and adhere well to the metal surface. These salts limit the access of dissolved oxygen to the metal surface and polarize the cathodic areas. This theory was confirmed by the work of Szklarska-Smialowska and Mankowsky [61]. 2.3.3.2 Types of Zinc Phosphates Because so many variations of zinc phosphates are available, it is convenient to divide them into groups for discussion. Although no formal classes of zinc phosphates exist, they have been divided here into groups or generations, more or less by chronological development. 2.3.3.2.1 First Generation The simplest, or first generation, zinc phosphate is made by either mixing disodium phosphate and zinc sulfate solutions at boiling temperature or saturating a 68% phosphoric acid solution with zinc oxide, also at boiling temperature. Both methods give a precipitate with an extremely coarse crystalline structure. Further treatment yields Zn3(PO4) 2 •4H2O, first generation zinc phosphate [37]. The usefulness of first-generation zinc phosphate is limited by its low solubility [62]; only a small concentration of phosphate ions is available to protect the metal. This is a problem because corrosion inhibition by phosphates takes place only when the anion concentration is higher than 0.001M in a salt solution at pH 5.5 to 7.0 [57]. 2.3.3.2.2 Second Generation Zinc phosphates can be modified to increase their solubility in water or to add other functional groups that can also act as inhibitors. This is usually achieved by adding an organic surface treatment to the pigment or blending other inorganic inhibitors with the zinc phosphate [23]. Table 2.4 shows the amount of phosphate ions in milligrams-per-liter water obtained from various first-generation and subsequent generation zinc phosphates [63]. It can be clearly seen why modifying phosphate © 2006 by Taylor & Francis Group, LLC
34 Corrosion Control Through Organic Coatings TABLE 2.4 Relative Solubilities in Water of Zinc Phosphate and Modified Zinc Phosphate Pigments Pigment Water-soluble matter (mg/l) (ASTM D 2448-73, 10 g pigment in 90 ml water) Total Zn +2 PO 4 −3 MoO 4 −2 Zinc phosphate 40 5 1 -- Organic modified zinc phosphate Aluminum zinc phosphate Zinc molybdenum phosphate 300 80 1 -- 400 80 250 -- 200 40 0.3 17 Source: Bittner, A., J. Coat. Technol., Vol. 61, No. 777, p. 111, Table 2, with permission. pigments is an area of great interest: aluminum zinc phosphate provides 250 times the amount of dissolved phosphate as first-generation zinc phosphate. Second-generation zinc phosphates can be divided into three groups: basic zinc phosphate, salts of phosphoric acid and metallic cations, and orthophosphates. First-generation zinc phosphate, Zn 3(PO 4) 2 •4H 2O, is a neutral salt. Basic zinc phosphate, Zn 2(OH)PO 4•2H 2O, yields a different ratio of Zn 2+ and PO 4 3− ions in solution and has a higher activity than the neutral salt [39]. It has been reported that basic zinc phosphate is as effective a corrosion inhibitor as zinc phosphate plus a mixture of pigments containing water-soluble chromates [64–66]. Another group of second-generation phosphate pigments includes salts formed between phosphoric acid and different metallic cations, for example, hydrated modified aluminium-zinc hydroxyphosphate and hydrated zinc hydroxymolybdate phosphate. Trials using these salts in alkyd binders indicate that pigments of this type can provide corrosion protection comparable to that of zinc yellow [67–69]. Orthophosphates, the third type of second-generation zinc phosphates, are prepared by reacting orthophosphoric acid with alkaline compounds [38]. This group includes: • Zinc aluminum phosphate. It is formed by combining zinc phosphate and aluminum phosphate in the wet phase; the aluminum ions hydrolyze, causing acidity, which in turn increases the phosphate concentration [38,70,71]. Aluminum phosphate is added to give higher phosphate content. • Organically modified basic zinc phosphates. An organic component is fixed onto the surface of basic zinc phosphate particles, apparently to improve compatibility with alkyd and physically drying resins. • Basic zinc molybdenum phosphate hydrate. Zinc molybdate is added to basic zinc phosphate hydrate so it can be used with water-soluble systems, © 2006 by Taylor & Francis Group, LLC
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Page 3 and 4: Corrosion of Ceramic and Composite
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Page 7 and 8: Preface This book has been written
Page 9 and 10: Author Amy Forsgren received her ch
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Page 15 and 16: 6.2.4.1 Alkaline Blistering........
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Page 21 and 22: Introduction 5 1.2.2 ELECTROLYTIC R
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5 Abrasive Blasting and Heavy-Metal
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