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Composition of the Anticorrosion Coating 35 for example, styrene-modified acrylic dispersions [38]. The pigment produces a molybdate anion (MoO 4 −2 ) that is an effective anodic inhibitor; its passivating capacity is only slightly less than that of the chromate anion [37]. 2.3.3.2.3 Third Generation The third generation of zinc phosphates consists of polyphosphates and polyphosphate silicates. Polyphosphates — molecules of more than one phosphorous atom together with oxygen — result from condensation of acid phosphates at higher temperatures than used to produce orthophosphates [38]. This group includes: • Zinc aluminum polyphosphate. This pigment contains a higher percentage of phosphate, as P 2O 5, than zinc phosphate or modified zinc orthophosphates. • Strontium aluminum polyphosphate. This pigment also has greater phosphate content than first-generation zinc phosphate. The solubility behavior is further altered by inclusion of a metal whose oxides react basic compared to amphoteric zinc [38]. • Calcium aluminum polyphosphate silicate. This pigment exhibits an altered solubility behavior due to calcium. The composition is interesting: active components are fixed on the surface of an inert filler, wollastonite. • Zinc calcium strontium polyphosphate silicate. In this pigment, the electrochemically active compounds are also fixed on the surface of wollastonite. 2.3.3.3 Accelerated Testing and Why Zinc Phosphates Commonly Fail Although zinc phosphates show acceptable performance in the field, they commonly show inferior performance in accelerated testing. This response is probably affected by their very low solubility. In accelerated tests, the penetration rate of aggressive ions is highly speeded up, but the solubility of zinc phosphate is not. The amount of aggressive ions thus exceeds the protective capacity of both the phosphate anion and the iron oxide layer on the metal substrate [37]. Bettan has postulated that there is an initial lag time with zinc phosphates because the protective phosphate complex forms slowly on steel’s surface. Because the amount of corrosion-initiating ions is increased from the very beginning of an accelerated test, corrosion processes can be initiated during this lag time. In field exposure, lag time is not a problem, because the penetration of aggressive species usually also has its own lag time. Angelmayer has supported this explanation also [66,72]. Romagnoli [37] also points out that researcher findings conflict and offers some possible reasons why: • Experimental variables of the zinc phosphate pigments may differ. One example is distribution of particle diameter; smaller diameter means increased surface area, which increases the amount of phosphate leaching from the pigment. The more phosphate anion in a solution, the better the © 2006 by Taylor & Francis Group, LLC
36 Corrosion Control Through Organic Coatings anticorrosion protection. Pigment volume concentration (PVC) and critical PVC (CPVC) for the particular paint formulations used are also important and frequently neglected. And, of course, because the term zinc phosphate applies to both a family of pigments and a specific formula, the exact type of zinc phosphate is important. • Binder type and additives are not the same. In accelerated testing, the type of binder is usually the most important factor because of its barrier properties. Only after the binder barrier is breached does effect of pigment become apparent. 2.3.3.4 Aluminum Triphosphate Hydrated dihydrogen aluminium triphosphate (AlH 2P 3O 10•2H 2O) is an acid with a dissociation constant, pKa, of approximately 1.5 to 1.6. Its acidity per unit mass is approximately 10 to 100 times higher than other similar acids, such as aluminium and silicon hydroxides. When dissolved, aluminium triphosphate dissociates into triphosphate ions: AlH 2P 3O 10 → Al 3+ + 2H + + [P 3O 10] 5− Beland suggests that corrosion protection comes both from the ability of the tripolyphosphate ion to chelate iron ions (passivating the metal) and from tripolyphosphate ions’ ability to depolymerize into orthophosphate ions, giving higher phosphate levels than zinc or molybdate phosphate pigments [23]. Chromy and Kaminska attribute the corrosion protection entirely to the triphosphate. They suggest that the anion (P 3O 10) 5– reacts with anodic iron to yield an insoluble layer, which is mainly ferric triphosphate. This phosphate coating is insoluble in water, is very hard, and exhibits excellent adhesion to the substrate [39]. Aluminum triphosphate has limited solubility in water and is frequently modified with either zinc or silicon to control both solubility and reactivity [23,29]. Researchers have demonstrated that aluminium triphosphate is compatible with various binders, including long-, medium-, and short-oil alkyds; epoxies; epoxy-polyesters; and acrylicmelamine resins [73–76]. Chromy notes that it is particularly effective on rapidly corroding coatings; it may therefore be useful in overcoating applications [39]. Nakano has found that aluminium triphosphate can outperform zinc chromate and calcium plumbate pigments in a chlorinated rubber vehicle. Testing in this study involved only salt spray, no field exposure. The substrate was galvanized steel, and the pigments were used in both chlorinated rubber and an air-drying alkyd. Aluminium triphosphate performed better in the chlorinated rubber [74]. Noguchi has seen that aluminium triphosphate in an alkyd vehicle performed better than zinc chromate and zinc phosphate, again using salt spray testing only [77]. 2.3.3.5 Other Phosphates Phosphate pigments other than zinc and aluminium phosphates have received much less attention in the technical literature. This group includes phosphates, hydroxyphosphates, and acid phosphates of the metals iron, barium, chromium, cadmium, © 2006 by Taylor & Francis Group, LLC
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Page 3 and 4: Corrosion of Ceramic and Composite
Page 5 and 6: Published in 2006 by CRC Press Tayl
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 17 and 18: 1 Introduction This book is not abo
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Page 21 and 22: Introduction 5 1.2.2 ELECTROLYTIC R
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