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106 Corrosion Control Through Organic Coatings Coating FIGURE 6.1 Peel forces at the edge of a blister 6.2.4.1 Alkaline Blistering Alkaline blistering occurs when cations, such as sodium (Na + ), migrate along the coating-metal interface to cathodic areas via coating defects, such as pores or scratches. At the cathodic areas, the cations combine with the hydroxyl anions produced by corrosion to form sodium hydroxide (NaOH). The result is a strongly alkaline aqueous solution at the cathodic area. As osmotic forces drive water through the coating to the alkaline solution, the coating is deformed upward — a blister begins. At the coating-metal solution interface, the coating experiences peel forces, as shown in Figure 6.1. It is well established that the force needed to separate two adhering bodies is much lower in peel geometry than in the tensile geometry normally used in adhesion testing of coatings. At the edge of the blister, the coating may be adhering as tightly as ever to the steel. However, because the coating is forced upward at the blister, the coating at the edge is now undergoing peeling and the force needed to detach the coating in this geometry is lower than the forces measured in adhesion tests. This facilitates growth of the blisters until (probably) the solution is diluted with water and the osmotic forces have decreased. Leidheiser and colleagues [27] have shown that cations diffuse laterally via the coating-metal interface, rather than through the coating. Their elegantly simple experiment demonstrating this is shown in Figure 6.2. Adhesion is significantly less under wet conditions (see “Wet Adhesion” in Chapter 1), making ion migration along the interface easier. 6.2.4.2 Neutral Blistering Metal Neutral blisters contain solution that is weakly acid to neutral. No alkali cations are involved. The first step is undoubtedly reduction of adhesion due to water clustering at the coating-metal interface. Funke [28] postulates that differential aeration is responsible for neutral blistering. The steel under the water does not have as ready access to oxygen as the adjacent steel, and polarization arises. The oxygen-poor center of the blister becomes anodic and the periphery is cathodic. Funke’s mechanism of neutral blistering is shown in Figure 6.3. © 2006 by Taylor & Francis Group, LLC Peel
Weathering and Aging of Paint 107 Salt solution No blisters Blisters Some blisters No delamination Delamination Delamination (a) (b) (c) FIGURE 6.2 Experiment of Leidheiser et al. establishing route of cation diffusion Source: Leidheiser, H., Wang, W., and Igetoft, L., Prog. Org. Coat., 11, 19, 1983. 6.3 TEMPERATURE Water Water In general, ambient temperature changes can alter: • Balance of stresses in the coating/substrate system • Mechanical properties of the viscoelastic coating • Diffusion (usually of water) through the coating Salt solution Salt solution Salt solution The balance of stresses is affected in various ways. At slightly elevated temperatures, crosslinking of the polymer can continue far beyond what is desirable; the paint becomes too stiff and cracks with minimal amount of mechanical stress. Even if undesired crosslinking does not occur, bonds that are needed begin to break at higher temperatures, and the polymer is weakened. Differences in coefficients of thermal expansion also cause thermal stress; epoxies or alkyds, for example, typically H 2 O O 2 Coating −OH H2O H H −OH O H H O −OH H H O Cathode A Secondary oxidation products FIGURE 6.3 Mechanism of neutral blistering Source: Funke, W., Ind. Eng. Chem. Prod. Res. Dev., 24, 343, 1985 © 2006 by Taylor & Francis Group, LLC A H 2 O H 2 O O 2 FE ++ FE ++ Anode − OH H H O
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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
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Preface This book has been written
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Author Amy Forsgren received her ch
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Contents Chapter 1 Introduction ...
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2.4.2 Reactive Reagents ...........
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6.2.4.1 Alkaline Blistering........
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1 Introduction This book is not abo
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Introduction 3 • A dissolution pr
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Introduction 5 1.2.2 ELECTROLYTIC R
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Introduction 7 to the metal is a ne
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Introduction 9 9. Thomas. N.L., Pro
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12 Corrosion Control Through Organi
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