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Corrosion Testing — Practice 139 TABLE 8.3 Evaluation of the Amount of Flaking Grade Description 0 Very sharp cuts. No material has flaked. 1 Somewhat uneven cuts. Detachment of small flakes of the coating at the intersections of the cuts. 2 Clearly uneven cuts. The coating has flaked along the edges and at the intersections of the cuts. 3 Very uneven cuts. The coating has flaked along the edges of the cuts partly or wholly in large ribbons and it has flaked partly or wholly on different parts of the squares. A cross-cut area of no more than 35% may be affected. 4 Severe flaking of material. The coating has flaked along the edges of the cuts in large ribbons and some squares have been detached partly or wholly. A cross-cut area of no more than 65% may be affected. 5 A cross-cut area greater than 65% is affected. tip depends not only on the energy with which the cuts are made (i.e., force and speed of scribing) but also on the mechanical properties — plastic versus elastic deformation — of the coating. For example, immediately in front of the knife-edge, the upper surface of the paint undergoes plastic deformation. This deformation produces a shear force down at the coating-metal interface, underneath the rim of indentation in front of the knife-edge [11]. A major drawback to methods using lateral stresses is that they are extremely dependent on the mechanical properties of the coating, especially how much plastic and elastic deformation the coating undergoes. Paul has noted that many of these tests result in cohesive cracking of coatings [11]. Coatings with mostly elastic deformation commonly develop systems of cracks parallel to the metal-coating interface, leading to flaking at the scribe and poor test results. Coatings with a high proportion of plastic deformation, on the other hand, perform well in this test — even though they may have much poorer adhesion to the metal substrate than do hard coatings. Elastic deformation means that little or no rounding of the material occurs at the crack tip during scribing. Almost all the energy goes into crack propagation. As the knife blade moves, more cracks in the coating are initiated further down the scribe. These propagate until two or more cracks meet and lead to flaking along the scribe. The test results can be misleading; epoxies, for example, usually perform worse than softer alkyds in cross-cut testing, even though, in general, they have much stronger adhesion to metal. For very hard coatings, scribing down to the metal may not be possible. Use of the cross-cut test appears to be limited to comparatively soft coatings. Because the test is very dependent on deformation properties of the coatings, comparing crosscut results of different coatings to each other is of questionable value. However, the test may have some value in comparing the adhesion of a single coating to various substrates or pretreatments. © 2006 by Taylor & Francis Group, LLC
140 Corrosion Control Through Organic Coatings 8.2.2.4 Important Aspects of Adhesion The failure loci — where the failure occurred — can yield very important information about coating weaknesses and eventual failures. Changes in failure loci related to aging of a sample are especially revealing about what is taking place within and under a coating system. Adhesion measurements are performed to gain information regarding the mechanical strengths of the coating-substrate bonds and the deterioration of these bonds when the coatings undergo environmental stresses. A great deal of work has been done to develop better methods for measuring the strengths of the initial coating-substrate bonds. By comparison, little attention has been given to using adhesion tests to obtain information about the mechanism of deterioration of either the coating or its adhesion to the metal. This area deserves greater attention because studying the failure loci in adhesion tests before and after weathering can yield a great deal of information about why coatings fail. Finally, it is important to remember that adhesion is only one aspect of corrosion protection. At least one study shows that the coating with the best adhesion to the metal did not provide the best corrosion protection [12]. Also, studies have found that there is no obvious relationship between initial adhesion and wet adhesion [13]. 8.2.3 BARRIER PROPERTIES Coatings, being polymer-based, are naturally highly resistant to the flow of electricity. This fact is utilized to measure water uptake by and transport through the coating. The coating itself does not conduct electricity; any current passing through it is carried by electrolytes in the coating. Measuring the electrical properties of the coating makes it possible to calculate the amount of water present (called water content or solubility) and how quickly it moves (called diffusion coefficient). The technique used to do this is EIS. An intact coating is described in EIS as a general equivalent electrical circuit, also known as the Randles model (see Figure 8.2). As the coatings become more porous or local defects occur, the model becomes more complex (see Figure 8.3). R sol C paint R paint FIGURE 8.2 Equivalent electric circuit to describe an intact coating. R sol is the solution resistance, C paint is the capacitance of the paint layer, and R paint is the resistance of the paint layer. © 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
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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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4 Blast Cleaning and Other Heavy Su
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5 Abrasive Blasting and Heavy-Metal
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© 2006 by Taylor & Francis Group, LLC

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