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Corrosion Testing — Practice 137 modes and can vary greatly across the interface. This leads to an interesting conundrum: the fundamental understanding of the wetting of a substrate by a liquid coating, and the subsequent adhesion of the cured coating to the substrate is one of the bestdeveloped areas of coatings science, yet methods for the practical measurement of adhesion are comparatively crude and unsophisticated. It has been shown that experimentally measured adhesion strengths consist of basic adhesion plus contributions from extraneous sources. Basic adhesion is the adhesion that results from intermolecular interactions between the coating and the substrate; extraneous contributions include internal stresses in the coating and defects or extraneous processes introduced in the coating as a result of the measurement technique itself [11]. To complicate matters, the latter can decrease basic adhesion by introducing new, unmeasured stresses or can increase the basic adhesion by relieving preexisting internal stresses. The most commonly used methods of detaching coatings are applying a normal force at the interface plane or applying lateral stresses. 8.2.2.2 Direct Pull-off Methods Direct pull-off (DPO) methods measure the force-per-unit area necessary to detach two materials, or the work done (or energy expended) in doing so. DPO methods employ normal forces at the coating-substrate interface plane. The basic principle is to attach a pulling device (a stub or dolly) to the coating by glue, usually cyanoacrylates, and then to apply a force to it in a direction perpendicular to the painted surface, until either the paint pulls off the substrate or failure occurs within the paint layers (see Figure 8.1). An intrinsic disadvantage of DPO methods is that failure occurs at the weakest part of the coating system. This can occur cohesively within a coating layer; adhesively between coating layers, especially if the glue has created a weak boundary layer within the coating; or adhesively between the primer layer and the metal Glue Coating FIGURE 8.1 Direct pull-off adhesion measurement. © 2006 by Taylor & Francis Group, LLC Weak boundary layer Metal substrate Metal dolly Glue Coating
138 Corrosion Control Through Organic Coatings substrate, depending on which is the weakest link in the system. Therefore, adhesion of the primer to the metal is not necessarily what this method measures, unless it is at this interface that the adhesion is the weakest. DPO methods suffer from some additional disadvantages: • Tensile tests usually involve a complex mixture of tensile and shear forces just before the break, making interpretation difficult. • Stresses produced in the paint layer during setting of the adhesive may affect the values measured (a glue/paint interactions problem). • Nonuniform tensile load distributions over the contact area during the pulling process may occur. Stress concentrated in a portion of the contact area leads to failure at these points at lower values than would be seen under even distribution of the load. This problem usually arises from the design of the pulling head. Unlike lateral stress methods, DPO methods can be used on hard or soft coatings. As previously mentioned, however, for a well-adhering paint, these methods tend to measure the cohesive strength of the coating, rather than its adhesion to the substrate. With DPO methods, examination of the ruptured surface is possible, not only for the substrate but also for the test dolly. A point-by-point comparison of substrate and dolly surfaces makes it possible to fairly accurately determine interfacial and cohesive failure modes. 8.2.2.3 Lateral Stress Methods Methods employing lateral stresses to detach a coating include bend or impact tests and scribing the coating with a knife, as in the cross-cut test. In the cross-cut test, which is the most commonly used of the lateral stress methods, knife blades scribe the coating down to the metal in a grid pattern. The spacing of the cuts is usually determined by the coating thickness. Standard guidelines are given in Table 8.2. The amount of paint removed from the areas adjacent to, but not touched by, the blades is taken as a measurement of adhesion. A standard scale for evaluation of the amount of flaking is shown in Table 8.3. Analysis of the forces involved is complex because both shear and peel can occur in the coating. The amount of shearing and peeling forces created at the knife © 2006 by Taylor & Francis Group, LLC TABLE 8.2 Spacing of Cuts in Cross-Cut Adhesion Coating thickness Spacing of the cuts Less than 60 µm 1 mm 60 µm–120 µm 2 mm Greater than 120 µm 3 mm
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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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© 2006 by Taylor & Francis Group, LLC

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