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Corrosion Testing — Practice 143 SCI −333 mV −878 mV 200 mV −333 −515 −696 −878 (mV) 22.00 mm i 5 p/mm FIGURE 8.5 Volta distribution (mV) of coated steel before (top) and after (bottom) 5 weeks’ weakly accelerated field exposure. Source: Forsgren, A. and Thierry, D., SCI Rapport 2001:4E. Swedish Corrosion Institute (SCI), Stockholm, 2001. Photo courtesy of SCI. figure, a large zone at low potential (–850 to –750 mV/NHE) can be clearly seen. Delamination, corrosion, or both is occurring at the transition area between the “intact” metal/polymer interface (zones at higher potential values, –350 to –200 mV/NHE) and more negative electrode potentials. The corrosion that is starting here after 5 weeks will not be visible as blisters for nearly 2 years at the Bohus-Malmön coastal station in Sweden [25]. 8.2.5 SCANNING VIBRATING ELECTRODE TECHNIQUE R61C 8.00 mm 2 p/mm The scanning vibrating electrode technique (SVET) is used to quantify and map localized corrosion. The instrument moves a vibrating probe just above (100 µm or less) the sample surface, measuring and mapping the electric fields that are generated in the adjacent electrolyte as a result of localized electrochemical or corrosion activity. It is a well-established tool in researching localized events, such as pitting corrosion, intergranular corrosion, and coating defects. The SVET, which gives a two-dimensional distribution of current, is similar in many respects to the SKP; in fact, some instrument manufacturers offer a combined SVET/SKP system. © 2006 by Taylor & Francis Group, LLC
144 Corrosion Control Through Organic Coatings 8.2.6 ADVANCED ANALYTICAL TECHNIQUES For the research scientist or the well-equipped failure analysis laboratory, several advanced analytical techniques can prove useful in studying protective coatings. Many such techniques are based on detecting charged particles that come from, or interact with, the surface in question. These require high (10 –5 or 10 –7 torr) or ultrahigh vacuum (less than 10 –8 torr), which means that samples cannot be studied in situ [26]. 8.2.6.1 Scanning Electron Microscopy Unlike optical microscopes, SEM does not use light to examine a surface. Instead, SEM sends a beam of electrons over the surface to be studied. These electrons interact with the sample to produce various signals: x-rays, back-scattered electrons, secondary electron emissions, and cathode luminescence. Each of these signals has slightly different characteristics when they are detected and photographed. SEM has very high depth of focus, which makes it a powerful tool for studying the contours of surfaces. Electron microscopes used to be found only in research institutes and more sophisticated industrial laboratories. They have now become more ubiquitous; in fact, they are an indispensable tool in advanced failure analysis and are found in most any laboratory dealing with material sciences. 8.2.6.2 Atomic Force Microscopy AFM provides information about the morphology of a surface. Three-dimensional maps of the surface are generated, and some information of the relative hardness of areas on the surface can be obtained. AFM has several variants for different sample surfaces, including contact mode, tapping mode, and phase contrast AFM. Soft polymer surfaces, such as those found in many coatings, tend to utilize tapping mode AFM. In waterborne paint research, AFM has proven an excellent tool for studying coalescence of latex coatings [27-30]. It has also been used to study the initial effect of waterborne coatings on steel before film formation can occur, as shown in Figures 8.6 and 8.7 [31]. 8.2.6.3 Infrared Spectroscopy Infrared spectroscopy is a family of techniques that can be used to identify chemical bonds. When improved by Fourier transform mathematical techniques, the resulting test is known as FTIR. An FTIR scan can be used to identify compounds rather in the same way as fingerprints are used to identify humans: an FTIR scan of the sample is compared to the FTIR scans of “known” compounds. If a positive match is found, the sample has been identified; an example is shown in Figure 8.8. Not surprisingly, FTIR results are sometimes called “fingerprints” by analytical chemists. © 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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Abrasive Blasting and Heavy-Metal C
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