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Corrosion Testing — Practice 141 R sol R sol R paint R paint C paint C paint R ct R ct C dl FIGURE 8.3 Equivalent electric circuits to describe a defective coating. C dl is the double layer capacitance, R ct is the charge transfer resistance of the corrosion process, Q dl is the constant phase element, C diff is the diffuse layer capacitance, and R diff is the diffuse layer resistance. Circuit A in Figure 8.3 is the more commonly used model; it is sometimes referred to as the extended Randles model [12, 14, 15]. EIS is an extremely useful technique in evaluating the ability of a coating to protect the underlying metal. It is frequently used as a “before-and-after” test because it is used to compare the water content and diffusion coefficient of the coating before and after aging (accelerated or natural exposure). Krolikowska [16] has suggested © 2006 by Taylor & Francis Group, LLC Q dl (a) (b) C diff R diff
142 Corrosion Control Through Organic Coatings that for a coating to provide corrosion protection to steel, it should have an initial impedance of at least 10 8 /cm 2 , a value also suggested by others [15], and that after aging, the impedance should have decreased by no more than three orders of magnitude. Sekine has reported blistering when the coating resistance falls below 10 6 /cm 2 , regardless of coating thickness [17-19]. For more in-depth reviews of the fundamental concepts and models used in EIS to predict coating performance, the reader is directed to the research of Kendig and Scully [20] and Walter [21-23]. 8.2.4 SCANNING KELVIN PROBE The scanning Kelvin probe (SKP) provides a measure of the Volta potential (work function) that is related to the corrosion potential of the metal, without touching the corroding surface [24]. The technique can give a corrosion potential distribution, with a spatial resolution of 50 to 100 µm, below highly isolating polymer films. The SKP is an excellent research tool to study the initiation of corrosion at the metal/polymer interface. Figures 8.4 and 8.5 show the Volta potential distribution for a coil-coated sample before and after 5 weeks of weakly accelerated field testing [25]. In the “after” SCI −293 mV −510 mV 100 mV −293 −365 −438 −510 (mV) 28.00 mm i 5 p/mm R612 10.00 mm 2 p/mm FIGURE 8.4 Volta distribution (mV) of coated steel before exposure. Source: Forsgren, A. and Thierry, D., SCI Rapport 2001:4E. Swedish Corrosion Institute (SCI), Stockholm, 2001. Photo courtesy of SCI. © 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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Blast Cleaning and Other Heavy Surf
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5 Abrasive Blasting and Heavy-Metal
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Abrasive Blasting and Heavy-Metal C
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