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118 Corrosion Control Through Organic Coatings thicknesses were tested at R dry = 0, 50, and 93.8%. For all four substrates, the highest amount of steel weight loss was seen at R dry = 50%. In summary, corrosion on both steel and zinc-coated steel substrates is slower if no drying occurs. This finding seems reasonable because, as the electrolyte layer becomes thinner while drying, the amount of oxygen transported to the metal surface increases, enabling more active corrosion [11, 12]. A similar highly active phase can be expected to occur during rewetting under cyclic conditions. Readers interested in a deeper understanding of this process may find the works of Suga [13] and Boocock [14] particularly helpful. 7.2.3.2 Zinc Corrosion — Atmospheric Exposure vs. Wet Conditions A drying cycle is an absolute must if zinc is involved either as pigment or as a coating on the substrate. The corrosion mechanism that zinc undergoes in constant humidity is quite different from that observed when there is a drying period. In field service, alternating wet and dry periods is the rule. Under these conditions, zinc can offer extremely good real-life corrosion protection — but this would never be seen in the laboratory if only constant wetness is used in the accelerated testing. This apparent contradiction is worth exploring in some depth. Although this is a book about paints, not metallic corrosion, it becomes necessary at this point to devote some attention to the corrosion mechanisms of zinc in dry versus wet conditions. The reason for this is simple: zinc-coated steel is an important material for corrosion prevention, and it is frequently painted. Accelerated tests are therefore used on painted, zinc-coated steel. In order to obtain any useful information from accelerated testing, it is necessary to understand the chemistry of zinc in dry and wet conditions. In normal atmospheric conditions, zinc reacts with oxygen to form a thin oxide layer. This oxide layer in turn reacts with water in the air to form zinc hydroxide (Zn[OH] 2), which in turn reacts with carbon dioxide in air to form a layer of basic zinc carbonate [15-17]. Zinc carbonate serves as a passive layer, effectively protecting the zinc underneath from further reaction with water and reducing the amount of corrosion. When zinc-coated steel is painted and then scribed to the steel, the galvanic properties of the zinc-steel system determine whether, and how much, corrosion will take place under the coating. Two mechanisms cause the growth of red rust and undercutting from the scribe [1, 6, 18-21]: 1. The first reaction is a galvanic cell located at the scribe. The anode is the metal exposed in the scribe, and the cathode is the adjacent zinc layer under the paint. 2. The second reaction is located not at the scribe but rather at the leading edge of the zinc corrosion front. Anodic dissolution of zinc occurs from the top of the zinc layer and works downward to the steel. Ito and colleagues have postulated that the magnitudes and the comparative ratio of these two mechanisms changes with the amount of water available. When they repeated their experiments with R dry on painted, cold-rolled and galvanized steels, © 2006 by Taylor & Francis Group, LLC
Corrosion Testing — Background and Theoretical Considerations 119 ln (D) 2 1 0 0 20 40 60 80 100 −1 −2 Rdry CRS, 0 Zn EGS, 20 Zn EGS, 40 Zn HDG, 90 Zn FIGURE 7.2 Natural log of underfilm corrosion, as a function of drying ratio for cold-rolled steel, electrogalvanized (20 g/m 2 Zn and 40 g/m 2 Zn), and hot-dipped galvanized (90 g/m 2 Zn). Data from: Ito, Y., Hayashi, K., and Miyoshi, Y., Iron Steel J., 77, 280, 1991. an interesting pattern emerged. Instead of measuring weight of metal lost, they measured the distance of underfilm corrosion from the scribe. In Figure 7.2, the natural logarithm of the length of underfilm corrosion D, measured by Ito and colleagues, is plotted against the R dry for each of the four coating weights. The relationship between zinc coating thickness, drying ratio, and underfilm corrosion distance is fairly distinct when presented thus. Ito and colleagues have also proposed that under wet conditions, (i.e., low R dry), more underfilm corrosion is seen on zinc-coated steel than on cold-rolled steel because the following two reactions at the boundary between paint and zinc layer dominate the corrosion: 1. Zinc dissolves anodically at the front end of corrosion. 2. In the blister area behind the front end of corrosion, zinc at the top of the zinc layer dissolves due to OH, which is generated by cathodic reaction. However, if conditions include high R dry, then underfilm corrosion is less on galvanized steel than on cold-rolled steel, for the following reasons: 1. The total supply of water and chloride (Cl – ) is reduced, limiting cell size at front end and zinc anodic dissolution area. 2. The electrochemical cell at the scribe is reduced. 3. Zinc is isolated from the wet corrosive environment fairly early. A protective film can form on zinc in dry atmosphere. The rate of zinc corrosion is suppressed in further cycling. 4. The zinc anodic dissolution rate is reduced because the Cl – concentration at the front end is suppressed. © 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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Page 81 and 82: 4 Blast Cleaning and Other Heavy Su
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