© 2006 by Taylor & Francis Group, LLC

More documents

Recommendations

Info

Composition of the Anticorrosion Coating 27 2.3 CORROSION-PROTECTIVE PIGMENTS 2.3.1 TYPES OF PIGMENTS Pigments come in three major types: inhibitive, sacrificial, and barrier. Coatings utilizing inhibitive pigments release a soluble species, such as molybdates or phosphates, from the pigment into any water that penetrates the coating. These species are carried to the metal surface, where they inhibit corrosion by encouraging the growth of protective surface layers [22]. Solubility and reactivity are critical parameters for inhibitive pigments; a great deal of research is occupied with controlling the former and decreasing the latter. Sacrificial pigments require zinc in large enough quantities to allow the flow of electric current. When in electrical contact with the steel surface, the zinc film acts as the anode of a large corrosion cell and protects the steel cathode. Both inhibitive and sacrificial pigments are effective only in the layer immediately adjacent to the steel (i.e., the primer). Barrier coatings are probably the oldest type of coating [22] and the requirements of their pigments are completely different. Specifically, chemical inertness and a flake- or plate-like shape are the requirements of barrier pigments. Unlike inhibitive or sacrificial coatings, barrier coatings can be used as primer, intermediate coat, or topcoat because their pigments do not react with metal. 2.3.1.1 A Note on Pigment Safety The toxicity of lead, chromium, cadmium, and barium has made the continued use of paints containing these elements highly undesirable. The health and environmental problems associated with these heavy metals are serious, and new problems are discovered all the time. To address this issue, pigment manufacturers have developed many alternative pigments, such as zinc phosphates, calcium ferrites, and aluminum triphosphates, to name a few. The number of proposed alternatives is not lacking; in fact, the number and types available are nearly overwhelming. This chapter explores the major classes of pigments currently available for anticorrosion coating. The alert reader will quickly note that lead and barium are described here, although use of these elements can no longer be recommended. This discussion is included for two reasons. First, the protective mechanism of red lead is highly relevant to evaluating new pigments because new pigments are inevitably compared to lead. Second, the toxicity of soluble barium is less widely known than the toxicities of lead, chromium, and cadmium; therefore, barium is included here to point out that it should be avoided. 2.3.2 LEAD-BASED PAINT The inhibitive mechanism of the red lead found in lead-based paint (LBP) is complex. Lead pigments may be thought of as indirect inhibitors because, although they themselves are not inhibitive, they undergo a reaction with select resin systems and this reaction can form by-products that are active inhibitors [23]. © 2006 by Taylor & Francis Group, LLC
28 Corrosion Control Through Organic Coatings 2.3.2.1 Mechanism on Clean (New) Steel Appleby and Mayne [24,25] have shown that formation of lead soaps is the mechanism used for protecting clean (or new) steel. When formulated with linseed oil, lead reacts with components of the oil to form soaps in the dry film; these soaps degrade to, among other things, the water-soluble salts lead of a variety of mono- and di-basic aliphatic acids [26,27]. Mayne and van Rooyen also showed that the lead salts of azelaic, suberic, and pelargonic acid were inhibitors of the iron corrosion. Appleby and Mayne have suggested that these acids inhibit corrosion by bringing about the formation of insoluble ferric salts, which reinforce the air-formed oxide film until it becomes impermeable to ferrous ions. This finding was based on experiments in which pure iron was immersed in a lead azelate solution, with the thickness of the oxide film measured before and after immersion. They found that the oxide film increased 7% to 17% in thickness upon immersion [25,28]. The lead salt of azelaic acid dissociates in water into a lead ion and an azelate ion. To determine which element was the key in corrosion inhibition, Appleby and Mayne also repeated the experiment with calcium azelate and sodium azelate [24,132]. Interestingly, they did not see a similar thickening of the oxide film when iron was immersed in calcium azelate and sodium azelate solutions, demonstrating that lead itself — not just the organic acid — plays a role in protecting the iron. The authors note that 5 to 20 ppm lead azelate in water is enough to prevent attack of pure iron immersed in the solution. They note that, at this low concentration, inhibition cannot be caused by the repair of the air-formed oxide film by the formation of a complex azelate, as is the case in more concentrated solutions; rather, it appears to be associated with the thickening of the air-formed oxide film. ‘‘It seems possible that, initially, lead ions in solution may provide an alternative cathodic reaction to oxygen reduction, and then, on being reduced to metallic lead at the cathodic areas on the iron surface, depolarize the oxygen reduction reaction, thus keeping the current density sufficiently high to maintain ferric film formation. In addition any hydrogen peroxide so produced may assist in keeping the iron ions in the oxide film in the ferric condition, so that thickening of the air-formed film takes place until it becomes impervious to iron ions” [25]. 2.3.2.2 Mechanism on Rusted Steel Protecting rusted steel, rather than clean or new steel, may demand of a paint a different corrosion mechanism, simply because the paint is not applied directly to the steel that must be protected but rather to the rust on top of it. Inhibitive pigments in the paint that require intimate contact with the metallic surface in order to protect it may therefore not perform well when a layer of rust prevents that immediate contact. Red-lead paint, however, does perform well on rusted steel. Several theories about the protective mechanism of red-lead paint on rusted steel exist. 2.3.2.2.1 Rust Impregnation Theory According to this theory, the low viscosity of the vehicle used in LBP allows it to penetrate the surface texture of rust. This would have several advantages: • Impregnation of the rust means that it is isolated and thereby inhibited in its corroding effect. © 2006 by Taylor & Francis Group, LLC
Page 1 and 2: © 2006 by Taylor & Francis Group,
Page 3 and 4: Corrosion of Ceramic and Composite
Page 5 and 6: Published in 2006 by CRC Press Tayl
Page 7 and 8: Preface This book has been written
Page 9 and 10: Author Amy Forsgren received her ch
Page 11 and 12: Contents Chapter 1 Introduction ...
Page 13 and 14: 2.4.2 Reactive Reagents ...........
Page 15 and 16: 6.2.4.1 Alkaline Blistering........
Page 17 and 18: 1 Introduction This book is not abo
Page 19 and 20: Introduction 3 • A dissolution pr
Page 21 and 22: Introduction 5 1.2.2 ELECTROLYTIC R
Page 23 and 24: Introduction 7 to the metal is a ne
Page 25 and 26: Introduction 9 9. Thomas. N.L., Pro
Page 27 and 28: 12 Corrosion Control Through Organi
Page 41: 26 Corrosion Control Through Organi
Page 81 and 82: 4 Blast Cleaning and Other Heavy Su
Page 83 and 84: Blast Cleaning and Other Heavy Surf
Page 93 and 94:
Blast Cleaning and Other Heavy Surf
Page 95 and 96:
Page 97 and 98:
Page 99 and 100:
5 Abrasive Blasting and Heavy-Metal
Page 101 and 102:
Abrasive Blasting and Heavy-Metal C
Page 103 and 104:
Page 105 and 106:
Page 107 and 108:
Page 109 and 110:
Page 111 and 112:
Page 113 and 114:
6 Weathering and Aging of Paint Thi
Page 115 and 116:
Weathering and Aging of Paint 101 c
Page 117 and 118:
Weathering and Aging of Paint 103 I
Page 119 and 120:
Weathering and Aging of Paint 105 c
Page 121 and 122:
Weathering and Aging of Paint 107 S
Page 123 and 124:
Weathering and Aging of Paint 109 e
Page 125 and 126:
Weathering and Aging of Paint 111 I
Page 127 and 128:
7 Corrosion Testing — Background
Page 129 and 130:
Corrosion Testing — Background an
Page 131 and 132:
Page 133 and 134:
Page 135 and 136:
Page 137 and 138:
Page 139 and 140:
Page 141 and 142:
Page 143 and 144:
130 Corrosion Control Through Organ
Page 145 and 146:
Page 147 and 148:
Page 149 and 150:
Page 151 and 152:
Page 153 and 154:
Page 155 and 156:
Page 157 and 158:
Page 159 and 160:
Page 161 and 162:
Page 163 and 164:
Page 165 and 166:
Page 167:
show all

© 2006 by Taylor & Francis Group, LLC

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?