© 2006 by Taylor & Francis Group, LLC

More documents

Recommendations

Info

Composition of the Anticorrosion Coating 47 There are two types of aluminum pigment: leafing and nonleafing. Leafing pigment orients itself parallel to the substrate at the top of the coating; this positioning enables the pigment to protect the binder against UV damage but may not be the best location for maximizing barrier properties. Leafing properties depend on the presence of a thin fatty acid layer, commonly stearic acid, on the flakes. Nonleafing aluminum pigments have a more random orientation in the coating and are very effective in barrier coatings [109]. De and colleagues, for example, have obtained favorable results with aluminum in a chlorinated rubber vehicle in seawater trials in India [126]. 2.3.8.4.2 Zinc Flakes Zinc flakes should not to be confused with the zinc dust used in zinc-rich coatings: the size is of a different magnitude altogether. Some research suggests that zinc flakes could give both the cathodic protection typical of zinc dust and the barrier protection characteristic of lamellar pigments [109]. However, in practice, this could be very difficult to achieve because the zinc dust particles in zinc-rich paints have to be in electrical contact to obtain cathodic protection. Designing a coating in which the zinc particles are in intimate contact with each other and with the steel, and yet completely free of gaps between pigment and binder or between pigment particles, is difficult. The lack of any gaps is critical for a barrier pigment, because it is precisely these gaps that provide the easy route for water and oxygen to reach the metal surface. In fact, Hare and Wright’s [127] research shows that zinc flakes undergo rapid dissolution in corrosive environments when they are used as the sole pigment in paints; their coatings are prone to blistering. 2.3.8.4.3 Other Metallic Pigments Other metallic pigments, such as stainless steel, nickel, and copper, have also been used in recent years. Their use in coatings of metals with more noble electrochemical potential than carbon steel entails a certain risk of galvanic corrosion between the coating and the substrate. The pigment volume concentrations in such paints must be kept well below the levels at which the metallic pigment particles are in electrical contact with each other and the carbon steel. If this condition is not met, pitting follows. Bieganska recommends using a nonconducting primer as an insulating layer between the steel substrate and the barrier coating, if it is necessary to use a strong electropositive pigment in the barrier layer [109]. The same author also warns that, although the mechanical durability and high-temperature resistance of stainless steel flake makes this type of pigment desirable, it is not suited to applications where chlorides are present [109]. Nickel flake-filled coatings can be useful for strongly alkaline environments. Cupronickel flakes (Cu – 10% Ni – 2% Sn) are used in ship protection because of their outstanding antifouling properties. The alloy pigment is of interest in this application because its resistance to leaching is better than that of copper itself [109]. 2.3.9 CHOOSING A PIGMENT Before choosing a pigment and formulating paint, one question must be answered: will an active or a passive role be required of the pigment? The role of the pigment — active or passive — must be decided at the start for the fairly straightforward © 2006 by Taylor & Francis Group, LLC
48 Corrosion Control Through Organic Coatings reason that only one or the other is possible. Many of the pigments that actively inhibit corrosion, such as through passivation, must dissolve into anions and cations; ion species can then passivate the metal surface. Without water, these pigments do not dissolve and the protection mechanism is not triggered. And, of course, it is the express purpose of barrier coatings to prevent water from reaching the coating-metal interface. Once the role of the pigment has been decided, choice of pigment depends on such factors as: • Price. Many of the newer pigments are expensive. The amounts necessary in a coating, and the respective impact on price, plays a large role in determining whether the pigment is economically feasible. • Commercial availability. Producing a few hundred grams of a pigment in a laboratory is one thing; however, it is quite another to generate a pigment in hundreds of kilograms for commercial paints. • Difficulty of blending into a real formulation. Pigments must do more than just protect steel. They have to disperse in the wet paint, rather than stay clumped together. They also have to be well attached to the binder so that water cannot penetrate through the coating via gaps between pigment particles and the binder. In many cases, the surfaces of pigments are chemically treated to avoid these problems; however, it must be possible to treat pigments without changing their essential properties (solubility, etc.). • Suitability in the binders that are of interest. A coating does not, of course, consist merely of a pigment; the binder is of equal importance in determining the success of a paint. The pigments chosen for further study must be compatible with the binders of interest. • Resistance to heat, acids or alkalis, and/or solvents, as needed. 2.4 ADDITIVES For corrosion-protective purposes, the most important components of a coating are the binder and the anticorrosion pigment. Additives are necessary for the manufacture, application, and cure of a coating; however, with the exception of corrosion inhibitors, they play a relatively minor role in corrosion protection. This section presents a brief overview of some of the additives found in modern anticorrosion coatings. The field of coating composition is too complex to be covered in any depth in the following sections and, in any case, numerous texts devoted to the science — or art — of coating formulation already exist. 2.4.1 FLOW AND DISPERSION CONTROLLERS Flow and dispersion controllers are used to control the behavior of the wet paint, either in the paint can during mixing and application or between application and cure. This group of additives includes thixotropic agents, surfactants, dispersants, and antiflooding/antifloating agents. Thixotropic agents and surfactants are the most important of the flow and dispersion controllers. © 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 61: 46 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 99 and 100: 5 Abrasive Blasting and Heavy-Metal
Page 101 and 102: Abrasive Blasting and Heavy-Metal C
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

Create successful ePaper yourself

Delete template?

Save as template?