© 2006 by Taylor & Francis Group, LLC

More documents

Recommendations

Info

Composition of the Anticorrosion Coating 41 basic magnesium chromate, calcium chromate, and ammonium dichromate; however, because they are used to a much lesser extent, they are not discussed here. Zinc potassium chromate is the product of inhibitive reactions among potassium dichromate, zinc oxide, and sulfuric acid. Zinc chromates are effective inhibitors even at relatively low loading levels [23]. Strontium chromate, the most expensive chromate inhibitor, is mainly used on aluminium. It is used in the aviation and coil-coating industries because of its effectiveness at very low loadings. Zinc tetroxychromate, or basic zinc chromate, is commonly used in the manufacture of two-package polyvinyl butyryl (PVB) wash primers. These consist of phosphoric acid and zinc tetroxychromate dispersed in a solution of PVB in alcohol. These etch primers, as they are known, are used to passivate steel, galvanized steel, and aluminium surfaces, improving the adhesion of subsequent coatings. They tend to be low in solids and are applied at fairly low film thicknesses [23]. 2.3.6.3 Solubility Concerns The ability of a chromate pigment to protect a metal lies in its ability to dissolve and release chromate ions. Controlling the solubility of the pigment is critical for chromates. If the solubility is too high, other coating properties, such as blister formation, are adversely affected. A coating that uses a highly soluble chromate pigment under long-term moisture conditions can act as a semipermeable membrane: with water on one side (at the top of the coating) and a saturated solution of aqueous pigment extract on the other (at the steel-coating interface). Significant osmotic forces thus lead to blister formation [90]. Chromate pigments are therefore not suitable for use in immersion conditions or conditions with long periods of condensation or other moisture exposure. 2.3.7 OTHER INHIBITIVE PIGMENTS Other types of inhibitive pigments include calcium-exchanged silica, barium metaborate, molybdates, and silicates. 2.3.7.1 Calcium-Exchanged Silica Calcium-exchanged silica is prepared by ion-exchanging an anticorrosion cation, calcium, onto the surface of a porous inorganic oxide of silica. The protection mechanism is ion-exchange: aggressive cations (e.g., H + ) are preferentially exchanged onto the pigment’s matrix as they permeate the coating, while Ca 2+ ions are simultaneously released to protect the metal. Calcium does not itself passivate the metal or otherwise directly inhibit corrosion. Instead, it acts as a flocculating agent. The small amounts (circa 120 µm /ml H 2O at pH ≈ 9) of silica in solution flocculate around the Ca 2+ ion. The Ca–Si species has a small δ+ or δ− charge, which drives it toward the metal surface (due to the potential drop across the metal/solution interface). Particles of silica and calcium agglomerate at the paint/metal interface. There the alkaline pH causes spontaneous coalescing into a thin film of silica © 2006 by Taylor & Francis Group, LLC
42 Corrosion Control Through Organic Coatings and calcium [93]. The major benefit of this inorganic film seems to be that it prevents Cl − and other corrosion-initiating species from reaching the metal surface. The dual action of entrapment of aggressive cations and release of inhibitor gives calcium-exchange silica two advantages over traditional anticorrosion pigments: 1. The “inhibitor” ion is only released in the presence of aggressive cations, which means that no excess of the pigment to allow for solubility is necessary. 2. No voids are created in the film by the ion-exchange; the coating has fairly constant permeability characteristics [38,93–95]. 2.3.7.2 Barium Metaborate Barium metaborate is a pigment to avoid. It contains a high level of soluble barium, an acute toxicant. Disposal of any waste containing this pigment is likely to be expensive, whether that waste is produced in the manufacture or application of the coatings or much later when preparing to repaint structures originally coated with barium metaborate. Barium metaborate creates an alkaline environment, inhibiting the steel; the metaborate ion also provides anodic passivation [23]. The pigment requires high loading levels, up to 40% of coating weight, according to Beland. It is highly soluble and fairly reactive with several kinds of binders; this leads to stability problems when formulated with such products as acidic resins, high-acid number resins, and acid-catalyzed baking systems. A modified silica coating is often used to reduce and control solubility. One way to decrease its reactivity and, therefore, increase the number of binders with which it can be used, is to modify it with zinc oxide or a combination of zinc oxide and calcium sulphate [23]. The high loading level required for heavy-duty applications implies that careful attention must be paid to the PVC/CPVC ratio when formulating with this pigment. Information regarding actual service performance of barium metaborate coatings is scarce, and what does exist does not seem to justify the use of this pigment. In the early 1980s, the state of Massachusetts repair-painted a bridge with barium metaborate pigment in a conventional oil/alkyd vehicle. The result was not satisfactory: after six years, considerable corrosion had occurred at the beam ends and on the railings above the road [22]. It should perhaps be noted that an alkyd vehicle is not the ideal choice for a pigment that generates an alkaline environment; better results may perhaps have been obtained with a higher-performance binder. However, because of the toxicity problems associated with soluble barium, further work with barium metaborate does not seem to be warranted. 2.3.7.3 Molybdates Molybdate pigments are calcium or zinc salts precipitated onto an inert core such as calcium carbonate [47,96–98]. They prevent corrosion by inhibiting the anodic corrosion reaction [47]. The protective layer of ferric molybdate, which these pigments form on the surface of the steel, is insoluble in neutral and basic solutions. © 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 55: 40 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 107 and 108:
Abrasive Blasting and Heavy-Metal C
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?