© 2006 by Taylor & Francis Group, LLC

More documents

Recommendations

Info

104 Corrosion Control Through Organic Coatings coating in the dissolved state. Diffusion is how mobile the water molecules are in the coating [15]. The permeability coefficient, P, is the product of the diffusion coefficient, D, and the solubility, S [16]: P = D × S In accelerated testing, the difference in absorption and desorption rates of water for various coatings is also important (see Chapter 7). The uptake of water affects the coating in several ways [17]: • Chemical breakdown • Weathering interactions • Hygroscopic stress • Blistering/adhesion loss 6.2.1 CHEMICAL BREAKDOWN Water is an excellent solvent for atmospheric contaminants, such as salts, sulfites, and sulphates. Airborne contaminants would probably never harm coated metals, if not for the fact that they so easily become Cl − or SO 4 2− ions in water. The water and ions, of course, fuel corrosion beneath the coating. Water can also be a solvent for some of the additives in the paint, causing them to dissolve or leach out of the cured film. And finally, it can act as a plasticizer in the polymeric network, softening it and making it more vulnerable to mechanical damages. Lefebvre and colleagues [18], working with epoxy films, have proposed that each coating had a critical RH. Above the critical RH, water condensed on the OH groups of the polymer, breaking interchain hydrogen bonds and displacing adsorbed OH groups from the substrate surface. The loss of adhesion resulting from this was reversible. However, an irreversible effect was the reaction of the water with residual oxirane rings in the coating to form diols. This led to an irreversible increase in solubility and swelling of the film. 6.2.2 WEATHERING INTERACTIONS As previously noted, the major weathering stresses interact with each other. Perera and colleagues have shown that temperature effects are inseparable from the effects of water [19, 20]. The same is even more true for chemical effects (see Section 6.4). The effects of UV degradation can be worsened by the presence of moisture in the film [1]. As a binder breaks down due to UV radiation, water-soluble binder fragments can be created. These dissolve when the film takes up water, are removed from the film upon drying, and add to the decrease in film density or thickness. 6.2.3 HYGROSCOPIC STRESS This section focuses on the changes in the coating’s internal stresses — both tensile and compressive — caused by wetting and drying the coating. As a coating takes up water, it swells, causing compressive stresses in the film. As the coating dries, it © 2006 by Taylor & Francis Group, LLC
Weathering and Aging of Paint 105 contracts, causing tensile stress. These compression and tension forces have adverse effects on the film’s cohesive integrity and on its adhesion to the substrate. Of the two types of stresses, the tensile stresses formed as the coating dries have the greater effect [9, 11, 21]. Coating stress is a dynamic phenomenon; it changes drastically during water uptake and desorption. Sato and Inoue [22] have reported that the initial tensile stresses (left over from shrinkage during film formation) of the dry film decrease to zero as moisture is absorbed. Once the initial tensile stresses have been negated by water uptake, further uptake leads to build-up of compressive stresses. If the film is dried, tensile (shrinkage) stresses redevelop, but to a lower degree than originally seen. Some degree of permanent creep was seen in Sato and Inoue’s study; it was attributed to breaking and reforming valency associations in the epoxy polymer. The same trend of initial tensile stress reduction, followed by compressive stress buildup was seen by Perera and Vanden Eynde [23] with a polyurethane and a thermoplastic latex coating. Hygroscopic stresses are interrelated with ambient temperature [11, 20]. They also depend heavily on the glass transition temperature (T g) of the coating [24]. In immersion studies, Perera and Vanden Eynde examined the stress of an epoxy coating whose T g was near — even below — the ambient temperature [25]. The films in question initially had tensile stress from the film formation. Upon immersion, this stress gradually disappeared. As in the previously cited studies, compressive stresses built up. The difference was that these stresses then dissipated over several days even though immersion continued. Hare also noted dissipation of compressive stresses as the difference between T ambient and T g is reduced; he attributes it to a reduced modulus and a flexibilizing of the film [11]. Because of the low T g of the film, stress relaxation occurred and the compressive stresses due to water uptake disappeared. Hygroscopic stresses have a very real effect on coating performance. If a coating forms high levels of internal stress during cure — not uncommon in thick, highly crosslinked coatings — then applying other stresses during water uptake or desorption can lead to cracking or delamination. Hare has reported another problem: cases where the film expansion during water uptake created a strain beyond the film’s yield point. Deformation here is irreversible; during drying, permanent wrinkles are left in the dried paint [17]. Perera has pointed out that hygroscopic stress can be critical to designing accelerated tests for coatings. For example, a highly crosslinked coating can undergo more damage in the few hours it dries after the salt-spray test has ended than it did in the entire time (hundreds of hours) of the test itself [26]. 6.2.4 BLISTERING/ADHESION LOSS Blistering is not, strictly speaking, brought about by aging of the coating. It would be more correct to say that blistering is a sign of failure of the coating-substrate system. Blistering occurs when moisture penetrates through the film and accumulates at the coating-metal interface in sufficient numbers to force the film up from the metal substrate. The two types of blistering in anticorrosion paints — alkaline and neutral — are caused by different mechanisms. © 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 29 and 30:
Page 31 and 32:
Page 33 and 34:
Page 35 and 36:
Page 37 and 38:
Page 39 and 40:
Page 41 and 42:
Page 43 and 44:
Page 45 and 46:
Page 47 and 48:
Page 49 and 50:
Page 51 and 52:
Page 53 and 54:
Page 55 and 56:
Page 57 and 58:
Page 59 and 60:
Page 61 and 62:
Page 63 and 64:
Page 65 and 66:
Page 67 and 68: 52 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: Weathering and Aging of Paint 103 I
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 143 and 144: 130 Corrosion Control Through Organ
Page 167: 154 Corrosion Control Through Organ
show all

© 2006 by Taylor & Francis Group, LLC

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

Delete template?

Save as template?