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Corrosion Testing — Practice 135 8.2.1 GENERAL CORROSION Direct evidence of corrosion can be obtained by macroscopic measurement of creep from scribe, rust intensity, blistering, cracking, and flaking. 8.2.1.1 Creep from Scribe If a coating is properly applied to a well-prepared surface and allowed to cure, then general corrosion across the intact paint surface is not usually a major concern. However, once the coating is scratched and metal is exposed, the situation is dramatically different. The metal in the center of the scratch has the best access to oxygen and becomes cathodic. Anodes arise at the sides of the scratch, where paint, metal, and electrolyte meet [9]. Corrosion begins here and can spread outward from the scratch under the coating. The coating’s ability to resist this spread of corrosion is a major concern. Corrosion that begins in a scratch and spreads under the paint is called creep or undercutting. Creep is surprisingly difficult to quantify, because it is seldom uniform. Several methods are acceptable for measuring it, for example: • Maximum one-way creep (probably the most common method), which is used in several standards, such as ASTM S1654 • Summation of creep at ten evenly spaced sites along the scribe • Average two-way creep None of these methods is satisfactory for describing filiform corrosion. The maximum one-way creep and the average two-way creep methods allow measurement of two values: general creep and filiform creep. 8.2.1.2 Other General Corrosion Blistering, rust intensity, cracking, and flaking are judged in accordance with the standard ISO 4628 or the comparable standard ASTM D610. In these methods, the samples to be evaluated are compared to a set of standard photographs showing various degrees of each type of failure. For face blistering, the pictures in the ISO standard represent blister densities from 2 to 5, with 5 being the highest density. Blister size is also numbered from 2 to 5, with 5 indicating the largest blister. Results are reported as blister density followed in parentheses by blister size (e.g., 4(S2) means blister density = 4 and blister size = 2); this is a way to quantify the result, “many small blisters.” For degree of rusting, the response of interest is rust under the paint, or rust bleed-through. Areas of the paint that are merely discolored on the surface by rusty runoff are not counted if the paint underneath is intact. The scale used by ISO 4628 in assigning degrees of rusting is shown in Table 8.1 [10]. Although the ASTM and ISO standards are comparable in methodology, their grading scales run in opposite directions. In measuring rust intensity or blistering, © 2006 by Taylor & Francis Group, LLC
136 Corrosion Control Through Organic Coatings the ASTM standard uses 10 for defect-free paint and 0 for complete failure. The ISO standard uses 0 for no defects and the highest score for complete failure. These standards have faced some criticisms, mainly the following: • They are too subjective. • They assume an even pattern of corrosion over the surface. Proposals have been made to counter the subjective nature of the tests by, for example, adding grids to the test area and counting each square that has a defect. The assumption of an even pattern of corrosion is questioned on the grounds that corrosion, although severe, can be limited to one region of the sample. Systems have been proposed to more accurately reflect these situations, for example, reporting the percentage of the surface that has corrosion and then grading the corrosion level within the affected (corroded) areas. For more information on this, the reader is directed to Appleman’s review [2]. 8.2.2 ADHESION TABLE 8.1 Degrees of Rusting Degree Area Rusted (%) Ri 0 0 Ri 1 0.05 Ri 2 0.5 Ri 3 1 Ri 4 8 Ri 5 40–50 Source: ISO 4628/3-1982, Designation of degree of rusting, International Organization for Standardization, Geneva, 1982. Many methods are used to measure adhesion of a coating to a substrate. The most commonly used methods belong to one of the following two groups: direct pull-off methods (e.g., ISO 4624) or cross-cut methods (e.g., ISO 2409). The test method must be specified; details of pull-stub geometry and adhesive used in direct pull-off methods are important for comparing results and must be reported. 8.2.2.1 The Difficulty of Measuring Adhesion It is impossible to mechanically separate two well-adhering bodies without deforming them; the fracture energy used to separate them is therefore a function of both the interfacial processes and bulk processes within the materials [11]. In polymers, these bulk processes are commonly a complex blend of plastic and elastic deformation © 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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