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86 Corrosion Control Through Organic Coatings 5.1.1 CHEMICAL ANALYSIS TECHNIQUES FOR HEAVY METALS Several techniques are available for determining whether or not toxic metals, such as lead and chromium, exist in paint. Some well-established methods, particularly for lead, are atomic absorption (AA) and inductively coupled plasma atomic emission spectroscopy (ICP or ICP-AES). Energy-dispersive x-ray in conjunction with scanning electron microscopy (EDX-SEM) is a somewhat newer technique. In the AA and ICP-AES methods, paint chips are dissolved by acid digestion. The amount of heavy metals in the liquid is then measured by AA or ICP-AES analysis. The amount of lead, cadmium, and other heavy metals can be calculated — with a high degree of accuracy — as a total weight percent of the paint. A very powerful advantage of this technique is that it can be used to analyze an entire coating system, without the need to separate and study each layer. Also, because the entire coating layer is dissolved in the acid solution, this method is unaffected by stratification of heavy metals throughout the layer. That is, there is no need to worry about whether the lead is contained mostly in the bulk of the layer, at the coatingmetal interface, or at the topmost surface. EDX-SEM can be used to analyze paint chips quickly. The technique is only semiquantitative: it is very capable of identifying whether the metals of interest are present but is ineffective at determining precisely how much is present. Elements from boron and heavier can be detected. EDX-SEM examines only the surface of a paint chip, to a depth of approximately 5 µm. This is a drawback because the surface usually consists of only binder. It may be possible to use very fine sandpaper to remove the top layer of polymer from the paint; however, this would have to be done very carefully so as not to sand away the entire paint layer. Of course, if the coating has aged a great deal and is chalking, then the topmost polymer layer is already gone. Therefore, analyzing cross-sections of paint chips is unnecessary in many cases, particularly for systems with two or more coats. Because coatings are not homogeneous, several measurements should be taken. 5.1.2 TOXICITY CHARACTERISTIC LEACHING PROCEDURE Toxicity characteristic leaching procedure (TCLP) is the method mandated by the U.S, Environmental Protection Agency (EPA) for determining how much toxic material is likely to leach out of solid wastes. A short description of the TCLP method is provided here. For an exact description of the process, the reader should study Method 1311 in EPA Publication SW-846 [5]. In TCLP, a 100g sample of debris is crushed until the entire sample passes through a 9.5 mm standard sieve. Then 5 g of the crushed sample are taken to determine which extraction fluid will be used. Deionized water is added to the 5g sample to make 100 ml of solution. The liquid is stirred for 5 minutes. After that time, the pH is measured. The pH determines which extraction fluid will be used in subsequent steps, as shown in Table 5.1. The procedure for making the extraction fluids is shown in Table 5.2. The debris sample and the extraction fluid are combined and placed in a special holder. The holder is rotated at 30 ± 2 RPM for 18 ± 2 hours. The temperature is maintained at 23 ± 2°C during this time. © 2006 by Taylor & Francis Group, LLC
Abrasive Blasting and Heavy-Metal Contamination 87 TABLE 5.1 pH Measurement to Determine TCLP Extraction Fluid If the first pH measurement is: …then < 5.0 Extraction fluid #1 is used. > 5.0 Acid is added. The solution is heated and then allowed to cool. Once the solution cools, pH is measured again (see below). If the second pH measurement is: …then < 5.0 Extraction Fluid #1 is used. > 5.0 Extraction Fluid #2 is used. The liquid is then filtered and analyzed. Analysis for lead and heavy metals is done with AA or ICP-AES. TCLP is an established procedure, but more knowledge about the chemistry involved in spent abrasive disposal is still needed. Drozdz and colleagues have reported that, in the TCLP procedure, the concentrations of lead in basic lead silico chromate are suppressed below the detection limit if zinc potassium chromate is also present. The measured levels of chromium are also suppressed, although not below the detection limit. They attribute this reduction to a reaction between the two pigments that produces a less-soluble compound or complex of lead [6]. 5.2 MINIMIZING THE VOLUME OF HAZARDOUS DEBRIS In chapter 4, we mentioned that choosing an abrasive that could be recycled several times could minimize the amount of spent abrasive. The methods described here attempt to further reduce the amount that must be treated as hazardous debris by TABLE 5.2 Extraction Fluids for TCLP Procedure Extraction Fluid #1 Extraction Fluid #2 Step 1 5.7 ml glacial acetic acid is added 5.7 ml glacial acetic acid is added to water (water to 500 ml water. volume < 990 ml). Step 2 64.3 ml sodium hydroxide is added. Water is added until the volume is 1 L. Step 3 Water is added until the volume is 1 L. Final pH 4.93 ± 0.05 2.88 ± 0.05 Note: Water used is ASTM D-1193 Type II. © 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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