© 2006 by Taylor & Francis Group, LLC

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