© 2006 by Taylor & Francis Group, LLC

80 Corrosion Control Through Organic Coatings
Other methods that are currently being developed for detecting oils include [22]:
• Iodine with the Bresle patch. Sampling is performed according to the
Bresle method (blister patch and hypodermic), but with different leaching
liquids. The test surface is first prepared with an aqueous solution
of iodine and then washed with distilled water. Extraction of the dissolved
iodine in oil on the surface is thereafter made by the aid of a
potassium iodide solution. After extraction of the initially absorbed
iodine from the contaminated surface, starch is added to the potassium
iodide solution. Assessment of the amount of iodine extracted from the
surface is then determined from the degree of blue coloring of the
solution. Because the extracted amount of iodine is a measure of the
amount of oil residues on the surface, the concentration of the oil on
the surface can be determined.
• Fingerprint tracing method. Solid sorbent of aluminum oxide powder
is spread over the test surface. After heat treatment, the excess of sorbent
not strongly attached to the contaminated surface is removed. The amount
of attached sorbent is thereafter scraped off the surface and weighed. This
amount of sorbent is a measure of the amount of oil or grease residues
on the surface.
• Sulfuric acid method. For extraction of oil and grease residues from the
surface, a solid sorbent aluminum oxide is used here, too. However,
concentrated sulfuric acid is added to the aluminum oxide powder that is
scraped off from the contaminated surface. The sulfuric acid solution with
the extracted oil and grease residues is then heated. From the coloring of
the solution, which varies from colorless to dark brown, the amount of
oil and grease residues can be determined.
4.5.3 DUST
Dust comes from the abrasive used in blasting. All blasting abrasives break down
to some extent when they impact the surface being cleaned. Larger particles fall to
the floor, but the smallest particles form a dust too fine to be seen. These particles
are held on the surface by static electricity and, if not removed before painting,
prevent the coating from obtaining good adhesion to the substrate.
Examining the surface for dust is straightforward: wipe the surface with a clean
cloth. If the cloth comes away dirty, then the surface is too contaminated to be
painted. Another method is to apply tape to the surface to be coated. If the tape,
when pulled off, has an excessive amount of fine particles attached to the sticky
side, then the surface is contaminated by dust. It is a judgment call to say whether
a surface is too contaminated because, for all practical purposes, it is impossible to
remove all dust after conventional abrasive blasting.
Testing for dust should be done at every step of the paint process because
contamination can easily occur after a coating layer has been applied, causing the
paint to become tack-free. This would prevent good adhesion of the next coating
layer.
© 2006 by Taylor & Francis Group, LLC