© 2006 by Taylor & Francis Group, LLC

Blast Cleaning and Other Heavy Surface Pretreatments 79
rusted steel, the meter cannot distinguish how much of the conductivity is due to
chlorides and how much is due simply to ferrous ions in the test water.
The Bresle method is robust; it can be used on very uneven or curved surfaces.
The technique is easy to perform, and the equipment inexpensive. Its major drawback
is the time it requires; 10 minutes for a test is commonly believed to be too long,
and there is a strong desire for something as robust and reliable — but faster.
The filter-paper technique is much faster. A piece of filter paper is placed on the
surface to be tested, and deionized water is squirted on it until it is saturated. The
wet paper is then placed on an instrument (such as the SCM-400 from Elcometer)
that measures its resistivity. As in the conductivity measurements discussed above,
when this is used for repainting applications, it is not certain how much of the
resistivity of the paper is due to chlorides and how much is due simply to rust in
the test water. In all, the technique is reliable and simple to implement, although
initial equipment costs are rather high.
Neither technique measures all the chlorides present in steel. The Bresle technique
is estimated to have around 50% leaching efficiency; the filter paper technique
is somewhat higher. One could argue, however, that absolute values are of very
limited use; if chlorides are present in any quantity, they will cause problems for
the paint. It does not perhaps matter at all that a measurement technique reports
200 mg/m 2 , when the correct number was 300 mg/m 2 . Both are far too high. This,
indeed, is a weakness in the field of pretreatment quality control; it is not known
how much chlorides is too much. Although there is some consensus that the acceptable
amount is very low, there is no consensus on what the cut-off value is [18–20].
4.5.2 HYDROCARBONS
Like salts, hydrocarbons in the form of oils and grease also come from a variety of
sources: diesel fumes, either from passing traffic or stationary equipment motors;
lubricating oils from compressors and power tools; grease or oil in contaminated
blasting abrasive; oil on operators’ hands; and so on. As mentioned above, the
presence of oils and grease on the surface to be painted prevents good adhesion.
Testing for hydrocarbons is more complex than is testing for salts for two
reasons. First, hydrocarbons are organic, and organic chemistry in general is much
more complex than the inorganic chemistry of salts. A simple indicator kit of reagents
is quite tricky to develop when organic chemistry is involved. Second, a vast range
of hydrocarbons can contaminate a surface, and a test that checks for just a few of
them would be fairly useless. What is needed, then, is a test simple enough to be
done in the field and powerful enough to detect a broad range of hydrocarbons.
Ever game, scientists have developed a number of approaches for testing for
hydrocarbons. One approach is ultraviolet (UV) light, or black lights. Most hydrocarbons
show up as an unappetizing yellow or green under a UV lamp. This only
works, of course, in the dark and, therefore, testing is done under a black hood,
rather like turn-of-the-century photography. Drawbacks are that lint and possibly
dust show up as hydrocarbon contaminations. In addition, some oils are not detected
by black lights [21]. In general, however, this method is easier to use than other
methods.
© 2006 by Taylor & Francis Group, LLC