© 2006 by Taylor & Francis Group, LLC

120 Corrosion Control Through Organic Coatings
It should be noted that the 90 g/m 2 zinc coating in this study is hot-dipped
galvanized, and the two thinner coatings are electrogalvanized. It may be that differences
other than zinc thickness — for example, structure and morphology of the
zinc coating — play a not yet understood role. Further research is needed in this
area, to understand the role played by zinc layer structure and morphology in
under–cutting.
7.2.3.3 Differences in Absorption and Desorption Rates
The rate at which a coating absorbs water is not necessarily the same as the rate at
which it dries out. Some coatings have nearly the same absorption and desorption
rates, whereas others show slower drying than wetting, or vice versa.
In constant stress testing, in which samples are always wet or always dry, this
difference does not become a factor. However, as soon as wet-dry cycles are introduced,
the implications of a difference between absorption and desorption rates
becomes highly important. Two coatings with roughly similar absorption rates can
have vastly different desorption rates. The duration of wet and dry periods in modern
accelerated tests is measured in hours, not days, and it is quite possible that, for a
coating with a slower desorption rate, the drying time in each cycle is shorter than
the time needed by the coating for complete desorption. In such cases, the coating
that desorbs more slowly than it absorbs can accumulate water.
The problem is not academic. Lindqvist [22] has studied absorption and
desorption rates for epoxy, chlorinated rubber, linseed oil, and alkyd binders, using
a cycle of 6 hours of wet followed by 6 hours of drying. An epoxy coating took up
100% of its possible water content in the wet periods but never dried out in the
drying periods. Conversely, a linseed oil coating in this study never reached its full
saturation during the 6-hour wet periods but dried out completely during the drying
periods.
Lindqvist has pointed out that the difference in the absorption and desorption rates
of a single paint, or of different types of paint, could go far in explaining why cyclic
accelerated tests often do not produce the same ranking of coatings as does field
exposure. There is a certain risk to subjecting different types of coatings with unknown
absorption and desorption characteristics to a cyclic wet-dry accelerated regime. The
risk is that the accelerated test will produce a different ranking from that seen in reality.
It could perhaps be reduced by some preliminary measurements of water uptake and
desorption; an accelerated test can then be chosen with both wet times and drying
times long enough to let all the paints completely absorb and desorb.
7.2.4 TEMPERATURE
Temperature is a crucial variable in any accelerated corrosion testing. Higher temperature
means more energy available, and thus faster rates, for the chemical processes
that cause both corrosion and degradation of cured films. Increasing the
temperature — within limits — does not alter the corrosion reaction at the metal
surface; it merely speeds it up. A potential problem, however, is what the higher
© 2006 by Taylor & Francis Group, LLC