© 2006 by Taylor & Francis Group, LLC

Corrosion Testing — Background and Theoretical Considerations 117
the binder and the particle may be in extremely close physical proximity
but are not chemically bonded. This area between binder and pigment can
be a potential route for water molecules to slip through the cured film.
Ström and Ström [1] have offered a definition of wetness that may be useful in
weighing vapor versus liquid water. They have pointed out that NaCl liquidates at
76% RH, and CaCl 2 liquidates at 35% to 40% RH (depending on temperature). NaCl
is by far the most commonly used salt in corrosion testing. It seems reasonable to
assume that, unless the electrolyte spray/immersion/mist step in an accelerated test
is followed by a rinse, a hygroscopic salt residue will exist on the sample surface.
At conditions below condensing but above the liquidation point for NaCl, the
hygroscopic residue can give rise to a thin film of moisture on the surface. Therefore,
conditions at 76% RH or more should be regarded as wet. Time of wetness (TOW)
for any test would thus be the amount of time in the cycle where the RN is at 76%
or higher.
7.2.3 DRYING
A critical factor in accelerated testing is drying. Although commonly ignored, drying
is as important as moisture. The temptation is to make the corrosion go faster by
having as much wet time as possible (i.e., 100% wet). However, this approach poses
two problems:
1. Studies indicate that corrosion progresses most rapidly during the transition
period from wet to dry [6–10].
2. The corrosion mechanism of zinc in 100% wet conditions is different
from that usually seen in actual service.
7.2.3.1 Faster Corrosion during the Wet–Dry Transition
Stratmann and colleagues have shown that 80% to 90% of atmospheric corrosion
of iron occurs at the end of the drying cycle [7]; similar studies exist for carbon
steel and zinc-coated steel. Ström and Ström [1] have reported that the effect of
drying may be even more pronounced on zinc than on steel. Ito and colleagues [6]
have provided convincing data of this as well. In their experiments, the drying time
ratio, R dry, was defined as the percentage of the time in each cycle during which the
sample is subjected to low RH:
R
dry
Tdrying
= •100%
T
cycle
The drying condition was defined as 35°C and 60% RH; the wet condition was
defined as 35°C and constant 5% NaCl spray (i.e., salt spray conditions). T cycle is
the total time, wet plus dry, of one cycle, and T drying is the amount of time at 60% RH,
35°C during one cycle. Cold-rolled steel and galvanized steels with three zinc-coating
© 2006 by Taylor & Francis Group, LLC