© 2006 by Taylor & Francis Group, LLC

Corrosion Testing — Practice 151
• Water uptake and hydrolysis are greater than in the field.
• A constant water film with high conductivity is present, which does
not happen in the field.
b. Elevated temperature
• Water, oxygen, and ion transport are greater than in the field.
• For some paints, the elevated temperature of the test comes close to
the glass transition temperature of the binder.
c. High chloride concentration (effect on corrosion depends on the type
of protection the coating offers)
• For sacrificial coatings, such as zinc-rich primers, the high chloride
content together with the constant high humidity means that the zinc
is not likely to form a passive film as it does in the field.
• For inhibitive coatings, chlorides adsorb on the metal surface, where
they prevent passivation.
• For barrier coatings, the osmotic forces are much less than in the field;
in fact, they may be reversed completely from that which is seen in
reality. In the salt spray test, corrosion at a scribe or defect is exaggeratedly
aggressive compared with a scribe under intact paint.
Lyon, Thompson, and Johnson [56] point out that the high sodium chloride content
of the salt spray test can result in corrosion morphologies and behaviors that are not
representative of natural conditions. Harrison has pointed out that the test is inappropriate
for use on zinc — galvanized substrates or primers with zinc phosphate pigments,
for example — because, in the constant wetness of the salt spray test, zinc undergoes
a corrosion mechanism that it would not undergo in real service [57]. This is a wellknown
and well-documented phenomenon and is discussed in depth in chapter 7.
8.4.3 IMPORTANCE OF WET/DRY CYCLING
Skerry, Alavi, and Lindgren have identified three factors of importance in the degradation
and corrosion of painted steel that are not modeled by the salt spray test:
wet/dry cycling, a suitable choice of electrolyte, and the effects of UV radiation
(critical because of the breakdown of polymer bonds in the paint) [3].
Lyon, Thompson, and Johnson explain why wet/dry cycles are an important
factor in an accelerated test method [56]:
Many studies have shown the specific importance of wetting and drying on atmospheric
corrosion... On a dry metal surface, as the relative humidity (RH) is increased, the
corrosion rate initially rises, then decreases to a relatively constant value which becomes
greater as the RH is increased. A similar effect is observed during physical wetting
and drying of a surface. Thus, on initial wetting, the corrosion rate rises rapidly as
accumulated surface salts first dissolve. The rate then decreases as the surface electrolyte
dilutes with continued wetting. The corrosion rate also rises significantly during
drying because of both the increasing ionic activity as the surface electrolyte concentrates
© 2006 by Taylor & Francis Group, LLC