Page 697 - Corrosion Engineering Principles and Practice

650 C h a p t e r 1 4 P r o t e c t i v e C o a t i n g s 651

Salt-spray testing: The salt-spray test, which was originally designed
to test coatings on metals, has been widely used to evaluate the
resistance of metals to corrosion in marine service or on exposed
shore locations [19;20]. However, extensive experience has shown
that, although salt-spray tests yield results somewhat similar to
those exposed in marine environments, they do not reproduce all
the factors causing corrosion in marine service. Salt-spray tests
should thus be considered to be arbitrary performance tests and
their validity dependent on the extent to which a correlation has
been established between the results of the test and the behavior
under expected conditions or service. Despite the current wide-
spread use of continuous salt-spray methods, their unrealistic
simulation of outdoor environments is a serious shortcoming.

Cathodic protection exposures: One accelerated test involves subject-
ing the material to which the coating has been applied to various
voltages of impressed current, often in excess of that normally
required to achieve cathodic protection, with the aim of demon-
strating the ability of the coatings to resist the disbonding effect of
the cathodic current or to resist electroendosmosis (Fig. 14.30).
Condensation apparatus: Relying on the known susceptibility of
coatings to permeation by condensed water, a screening test is
available in a laboratory-type cabinet which results in condensa-
tion of water on the painted surface. Disbonding of poorer coat-
ings occurs.

FIGURE 14.30 Laboratory setup to test for disbonding during the application
of cathodic protection. (Courtesy of Corrpro)