© 2006 by Taylor & Francis Group, LLC

130 Corrosion Control Through Organic Coatings
In addition, some of the tests used in the automotive industry are described. These
are tests with proven correlation to field service for car and truck paints, which may,
with adaptations, prove useful in heavier protective coatings.
8.1.1 GENERAL CORROSION TESTS
A general accelerated test useful in predicting performance for all types of coatings, in
all types of service applications, is the ‘‘Holy Grail” of coatings testing. No test is there
yet, and none probably ever will be (see Chapter 7). However, some general corrosion
tests can still be used to derive useful data about coating performance. The two allpurpose
tests recommended here are the ASTM D5894 test and the NORSOK test.
8.1.1.1 ASTM D5894
ASTM D5894, “Standard Practice for Cyclic Salt Fog/UV Exposure of Painted Metal
(Alternating Exposures in a Fog/Dry Cabinet and a UV/Condensation Cabinet),” is
also called “modified Prohesion” or “Prohesion UV.” This test, incidentally, is
sometimes mistakenly referred to as ‘‘Prohesion testing.” However, the Prohesion
test does not include a UV stress; it is simply a cyclic salt fog (1 hour salt spray,
with 0.35% ammonium sulphate and 0.05% sodium chloride [NaCl], at 23°C, alternating
with one drying cycle at 35°C). The confusion no doubt arises because the
original developers of ASTM D5894 referred to it as ‘‘modified Prohesion.”
This test is can be used to investigate both anticorrosion and weathering characteristics.
The test’s cycle is 2 weeks long and typically runs for 6 cycles (i.e., 12
weeks total). During the first week of each cycle, samples are in a UV/condensation
chamber for 4 hours of UV light at 60°C, alternating with 4 hours of condensation
at 50°C. During the second week of the cycle, samples are moved to a salt-spray
chamber, where they undergo 1 hour of salt spray (0.05% NaCl + 0.35% ammonium
sulphate, pH 5.0 to 5.4) at 24°C, alternating with 1 hour of drying at 35°C.
The literature contains warnings about too-rapid corrosion of zinc in this test;
therefore, it should not be used for comparing zinc and nonzinc coatings. If zinc
and nonzinc coatings must be compared, an alternate (i.e., nonsulphate) electrolyte
can be substituted under the guidelines of the standard. This avoids the problems
caused by the solubility of zinc sulphate corrosion products. It has also been noted that
the ammonium sulphate in the ASTM D5894 electrolyte has a pH of approximately 5; at
this pH, zinc reacts at a significantly higher rate than at neutral pH levels. The zinc
is unable to form the zinc oxide and carbonates that give it long-term protection.
8.1.1.2 NORSOK
NORSOK is suitable for both corrosion and weathering testing. Its cycle is 168
hours long, and it runs for 25 cycles (i.e., 25 weeks total). Each cycle consists of
72 hours of salt spray, followed by 16 hours drying in air, and then 80 hours of UV
condensation (ASTM G53).
The NORSOK test was developed for the offshore oil industry, particularly the
conditions found in the North Sea. The test is part of the NORSOK M-501 standard,
© 2006 by Taylor & Francis Group, LLC