© 2006 by Taylor & Francis Group, LLC

122 Corrosion Control Through Organic Coatings
From this study, it can be seen that raising the chloride load has a much stronger
effect on painted zinc-coated substrates than on painted carbon-steel substrates. It is
known that for bare metals, the zinc corrosion rate is more directly dependent than
the carbon steel corrosion rate on the amount of pollutant (NaCl in this case). This
relationship may be the cause of the results in the table above. In addition, higher salt
levels leave a heavier hygroscopic residue on the samples (see Section 7.2.3); this may
have caused a thicker moisture film at RH levels above 76%.
Boocock [23] reports another problem with high NaCl levels in accelerated tests:
high saponification reactions, which are not seen in the actual service, can occur at
high NaCl loads. Coatings that give good service in actual field exposures can
wrongly fail an accelerated test with a 5% NaCl load.
Increasing the level of NaCl increases the rate of corrosion of painted samples,
but the amount of acceleration is not the same for different substrates. As the NaCl
load is increased, the range of substrates or coatings that can be compared with each
other in the test must narrow. A low salt load is recommended for maximum
reliability.
Another approach is to reduce the frequency of salt stress. Most cyclic tests call
for salt stress between 2 and 7 times per week. Smith [24], however, has developed
a cyclic test for the automotive industry that uses 5-minute immersion in 5% NaCl
once every 2 weeks. The high salt load — typical for when the test was developed
— is offset by the low frequency.
How much salt is too much? There is no consensus about this, but several agree
that the 5% NaCl used in the famous salt spray test is too high for painted samples.
Some workers suggest that 1% NaCl should be a natural limit. Some of the suggested
electrolyte solutions at lower salt loads (using water as solvent) are:
0.05% (wt) NaCl and 0.35% ammonium sulfate, (NH 4) 2SO 4 [25]
0.5% NaCl + 0.1% CaCl 2 + 0.075% NaHCO 3 [26]
0.9% NaCl + 0.1% CaCl 2 + 0.25% NaHCO 3 [27]
7.2.6 ABRASION AND OTHER MECHANICAL STRESSES
While in service, coatings undergo external mechanical stresses, such as:
• Abrasion (also called sliding wear)
• Fretting wear
• Scratching wear
• Flexing
• Impingement or impact
These stresses are not of major importance in corrosion testing. Even though some
damage to the coating is usually needed to start corrosion, such as a scribe down to
the metal, the mechanical damage in and of itself does not cause corrosion. This is
© 2006 by Taylor & Francis Group, LLC