© 2006 by Taylor & Francis Group, LLC

114 Corrosion Control Through Organic Coatings
The difference may seem unimportant; however, there are advantages to separating
the two questions. Testing a coating for initial corrosion protection is relatively
inexpensive and straightforward. The stresses — water, heat, electrolyte — that
cause corrosion of the underlying metal are exaggerated and then the metal under
the coating is observed for corrosion. However, trying to replicate the aging process
of a coating is expensive and difficult for several reasons:
1. Coatings of differing type cannot be expected to have a similar response
to an accentuated stress.
2. Scaling down wet-dry cycles changes mass transport phenomena.
3. Climate variability means that the balance of stresses, and subsequent
aging, is different from site to site.
7.2 WHAT FACTORS SHOULD BE ACCELERATED?
The major weathering stresses that cause degradation of organic coatings are:
• UV radiation
• Water and moisture
• Temperature
• Ions (salts such as sodium chloride and calcium chloride) and chemicals
The first of these weathering factors is unique to organic coatings; the latter three
are also major causes of corrosion of bare metals. Most testing tries to reproduce
natural weathering and accelerate it by accentuating these stresses. However, it is
critically important to not overaccentuate them. To accelerate corrosion, we scale
up temperature, salt loads, and frequency of wet-dry transitions; therefore, we must
scale down the duration of each temperature–humidity step. The balances of mass
transport phenomena, electrochemical processes, and the like necessarily change
with every accentuation of a stress. The more we scale, the more we change the
balances of transport and chemical processes from that seen in the field and the
farther we step from real service performance. The more we force corrosion in the
laboratory, the less able are we to accurately predict field performance.
For example, a common method of increasing the rate of corrosion testing is to
increase the temperature. For certain coatings, the transport of water and oxygen
increase markedly at elevated temperature. Even a relatively small increase in temperature
above the service range results in large changes in these coating properties.
Such coatings are especially sensitive to artificially elevated temperatures in accelerated
testing, which may never be seen in service. Other coatings, however, do not
see strongly increased oxygen and water transport at the same elevated temperature.
An accelerated test at elevated temperatures of these two coatings may falsely show
that one was inferior to the other, when in reality both give excellent service for the
intended application.
And, of course, interactions between stresses are to be expected. Some major
interactions that the coatings tester should be aware of include:
• Frequency of temperature/humidity cycling. Because the corrosion reaction
depends on supplies of oxygen and water, the accelerated test must
© 2006 by Taylor & Francis Group, LLC