The Value of Enhanced Weathering Tests
The recent developments of electrochemical testing procedures, of which we have
discussed two, may result in better system lifetime prediction methods and monitoring
techniques for coating systems than are now available. Many new coating systems
are being developed as a result of the various environmental demands and legislation
together with demands for high quality for improved lifetime.
The industrial need for fast testing methods is therefore enormous because
testing by, e.g., atmospheric exposure of the systems takes too much time. Many
fast testing procedures exist. One of the first enhanced weathering tests was the
exposure of coated panels in Florida, where 3 years of exposure would be more or
less similar to 10 years of exposure in Europe because of the high temperature and
humidity in Florida. However, many new tests have been developed, and the exposure
in Florida is now not regarded as a fast test. Shortening the test period means that
various parameters determining the corrosion initiation and propagation processes
are considerably changed compared with natural conditions. These include:
The temperature
The composition of the corrosive including NaCl, pH, SO
2
, and additions such as
copper
The relative humidity
UV radiation
For modern cyclic testing some of these parameters are changed in time at regular
intervals, simulating more natural exposure conditions.
The aim of any testing procedure is to find a simple, perferably linear relation
between natural exposure and enhanced testing results. However, according to
many authors there is no such simple relation [101]. Some authors state that
enhanced weathering just gives some indication of the relative quality of coating
during testing, without any predictive value for the lifetime. Even standard tests
(e.g., ASTM B117) emphasize the lacking relation between, e.g., the salt spray test
and corrosion protection in different surroundings due to the enormous differences
in the reaction mechanisms. It has been stated that coated galvanized steel always
performs worse in a salt spray test than normal cold-rolled steel, whereas in reality
it always performs better [102]. Cyclic testing improves the relation between test
and practice [103].
It has been reported [104] that standard salt spray tests give reasonable results
for coatings in use in surroundings with a high salt concentration (seawater, deicing
salt) and high RH but not for other applications. The high salt concentration would
be responsible for decreased action of the anticorrosive pigments, which perform well
under natural low-salt conditions. Also, salt spray tests often give blistering, which
is often not relevant except for marine applications. Cyclic testing again results in less
blistering, which at least seems to indicate a better simulation of natural conditions.
A comparison of a cyclic test newly developed by Hoogovens for automotive
panels with the salt spray test, the SCAB (simulated corrosive atmospheric
breakdown) test, and the VDA(Verein der Deutschen Automobilindustrie) test showed
that some differences can be observed in the test results for the ranking as far as
undermining is concerned. A negative relation exists between the results of the new
cyclic test and the salt spray test. The new cyclic test shows reasonable agreement in
Organic Coatings 723
Copyright © 2002 Marcel Dekker, Inc.