New, High Tg Internal Pipe Coating System
December 2016 www.coatingsworld.com Coatings World | 37
also compared to the results of post-autoclave exposure testing on a commercially
available product as a control.
Results Summary
Figure 1 (coating “C”) and Figure 2 (
experimental Axalta coating) contain photographs of both coatings before and
after autoclave exposure. The newly
developed Axalta coating exhibited excellent adhesion, no blistering, and no
swelling. “Coating C” was completely
disbonded from the steel test substrate
and no further evaluation was possible.
Understanding EIS Testing
Organic coatings provide corrosion protection by isolating the corrosive environment from the steel structure to which
they are applied. In general, a coating
needs to have good barrier properties to
provide good protection. Low permeability to water, ions, gases, and other
corrosives is essential to the success of a
coating system at higher temperatures.
Field experience and laboratory research have shown that highly protective
coatings with good barrier properties
have a high electrical resistance. EIS is
now a well-established laboratory technique for evaluating the corrosion protection of organic coatings because EIS
allows for simultaneous measurement of
the degradation of a coating caused by
exposure to an electrolyte and the change
in the corrosion rate of the substrate
caused by coating perforation.
During the test, an AC voltage of varying frequency is applied to the sample,
which allows collection of information
about the electrochemical reactions. The
capacitance of the coating will change
during the experiment as a result of water
swelling or absorption. Typically, a three-electrode arrangement with a working
electrode, a reference electrode, and a
counter-electrode is used to perform EIS
in aqueous solutions.
As demonstrated in Figure 3, the corrosion protection from a coating, expressed in terms of Log Z, increases as its
impedance increases. Therefore, a newly
applied, high-performance coating will
have a high impedance in the range of
Log Z= 9 to 11.
Post-Autoclave Exposure
EIS Results
Figure 4 is a graph representing the results
of the post-autoclave exposure EIS testing
of the experimental Axalta FBE. The Axalta
FBE coating system exhibited excellent
barrier properties in EIS evaluation. The
pre-autoclave exposure Log Z of the Axalta
FBE was measured as 11.09. After expo-
sure, the Log Z observed within the water
phase was 10.66, within the hydrocarbon
phase was 10.92, and within the gas phase
Figure 3. Evaluation of Coating Corrosion Protection Using EIS.
Figure 4. Post-autoclave exposure EIS testing of the experimental Axalta FBE coating system.
