Coatings tests using
standard methods can take from 30 to 90 days for a single
test. Efird Corrosion International has developed acoustic
emission techniques to detect failure of a coating during
testing before the failure can be observed visually. This
significantly shortens the time required for a test, provides
information on the nature of the substrate/coating/environment
interaction, and adds a great deal of flexibility to the
design of the test apparatus. Acoustic emission is unique
as a test monitoring technique in that it originates in
the material, and does not require an external source of
energy as do ultrasonic or electrochemical techniques.
monitoring of a coating in a specific environment allows
determination of the failure temperature, and the type of
failure occurring. Each coating failure mechanism possesses
unique acoustic emission characteristics. Analysis of these
characteristics at each temperature reveals what is happening
to the coating at that temperature, determines if the coating
will fail at that temperature, and evaluates the mode of
The most widely used
acoustic emission signal measurement parameters are counts,
amplitude, rise time, and the measured area under the rectified
signal envelope (MARSE) sometimes referred to as energy
counts. The relationship of these parameters is shown in
Figure 1. Commonly measured
parameters of an acoustic emission signal.
The acoustic emission
stress source with respect to coatings is a result of the
penetration of water, gasses, and ions through the coating
and their interaction with the coating/substrate interface.
This interaction at the interface can result in coating
disbondment and blistering, which are acoustic emission
sources. A second source of acoustic emissions is the corrosion
process on the steel surface under the coating.
setup for acoustic emission monitoring for a cathodic disbondment
test of external pipeline coatings is shown in Figure 2.
This is a modification of ASTM G 95, "Standard Test Method
for Cathodic Disbondment Test of Pipeline Coatings (Attached
Cell Method." The test section can be either a coated plate
of a section of coated pipe.
Test apparatus for monitoring acoustic emission activity
for external pipeline coatings in a cathodic disbondment
This method can also
be used to test internal tubular coatings or internal tank
and vessel coatings according to ASTM. This makes the technique
extremely versatile, and allows testing of a wide range
of coatings in a wide range of applications. Details of
the ASTM C868 Atlas test cell and the internal tubular coating
test cell are shown in Figures 3 and 4, respectively. The
internal tubular test cell is shown with thermal insulation
covering the test sample to simulate downhole conditions.
Removal of the insulation allows investigation into the
results of a pronounced cold wall effect on the coating
Atlas test cell for monitoring acoustic emission activity
as a function of temperature for internal tank and vessel
Figure 4. Test
cell for monitoring acoustic emission activity as a
function of temperature for internal tubular coatings.
The external insulation is omitted to allow study of
the cold wall effect.
monitoring of coatings in an atlas test cell demonstrated
that acoustic emission provided an indication of coating
failure within 48 hours during environmental exposure at
a test temperature and pressure. This permits a test procedure
where the test temperature is stepped at 48-hour intervals
to determine the failure temperature of a specific coating
for a specific set of environmental conditions. Differences
in the acoustic emission signature allow evaluation of the
coating failure mode, i.e., disbonding, blistering and/or
Examples of the results
are given in Figures 5 and 6. Acoustic emission tests were
conducted on an amine epoxy coating exposed to 3-phase environments
(water/oil/gas). The water phase was a buffered NaCl solution
(Figure 5) or distilled water (Figure 6). Exposures were
for 48 hours at incremental temperatures of 60°C to 100°C.
Figure 5. Acoustic
emission activity as a function of temperature for an
amine epoxy coated plate with 3% NaCl + 1000 ppm HCO3
as the aqueous phase.
The coating in the
buffered NaCl solution (Figure 5) showed increased acoustic
emission activity at 100°C. Post test examination found
disbondment between the coating and the steel substrate.
With distilled water as the aqueous phase (Figure 6), increased
acoustic emission activity began at 70°C, increased at 80°C,
and was extremely active at 90°C. Post test examination
of the test sample from this experiment found ruptured blisters
and extensive disbonding between the coating and the steel
substrate. This acoustic emission behavior is typical of
that observed in other coatings in similar test environments.
Figure 6. Acoustic
emission activity as a function of temperature for an
amine epoxy coated plate with distilled water as the
The Bottom Line
The use of acoustic
emission methods developed by Efird Corrosion International
to monitor coating performance during testing provides significant
advantages over standard test methods:
- It significantly shortens
the time required for a test sequence.
- It is not restricted to
any particular type of test cell, and can be used
with any configuration that allows detection of the
- It provides information
on the nature of the substrate/coating/environment
interaction and the coating failure mode.
- It does not require an outside
perturbation to the coating that can affect the test