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Combatting Liquid Metal Attack by Mercury in Ethylene and Cryogenic Gas PlantsTask 1 - Non-Destructive Testing
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Presented herein are the results from the Task 1 nondestructive testing studies conducted under the a research program entitled, "Combatting Liquid Metal Attack of Aluminum Alloys by Mercury in Ethylene and Cryogenic Gas Plants". This program was organized and conducted by CLI International, Inc., under industrial multiclient support. The goal of the program was to develop preventative and remedial measures and detection and monitoring methods for mercury (Hg) attack of Al-alloys commonly used in the ethylene and cryogenic gas plant service.
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Presented
herein are the results from the Task 1 nondestructive testing studies conducted
under the a research program entitled, "Combatting Liquid Metal Attack of
Aluminum Alloys by Mercury in Ethylene and Cryogenic Gas Plants". This
program was organized and conducted by CLI International, Inc., under
industrial multiclient support. The goal of the program was to develop
preventative and remedial measures and detection and monitoring methods for
mercury (Hg) attack of Al-alloys commonly used in the ethylene and cryogenic
gas plant service. This
report summarizes the results of tests conducted in Task 1 of the
abovementioned program. It also serves to document experimental and analytical
procedures used in the nondestructive testing of Al specimens exposed to
Hg-containing environments under various loading configurations. Laboratory
tests were conducted to evaluate the acoustical emission (AE) from
Hg-contaminated and control (Hg-free) specimens of Al-alloy 5083-0. These tests
were conducted on compact tension (CT) specimens under either dead weight and
hydraulic loading and on welded pipe specimens under internal pressure. Both
types of specimens showed evidence of corrosive attack on fresh metal surfaces
and liquid metal embrittlement (LME) in the presence of Hg. Computerized AE
monitoring equipment was utilized to record and analyze the AE response of these
specimens under various conditions of loading and pressurization. These
conditions included increasing and decreasing load (pressure) as well as hold
periods at constant load or pressure. Characteristic
AE response from both control and Hg-contaminated specimens was obtained. The
control specimens were found to have increasing AE with rising load (pressure)
with rapid attenuation during periods of constant load. High amplitude (>50
dB) AE from these specimens showed the Kaiser effect. Hg-contaminated specimens
shoved a more complicated AE response than the control specimens. The response
from contaminated specimens was characterized by a substantial increase in low
amplitude (<50 dB) AE over the control specimens which appeared to occur
readily at low pressures (0 to 500 psi) in pipe specimens and at low loads in
CT specimens. Prolonged high amplitude (>50 dB) AE was observed in most
cases during hold periods at constant load. While substantial low amplitude AE
was found to occur at loads prior to the previous load maximum, this behavior
was not completely characteristic of the Felicity effect. Acoustic emission (AE) testing has many natural advantages
for locating damage associated with Hg attack in Al-alloys used for cryogenic
gas service. It is non-intrusive, rapid and can potentially locate defects from
remote sensors. Therefore, if the characteristic AE associated with Hg attack
of Al-alloys can be defined in laboratory tests, better and more complete
inspection of process equipment used in ethylene plant service may be
performed. Not
all liquid metals result in LME of metal substrates. As shown in Figure
2, it is related to the solubility of the liquid metal into the substrate
material along with its ability to reduce the fracture surface energy.
Consequently, solid-liquid metal couples can be categorized as shown in this
figure. [1] One
very important aspect of LME is that typically once a LME crack is initiated,
crack propagation rates tend to be very rapid (see Figure
3). [2] Hence lies the serious concern with using AE test methods for the
detection of LME. AE field or in-plant test methods usually involve evaluation
the vessel or component at a stress or pressure higher than those found during
its normal operating conditions. Since LME may result in rapid, uncontrollable
propagation of brittle cracks, it is important to know if AE can obtain meaningful
information at low enough stresses or pressures where catastrophic failure of
the vessel or component is not a major concern. The
present study attempts to evaluate the use of AE techniques to identify LME of
Al-alloys produced by exposure of specimens to Hg under various conditions of
stress and pressure. The experimental methods employed involve AE monitoring of
stressed fracture mechanics specimens and pressurized vessels made from welded
pipe exposed to both inert and Hg contaminated environments in the laboratory. When
metals are exposed to a corrosive environment, several processes can occur
which result in detectable AE (see Figure
4). [3] These chemical processes involve anodic and cathodic reactions and
film rupture as well as mechanical processes such as plastic deformation (i.e.
dislocation motion and twinning), crack propagation, if residual or applied
stresses are present, and phase transformations. Typically, the AE from
chemical sources are of low amplitude and do not contribute greatly to overall
AE during plant and field inspections. Only in cases where extensive corrosion
product formation, gas generation and spalling occur do corrosion processes
themselves play an important role in AE inspection. More
importantly, the combination of corrosion and mechanical processes can produce
crack initiation and propagation events typical of HEC, SCC and LME in
susceptible materials. These generally would be expected to produce high
amplitude AE which can be readily observed in AE inspection. Consequently, AE
inspection of plant vessels to identify SCC damage is fairly routine. [4] AE
has been employed to evaluate Al and Al-alloys in a number of applications.
Typically, in non-corrosive systems, tensile straining causes AE to increase
dramatically at the beginning of plastic deformation by the production and
movement of dislocations corresponding to the onset of macroscopic yielding. The
AE will then decrease somewhat with subsequent plastic deformation (see Figure
5). [5,6] Upon further straining, AE can be directly linked to crack
initiation and growth processes. Typically, AE amplitude increases with the
grain size of the material. [7] Under
conditions of corrosion, AE has been found to be useful in the evaluation of
crack growth by either intergranular or transgranular SCC in metals. [6-10] However,
as shown in Table
1, intergranular SCC and HEC typically provide higher amplitude and more
easily monitored AE response than transgranular SSC. [6] Studies have shown
that for intergranular SCC, the primary source of AE during crack growth was
from the failure of ligaments behind the advancing crack front. [9] This
probably results in highly localized deformation of the material which provides
high AE intensity. It has also been observed that AE associated with SCC in
Al-alloys is characterized by the correlation between AE activity and crack
velocity as shown in Figure
6. [10] Interesting
effects have also been found in material systems where HEC is operating.
[11,12] Specimens under cathodic charging were found to exhibit AE bursts which
were related to the formation of hydrogen cracking in the material. Tensile
testing conducted on specimens following hydrogen charging showed increased AE
over uncharged specimens due to the effects of internal hydrogen on the
deformation processes and possible cracking produced in the material during
charging. Additionally, when these specimens were unloaded and loaded a second
time, the uncharged specimens did not exhibit significant AE until reaching the
stress used in the first cycle. This behavior is referred to as the Kaiser
effect. However, the hydrogen charged specimens, exhibited AE at stresses below
that used in the first cycle. This lack of Kaiser effect is referred to as a
Felicity effect. The Felicity effect has been attributed to the formation of
subcritical cracks in the lattice in this case caused by the hydrogen in the lattice
and the stress from the first cycle. The
experimental program in the present study was developed to determine the
ability of AE to detect LME in Al-alloys. Four series of tests were performed
as shown below: Series 1 - | Evaluation of Compact Tension Specimens for AE Response in
Hg. | Series 2 - | Evaluation of Pipe Specimens for AE with Internal Pressure
and Hg. | Series 3 - | Evaluation of Pipe Specimens for AE Using Pressure Cycles
and Hg. | Series 4 - | Evaluation of Kaiser/Felicity Effects with Compact Tension
and Pipe Specimens with Hg. |
These
tests started in a very exploratory manner to observe the AE response of both
compact tension (CT) fracture mechanics specimens and pressurized pipe specimens
in both inert and Hg-contaminated environments. Once the exploratory tests were
completed, a more detailed engineering study was conducted which tried to
characterize the AE response from LME and determine the practical feasibility
of using AE as a nondestructive testing tool in the evaluation of Al-alloy
components in ethylene plant service. The
alloy used in this study was alloy 5083-0. The chemical composition is provided
in Table
2 along with its nominal mechanical properties. Al-5083-0 is commonly used
in the construction of cryogenic equipment in ethylene plant service. Pipe
and plate material was obtained for this study and was evaluated in both welded
and non-welded condition. Welding procedures were typical of those used in
fabricating plant equipment. The
1T compact tension (CT) specimens were machined per ASTM E-399 to the
dimensions given in Figure
7. A nominal specimen thickness of 0.25 inch was used for these specimens.
Side grooves were machined in the specimens in an attempt to enforce planar
crack growth and test validity. Both welded and non-welded CT specimens were
employed in this program. The welded specimens were oriented so that the crack
would be growing in the weld metal parallel to the direction of the weld. Pipe
specimens were fabricated from nominal 5-inch O.D. pipe. To fabricate the pipe
samples, sections of pipe were butt welded together using a full penetration
single V weld. Samples were tested both with and without aluminum backing rings
on the inside of the pipe at the weldment. As shown in Figure
8, the pipe specimens were machined on the ends to provide a true O.D. on
which the pressurizing fixture could seal. A
complete report detailing the Series 1 AE test procedures and results is given
in Appendix I. Series 2 - Evaluation of Pipe Specimens for AE with Internal Pressure
and Hg Pipe
specimens were monitored using the same type of AE sensors and instrumentation
described in Series 1. For these tests, however, two sensors were used. The
test set-up used in Series 2 is shown in Figure
10. It included the pipe specimen and end fixture which allowed for
internal pressurization without end constraint. The AE sensors were placed on
either side of the butt weld. The pipe specimen and sensors were placed in a
plastic containment vessel to prevent contamination of the laboratory with Hg
during failure of the pipe specimen. A manually operated, air driven hydraulic
pump was used to hydrostatically pressurize the specimens to failure. The
specimens were pressurized with either oil or water both with and without Hg
and with and without backing rings inside the pipe. For the Hg tests, a total
of 1875 grams of Hg were added to the inside of the pipe sample prior to sealing.
The Hg settled to the bottom (6 O'clock) position in the pipe during testing.
The pressurized specimens were monitored using AE both during pressurization
and during selected hold periods at constant pressure. The specimens were then
pressured to failure. A
complete report detailing the Series 2 AE pipe test procedures and results is
given in Appendix II. Series 3 - Evaluation of Pipe Specimens for AE Usincr Pressure Cycles
and Hg The
pipe specimens in Series 3 were tested using the same AE sensor configuration
used in Series 2 except that a linear array of four sensors were used. These
sensors were labeled 1 through 4 and were located along the length of the
specimen. Two sensors were on one side of the butt weld and two were on the
other side of the weld. Additionally, the containment and pressurization
equipment were also the same as described previously in the Series 2 tests. The
main difference in Series 2 and 3 tests were in the pressure cycles employed.
The pipe specimens were initially pressured to 450 psi for tan minutes. This
pressure corresponded to approximately the normal working pressure of the Al
pipe specimens. After this initial hold, the pressure was varied and held
according to the schedule given below: Step
1
2
3
4
5
6 | Pressure (gsi)
500
525
450
500
525
550 | Hold Time (min)
10
30
10
10
10
30 |
The
pressures used in this sequence corresponded to approximately 110 and 120
percent of the original working pressure. Following
the pressure/hold sequence, the pipe specimens were subjected to rapid cyclic
pressurization at one or more pressure levels in an attempt to initiate liquid
metal attack in the pipe specimen. The initial pressure sequence was then
repeated again followed by pressurization of the pipe specimens to failure.
Pipe specimens were tested both with and without Hg contamination. The Hg tests
were conducted with approximately 1000 grams Hg on the inside of the pipe. A
MONPAC severity analysis was conducted on the AE data. A complete report from
detailing the Series 3 AE pipe test procedures and results is given in Appendix
III. Series 4 - Evaluation of Kaiser/Felicity Effects with Compact Tension
and Pipe Specimens with Hg Series
4 AE tests were conducted using the same AE instrument settings and similar
laboratory techniques developed in the previous three series of tests. Series 4
tests involved experiments on both CT and pipe specimens using multiple load or
pressure cycles. The typical cycle configuration consisted of a ramp to a
designated load or pressure followed by a hold period until the AE signal
subsided. Then, the load or pressure was decreased to approximately one half
the previous maximum value and increasing again to a higher value of load or
pressure. This sequence was continued until failure of the specimen was
obtained. During
these load and pressure cycles, careful monitoring of the AE response of the
specimen was conducted to observe whether Kaiser or Felicity effects were
present. The presence of a Felicity effect could be evidence of LME attack of
the specimen. Additionally, specimens were tested both with an inert
environment (no Hg) and with Hg contamination. This allowed for a direct
comparison of AE from the Al alloy both with and without the presence of Hg.
The purpose of these observations was to determine specifically if an enhanced
AE is obtained from the Al alloy when exposed to Hg as opposed to the same
specimen tested in the inert environment. A
MONPAC severity analysis was conducted on the AE data. A complete report from
detailing the Series 4 CT and pipe test procedures and results is given in
Appendix IV.
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