In-situ visualization of CRA corrosion in chloride containing conditions at high T and high P

Abstract

In-situ micro-visualization was used to study the pitting corrosion of a 25 Cr Duplex Stainless Steels in chloride containing aqueous solutions containing CO2 gas at high temperatures and high pressures. A small area on the surface of the sample was imaged and video recorded (in real time) during the anodic polarization in the gas/liquid environment. The video and images obtained during the corrosion process were used to confirm the potential under which the transition from passivity to pitting corrosion could take place in this alloy. This methodology allowed the observation of initiation sites for pitting corrosion on the 25 Cr DSS sample.

Full text

In-situ Visualization of CRA Corrosion in Chloride Containing
Conditions at High T and High P
Eric Ramirez and L. F. Garfias-Mesias
INTERTEK - Asset Integrity Management
16100 Cairnway Dr., Suite 310, Houston, Texas 77084, USA
ABSTRACT
In-situ micro-visualization was used to study the pitting corrosion of a 25 Cr Duplex Stainless Steels in
chloride containing aqueous solutions containing CO2 gas at high temperatures and high pressures. A
small area on the surface of the sample was imaged and video recorded (in real time) during the anodic
polarization in the gas/liquid environment. The video and images obtained during the corrosion process
were used to confirm the potential under which the transition from passivity to pitting corrosion could
take place in this alloy. This methodology allowed the observation of initiation sites for pitting corrosion
on the 25 Cr DSS sample.
Key words: In-situ, Electrochemistry, DSS, Pitting, CPT, Ferrite, Austenite, Inclusions, Precursor Sites
INTRODUCTION
In-situ micro visualization of the surface changes during the corrosion process of corrosion resistant
alloys (CRAs) can have a wide range of applications in the modern industry. This type of
characterization can be used to improve an alloy, by understanding the weak (and susceptible to
corrosion) areas in the microsctructure1. It can also be used to study how changes in the processing
conditions (in a given environment) can help reduce the corrosion rate of these materials. It is also a
good tool to qualify new materials for their use in highly corrosive environments or to study their
performance under accelerated conditions.
Pitting corrosion and the effect of initiation sites for pitting in commercial stainless steels has been
documented2-6. However, highly alloyed materials, like Duplex Stainless Steels or Ni alloys (CRAs),
have different susceptibilities to corrosion, which is typically determined in laboratory tests at higher
temperatures (mainly because these materials will not pit at room temperature). Pitting corrosion of
CRAs in certain environments may only corrode at high temperatures, where the combination of the
liquid environment and a corrosive gas can initiate the corrosion process. Additionally, pitting corrosion
of CRAs in harsh gas/liquid environments may be more complex, due to the fact that several initiation
sites can be found in the same sample. The environmental conditions may also play an important role
©2013 by NACE International.
Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to
NACE International, Publications Division, 1440 South Creek Drive, Houston, Texas 77084.
The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.
1
Paper No.
2778

in the initiation and further development of pits. For example, some initiation sites may trigger corrosion
events at moderate temperatures in chloride containing environments like those found in splash zone.
However, other initiation sites may be more susceptible to pitting corrosion in more aggressive
conditions that include higher temperature, higher pressure and with a high concentration of aggressive
species such as H2S and CO2.
In the present work, we show that by using a novel setup that includes a mini-autoclave with a small
window coupled to a potentiostat and a stereomicroscope it is possible to image, in real time, the
surface of CRAs at high temperature and high pressure in the presence of a liquid/gas mixture. This
methodology has allowed the observation of metastable pitting and its transition to stable pitting in this
alloy (UNS S32550) above the CPT, in a chloride containing environment with air and CO2.
EXPERIMENTAL PROCEDURE
Materials
All samples used in the present research were cut from commercially available 25Cr DSS of the type
UNS S32550, which were obtained in wrought form (bars of 10 mm diameter); its diameter was further
decreased to about 5 mm. Table 1 shows the composition of the alloy.
Table 1
UNS S32550 alloy used in the present work
Composition (wt%)
Cr
Ni
Mo
Cu
N
Mn
Si
C
S
P
25.92
5.9
3.19
1.62
0.2
0.96
0.47
0.03
0.007
0.024
In order to modify the microstructure of the material, two different annealing procedures (at 1020oC or
1140oC) following by water quenching were selected.
Annealing this material at 1020oC has been shown to yield a microstructure that contains roughly 47%
ferrite and 53% austenite7. Pitting corrosion studies of this material after annealing at 1020oC followed
by water quenching in chloride containing environments has been reported7. The Critical Pitting
Temperature (CPT) of this material was determined using a ZRA procedure in a Ferric Chloride
environment using a modified version of the ASTM G-48 procedure8. The CPT of this material was
found to be equal to 62oC7.
Annealing this alloy at 1140oC has been shown to yield a microstructure that contains roughly 64%
ferrite and 36% austenite7. Pitting corrosion studies of this material after annealing at 1140oC followed
by water quenching in chloride containing environments has been also reported7. The Critical Pitting
Temperature (CPT) of this material obtained using the same procedure described above yielded a CPT
equal to 42oC7.
The chemical compositions of both ferrite and austenite phases for both heat treatments 1020oC and
1140oC followed by water quenching have been determined quantitatively by Electron Probe
Microanalysis (EPMA) 7, as described in previous publications by one of the authors7 and are presented
in Table 2.
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2

Table 2
Average composition of Ferrite and Austenite for the 2 UNS S32550 alloys (Annealed at 1020oC
and 1140oC followed by water quenching)
Composition (wt %)
Annealing Temp.
Phase
%
Cr
Ni
Mo
N1
Cu
P
1020oC
Ferrite
47
26.36
4.63
3.69
0.05
1.27
0.043
Austenite
53
23.14
7.36
2.35
0.33
1.95
0.025
1140oC
Ferrite
64
26.37
5.05
3.36
0.05
1.36
0.038
Austenite
36
24.04
7.15
2.33
0.47
1.83
0.026
1 Nitrogen in ferrite was estimated to be at the saturation value of ~0.05%, the rest partitions to austenite
Sample Preparation
The samples were cut into 2 mm thick small discs. The discs were then successively grinded with 200
Grit, 600 grit and 800 grit SiC polishing paper, followed by ultrasonic cleaning in DI water. Further
polishing with 6 micron diamond paste and 1 micron diamond paste was done using polishing oil. All
the samples were cleaned carefully to avoid contact with any particulate other than the diamond
polishing pastes and the cleaning solvents (acetone, isopropyl alcohol and DI water). At the end of the
polishing procedure, the samples were rinsed with acetone, isopropyl alcohol and DI water, followed by
drying in the presence of high purity compressed air.
The samples were etched lightly electrolytically for 3-5 seconds in a solution prepared with 20% KOH at
an applied voltage of 2.5 Volts, to allow the ferrite and austenite to be distinguished under the
microscope. The etching also allowed for the removal of the air formed oxides, thus providing a fresh
surface. It also facilitated finding grains and inclusions before and after each test.
After etching, the whole sample was covered with an insulating lacquer except for the area of interest
which consisted of rectangles with an area approximately 200 micrometers by 300 micrometers. No
crevices were ever observed at the edges of the lacquer and this procedure allowed us to isolate a
small part of the surface and consequently to keep the background passive current very low.
Particle Analysis
Following polishing and etching, the samples were imaged using a conventional optical microscope to
select the areas containing inclusions to be analyzed. The ferrite matrix and austenite grains as well as
the inclusions were analyzed by using a Scanning Electron Microscope, equipped with an Energy
Dispersive Spectrometer (EDS), which provides X-ray mapping and particle analysis.
Figure 1 shows the Scanning Electron Microscopy (SEM) image of sample 4 (annealed at 1140oC) in
the area selected (Area 2) for SEM and particle analysis prior to the test. The dark interconnected
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3

areas correspond to the ferrite matrix, whereas the light discrete areas correspond to the austenite
grains. There are several inclusions in the surface of the sample which were circled within the image.
The inclusions are typically in the order of 5 to 10 micrometers. However, larger inclusions as large as
20 micrometer across have been found, for example the inclusions that appear in the red rectangle.
Figure 1: Scanning Electron Microscopy (SEM) of sample 4 (annealed at 1140oC), Area 2. Dark
gray areas correspond to the ferrite matrix, whereas light areas correspond to the austenite
grains.
Figure 2 shows the Energy Dispersive Spectrum (EDS) as well as the image of sample 4 annealed at
1140oC in the area selected (Area 2) for particle analysis (red rectangle in Figure 1). Again, the
interconnected areas correspond to the ferrite matrix, whereas the light discrete areas correspond to
the austenite grains. Notice that the largest area with the higher density of inclusions (from Figure 1)
was selected for analysis using the EDS. The composition of three different inclusions as well as the
composition of an area containing grains of ferrite and austenite were analyzed, see yellow square in
figure 2. Notice that the area highlighted in yellow inside the spectrum corresponds to the main
elements present in the alloy (Cr, Mn, Fe, Ni, Cu and Mo). Whereas the elements that are associated
with the inclusions (O, Mg, Al, Si, S, Ca and Ti) are highlighted in different shades of blue. Table 3
shows the chemical composition of each of the three inclusions as well as the overall composition of
the square area highlighted in yellow in Figure 2.
4
©2013 by NACE International.
Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to
NACE International, Publications Division, 1440 South Creek Drive, Houston, Texas 77084.
The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.
4

12345678910
keV
0
10
20
30
40
50
60
cps /eV
Fe
Fe
Mn
Mn
Cr
Cr
Ti
Al
Mg
O
Ni
Ni S
Mo
Ca
Si
Cu
Cu
Figure 2: SEM image (top) and Energy Dispersive Spectrum (EDS) of sample 4 (annealed at
1140oC), Area 2. Dark gray areas correspond to the ferrite matrix, whereas light areas
correspond to the austenite grains.
Table 3 shows the composition obtained by using Energy Dispersive Spectrometry (EDS) of the three
different inclusions as well as the composition of the area containing grains of ferrite and austenite, see
yellow square in figure 2. The highlighted elements in Table 3 show those elements that are present in
the inclusions. The inclusions have different concentrations of the following elements: O, Mg, Al, Si, S,
Ca and Ti. They also contain significant amounts of Fe, Cr and Mn. Some of them have a relatively high
concentration of oxygen, must likely because they contain metallic oxides.
©2013 by NACE International.
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5

The inclusions can be located either in the austenite grains, the ferrite matrix or in between both of
them.
Table 3
Chemical composition obtained by using Energy Dispersive Spectrometry (EDS) of the 3
inclusions and the clean area in sample 4 (annealed at 1140oC), Area 2, shown in Figure 2.
* = Not detected or below the detection limit
Figure 3 shows the optical image of sample 4 annealed at 1140oC in the area selected (Area 2) for the
test. Again, the interconnected areas correspond to the ferrite matrix, whereas the light discrete areas
correspond to the austenite grains. Notice that although an effort was made to isolate the exact area
that was analyzed using the SEM and EDS (Figure 1 and 2), it is quite difficult to isolate such a small
area and a larger area was isolated for the test which will be discussed below. Also notice that the area
inside the red rectangle shows the area analyzed using the EDS.
Figure 3: Optical image of sample 4 (annealed at 1140oC), Area 2. The black rectangle in the
figure on the left (obtained while the sample was dry) shows the area of the sample that was
isolated and used for the test inside the mini-autoclave. The image in the right, shows the area
isolated and used for the test inside the autoclave while exposed to the liquid/gas environment.
The red rectangle in the left image shows the area analyzed using EDS (Figure 2)
In-situ Microscopy with Concurrent Electrochemistry (EC) at HT/HP
Anodic polarization was conducted at high temperature and high pressure in 3.5% NaCl solution with a
~2 bar partial (200 kPa) pressure of CO2 in the mini-autoclave setup shown in Figure 4.
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6

Figure 4: Experimental setup showing the mini-autoclave and hardware used in the present
work
A stereo microscope was used to image the samples through a quartz window that allowed capturing
the images and real time videos of the surface of the sample while immersed in the liquid/gas
environment.
Figure 4 shows the different parts of the setup used in the present study. The sample (working
electrode), pseudo reference electrode (Pt wire) and counter electrode (Pt wire) are all inside the mini-
autoclave. They are connected to the potentiostat, which is connected and operated using a laptop
computer. Also, the stereomicroscope is connected to the laptop computer and proprietary software
allows capturing in real time both the image on the surface of the sample as well as the anodic
polarization from the proprietary software that controls the potentiostat.
This configuration and methodology allowed the observation of the inclusions, which are precursor sites
for pitting, in real time () in the alloy while exposed to the liquid/gas mixture. Before the tests, Open
Circuit Potential (OCP) measurements were continuously taken until stable OCP signals were
achieved. The fluctuation on the measured value was below 20 mV. After the OCP measured was
stable, anodic polarizations and real time video of the surface of the sample were collected using the
laptop computer.
RESULTS
Microscopy and Anodic Polarization at 100oC in 3.5% NaCl and 120 psi (827 kPa)
Figure 5 shows a snap shot of the video obtained during these tests. The window in the left shows the
image of the surface of the sample. The window in the right shows the plot obtained using the
proprietary software that controls the potentiostat. The X axis in the plot corresponds to the potential
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7

applied to the sample (in Volts). The Y axis corresponds to the measured current (in Amps). The small
rectangle in the middle at the bottom of the image shows some of the data related to the video. In this
case, it shows the number of frames and the time elapsed since the beginning of the test (66 seconds).
The plot in Figure 5 shows the Potentiodynamic scan in the positive direction (Cathodic Part of the
curve) of Sample 4 (annealed at 1140oC), Area 2 obtained at 100oC in 3.5% NaCl and 120 psi (827
kPa) total pressure. The left window shows the image of the surface of the sample including the grain
boundaries and some of the inclusions (shown in Figures 1 to 3).
Figure 5: Anodic Polarization (Cathodic Part of the curve) of the sample annealed at 1020oC
while immersed in a solution of 3.5% NaCl at 100oC and 120psi (827 kPa).
Figure 6 shows the anodic polarization on Area 2 of Sample 4 annealed at 1140oC just before
metastable pitting starts, obtained at 100oC in 3.5% NaCl and 120 psi (827 kPa) total pressure. Again,
the left window shows the image of the surface of the sample with no corrosion activity, whereas the
right window shows that the sample is passive at a potential of +300 mV.
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8

Figure 6: Anodic Polarization (just before stable pitting starts) of the sample annealed at 1020oC
while immersed in a solution of 3.5% NaCl at 100oC and 120psi (827 kPa).
Figure 7 shows the anodic polarization on Area 2 of Sample 4 annealed at 1140oC at the beginning of
the pitting event , obtained at 100oC in 3.5% NaCl and 120 psi (827 kPa) total pressure. Notice that the
left window shows the image of the surface of the sample with the initial breakdown in an area
containing an inclusion. The right window shows the transition from passivity to pitting; as indicated by
the sudden increase of the current at +350 mV.
Figure 7: Anodic Polarization (just after pitting initiated) of the sample annealed at 1020oC while
immersed in a solution of 3.5% NaCl at 100oC and 120psi (827 kPa).
Figure 8 shows the anodic polarization on Area 2 of Sample 4 annealed at 1140oC after the pit has
increased in size ,obtained at 100oC in 3.5% NaCl and 120 psi (827 kPa) total pressure. Notice that the
left window shows the image of the surface of the sample with a fully developed pit. The right window
shows the increase of the current at +350 mV. The increase in current following the developing of the
pit was confirmed visually in the surface of the sample.
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9

Figure 8: Anodic Polarization (pit has increased in size) of the sample annealed at 1020oC while
immersed in a solution of 3.5% NaCl at 100oC and 120psi (827 kPa).
The maximum current measured during these experiments was not allowed to increase beyond 100
microamperes, mainly to avoid destroying most of the surface of the sample following the pitting events.
Figure 9 shows the image of the surface of the sample before (left) and after the test was completed;
with a fully developed pit in the left side of the sample where an inclusion was present.
Figure 9: Optical image of sample 4 (annealed at 1140oC), Area 2. The green circle shows the
area with an inclusion before and after the test.
Further work is being carried out trying to decrease the size of the area exposed to the environment.
However, as the size of the exposed area decreases, imaging of the grains and inclusions becomes
more challenging, since the amount of light that arrive to the surface of the exposed sample is smaller.
This work was done on an area approximately 200 by 300 micrometers in each side.
CONCLUSIONS
A new mini-autoclave that allows the in-situ micro visualization in real time of pitting of CRAs at high
temperature and high pressure was developed.
This methodology allowed real-time in-situ observation of pitting in a 22Cr DSS UNS32550. The
increase in the current measured was accompanied by visual confirmation of stable pitting that
developed in an inclusion on the surface of the sample.
Small metastable events could not be fully resolved in the surface of the sample because of their size
and the final lateral resolution of the camera. Future work will aim at imaging metastable events as well
as analyzing smaller areas without losing the optimal resolution in the optical image.
The maximum Temperature (400oC) and Pressure (4000 psi or 27,579 kPa) rating for the window and
the experimental setup can allow in-situ studies in more aggressive conditions.
©2013 by NACE International.
Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to
NACE International, Publications Division, 1440 South Creek Drive, Houston, Texas 77084.
The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.
10

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©2013 by NACE International.
Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to
NACE International, Publications Division, 1440 South Creek Drive, Houston, Texas 77084.
The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.
11