Electrochemical Corrosion in H S Containing Alkaline...
Transcript of Electrochemical Corrosion in H S Containing Alkaline...
Electrochemical Corrosion in H2S
Containing Alkaline Brines –
Experimental Measurements and OLI
Simulations
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R. Fenga,b, J. Becka,b, S. Lvova,b,c, and
M. Ziomek-Morozd
aThe EMS Energy Institute bDepartment of Energy and Mineral Engineering
cDepartment of Materials Science and Engineering
The Pennsylvania State University, University Park, PA 16802, USA
dU.S. Department of Energy
National Energy Technology Laboratory, Albany, OR 97321, USA
OLI Simulation Conference 2014
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HIC/ SSC:
H2S(aq) = H2S(ad)
H2S(ad) +e- = H (ad) + HS-(ad)
H (ad) + H (ad) = H2 (ad)
HS-(ad)+H3O+(aq)=H2O(l)+H2S(ad)
(catalytic effect in acidic condition)
Iron Sulfide:
Fe(s) = Fe2+(aq) + 2e-
Fe2+(aq)+HS-(aq)=FeS(s)+H+(aq)
SSC
Sulfide Stress Cracking. Modified from
http://www.tenaris.com/en/Products/OCTG/SteelGrades/SourService.aspx
H2S can cause hydrogen induced cracking (HIC)
• Sulfur is believed to be a hydrogen
recombination poison, increasing the
risk of HIC.
• Dissolved H2S can decrease the
fatigue limit of carbon steel by 20%.
The presence of dissolved H2S can provide an
alternative cathodic mechanism, boosting
corrosion rates.
Mehdizadeh, P., 1966.
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1- Heating mantle; 2- Autoclave stainless steel vessel;
3- PTFE-coated thermo couple; 4- Ag/AgCl reference
electrode; 5- PTFE assembly; 6- Working electrode;
7- Pt counter electrode; 8- H2S/N2 cylinder Home-made reference electrode
Three-electrode system in an autoclave Three-electrode system in a Parr autoclave
• Can operate up to 250 oC and 3000 psi
• Be able to study corrosion due to CO2
and H2S in deaerated conditions.
The electrode assembly with working and
counter electrodes.
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Test conditions and preparation of the steel samples
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Background
(Buffer) Soln.
t (ºC) NaCl NaHCO3 Na2CO3 NaOH pH
#1 85 5% wt 0.5 mol/kg 0 mol/kg 0 mol/kg 7.5
#2 85 5% wt 0 mol/kg 1 mol/kg 0 mol/kg 10.6
#3 85 5% wt 0 mol/kg 1 mol/kg 1 mol/kg 12.3
PH2S (psi) [H2S] (mol/kg) pH (OLI)
Soln. 1 Soln. 2 Soln. 3
0 0 7.53 10.60 12.26
0.12 2.63×10-4 7.53 10.60 12.26
1.2 2.63×10-3 7.52 10.57 12.26
10 2.19×10-2 7.21 10.36 12.24
• Electrodes were grounded by 600 (16 micron) and 800 (12 micron) grits SiC papers
• Polished by 1 and 0.05 micron alumina polishing solution until mirror-smooth.
• Washed by isopropanol (IPA) and distilled water and wiped dry.
• The concentration of H2S
was calculated using a
corresponding PH2S.
• Adding H2S slightly
decreased the pH.
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Chemical compositions (wt.%) and mechanical properties
of samples
5 K. A. Rozman et al., 2010.
Property S-135 UD-165
Yield Strength (MPa)
1072 1183
Ultimate Tensile Strength (MPa)
1163 1257
Hardness (HRC) 35.1±0.1 40.1±0.2
Microstructures of a) S-135 and b) UD-165 etched with Nital (2wt.% nitric acid in ethanol).
• The solutions were deaerated with N2 for an hour before corrosion tests.
• In-situ electrochemical measurements were performed with the time interval of one hour over the
exposure period. Around 60 h was needed to reach a steady-state.
• Electrochemical test methods used:
Linear Polarization Resistance (LPR): Rpol (corrected for Rsol from EIS)
• |η|<10 mV, j= η/Rpol
Electrochemical Frequency Modulation (EFM): Tafel slops
• Direct calculation of Tafel slops without damaging the sample surface.
Electrochemical Impedance Spectroscopy (EIS) within 5mHz-300kHz range
Stern-Geary equation was used to calculate the in-situ corrosion rate from Rpol:
icorr =1/(Rpol B’ ) where Rpol =Rmeas – Rsol, and, B’=(ln10)(bc+ba)/(bc ba)
• Post-corrosion surface investigations:
Optical and scanning electron microscopy (SEM) for morphology
Energy dispersive X-ray spectroscopy (EDS) for the chemical composition of corrosion product.
Wavelength dispersive X-ray spectroscopy (WDS) for sulfur detection.
Experimental procedures
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In H2S free solutions, the concentrations
of CO2(aq) and HCO3- (aq) is decreased
with pH increase.
Calculated (OLI) primary species
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1.0E-12
1.0E-10
1.0E-08
1.0E-06
1.0E-04
1.0E-02
1.0E+00
7 8 9 10 11 12 13
Con
cen
trati
on
(m
ol/
kg H
2O
)
pH
Solutions 1, 2, and 3 with H2S
CO2 (aq)
HCO3-
CO3--
H2S (aq)
HS-
S--
1.0E-12
1.0E-10
1.0E-08
1.0E-06
1.0E-04
1.0E-02
1.0E+00
7 8 9 10 11 12 13
Con
cen
trati
on
(m
ol/
kg H
2O
)
pH
Solutions 1, 2, and 3 without H2 S
CO2 (aq)
HCO3-
CO3--
Adding 2.63×10-3 mol/kg H2S
(PH2S=1.2 psi) the carbonate species
were almost the same and
concentation of S2- is incresed with
pH increased.
LPR increased with time then reached
the steady state around 60 hours.
LPR and EIS over time
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0
1,000
2,000
3,000
4,000
0:00 12:00 24:00 36:00 48:00 60:00 72:00 Lin
ear
Pola
riza
tion
Res
ista
nce
(oh
m)
Time (hh:mm)
-500
0
500
1,000
1,500
2,000
2,500
3,000
3,500
4,000
4,500
0 1,000 2,000 3,000 4,000
-Zim
g (
oh
m)
Zreal (ohm)
0:30
12:30
48:30
64:00
Conditions: UD-165 at [H2S]=2.63 × 10-3 mol/kg and pH 10.6
Below 48 h the low frequency impedance in
EIS increased with time and then got a
steady state.
• The corrosion rates is generally decreased at high pH 12.3.
• Several cases showed that CR was the highest at pH 10.5.
• At some pH the addition of H2S increases corrosion rate.
Corrosion rates (CR) from LPR (+EFM,+EIS) results
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1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+01
7 8 9 10 11 12 13
Corr
osi
on
Rate
(m
m/y
r)
pH
UD165-0
UD165-0.12
UD165-1.2
UD165-10
1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+01
7 8 9 10 11 12 13
Corr
osi
on
Rate
(m
m/y
r)
pH
S135-0
S135-0.12
S135-1.2
S135-10
UD-165 S-135
• Magnetic stir 100% output was 700 RPM.
• In H2S free tests, decreasing stir rate resulted in
decreasing the low frequency impedance,
indicating the effect of mass transport in the
solution.
• With [H2S]=2.63×10-3 mol/kg , the EIS plots
overlapped when decreasing stir rate, indicating
that the corrosion was not controlled by mass
transport in the solution.
• It is speculated that the corrosion rate was
limited by an adsorption/desorption process with
the presence of H2S.
EIS Results: Effect of stir rate
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-500
0
500
1,000
1,500
2,000
2,500
3,000
3,500
4,000
4,500
0 1,000 2,000 3,000 4,000 5,000
-Zim
g (
oh
m)
Zreal (ohm)
UD-165 at pH 10.6, no H2S
75%
50%
25%
0%
-500
0
500
1,000
1,500
2,000
2,500
3,000
3,500
4,000
4,500
0 1,000 2,000 3,000 4,000 5,000
-Zim
g (
oh
m)
Zreal (ohm)
UD-165 at pH 10.6 with H2S
75%
50%
25%
0
1000
2000
3000
4000
5000
0% 25% 50% 75% 100%
Zre
al
(oh
m)
Stir Rate (%output)
Low frequency impedance
with no H2S
• The OLI calculation was with scales and passivating film included.
• The corrosion potentials or open circuit potential (OCP) generally became more
negative with pH, according to both the measurements and OLI calculation.
• At PH2S=10 psi, measured OCP matched the corrosion potentials calculated
with OLI very well.
• The corrosion current density at pH 12.3 agreed with OLI calculation, but
diverged at pH 7.5 and 10.5.
Corrosion potential and current density:
Experiment vs. OLI simulations
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-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
7 8 9 10 11 12 13
Co
rro
sio
n P
ote
nti
al
(V,
vs S
HE
)
pH
UD 165
OCP-0.12
OCP-10
OLI-0.12
OLI-10
1.0E-07
1.0E-06
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
7 8 9 10 11 12 13
Cu
rren
t d
en
sit
y (
A/c
m2)
pH
UD 165
jcorr-0.12
jcorr-10
OLI-0.12
OLI-10
Stream Parameters
• Stream Amount (mol): the total amount of all the substances in the solution. Automatically calculated by OLI after the input of chemicals.
• Temperature: 85 oC.
• Pressure: 113 psi
Calculation Parameters
• Flow Type: Complete Agitation was chosen.
• Ignore Influence of Scales: “Scales included-passivating films included.”
Inflows(mol)
• Water: 55.51 mol
• Sodium chloride: 0.9006 mol/kg (5%wt.)
• Sodium carbonate: 1 mol (e.g. Soln. #2, pH ~10.3)
• Hydrogen sulfide: 2.19 ×10-2 mol/kg (equivalent to 10 psia H2S at 85oC)
• Iron: 0 (to calculate the initial CR)
Contact Surface
• Fe (pure) or Carbon steel G10100(generic)
was used to select iron redox couple.
Corrosion rate - OLI settings
OLI Analyzer interface
• In most cases, the OLI calculated CRs were higher when scales were excluded
than included excepted for that at PH2S=10 psi and pH 10.4.
• The experimental CRs were closer to the CRs calculated with scales included
than excluded, except for that at PH2S=0.12 psi and pH 10.6.
Corrosion rates with scales included and excluded
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1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+01
1.0E+02
7 8 9 10 11 12 13
Corr
osi
on
Rate
(m
m/y
r)
pH
UD165-0.12
OLI-0.12
OLI-0.12ex
1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+01
1.0E+02
7 8 9 10 11 12 13
Corr
osi
on
Rate
(m
m/y
r)
pH
UD165-10
OLI-10
OLI-10ex
PH2S=0.12 psi, [H2S]=2.63 ×10-4 mol/kg PH2S=10 psi, [H2S]=2.19 ×10-2 mol/kg
17Fe14C67O, 19Fe45S19O,
34Fe32S33O
Surface analysis of UD-165 with [H2S]=2.63 ×10-3 mol/kg
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22Fe17C56O, 28Fe62O 32Fe60O, 75Fe16O
• At pH 7.5, the steel sample was corroded more severely.
• There is significant amount (25at%~70at%) of sulfur detected by EDS at pH 7.5.
• Corrosion products changed from Fe, S-rich to oxides with increasing pH, which
agrees with Pourbaix diagram.
pH 7.5 pH 10.6 pH 12.3
Pourbaix diagram of Fe-H2S-H2O-CO2 system
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The thermodynamically predominant oxidized Fe species were simulated to change
from FeS(s) / FeS2 (s) / FeCO3 (aq) to FeS (s) / Fe3O4 (s) to HFeO2-
(aq) / Fe3O4 (s) when pH
increased from 7.5 to 12.3.
Condition: 85 ºC, 26.696 psia. [H2S]= 2.63 ×10-3 mol/kg. Titrant: NaOH and HCl. (OLI analyzer 9.0)
Soln. #1 Soln. #2 Soln. #3
CR as a function of [Fe] - Soln. #2, PH2S=10 psi, scales
included
Fe concentration in the solution can significantly affect CR taking into
account the scale formation. Measurement of Fe(aq) concentration is needed.
18 Fe concentration in the solution can slightly affect CR without taking into
account the scale formation. Measurement of Fe(aq) concentration is needed.
CR as a function of [Fe] - Soln. #2, PH2S=10 psi, scales exclude
Summary
• In-situ corrosion rates of ultra-deep drilling carbon steels were measured with
electrochemical methods in a HPHT autoclave simulating a brine with different
concentrations of H2S and pH at 85 oC and 113 psi.
• Increasing pH up to 12.3 generally decreased the corrosion rates despite of PH2S.
Several cases showed that CR was the highest at pH 10.5. This could be related
with pitting corrosion, the nature of corrosion products, and the concentration of
dissolved Fe in the closed autoclave system.
• Unlike the H2S free tests, the corrosion rates with [H2S]=2.63×10-3 mol/kg
(PH2S=1.2 psi) were not limited by the mass transport in the solution at steady
state.
• With H2S concentration of 2.63×10-3 mol/kg (PH2S=1.2 psi), the corrosion
products changed from Fe, S-rich to oxides with pH increase, according to both
the Pourbaix diagram and the EDS surface analysis.
• The only way to properly study the corrosion phenomena in HPHT
environments is to combine (1) electrochemical experiment with different
techniques and sophisticated data treatment, (2) spectroscopic surface
analysis, and (3) computer simulations using OLI software.
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Acknowledgments
• This project was initiated in support of the National Energy Technology
Laboratory’s ongoing research in “Catalytic properties of H2S in corrosion
degradation of High Strength Steels”.
• The authors would like to thank Mr. Keith Collins for analytical support.
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Disclaimer
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This report was prepared as an account of work sponsored by an agency of
the United States Government. Neither the United States Government nor
any agency thereof, nor any of their employees, makes any warranty,
express or implied, or assumes any legal liability or responsibility for the
accuracy, completeness, or usefulness of any information, apparatus,
product, or process disclosed, or represents that its use would not infringe
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otherwise does not necessarily constitute or imply its endorsement,
recommendation, or favoring by the United States Government or any
agency thereof. The views and opinions of authors expressed herein do not
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agency thereof.