Electrochemical Corrosion in H2S

Containing Alkaline Brines –

Experimental Measurements and OLI

Simulations

1

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

2

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.

3

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.

4

Test conditions and preparation of the steel samples

4

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.

5

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

6

In H2S free solutions, the concentrations

of CO2(aq) and HCO3- (aq) is decreased

with pH increase.

Calculated (OLI) primary species

7

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

8

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

9

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

10

-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

11

-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

14

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

15

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

16

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.

19

20

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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21

Disclaimer

21

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

privately owned rights. Reference herein to any specific commercial

product, process, or service by trade name, trademark, manufacturer, or

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

necessarily state or reflect those of the United States Government or any

agency thereof.