Pit Metastability and Stress Corrosion Cracking ......Pit Metastability and Stress Corrosion Cracking Susceptibility Assessment of Austenitic Stainless Steels in Sour Gas Service Conditions

Pit Metastability and Stress Corrosion Cracking Susceptibility Assessment of Austenitic Stainless Steels

in Sour Gas Service Conditions

Raymundo P. Case

Texas A&M UniversityMaterials Science & Engineering

National Corrosion and Materials Reliability CenterCollege Station, TX 77843

• Pitting and environmentally assisted cracking (EAC) is the principal cause ofCRA failures.

• Pitting and EAC are related events, although the presence of pitting does notimply EAC.

• From both the Kondo and Tsujikawa criteria it can be inferred that Metastable pits are critically more relevant to EAC than stable pits.

• The Kondo condition: involves fracture mechanics and indicates that KI>KIEAC forEAC, where KI and KIEAC are the stress intensity and critical intensity

• The Tsujikawa condition: establishes that nucleated crack must be able to“outrun” the pit that is, Vcrack>Vpit, where Vcrack and Vpit are the crack propagationand pit penetration rates, respectively.

Introduction

Research Objectives

• The objective

is to obtain the information associated to the likelihood ofcracking and the estimation of the time to failure from theelectrochemical characterization of the pitting behavior.

Procedure

Pote

nti

ost

atic

tes

ts

Pit metastability –stability transition

Probability of cracking

Estimation of the time to failure

Data analysis algorithms& modeling

Stochastical modeling

Modeling of EAC Susceptibility

Active dissolution (enhanced by stress at the tip)

Passive dissolutionDiffusion

nctn

fn1

n00i

avi

i0

0

σ

Cogleton’s correlation

The stress effect term

•εf is the fracture strain for the oxide passive layer, which is taken to be εf=8x10-4 for stainless steel

•0 is the time of exposure of the bare metal surface between fracture events,

•n is the decay factor (for a diffusion control process n=1/2)

•i0 is the average pit dissolution current density

Modeling of EAC Susceptibility

To model crack initiation it is assumed that 0 can be equated to the propagationtime of a metastable pit, then a pitting event frequency 0 can be defined asω0=1/0

Thus it can be shown that from Cogleton’s correlation the following condition mustbe fulfilled by 0 in relation to the Tsujikawa and Kondo conditions:

2

3

0ATsujikawa,0 iC

6

IEAC0BKondo,0

KiC

For a typical 300 series stainless steel it can be shown that

0,Kondo < 0,Tsujikawa

Likelihood of Cracking

According the assumption of the EAC modeling, the cracks to propagatemust satisfy the Tsujikawa condition, thus a data analysis algorithm is usedbased on the following condition:

Critical condition for EAC propagation:

zF

Mi

zF

Mi

dt

da 0av

events

crackscrack

N

NP

By keeping the count of the number of events that satisfy the previouscondition the cracking probability can then be defined as:

0 5 10 15 20 250

100

200

300

400

500

600

700

Δt

!n

ett;nP

tn

The experimental evidence suggest thatthe distribution of repassivation timesfollow a Poisson distribution

SS 316, Brine + Selexol, 300ºF, 34 psi H2S, 100 ppm Cl-

Where is the repassivation constant,defined as:

crackexpP

Estimation of the time to failure

t1

t2t3

t4

exp is the assessed from the averagefrequency of pitting events recordedduring the potentiostatic experiment

Experimental Validation of the EAC Likelihood Model

• Previous results have shown that the pits in 316 (UNS S31603) and 304 (UNS S30403) stainless steels

exposed to H2S containing brines exhibit metastable propagation along a wide range of potentials.

• The proposed model is developed to evaluate the likelihood of EAC from the current transient values

measured in a potentiostatic electrochemical experiment.

• To test the validity of the assumptions considered, the output of the calculations in terms of the

probability of cracking and the expected time-to-failure will be compared to the information provided by

actual EAC cases reported from the operation of a natural gas processing plant that handles sour gas

(20% CO2, 12% H2S).

Experimental Validation of the EAC Likelihood Model

Experimental Parameters of the Different Test Conditions Studied

Condition Temperature (ºC)

Environment Acid Gas content

Material

1 65 Brine with 100 ppm Cl- 0.23 MPa H2S +0.013 Mpa CO2

316L (UNS S31603)

2 80 Polyethylene dimethyl ether+Brine with 780 ppm Cl-

None reported 316L (UNS S31603)

3 143 Polyethylene dimethy l ether+Brine with 780 ppm Cl-

None reported 304 (UNS S30403)

Test Matrix

From the cases where EAC was identified as a cause of failure, the test objective is:

• Reproduce the conditions using conventional laboratory techniques

• Verify the EAC susceptibility by comparing the likelihood of cracking and the time to failureforecast with the results obtained from the failure analysis investigation

Experimental Results

0.00E+00

1.00E-05

2.00E-05

3.00E-05

4.00E-05

5.00E-05

6.00E-05

0.00 2000.00 4000.00 6000.00 8000.00 10000.00 12000.00 14000.00

time (s)

cu

rre

nt

(A)

5 mV vs OCP

50 mV vs

OCP

Current Transient Curves Obtained from the Potentiostatic Tests ofUNS S31603 in the Brine Saturated at 34 psia H2S, 100 ppm Cl

- and65ºC.

The evaluation of the current transientsat 50 mV vs. OCP indicates that theaverage number of pitting events is0.88 pits/min, which corresponds to apit propagation time of 69 s.

Evaluation of the EAC Susceptibility for Condition 1



0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2

normalized stress (S/Sy)

Cra

ck

ing

Pro

ba

bilit

y

Actual threshold stress

50% of yield strength

Actual time to failure

reported 168 hrs

Behavior of the Cracking Probability and the Probability of Failure as a function of exposure time for theUNS S31603 in Brine, 34 psia H2S, 100 ppm Cl

-, 65ºC

At the reported value of the time-to-failure (168 hrs) of the actual component the likelihood of failure model

suggest a failure probability greater than 75%, depending on the stress applied, which suggests that the model

proposed can forecast the observed time-to-failure with high confidence

file:///C:/AppData/Project File Final EEF175/LCGP/Figure 6 v2.jpgfile:///C:/AppData/Project File Final EEF175/LCGP/Figure 6 v2.jpg



Sulfur

rich

Base metal

Elemental Distribution Obtained from EDS Scanning of theSurface from the Base Metal Pipe from study Condition #2,Showing the Concentration of S inside the Cracks

Experimental Sequence for the Electrochemical Tests Performed to Assess EAC Susceptibility in UNS S31603 Stainless Steel under the Conditions of Study Condition #2

Test sequence number

Environment

2a Fresh lean Polyethylene dimethyl ether / Brine

2b Lean Polyethylene dimethyl ether / Brine+ metal cations

2c Lean Polyethylene dimethyl ether / Brine + metal cations+ 1 g/l S

2d Lean Polyethylene dimethyl ether / Brine + metal cations + 8 g/l S

Based on these observations, a set of experiments was devisedto test EAC susceptibility on the UNS S31603 stainless steel, byintroducing contaminants.

The experiments were performed at 80°C and under constantN2 sparging to avoid oxygen contamination of the test solution



Observed Frequency of Pitting Events in the Different Experiments Performed to Evaluate the EAC Susceptibility of UNS S31603 Stainless Steel for Study Condition #2

Test condition Pitting frequency (pits/min)

Polyethylene dimethyl ether+ Brine +metal cations 16.4

Polyethylene dimethyl ether + Brine +metal cations + 1 g/L S 14.3

Polyethylene dimethyl ether+ Brine +metal cations + 8 g/L S 14.6

Chronoamperometric Values for the UNS S31603 Stainless Steelexposed to the study condition #2 (80ºC, sat. N2, OCP+50 mV)



0

0.02

0.04

0.06

0.08

0.1

0.12

0 0.2 0.4 0.6 0.8 1 1.2

S/Sy

P c

rack

Poly ethylene dimethyl +

Brine mixture


Brine mixture + metal

cations


Brine mixture +metal

cations + 1g/l S


Brine mixture + metal

cations + 8.1 g/l S

SEM Image of the Pits at the Metal Surface at 1 g/l, and thecorresponding EDAX Element Map for S

Crack Probability as a Function of the Applied Stress

the Probability of Failure as a Function of Exposure Time for Each ofthe Conditions Tested at 2/3 of the Nominal Yield Stress, where theDotted Line Represents the Actual Time to Failure Reported



Experimental Sequence for the Electrochemical Tests Performed to Assess SCC Susceptibility in the UNS S30403 Stainless Steel under Study Condition #3.

Test sequence number Environment

3a Fresh Polyethylene dimethyl ether /Brine

3b Fresh Polyethylene dimethyl ether /Brine+ 1 g/l S

• The experiments were performed at 80°C under constant N2 deaeration to avoid oxygen

contamination of the test solution.

•The test temperature was significantly lower than that in the reported operating condition; because of

the experimental limitations of the electrochemical instrumentation.

• To compensate for the lower temperature, the testing was performed at a polarization level of 150 mV

above the recorded OCP at 80 ºC , which is consistent with a 50 mV at 143 ºC .



Chronoamperometric Response for UNS S30403 Stainless Steel Polarized at differentpotentials above the OCP under study condition #3 (80°C, sat. N2).



Elemental Distribution Obtained from EDS Scanning of the PittedSurface of the UNS S30403 Stainless Steel Specimens Tested with thePolyethylene Dimethyl + Brine + 1 g/l of S at 80 ºF and 150 mV + OCP

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2

S /Sy

Cra

ckin

g p

rob

ab

ilit

y

Lean Poly ethylene dimethyl

Lean Poly ethylene dimethyl

+ 1g/L S

Results Obtained from the EAC susceptibility assessment based

on the Electrochemical Tests Performed on UNS S30403

Stainless Steel under the study condition #3.

A) Crack Probability as a Function of the Applied Stress

B) Probability of Failure as a Function of Exposure Time for each

of the Conditions Tested at 1/2 of the Nominal Yield Stress,

where the Dotted Line Represents the Actual Time-to-Failure

Reported (5760 hrs)

A

B

Pitting Event Frequency and EAC Susceptibility

Discussion

To test the validity of the hypothesis used in the modeling, it is necessary to verify that the experimental datafrom each test satisfy the relationship between the frequency of pitting events and the Tsujikawa condition

Calculation of the Minimum Pit Propagation Time to Induce EAC, for the Different Conditions Studied

Study Case Threshold stress applied

(/yield, nominal)

Pit current density, i0

(A/cm2)

Measured average pit propagation time,

t0, exp, (s)

Calculated minimum pit propagation time,t0, calc, (s)

1 0.5 1.60x10-05 68.6 51.9

2b 2/3 2.00x10-04 3.8 3.4

2c 2/3 2.20x10-04 4.2 3.1

2d 2/3 1.59x10-04 4.1 2.6

3a 0.5 9.36x10-04 4.9 1.2

3b 0.5 2.86x10-04 4.5 3.6

The results obtained indicate that in all of the conditions tested, the average pit propagation time that wasmeasured is greater than the minimum value calculated.

this result is consistent with the hypothesis considered and demonstrates that the metastable pitting eventsmeasured fulfill the Tsujikawa condition for EAC

Pitting Event Frequency and EAC Susceptibility

Discussion

To demonstrate that the metastable pitting observed in the different tests fulfill the Kondo condition is moredifficult because there are no reported values for the threshold stress intensity, KIEAC, for the materials andconditions evaluated.

However it is possible to estimate a minimum KIEAC value for each of the conditions tested.

Calculation of the Minimum Threshold Stress Intensity (KIEAC) to Induce EAC for the Different Conditions Studied

Study Case KIEAC calculated (MPa m ½)

1 1.4

2a 1.7

2b 1.8

2c 1.7

3a 1.7

3b 1.4

CONCLUSIONS

1. The modeling approach for estimating EAC susceptibility allows effective

forecasting of the expected time-to-failure at each of the conditions

studied.

2. The measured experimental frequencies of pitting events are consistent

with the conditions proposed by the modeling approach, demonstrating

that they fulfill the Tsujikawa criteria for EAC susceptibility.

3. The validation of the model by experimental simulation of the conditions

associated with EAC in stainless steels in sour service suggests that the

forecasting of EAC susceptibility can be accomplished from

electrochemical evaluation of the chronoamperometric transients typical of

metastable pitting.

Pit Metastability and Stress Corrosion Cracking ......Pit Metastability and Stress Corrosion Cracking Susceptibility Assessment of Austenitic Stainless Steels in Sour Gas Service Conditions

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