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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
Experimental Results
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
Evaluation of the EAC Susceptibility for Condition 2
Experimental Results
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
Evaluation of the EAC Susceptibility for Condition 2
Experimental Results
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)
Evaluation of the EAC Susceptibility for Condition 2
Experimental Results
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
Poly ethylene dimethyl +
Brine mixture + metal
cations
Poly ethylene dimethyl +
Brine mixture +metal
cations + 1g/l S
Poly ethylene dimethyl +
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
Evaluation of the EAC Susceptibility for Condition 3
Experimental Results
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 .
Evaluation of the EAC Susceptibility for Condition 3
Experimental Results
Chronoamperometric Response for UNS S30403 Stainless Steel Polarized at differentpotentials above the OCP under study condition #3 (80°C, sat. N2).
Evaluation of the EAC Susceptibility for Condition 3
Experimental Results
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.