THE EFFECT OF SENSITIZATION ON STRESS CORROSION CRACKING OF TYPE 316 AUSTENITIC STAINLESS STEEL IN

SULPHURIC ACID SOLUTION

Rokuro NISHIMURAa and Achmad SULAIMANb

a Department of Materials Science, graduate school of Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Sakai, Osaka 599-8531, Japan.

b Research and Development Centre for Metallurgy, Komleks PUSPITEK, Serpong, Tangerang, 15314, Indonesia.

The stress corrosion cracking ( SCC ) of a commercial austenitic stainless steel type 316 was investigated as functions of applied stress, sensitizing temperature and sensitizing time in 0.82 kmol/m3 sulphuric acid solution by using a constant load method. The applied stress dependence of the three parameters ( lss, steady state elongation rate, tss, transition time, tf , time to failure ) showed three regions irrespective of sensitizing temperature and sensitizing time; the stress- dominated, SCC- dominated and corrosion- dominated regions. In the SCC- dominated region, the logarithm of lss was a linear function of the logarithm of tf for each sensitized specimens, showing that lss became a relevant parameter for prediction of tf, although the slope depended upon sensitizing temperature and sensitizing time. At a sensitizing time of more than 360 ks, the SCC susceptibility was found to reduce because of Cr healing. Sulphate ions became more aggressive to SCC than chloride ions for the specimens with the severest sensitization. The effect of sensitization on SCC and the role of sulphate ions were discussed and further a qualitative IGSCC mechanism was proposed.

1. INTRODUCTION

In a series of papers[1,2], the stress corrosion cracking ( SCC ) of type 316 austenitic stainless steel, which was sensitized at various sensitizing temperatures for 86.4 ks[1] and with various sensitizing times at 923 K[2], was investigated as functions of applied stress (σ) and test temperature in 0.82 kmol/m3 hydrochloric acid ( HCl ) solution by using a constant load method. It was found that a steady state elongation rate ( lss/ms-1 ) obtained from a corrosion elongation curve ( elongation vs. time ) became a relevant parameter for the prediction of time to failure ( tf ) for the sensitized type 316 specimens irrespective of sensitizing temperature and sensitizing time in the solution as well as the solution annealed specimens[3- 5]. In addition, the maximum applied stress (σmax ), the value of tss/tf, the slope of the linear equation between log tf and log lss and the fracture appearance mode ( transgranular or intergranular ) in the SCC- dominated region depended upon sensitizing temperature and sensitizing time. From the results obtained, a

ECS Transactions, 1 (4) 401-411 (2006)10.1149/1.2215524, copyright The Electrochemical Society

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402

qualitative intergranular SCC ( IGSCC ) mechanism was proposed and the role of sulphate ions on SCC was discussed. Recently, it was conducted to investigate the effect of sensitization on the SCC behavior of type 316 specimens in 0.82 kmol/m3 H2SO4 solution[6,7]. The objectives of this work were to clarify the role of sulphate ions and the effect of sensitization on the SCC behavior of the specimens in 0.82 kmol/m3 H2SO4 solution in more details. In addition, a qualitative IGSCC mechanism was proposed to explain the results obtained.

Fig. 1 Corrosion elongation curves at various constant applied stresses for the specimens sensitized at 923 K

2. EXPERIMENTAL

The specimens used were a commercial type 316 austenitic stainless steel ( yield strength: 333 MPa and ultimate tensile strength: 647 MPa ). The geometry of the specimens for SCC experiments was as follows; the gauge length 25.6 mm, the width 5 mm and the thickness 1 mm. The chemical composition ( mass % ) was as follows: C 0.054, Si 0.67, Mn 1.38, P 0.030, S 0.005, Ni 11.16, Cr 17.21, Mo 2.21, Fe balance. The specimens were solution annealed at 1373 K for 3.6 ks under an argon atmosphere and then water quenched. The solution annealed specimens were sensitized in the temperature range of 800 K to 1300 K for 86.4 ks and in the time range of 0 to 1.08 Ms at a temperature of 923 K under the argon atmosphere and then water quenched. The solution annealed and sensitized specimens were polished to 1000 grit emery paper, degreased with acetone in an ultrasonic cleaner and washed with distilled water. The test solution was 0.82 kmol/m3 H2SO4 solution. The test temperature was 353±0.5 K. All experiments were carried out under an open circuit condition. A lever-type constant load apparatus ( lever ratio 1:10 ) to which three specimens can be separately and simultaneously attached was used with a cooling system on the top to avoid evaporation of the solution during experiments. The specimens were insulated from rod and grip by surface oxidized zirconium tube. A change in elongation of the specimens was measured by an inductive linear transducer with an accuracy of±0.01 mm.

3. RESULTS 3.1 Effect of Sensitizing Temperature Fig. 1 shows a representative example of the corrosion elongation curves for type 316 specimens sensitized at 923 K for 86.4 ks at various constant applied stresses in 0.82 kmol/m3

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H2SO4 solution with a test temperature of 353 K. Similar results were obtained for the other specimens sensitized at various sensitizing times and sensitizing temperatures. From the curves, three parameters can be obtained; (1) the steady state elongation rate, lss, (2) the transition time, tss, and (3) the time to failure, tf. If SCC does not occur within a laboratory time scale ( < 107 seconds ), only lss can be estimated. Fig. 2 shows a representative example of the applied stress (σ) vs. tf relationships for the specimens sensitized at the sensitizing temperatures of 923 K and 1123 K for 86.4 ks, where the dashed line is the result for the solution annealed specimens. The relationships for the sensitized specimens were divided into three regions designated by the arabic numerals 1 to 3 in Fig. 2, which correspond to the stress- dominated ( region 1 ), SCC- dominated ( region 2 ) and corrosion- dominated ( region 3 ) regions, respectively. The maximum applied stress (σmax ) in the SCC- dominated region at 923 K became smaller than for the solution annealed specimens, whileσmax at 1123 K became larger than for the solution annealed specimens. The minimum applied stress (σmin ) in the SCC- dominated region became smaller for their sensitized specimens than for the solution annealed specimens. Fig. 3 shows the relationships betweenσand lss for the specimens sensitized at 923 K and 1123 K, where the dashed line is the result for the solution annealed specimens. The relationships were also divided into three regions corresponding to those in Fig. 2. In addition, the clear distinction between regions 2 and 3 was observed in contrast to that in Fig. 2.

Fig. 2 The relationship betweenσand tf for Fig. 3 The relationship betweenσand lss for the specimens sensitized at sensitizing the specimens sensitized at sensitizing

temperatures of 923 K and 1123 K. temperatures of 923 K and 1123 K.

Fig. 4 shows the relationships betweenσand the ratio of tss to tf ( tss/tf ) for the specimens sensitized at 923 K and 1123 K, where the dashed line is the result for the solution annealed specimens. The value of tss/tf for the sensitized specimens in the SCC- dominated region held

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constant independent ofσand increased in order of 1373 K ( solution annealed specimens ) < 1123 K ( sensitized ) < 923 K ( sensitized ). The values of tss/tf in the stress- dominated and corrosion- dominated regions for the sensitized and solution annealed specimens became larger than that in the SCC- dominated region. Thus, the SCC- dominated region was evidently distinguished from the corrosion- dominated and the stress-dominated regions by obtaining the three parameters. Fig. 5 showsσmax andσmin in the SCC- dominated region as a function of sensitizing temperature. The decrease inσmax andσmin in comparison to those for the solution annealed specimens was observed for the specimens sensitized in a sensitizing temperature range of 900 K to 1250 K. However, when the test temperature of 323 K was used because of the reduction in corrosion rate, the increase inσmax for the specimens sensitized at 923 K was observed as well as that in 0.82 kmol/m3 HCl solution at 353 K[1,2], which was indicated by the dashed line in Fig. 5.

Fig. 4 The relationship betweenσand Fig. 5 The maximum applied stress and minimum

sitized applied stress in the SCC- dominated region tss/tf for the specimens sen

at sensitizing temperature of as a function of sensitizing temperature. 923 K and 1123 K. 3.2 Effect of Sensitizing Time

Fig. 6 shows the applied stress (σ) vs. t relationships for the specimens sensitized at the sensitizing times of 3.6 ks, 86.4 ks and 1.08 Ms with a sensitizing temperature of 923 K, where the dashed line is the result for the solution annealed specimens. The relationships for the sensitized specimens were also divided into three regions. Theσ in the SCC- dominated region became smaller for the specimens sensitized at 86.4 ks and 1.08 Ms than for the solution annealed specimens, while the specimens sensitized at 3.6 ks had almost the same value as that for the solution annealed specimens. Theσ

f

max

min in the SCC- dominated region became smaller for their nsitized specimens than for the solution annealed specimens. Here, it should be noted that tf in se

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the SCC- dominated region at a sensitizing time of 1.08 Ms became larger than that at a sensitizing time of 86.4 ks, which implies that SCC susceptibility was reduced. The relationships betweenσand lss for the specimens sensitized at 3.6 ks, 86.4 ks and 1.08 Ms were also divided into three regions corresponding to those in Fig. 6. Fig. 7 shows the relationships betweenσand the ratio of tss to tf ( tss/tf ) for

the specimens sensitized at 3.6 ks, 86.4 ks and 1.08 Ms. The value of tss/tf for the sensitized specimens in the SCC- dominated region held constant independent ofσand increased in order of 3.6 ks < 1.08 Ms < 86.4 ks in comparison to that for the solution annealed specimens, where the value at 3.6 ks was almost the same as that for the solution annealed specimens. Fig. 8 showsσmax andσmin in the SCC- dominated region as a function of sensitizing time. At a sensitizing time of more than 3.6 ks, theσmax andσmin become smaller than those for the solution annealed specimens. Theσmax was the smallest for the specimens sensitized at 86.4 ks, but tended to increase with the longer sensitizing time, while theσmin became constant at a sensitizing time of more than 86.4 ks.

betweenσand Fig. 8 The maximum applied stress and minimum tss/tf for the specimens sensitized applied stress in the SCC- dominated region

Fig. 6 The relationship betweenσand tf for the specimens sensitized at various sensitizing times.

Fig. 7 The relationship at various sensitizing times. as a function of sensitizing time.

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4. DISCUSSION

4.1 A Parameter for Prediction of Time to Failure Fig. 9 shows the relationships between log l and log t (σ: variable ) for the specimens sensitized at 923 K with the test temperatures of 323 K and 353 K, and at 1123 K with a test temperature of 353 K. The relationships in the SCC- dominated region became a good straight line. Such a straight line was applicable to the relationships for the sensitized specimens obtained as a function of sensitizing time. Hence, we got the following empirical equation for the sensitized specimens,

ss f

log lss = - B・log tf + Cb (1)

where the slope, - B, depends on sensitizing time and Ctime and test temperature. Hotemperature, changed from -2 for the solution aat the severest sensitization as the degree of sensitization incr(1) as a function of sensitizing temperature, where the solid circlespecimens. The severest sensitization was faround 900 K. The above linear equation implies that lpredicting tf even for the specimens sensitized as well lss can be obtained at a time within 10 to 20% of t

b is a constant depending on sensitizing

wever, their slopes, depending on sensitizing time and sensitizing nnealed specimens to about -1.2 for the specimens

eases. Fig. 10 shows the slope in eq. is that for the solution annealed

ound to take place at a sensitizing temperature of ss becomes the relevant parameter for

as the solution annealed specimens, because f from the corrosion elongation curve.

Fig.9 The relationship between lss and tf Fig. 10 The slope in eq.(1) as a function of

for the specimens sensitized at the sensitizing temperature, whereσ: sensitizing temperatures of 923 K variable, environment: constant. and 1123 K.

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4.2 Effect of Sensitization The parameters such as t l , t /t ,σ ,σ and the slope in the previous sections were found to depend upon sensitizing temperature and sensitizing time, where their changes are associated with that in the degree of sensitization. The increase in the degree of sensitization corresponds to that in the amounts of Cr carbides and Cr depletion zone along the grain boundaries. Therefore, the results obtained show that the severest sensitization takes place at a sensitizing temperature of around 923 K with

f, ss ss f max min

a sensitizing time of 86.4 ks. The increase in tss/tf, and the ecrease or the increase inσmax andσmin are explained as follows. The Cr carbides act as an

n the grains[8], by which the sensitized specimens would becom

he reason why the value of tss/tf increases with increasing the degree of

nsitization.

dobstacle of dislocation movement i

e harder than the solution annealed specimens. This leads to the increase in the ultimate tensile strength of the sensitized specimens to contribute to the increase inσmax. Although the present results showed the reduction inσmax, this is described in the next section. The increase in the ultimate tensile strength is associated with the increase in a true stress at tss (σtrue=σ/Ass ) under a constant applied stress condition by using an identical geometry of the specimens, where Ass is the cross sectional area at tss. For the increase inσtrue, the distance of the crack propagation up to tss ( Lss ) needs to increase for the reduction in Ass, which means that tss becomes close to tf. This would be tse 4.3 Aggressiveness of Sulphate Ions

The SCC susceptibility of the specimens sensitized at a sensitizing time of more than 86.4 ks was higher in 0.82 kmol/m H SO solution than in 0.82 kmol/m HCl solution. This implies that sulphate ions become more aggressive to SCC of the sensitized specimens than chloride ions, but not to that of the solution annealed specimens, where it should be noted that chloride ions themselves are much more aggressive to general corrosion of the stainless steels than sulphate ions. Therefore, the aggressiveness of sulphate ions to SCC of the sensitized specimens may be explained in terms of the difference in the corrosion behavior between the Cr depletion zone and the other surface. It is reasonable to consider that a corrosio

32 4

3

n rate of the Cr depletion zone in 0.82 kmol/m3 2SO4 solution becomes larger than that of the other surface on which a more protective film is H

formed. This should lead to the formation of a local electrochemical cell between the Cr depletion zone and the other surface. Such a local electrochemical cell would accelerate the corrosion rate of the Cr depletion zone even in the absence of applied stress. In contrast, chloride ions themselves are more aggressive to the general corrosion of the materials than sulphate ions, so that the formation of the protective film on the other surface becomes more difficult than that in 0.82 kmol/m3 H2SO4 solution. This suggests that the magnitude of the local electrochemical cell becomes smaller in 0.82 kmol/m3 HCl solution than in 0.82 kmol/m3 H2SO4 solution, and would be the reason why sulphate ions become more aggressive to SCC of the sensitized specimens than chloride ions. In addition, the magnitude of the local electrochemical cell would increase by the

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of Cr depletion zone becomes large by the combination of a high ess and a high test temperature ( 353 K ). The high corrosion rate caused by the

lowered even at a high plied stress. Consequently, the SCC susceptibility is observed at the high applied stress, that is ,

increase in applied stress and test temperature. The decrease inσmax compared to that of the solution annealed specimens may be explained as follows. The corrosion rateapplied strcombination would be mainly associated with a high dissolution current density ( jd ) followed by a small jf. Hence, the reduction in cross sectional area by jd would be superior to that by crack propagation even if a crack would be formed. This could lead to the intergranular corrosion and finally to a mechanical fracture, but not to the intergranular SCC. As applied stress is lowered, the corrosion rate by the combination is reduced and as a result the crack propagation would become predominant to show the SCC susceptibility at an applied stress of less thanσmax. In the case of the decrease in test temperature, showing the increase inσmax, the corrosion rate of Cr depletion zone would decrease, so that the effect of the combination isapthe increase inσmax. Next, consider the reduction inσmin with increasing the degree of sensitization. In the case of the solution annealed specimens in 0.82 kmol/m3 H2SO4 solution, the applied stress range in the SCC- dominated region becomes very narrow due to a large rate of film formation ( repassivation rate ) at crack tips. This is supported from the fact that the specimens did not failure within a laboratory time scale ( < 107 sec ) in the corrosion-dominated region. However, as the degree of sensitization increases, the repassivation rate at grain boundaries with Cr depletion zone reduces. As a result, the SCC of the specimens would begin to take place at a lower applied stress with increasing the degree of sensitization, that is, the decrease inσmin. 4.4 Application of the Intergranular SCC ( IGSCC ) Mechanism As for the IGSCC mechanism already reported[1-3], grain boundary sliding ( GBS ) becomes an important factor for the sensitized specimens instead of the dislocation movement in the grains for the solution annealed specimens. The IGSCC mechanism is based on a film rupture-formation event with a time interval of Δt at grain boundaries, which is originated from the interaction among the film formed, GBS and dissolution current density ( j ). For each sensitized specimens, it is as

d

sumed that the film rupture takes place when a net local stress at crack ps (σtip ) reaches a critical fracture stress (σF ), whereσtip is composed ofσapp caused byσand

eases with increasing applied ress. Therefore,σtip changes fromσapp toσF and vice versa during the cyclic film rupture-

( ln ) consists of the elongation related film rupture ( lr ) and the elongation caused by GBS during film formation up to the next film

tiσgbs caused by the interaction among the film, GBS and jd: σtip =σapp +σgbs. Theσapp holds constant under a constant applied stress condition, although it incrstformation events. Correspondingly, a net local elongation torupture: ln = lr + lgbs. A corrosion current density ( js ) at crack tips is composed of a film formation current density ( jf ) and jd: js = jf + jd , and a time variation of js ( jf or jd ) during one film rupture- formation event is provided as js ∝ j0（t/t0）- k and jｆ or jd ∝ j0(t/t0)- m , where t is a time

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during the film rupture- formation event, and

j0, t0, k and m are the constants, but depend on nsitizing temperature.

A rate of sliding displacement ( S = nb ) by GBS[9], dS/dt, is associated with jd, which is

here n is the number of dislocations in each pile- up on grain boundaries, b is the Burgers vector, tively. In

dition, a rate ofσtip, dσtip/dt, is considered to be proportional to dS/dt andτ( shear stress:σ= 2

ttained just after film rupture and then keep constant

constant applied stress condition, the following rela(4), dσgbs/dt ∝ dlgbs/dt ∝

Eq. (5) shows that dσgbs/dt and dlgbs/dt On the basis of the above considerations, susceptibility for each sensitized specim

e Fig. 12 Schematic representation of time ilm dependence of ln during the film ious rupture- formation events at various

constant applied stresses.

se connected to dn/dt by injection of defects into grain boundaries[10], dS/dt = bdn/dt ∝ jd/zF (3) wz is the average valence of dissolved metal ions and F is the Faraday constant, respecadτ in the case of a uniaxial tensile stress )[11], dσtip/dt ∝τdS/dt ∝τjd/zF (4) Theσapp and lr would be instantaneously aduring film formation, that is, dσapp/dt = dlr/dt = 0 during film formation. Therefore, under each

tionship would be established from eqs. (3) and

jd/zF ( during film formation ) (5)

depend on only jd, not τ( orσ).

the applied stress dependence of the SCCens is considered as follows. Under a constant

Fig. 11 Schematic representation of tim dependence ofσtip during the f rupture- formation events at var constant applied stresses.

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environmental condition, the increase inσleads to the increase inσapp. Hence the contribution ofσgbs toσtip lessens because ofσf = constant as shown in Fig. 11, considering that the behavior of dσgbs/dt or jd in eq. (5) is constant independent ofσ( even ifσgbs/dt or jd increases with increasingσ). In other words,σtip can reachσf by a small increment ofσgbs, asσincreases. This means that the increase in σ contributes to the decrease in Δt, that is, the increase in SCC susceptibilityCorrespondingly, Fig. 12 shows

ts. Asσincreases, lss is recognized to increase. runder thethe increa On the other hand, the aincrease isinceσapp

from the r ined it may be reasonable to consider that the contribution of the increase in jd to th cresult, the

The SCC characteristics of the sensitized type 316 specimens in 0.82 kmol/m3 H2SO4

solution are summarized as follows: elongation rate, lss, time to

he stress- dominated, , respectively.

tf obtained as functions of applied stress become a lopes depend on sensitizing time and sensitizing

ecomes the parameter for prediction of time to

ochemical cell between Cr

.

schematically the behavior of ln during the film rupture- formation even

Fu thermore, consider the degree of sensitization dependence of the SCC susceptibility constant environment and applied stress conditions. It is known that jd increases with se in the degree of sensitization, which leads to the decrease in Δt.

incre se in the degree of sensitization contributes to the increase inσmax, corresponding to the nσF. This means that Δt would increase with increasing the degree of sensitization, is constant and hence the contribution ofσgbs forσtip to reachσf increases. However, esults obta

e de rease in Δt would be superior to that of the increase inσF to the increase in Δt. As a SCC susceptibility increases with increasing the degree of sensitization.

5. CONCLUSIONS

(1) The curves of applied stress - three parameters ( steady statefailure, tf, and transition time tss ) are divided into three regions, tSCC- dominated and corrosion- dominated regions

(2) The relations between log lss and loggood straight line, although their stemperature. This implies that the lss bfailure tf.

(3) The sulphate ions become very aggressive to SCC of the specimens with the severest sensitization, which causes by the formation of the local electrdepletion and the other surface.

Fig. 13 Schematic representation of time dependence ring the film rupture- formation

events at various deofσtip du

grees of sensitization

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360 ks, the degree of sensitization begins to decrease

[1] R. Nishimura, I. Katim and Y. Maeda, Corrosion, 57, 853 (2001). [2] R. Nishimura, I. Musalam and Y. Maeda, Corros. Sci., 44, 1343 (2002). [3] R. Nishimura and K. Kudo, Corrosion, 45, 308 (1989). [4] R. Nishimura, Corrosion, 46, 311 (1990). [5] R. Nishimura, Corrosion, 49, 796 (1993). [6] R. Nishimura, Corros. Sci., 34, 1859 (1993). [7] R. Nishimura, K. Yamakawa, in Passivity and Localized Corrosion, M. Seo, B. MacDougall,

H. Takahashi, R.G. Kelly, Editors, PV 99-27, p.624, The Electrochemical Society Proceedings Series, Pennington, NJ, (2000).

[8] V. Cihal, Intergranular Corrosion of Steels and Alloys, Materials Science Monographs, Vol. 18, p.97, Elsevier, Amsterdam, (1984).

[9] H.E. Evans, Mechanisms of Creep Fracture, p.169, Elsevier, Amsterdam, (1984). [10] B. Gu, W.-Y. Chu, L.- J. Qiao, C.- M. Hsiao, Corros. Sci., 36, 1437 (1994). [11] H. L. Logan, J. Res. Natl. Bur. Stand., 48, 99 (1952).

(4) At a sensitizing time of more than due to Cr healing.

REFERENCES

ECS Transactions, 1 (4) 401-411 (2006)