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INTERGRANULAR CORROSION OF STAINLESS STEELS UNDER TRANSPASSIVE CONDITIONS :
STUDY OF SILICON SEGREGATION IN TILT BICRYSTALS
J. Stolarz, J. Le Coze
To cite this version:
J. Stolarz, J. Le Coze. INTERGRANULAR CORROSION OF STAINLESS STEELS UNDER TRANSPASSIVE CONDITIONS : STUDY OF SILICON SEGREGATION IN TILT BICRYSTALS.
Journal de Physique Colloques, 1990, 51 (C1), pp.C1-641-C1-645. �10.1051/jphyscol:19901101�. �jpa-
00230008�
COLLOQUE DE PHYSIQUE
Colloque Cl, suppl6ment au n o l , Tome 51, janvier 1990
INTERGRANULAR CORROSION OF STAINLESS STEELS UNDER TRANSPASSIVE CONDITIONS : STUDY OF SILICON SEGREGATION IN <001> TILT BICRYSTALS
J. STOLARZ and J. LE COZE
Ecole Nationale Superieure d e s Mines, 158, Cours F a u r i e l , F-42023 S a i n t - E t i e m e Cedex 2 , France
RCsumt5
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La corrosion intergranulaire transpassive des aciers inoxydables austinitiques dCpend fortement de leur teneur en silicium. Une relation IinCaire entre l'intensitk d'attaque intergranulaire dans les conditions klectmchirniques dCfinies et la teneur en silicium aux joints de grains a CtC trouvke.Le test klectrochimique de corrosion intergranulaire a 6t6 appliquC B l'ttude de la relation entre le niveau de sCgr6gation du silicium et l'angle de dksorientation, dans les joints symCtriques de flexion autour de
<001> dans racier inoxydable 17Cr-13Ni contenant 0,3 et 0,8% Si. Les resultats ont 6tk interpkt6s par la variation de la distance des plans d(hkl)la paralltles au plan du joint de grains, en fonction de l'angle de d6sorientation. Dans les joints CtudiCs, la sCgr6gation intergranulaire est d'autant plus forte que la distance d est plus Bev6e.
Abstract
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Intergranular corrosion of austenitic stainless steels under transpassive conditions depends strongly on silicon content of the steel. A linear relation between intergranular corrosion rate at constant electrochemical potential in the transpassive range and silicon content at grain boundaries has been found. The electrochemical corrosion test has been applied to study the misorientation dependence of silicon segregation in <001> symmetrical stainless steel tilt bicrystals containing 0.3 and 0.8 wt.% Si.The correlation segregation-structure has been explained by the variation of interplanar spacing of planes d(hk1)la parallel to the grain boundary with the misorientation angle. The intergranular segregation of silicon increases when the interplanar spacing d increases.
The level of intergranular segregation depends strongly on grain boundary structure. However, because of experimental difficulties related to bicrystal fabrication, almost all the works concerning this effect have been performed on polycrystalline materials /l/. The only systematical study of the relation between segregation level and misorientation angle was performed on <001> bismuth containing copper bicrystals
/U.
In Fe-Cr-Ni alloys and nickel, the strong influence of silicon on the intergranular corrosion is generally considered as resulting from segregation of this element to grain boundaries /3,4/.
The purpose of this work is:
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to show the relation between intergranular segregation of silicon and electrochemical corrosion of grain boundaries in a 17-13 stainless steel;-
to apply the potentiostatic aanspassive corrosion test to study the misorientation dependence of silicon segregation in <001> symmetrical tilt bicrystals of an austenitic stainless steel.2. EXPERMENTAL PROCEDURE
Stainless steels <001> tilt bicrystals containing 0.3 and 0.8 wt.% Si have been prepared by the Chalmers method in the Ecole des Mines de Saint-Etienne. Misorientation angles od grain boundaries are between 10 and 70". The mean chemical compositions of the bicrystals are listed in Table 1.
Table 1.
17-13-03Si 17-13-08Si
*
wt.%Cr*
15.8-17.5 16.2-17.9
Ni* S I * c** p** S** '
12.3-14.1
12.2-14.5 < l 0
< l 0 0.25-0.35
0.70-0.85 20-68 13-61)
c25 c25
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901101
Cl-642 COLLOQUE DE PHYSIQUE
An increase of chromium (+2%) and nickel (+1,5%) contents between the beginning and the end of the bicrystals has been measured. On the other hand, silicon and carbon are approximatively constants along the length of the crystals.
The samples are cut off from the bicrystals at an equal distance and are given a 24 hours treatment at 1200°C and water quenching followed by a 24 h annealing at 650°C.
The potentiostatic transpassive corrosion tests are performed in 2N sulphuric acid solution at 50°C at two electrochemical potentials (650 and 700 mV/Hg2SOq). The testing time is not decisive because the groove angles do not depend on time under transpassive conditions /5,6/.
The corrosion grooves have been measured on SEM micrographs on the face perpendicular to <001> axis, common for both parts of bicrystals.
In order to study the relation between intergranular corrosion and silicon segregation at gain boundaries, AES tests have been performed on bicrystalline samples. To obtain intergranular fracture, the samples have been submitted to hydrogen charging in a molten salt soIution 171 at 200°C during 4 hours.
The samples have been fractured and analysed in the Auger spectroscope (Riber). The incident beam diameter was of about 2 with the energy of 3 keV.
If silicon is considered as the only segregating element and its segregation takes place on the first atomic layer only, the approximative silicon segregation cgbSi at grain boundaries can be obtained from the heights of Auger pics of iron and silicon using the following relations 161:
with: H
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heights of Auger pics;a,k
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coefficients;cl
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atomic concentration of the first atomic layer;cv
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bulk concentration;cgb
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intergranular concentration.An electrochemical model of intergranular corrosion under transpassive conditions has been proposed by Beaunier and Frornent /4,5/. In this theoretical representation, the bulk material and intergranular layers are sllpposed to have different dissolution rates: v, and vgb (Fig.1). If vgb > v , , intergranular grooves appear. The groove angle do not depend on the testing time at a constant potential 141; it can then be considered as the only criterion of the intergranular corrosion rate.
Fig.1. Formation of intergranular grooves at a constant electrochemicalpotential in the transpassive range 1431
In the present work, the intergranular corrosion rate at a constant potential in the transpassive range is defined by the following equation 161:
v b - v , = 1 'J =
PVm
sin a / 2-
1with: ct
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angle of the intergranular corosion groove (Fig.1).In this way, the specific corrosion rate of grain boundaries
3
is equal to zero if no intergranular grooves are produced (v, = v&.Our electrochemical model of intergranular dissolution under transpassive conditions predicts the existence of a linear relation between the intergranular corrosion rate 'J and the concentration of segregating element at grain boundaries 161.
The experimental relation between the intergranular corrosion rate 'J and silicon segregation at grain boundaries was established by AES on some bicrystalline samples (Fig.2). The corrosion tests have been performed at two different potentials in the transpassive range: 650 and 700 mV/Hg2S04. This relation is linear according to the previsions of our electrochemical model of intergranular corrosion /6/.
Fig.2. Relation between intergranular corrosion at constant potential un&r transpassive conditions and silicon concentration at symmetrical tilt boundaries, measured by AES
It is then possible to compare the segregation lsvels of silicon in different bicrystals using the potentiostatic intergranular corrosion test under transpassive conditions. A comparision of segregation levels in different samples is only possible if corrosion tests are performed at a same electrochemical potential. Free corrosion tests 181 where the potential may vary in the time are not suitable for segregation studies in stainless steels. On the other hand, tests based on fixed dissolution current densities /9/ may lead to important errors if they are applied for comparing segregation levels in different samples. It can be shown that even for samples prepared from the same bicrystal, the variations of dissolution current density at a same potential may exceed 50% 161.
3.2. Misorientation devendence of intergranular segregation of silicon in stainless steels <001> tilt bicrvstals Figure 3 shows the results of transpassive corrosion tests performed in H2S04/2N at 650 rnVfHg2SOq on symmetrical tilt boundaries in 17Cr-13Ni stainless steel containing 0.3 and 0.8 wt.% Si.
The correlation between the intergranular corrosion rate J and silicon concentration at grain boundaries presented in the precedent paragraph, is used to transform these results in terms of intergranular segregation (Fig.4).
COLLOQUE DE PHYSIQUE
Fig.3. Misorientation dependence of intergranular corrosion of silicon containing steel bicrystals in H2SOq12N at 650 rnV/HgzS04
Fig.4. Misorientation dependence of silicon segregation in <001> symmetrical tilt boundaries in 17-13 stainless steel
The curve of silicon concentration at grain boundaries vs. misorientation angle exhibits maxima at 25(36.87'), X5(53.1l0) and X13(67.38O). It is clear that the intergranular segregation level do not depend in a simple way on grain boundary energy in pure metals. On the other hand, our results are very different from those concerning bismuth segregation in copper <001> tilt bicrystals 121.
The value of interplanar spacing of planes d(hk1)la parallel to the grain boundary has ben proposed by Bouchet and Priester 1101 to characterize sulphur segregation to general grain boundaries in nickel.
This criterion has been applied to the special boundaries investigated in the present work.
The variation of dla with the misorientation angle in <001> symmetrical tilt boundaries (Fig.5) exhibits three maxima which correspond to the same rnisorientation angles as for the silicon segregation (Fig.4).
In <001> symmetrical tilt bicrystals, the boundaries with the highest segregation level are those with the highest dla values.
Fig 5. Relation between planar spacing of planes d(hkl)la parallel to the grain boundary in symmetrical <001> tilt fcc bicrystals
Bouchet and Priester /10/ have studied the segregation at general grain boundaries in the Ni-S system. They conclude that the boundaries containing the highest level of sulphur segregation are those with the lowest density of the grain boundary plane. It seems that this concept is not sufficient to describe silicon segregation to
m
svmmetrical tilt boundaries in stainless steels wheren an inverse relation has been found (Fig.5). This difference can be related to the specific character of probable segregation sites in <001> tilt boundaries. Recent structure simulations and HREM studies /l l/ have shown that the structure of these boundaries could be represented by a sequence of three different structural units. Only one of these units seems to be favarable for silicon segregation.
The silicon content at the boundary depends then exclusively on the density of such units in the boundary. This concept, applied to the misorientation dependence of intergranular segregation in <001> tilt boundaries, has given a similar result to the one presented in Figure 5 161.
4. CONCLUSION
The purpose of this work was to developpe a potentiostatic corrosion test under transpassive conditions to study the misorientation dependence of silicon segregation in <001> symmetrical stainless steel bicrystals.
It has been shown that the corrosion test in sulphuric acis at a constant potential in the transpassive range can be used to characterize the segregation level of silicon at grain boundaries in stainless steels. A linear relation between the silicon concentration at grain boundaries and the intensity of intergranular corrosion measured by the groove angle has been found.
In stainless steel <001> symmetrical tilt bicrystals, the relation between the segregation level and the misorientation angle shows three maxima whicc correspond to the grain boundaries with the highest interplanar spacings dla.
REFERENCES
/l/ E.D. Hondros (1975). J.Phvs.C4,x, 117-135.
/2/ A. Fraczkiewicz and M. Biscondi (1985). J.Phvs.C4,&5,497-503.
13/ A. DCsestret, J. Femol and G . Vallier (1977). Matkr.Tech.(Paris), N09/10, 3-18.
/4/ L. Beaunier and M. Froment (1974). C.R.Acad.Sci.Paris C, 279.91-94.
/5/ M. Froment (1975). J.Phvs.C4,%, 372-385.
161 J. Stolarz (1989). Ph.D.Thesis, ENSM Saint-Etienne.
P/ A. Elkholy, J. Galland, P. Azon and P. Bastien (1977). C.R.Acad.Sci.Paris C, 284, 363-367.
181 T. Ogura, A. Makino and T. Masumoto (1981). J.Inst.Met.Journal,45, 1093-1 101.
B/ L. Beaunier, C. Chefi, A. Larere and C. Vignaud (1982). J.Microsc.Specuosc.Electron., 171-182.
/10/ D. Bouchet and L. Priester (1987). Scr.Metall., 2,475-478.
/l l/ T. Nowicki (1989). Ph.D.Thesis, ENSM Saint-Etienne
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