GRAIN BOUNDARY CORROSION, STRUCTURE AND SEGREGATION IN NICKEL BICRYSTALS

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GRAIN BOUNDARY CORROSION, STRUCTURE AND SEGREGATION IN NICKEL BICRYSTALS

C. Vignaud, L. Beaunier, M. Biscondi

To cite this version:

C. Vignaud, L. Beaunier, M. Biscondi. GRAIN BOUNDARY CORROSION, STRUCTURE AND

SEGREGATION IN NICKEL BICRYSTALS. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-

697-C1-702. �10.1051/jphyscol:19901111�. �jpa-00230019�

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C o l l o q u e Cl, suppl6ment au nol, Tome 51, j a n v i e r 1990

GRAIN BOUNDARY CORROSION, STRUCTURE AND SEGREGATION IN NICKEL BICRYSTALS

C. VIGNAUD, L. BEAUNIER and M. BISCONDI*

Laboratoire de Physique des Liquides et Electrochimie, LP15 du CNRS, Universite Paris V I , Tour 22, 4 Place Jussieu, F-75252 Paris Cedex 0 5 , France

" ~ c o l e Nationale Superieure des Mines de saint-~tienne, F-42023 Saint-Etienne Cedex 2. France

R6sumk

-

A I'aide de bicristaux de nickel d'axe de flexion <TOO>, nous etudions I'influence de la structure du joint de grains sur la tenue b la corrosion intergranulaire. Un test electrochimique permet de suivre I'evolution de la corrosion en fonction de la desorientation des grains et de la repartition d'elements ggr6ges pendant la croissance et / ou un traitement thermique b 8DO;C. L'analyse par spectrometrie d'electrons Auger (AES) et I'observation par microscopie electronique b balayage (SEMI des fractures intergranulaires permet une correlation corrosion-segregation-structure. Outre le role deja connu du phosphore et du soufre nous mettons en evidence I'effet nocif de I'oxyg8ne.

avec. de plus. un effet plus ou moins marque selon la desorientation.

Abstrad

-

Nickel bicrystals with tilt boundaries are used to study the influence of the structure of the grain boundaries on the intergranular corrosion. An electrochemical test allows to obtain the evolution of the corrosion versus the misorientation of grains and the repartition of elements segregated during the elaboration and / or heat treatment at 80OoC. Auger electron spectroscopy (AES) and scanning electron microscopy (SEMI on intergranular fracture surfaces allows a corrosion-segregation-structure correlation. With the well known effect of phosphorus and sulfur, we have pointed out the detrimental effect of oxygen depending on the misorientation of adjacent grains.

1

-

INTRODUCTION

Since twenty years, some studies have been devoted to the relation between corrosion processes and misorientation of grain boundaries thanks to bicrystals elaboration, models and theoretical structure calculations. Misorientation effect and mechanism of the intergranular corrosion related to active sites of dissolution at the emergence of grain boundaries are pointed out : aluminum bicrystals in pressurized water 111 and hydrochloric acid 121. stainless steels bicrystals /3,4/, nickel bicrystals 151.

However. the well known detrimental effect of segregated elements (S.P

...

^), has to be taken into account in these models. Corrosion-segregation relation has been studied on polycrystals: silicon in nickel 161, ferritic steels 171, silicon in stainless steels 181. In the case of corrosion-segregation-structure relation we note: Ni-S polycrystals 191. Si in poly- and bicrystals of stainless steels /10/, Ni-S in high coincidence and special grain boundaries l 1 l / , polycrystalline nickel 1\21. These studies try to correlate the measured segregation level and the intergranular corrosion with respect to the real misorientation of grain boundaries.

In the present work. w e intend to separate the effect of structure from the chemical effect of segregation on intergranular corrosion sensitivity. So. by analysing each bicrystal, we expect to follow chemical A.E.S.

parameters versus the geometrical known parameters imposed during elaboration.

2 - EXPERIMENTAL MFTHODS

Bicrystals are elaborated in an horizontal crucible by directional solidification (150 Torr Ar, 10% H2) The growth induces a gradient of impurities from the beginning (Part I) to the end (Part II) of the bicrystal (Fig.1).

Misorientations and bulk analysis are given in Table 1 and Table 2 respectively.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901111

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END

Fig. 1

-

Bicrystaldex:Qt~

INITIAL Part l

Table l Table 2

Specimens are cut off with an electrolytic saw and then paper polished and electropolished. These specimens are studied after elaboration and after heat treatment: a desulphuriiation effect is hoped to be obtained with 6 weeks at 8W°C under hydrogen flow in a quark tube/l3/.

The eiectrochemical etching is performed in H2S04-2N -25°C at a transpassive potential (1300 mV/SCE) and a constant quantity of electricity (450 c/cm2) for every specimenl3l. The intergranular grooving developed at the emergence of grain boundary is observed by SEM (S250 Cambridge Instrument) and the importance of the attack is estimated by penetration depth measurements (Fig.2).

groove

Fig 2 - Intergranular groove parameters

initial surface

--- .A:.- ^-

grain boundary

To achieve intergranular fracture, hydrogen cathodic charging in melted salts is carried out/l4/. In situ fractures are obtained by using a fitted tool in the Auger spectrometer. The quantitative treatment of the spectra has been made using relative sensitivity coefficients 1151 under two hypotheses: the segregation at grain boundaries is supposed on one atomic layer, all retrodiffusion factors are supposed equal

The vacuum chamber cannot be heated because of the rapid hydrogen desorption at the grain boundary.

Therefore, a contamination in oxygen and carbon is observed due to the 1 0 - ~ Pa vacuum. The only spectra which are retained are those obtained a few minutes after rupture (< 10 min).

3 - RESULTS AND DISCUSSION 3 - 1 - lnteraranular corrosion

Figure 3 presents the evolution of the depth of the corrosion penetration, D, versus the misorientation angle 8, for Part I of bicrystal. After elaboration, only a smal corrosive effect (about 5 pm) is observed for all misorientations except 40°~(50 pm). After heat treatment, an strong increase of the corrosion is showed for all specimens. particularly for 40° and 53" (012). This experimental result is strange because the heat treatment is expected to decrease the amount of impurities at nickel grain boundaries /13/.

just heat

elaborated treated

I U

without

precipitate

a ⁶³

^with

precipitate

Figure 3 : Evolution of the intergranular corrosion depth versus misorientation angle and thermal treatment.

Additionaly. on 40°n bicrystal an important gradient of corrosion between Part 1 (50 pm) and Part 11 (5.103im) is observed. For 40°~, 20". 53", 67" no gradient of corrosion is revealed along bicrystals. So, two questions have to be answered by Auger analysis:

(1) which species are segregated to the 40°A boundary and differ from 40°B?

(2) why does thermal treatment enhance the corrosion sensitivity in these tests?

3

-

2 -Characterization of arain boundarie~,

Same fractures have not been successful in the Auger spectrometer, so "ex situ" fractures in air just after hydrogen charging have been performed to complete the characterization of all grain boundaries. Four type of interaranular fracture surfaces are observed bv SEM (Fia. 4).

Fig. 4 - Morphology of intergranular fractures surfaces Type a

-

plane area with slip lines

Type b - plane area with slip lines and precipitates Type c - plane area with slip lines and cavities Type d

-

mixing of type a and type b

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Table 3 lists for all specimens corrosion depth, morphology, success or not in intergranular fracture in situ and quantitative segregation analysis. Two typical AES spectra are given (Fig. 5).

Table 3

I

Ni E(eV)

I I 1 -/

l00 300 500 900

Fig. 5

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AES spectra on intergranular fracture: a) on 40° B heat treated, out of cavities b) on 40°A heat treated Just after elaboration all grain boundaries, except 40°A, contain geometrical precipitates. The precipitates are aligned along the grain boundary with different densities. X ray energy dispersive spectrometry analysis in a scanning electron microscope (EDX-SEM) allows to identify these precipitates as nickel oxide (Fig.6). AES does not reveal any detectable segregation in those "with precipitate$ boundaries when the electron beam does not touch any precipitate and registers a lot of oxygen in the contrary case. In the "without precipitate"

4 0 " ~ grain boundary, a sulfur and phosphorus segregation is observed.

After heat treatment, all in situ fractures are successful. Precipitates have disappeared and have been replaced by cavities. An important point is that, in all cases, considerable amounts of oxygen has been detected on grain boundary planes or on cavities. In addition sulfur is always present in cavities where an extra peak located at the chlorine energy is sometimes obseived. Moreover, the 4 0 " ~

-

Part I

-

boundary has no sulfur but is enriched in oxygen.

Fig. 6 - X ray EDS analysis on infergranular fracture precipitate .

E k z E _{-_.} !

matrix

4 - CONCI USlON

Elaboration furnishes two types of bictystals:

a) 20". 40°B,

So,

67" grain boundaries are obtained under oxidizing conditions: until now. we can not conclude if oxygen is coming from atmosphere or from a reaction with the crucible. In such condition, grain boundaries exhibi nickel oxide precipitates and a clean interface without noticeable segregation.

b) 40°A : the absence of precipitate is balanced by a real sulfur and phosphorus segregation with a high gradient along the bicrystol.

The long heat treatment induces a general phenomenon: it allows always a noticeable intergranular oxygen segregation at a ievei of 20 at%, that is an enrichment superior to 104. Furthermore, desulfurization and dephosphorization occur when phosphorus and sulfur are initially present, like in 40°A (Part I and 11). The nickel oxide precipitates have been reduced to form H,O bubbles at high temperature, which manifest as cavities.

The relation between intergranular corrosion and segregation appears very clearly:

-phosphorus and sulfur

.

as already known, favour corrosion.

-oxygen segregation ( but not oxide crystals ) highly enhances corrosion.

Concerning intergranular structure, we can conclude that, for any misorientation 6. segregation absence leads to limited corrosion. The role of misorientation upon oxygen segregation cannot be extracted because the oxygen bulk content (about 5 at. ppm) is not well controlled.

When oxygen can segregate strongly and promote an enhanced intergranular corrosion, it appears that corrosion level depends furthermore on misorientation: 53" and 67" bicrystals present the same segregation oxygen level but a quite different corrosion sensitivity. Therefore, not only chemical factors but also structural effects and especially active sites have to be considered for grain boundary corrosion interpretation.

Cl-702 COLLOQUE DE PHYSIQUE

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