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THE ROLE OF GRAIN BOUNDARY STRUCTURE ON THE ELECTROCHEMICAL STABILITY OF
PASSIVE FILMS FORMED ON HIGH PURITY POLYCRYSTALLINE NICKEL
G. Palumbo, K. Aust
To cite this version:
G. Palumbo, K. Aust. THE ROLE OF GRAIN BOUNDARY STRUCTURE ON THE ELEC- TROCHEMICAL STABILITY OF PASSIVE FILMS FORMED ON HIGH PURITY POLY- CRYSTALLINE NICKEL. Journal de Physique Colloques, 1988, 49 (C5), pp.C5-569-C5-574.
�10.1051/jphyscol:1988570�. �jpa-00228067�
JOURNAL DE PHYSIQUE
Colloque C5, suppl&ment a u n O I O , Tome 49, octobre 1 9 8 8
THE ROLE OF GRAIN BOUNDARY STRUCTURE ON THE ELECTROCHEMICAL STABILITY OF PASSIVE FILMS FORMED ON HIGH PURITY POLYCRYSTALLINE NICKEL
G. PALUMBO and K.T. AUST
Department of Metallurgy and Materials Science, University of Toronto, Toronto M5S 1 A 4 , Canada
~ 6 s u m 6
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Les techniques Blectrochimiques 2 l'aide de la microscopic 61ectronique (SEM~ECP) sont utiliser pour examiner l'effet de la structure des joints de grain (i.e. CSL) sur la corrosion intergranulaire du nickel polycristallin de haute puretg submerge dans l'acide sulfurique (2N). Les potentiels 61ectrochimiques caracteristiques (Egb) de l'initiation de l'attaque intergranulaire sont obtenus dans le domain de potentiel passif-transpassif. Ces potentiels sont fortement dgpendant des parametres structuraux (i.e., E, 661, avec les joints ayant une desorientation proche2
cette de basse-E et de haute resistance2
l'attaque intergranulaire. Ces rgsultats sont considergs en relation avec la stabilitg des couches passive form6es sur defauts cristallins.Abstract
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Electrochemical techniques were utilized in conjunction with SEM/ECP to investigate the effects of grain boundary structure (i.e. CSL) on the intergranular corrosion behaviour of high purity polycrystalline Ni (99.999%) in 2 N H2S04.Characteristic electrochemical potentials (Egb) for the initiation of grain boun- dary corrosion were found to exist within the passive-transpassive potential range.
These potentials were determined to be strongly structure-dependent (i.e. C, b e ) with boundaries close to low-C CSL relationships displaying a high resistance to the initiation of localized attack (i.e. high Egb). These results are discussed in terms of the stability of passive films formed at crystalline defects.
1. INTRODUCTION
The effect of grain boundary structure on the intergranular corrosion behaviour of high purity metals has been investigated in several studies (1-9), and recently reviewed by the present authors (10). These investigations have primarily dealt with the propagation kinetics of intergranular attack under conditions of high general surface dissolution. Arora and Metzger (2) have discussed the possible role of passivation in leading to structure dependent intergranular corrosion;
however, the effect of grain boundary structural parameters on the stability of passive films has been largely neglected in previous studies.
The primary objective of this study is to investigate the 'structure-dependence of passive film stability at grain boundaries in high purity nickel. In this paper, some preliminary results concerning the morphology and structure-dependence of passive film breakdown are presented.
2. EXPERIMENTAL PROCEDURE 2.1. Sample Preparation
High purity nickel powder (99.999%) was induction melted in an alumina crucible under purified flowing argon to produce a 20 mm length of 10 mm diameter cast rod.
The nickel rod was then axially deformed 10% in compression and recrystallized at
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988570
C5-570 JOURNAL
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PHYSIQUE1450 K for 2 h and water quenched. This resulted in an average grain size of 750 Llm. The impurity content of the resulting material is listed in Table I.
After annealing, 1 m thick discs were spark-machined from the rod, with one face of each disc mechanically polished to a 114 p diamond finish. The discs were then briefly electropolished in a perchloric-ethanol solution, and argon atom- milled to remove any residual polishing films. Prior to electrochemical testing, the discs were degreased in trichloroethane and compression sealed in a PTFE holder to expose a circular 0.5 cm2 area.
TABLE I.
race
Element Analysis for 99.999% Ni (ppm-wt .)2.2. Electrochemical Testing
Electrochemical tests were conducted in 2 N H2SOr, prepared from deionized water and reagent grade sulphuric acid. Test solutions were maintained at 303 K, and deaerated by bubbling nitrogen into the test cell for one hour prior to sample immersion, and throughout the duration of the tests. Potentials were measured with reference to a saturated calomel electrode (SCE). Potentiostatic anodic polariza- tion tests were conducted under conditions of controlled potential (E) and charge transfer (Q) utilizing an electrochemical circuit consisting of a potentiostat, potential wave form generator, and digital coulometer.
Figure 1 schematically illustrates the test procedure used in this study. Upon immersion of the sample in the test solution, the open circuit potential (Eoc) was monitored until stable. The nickel electrode was then prepassivated (Ep) at +750 mV,SCE to a constant passive current density (ip) of 6 pA/cm2 (t = 2 h). (Anodic polarization at this potential for a duration of up to 2 weeks did not result in preferential corrosion at grain boundaries.) The system was then anodically polarized to preselected potentials (Eb) in the range 800 to 1400 mV,SCE and main- tained until a total charge transfer (Q) of 5 c/cm2 was obtained. Following the tests, samples were rinsed in ethanol and analyzed by SEM.
3
Fig. 1
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Schematic representation o s E b-
of the electrochemical test proce- dure used in this study. The
experimental parameters (i.e. E
%aE,
E
,
Eoc, i Q) are defined in tie tgxt. P'TIME
2.3. Orientation Analysis
The crystallographic orientation of individual grains in the corroded samples was determined by the selected area channeling technique (ECPISEM). Electron chan- neling patterns were recorded using the backscattered electron signal generated with an accelerating voltage of 30 keV and a minimum rocking angle of lo0.
Computer-aided rotation matrix analysis (11) was used to determine the orientation relationship!; between adjacent crystals and to calculate the angular deviation from coincidence site lattice (CSL) relationships in the range Z = 1 to 49. The clasal- fication of boundaries was in terms of the closest CSL relationship, and was based upon minimizing the ratio of the determined angular deviation ( A 0 ) to that allowed
d
on t h e b a s i s of Brandon's c r i t e r i o n ( 1 2 ) ( i . e . bec = 15 1 - l l 2 ( d e g r e e s ) . R a t i o v a l u e s g r e a t e r t h a n u n i t y c o r r e s p o n d t o " g e n e r a l " b o u n d a r i e s .
3. RESULTS
F i g u r e 2 shows a p o t e n t i o s t a t i c a n o d i c p o l a r i z a t i o n c u r v e f o r t h i s system. The p o t e n t i a l l i m i t s d e t e r m i n e d f o r t h e i n i t i a t i o n of l o c a l i z e d c o r r o s i o n a t g r a i n b o u n d a r i e s a r e i n d i c a t e d on t h e f i g u r e . Below t h e p o t e n t i a l of 1050 mV,SCE c o m p l e t e immunity t o i n t e r g r a n u l a r a t t a c k i s e v i d e n t . At o v e r p o t e n t i a l s g r e a t e r t h a n o r e q u a l t o 1250 mV,SCE, a l l g r a i n b o u n d a r i e s ( i n c l u d i n g t h e c o h e r e n t t w i n ) show e v i d e n c e of l o c a l i z e d c o r r o s i o n . W i t h i n t h e i n t e r m e d i a t e p o t e n t i a l r a n g e ( i . e . 1050 t o 1250 mV,SCE), g r a i n boundary s e l e c t i v i t y i n t e r m s of i n i t i a t i o n i s observed.
F i g . 2
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P o t e n t i o s t a t i c a n o d i c p o l a r i z a t i o n c u r v e of t h e p a s s i v e - t r a n s p a s s i v e p o t e n t i a l r e g i o n showing t h e p o t e n t i a l l i m i t s f o r g r a i n boundary s e l e c t i v i t y i n t h e i n i t i a t i o n of l o c a l i z e d c o r r o s i o n .10' r 99.999 Ni
E
2N H2S043Bc
.w
-
5 -
.
z w - O
a
-
a
-
3
-
U
-
25mvIsI.p
750 850 950 1050 1150 1150 I350 1450
POTENTIAL. mVx,
The t y p i c a l c o r r o s i o n morphology e v i d e n t w i t h i n t h i s p o t e n t i a l r a n g e i s shown i n F i g u r e 3. The i n i t i a t i o n of l o c a l i z e d c o r r o s i o n a t g r a i n b o u n d a r i e s i s c h a r a c - t e r i z e d by i s o l a t e d p i t t i n g ( F i g . 3 a ) . With i n c r e a s i n g o v e r p o t e n t i a l w i t h i n t h i s r a n g e , e v i d e n c e of i n c r e a s e d p i t d e n s i t y ( F i g . 3 b ) , p i t c o a l e s c e n c e ( F i g . 3 b ) , and g r o o v e development ( F i g . 3 c ) a r e observed.
Some examples of t h e s t r u c t u r e dependence of c o r r o s i o n morphology a r e d e m o n s t r a t e d i n F i g u r e 4 . B o u n d a r i e s d e v i a t i n g s l i g h t l y from low-E CSL r e l a - t i o n s h i p s t y p i c a l l y d i s p l a y l e s s s e v e r e i n t e r g r a n u l a r a t t a c k t h a n " g e n e r a l " boun-
F i g . 3
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Scanning e l e c t r o n m i c r o g r a p h s showing t h e p o t e n t i a l - d e p e n d e n c e of c o r r o - s i o n morphology f o r " g e n e r a l " g r a i n b o u n d a r i e s : ( a ) 1075 mV,SCE; ( h ) 1175 mV/SCE;( c ) 1200 mV,SCE.
JOURNAL DE PHYSIQUE
daries (Fig. 4a). At certain overpotentials, these "special" boundaries can demonstrate selective immunity to localized corrosion (Fig. 4b-d). Figure 5 sum- marizes the structural-dependence of corrosion morphology at various potentials within the selective initiation range. Decreasing overpotential from 1200 to 1150 mV,SCC r c a t L t s i i , an expanding structural field (C, A 8 ) of i~munity (Fig. 5a-c).
Below 1150 mV,SCE (Fig. 5d) little change in the structural limits of immunity are noted, with this field not extending beyond C25.
4. DISCUSSION,
The potential range for the initiation of grain boundary corrosion determined in this study (Figure 2) lies entirely within a transpassive potential regime where the nickel electrode is still predominantly covered by a passive surface film (13).
The initiation of localized corrosion within this potential range can thus be attributed to the local destabilization of the passive layer. This situation can
Fig. 4
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Scanning electron micrographs depicting the structure-dependence of corro- sion morphology evident at various applied potentials: (a) 1200 mV,SCE; (b) 1175 mV,SCE; (c),(d) 1150 mV,SCE, NOTE: 1. "R" denotes non-CSL related boundaries. 2.Arrows indicate the trace of unetched boundaries.
arise when the local rate of film dissolution exceeds the rate of repassivation (14). Preferential pit initiation at grain boundaries is thus a result of surface film instability at the boundary region, with this manifested through lower over- potential requirements for enhanced film dissolution. The determination of over- potential requirements for local film breakdown has been applied primarily to studies of environmental effects on localized corrosion (15), and in some cases (16,17) to the electrochemistry of defects.
The propagation of localized attack at grain boundaries is likely sustained by the local pit environment in the early stages of corrosion. This is evidenced by substantial penetration and absence of "spreading" along the boundary (Fig. 3a).
At later stages, the pits may undergo repassivation. This effect is demonstrated in Figure 3b where small pits have nucleated at the base of larger ones. The most significant effect of increasing overpotential is to increase the density of ini- tiation sites along the boundary, with this effect attributed to decreasing film stability with increasing potential. The propagation of attack at higher over- potentials is characterized by enhanced lateral spreading, ultimately leading to the development of grooves (Fig. 312). These effects are primarily due to the increased rate of general surface dissolution, which has been rigorously demonstrated (18) to affect the development of intergranular grooves.
The structural dependence evident in this study is characterized by an expanding structural field (C, AO) of immunity with decreasing overpotential (Fig.
5). This dependence on structural parameters indicates that the nature of the passive film formed at grain boundaries is highly sensitive to the structure (and chemistry) of the substrate interface. The observed limiting field of immunity
Fig. 5
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Summary of the structure dependence (E, 80) of corrosion morphology at various applied potentials: A grooving, 0 pitting,fl
immunity. The dashed lines outline the structural field of immunity.C5-5 74 JOURNAL
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PHYSIQUE( F i g . 5c and d ) o u t l i n e s a s u b s t a n t i a l l y n a r r o w e r r e g i o n (A8) t h a n t h a t d e f i n e d by B r a n d o n ' s c r i t e r i o n ( 1 2 ) . The l i m i t i n g C v a l u e of 25 o b s e r v e d i n t h i s s t u d y i s c o n s i s t e n t w i t h Watanabe's s u g g e s t i o n ( 1 9 ) of a n u p p e r l i m i t of C29 f o r s p e c i a l p r o p e r t i e s w i t h r e s p e c t t o i n t e r g r a n u l a r c o r r o s i o n and f r a c t u r e b e h a v i o u r .
The i n i t i a t i o n phenomena o b s e r v e d i n t h i s s t u d y c a n n o t be a t t r i b u t e d s o l e l y t o s t r u c t u r a l e f f e c t s s i n c e i t h a s been d e m o n s t r a t e d t h a t s t r u c t u r e - s e n s i t i v e i m p u r i t y s e g r e g a t i o n c a n i n f l u e n c e c o r r o s i o n p r o p e r t i e s even i n m a t e r i a l s of h i g h p u r i t y ( 2 0 ) . The major i m p u r i t y element of c o n c e r n i n t h e p r e s e n t s t u d y i s s u l p h u r ( 3 ppm) which has been shown t o i n h i b i t g e n e r a l p a s s i v i t y ( 2 1 ) , and s i g n i f i c a n t l y enhance i n t e r g r a n u l a r d i s s o l u t i o n r a t e s (22-24) i n n i c k e l . F u r t h e r work i s b e i n g c o n d u c t e d t o c l a r i f y t h e r o l e of s u l p h u r and t o i n v e s t i g a t e p o s s i b l e mechanisms f o r t h e boundary s t r u c t u r e dependence of i n t e r g r a n u l a r c o r r o s i o n o b s e r v e d i n t h i s s t u d y .
5. SUMMARY
The e f f e c t of t h e g r a i n boundary s t r u c t u r a l p a r a m e t e r s (C, A8) on t h e morpho- l o g y of i n t e r g r a n u l a r c o r r o s i o n a t v a r i o u s a p p l i e d p o t e n t i a l s h a s been i n v e s t i - g a t e d . A p o t : e n t i a l r a n g e (1050 t o 1250 mV,SCE) was d e t e r m i n e d i n which a
s i g n i f i c a n t s t r u c t u r e dependence f o r t h e i n i t i a t i o n of l o c a l i z e d c o r r o s i o n at g r a i n b o u n d a r i e s was o b s e r v e d . These phenomena were d i s c u s s e d i n t e r m s of p a s s i v e f i l m s t a b i l i t y a t g r a i n b o u n d a r i e s .
ACKNOWLEDGEMENTS
The a u t h o r s would l i k e t o t h a n k P r o f e s s o r T. Mimaki f o r many h e l p f u l
d i s c u s s i o n s . F i n a n c i a l s u p p o r t from t h e N a t u r a l S c i e n c e s and E n g i n e e r i n g R e s e a r c h C o u n c i l o f Canada i s g r a t e f u l l y acknowledged.
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