SOME METALLURGICAL, CHEMICAL AND ELECTROCHEMICAL FACTORS AFFECTING HYDROGEN EVOLUTION KINETICS ON STAINLESS ALLOY SURFACES IN ACID MEDIA

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SOME METALLURGICAL, CHEMICAL AND ELECTROCHEMICAL FACTORS AFFECTING

HYDROGEN EVOLUTION KINETICS ON STAINLESS ALLOY SURFACES IN ACID MEDIA

J. Bavay, E. Noga, J. Bricout, K. Vu Quang

To cite this version:

J. Bavay, E. Noga, J. Bricout, K. Vu Quang. SOME METALLURGICAL, CHEMICAL AND ELEC-

TROCHEMICAL FACTORS AFFECTING HYDROGEN EVOLUTION KINETICS ON STAIN-

LESS ALLOY SURFACES IN ACID MEDIA. Journal de Physique Colloques, 1984, 45 (C5), pp.C5-

471-C5-475. �10.1051/jphyscol:1984571�. �jpa-00224192�

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JOURNAL DE PHYSIQUE

Colloque C5, suppl6rnent a u n04, Tome 45, a v r i l 1984 page C5-471

S O M E METALLURGICAL, C H E M I C A L A N D ELECTROCHEMICAL F A C T O R S AFFECTING HYDROGEN EVOLUTION KINETICS O N S T A I N L E S S ALLOY S U R F A C E S IN ACID MEDIA

J.C. Bavay, E. Noga, J.P. ~ r i c o u t * and K. Vu ~ u a n g * *

Laboratoire de Physicochimie des SoZides, E. N. S . C . LiZZe, B. P. 108, 5 9 6 5 2 ViZZeneuve d f A s c q Cedex, France

*Laboratoire de Me'taZZurgie e t CristaZZographie, Universite' de VaZenciennes, Le Mont Houy, Valenciennes Cedex, France

**

Centre dfEtudes de Chimie Me'taZZurgique, C.N.R.S., 15 rue Georges Urbain, 94400 V i t r y , France

Resume

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La p e n t e de T a f e l cathodique e t l ' o r d r e de l a r e a c t i o n e l e c t r o c h i -

-

mique c a r a c t 6 r i s t i q u e s du degagement de 1 'hydrogsne o n t e t & d&termin&s en m i l i e u a c i d e s u l f u r i q u e s u r c e r t a i n s a l l i a g e s i n o x y d a b l e s

a

base de f e r e t de n i c k e l

.

La c o n f r o n t a t i o n des v a l e u r s e x p e r i m e n t a l e s aux v a l e u r s t h e o r i - ques c o r r e s p o n d a n t aux d i f f 6 r e n t s processus p o s s i b l e s rnontre que l e s p r o - d u i t s de c o r r o s i o n s u p e r f i c i e l s e t l e d e p b t d ' a r s e n i c e l e m e n t a i r e j o u e n t un r 6 1 e i m p o r t a n t s u r l e mecanisme de l a r e a c t i o n de degagement de 1 'hydrogene.

A b s t r a c t

-

C a t h o d i c T a f e l s l o p e and e l e c t r o c h e m i c a l r e a c t i o n o r d e r f o r t h e hydrogen e v o l u t i o n process on some i r o n and n i c k e l - b a s e a l l o y s i n s u l f u r i c s o l u t i o n s a r e p r e s e n t e d and d i s c u s s e d w i t h r e g a r d t o t h e p o s s i b l e r e a c t i o n mechanisms. C o r r o s i o n p r o d u c t s and elemental a r s e n i c d e p o s i t a r e shown t o p l a y an i m p o r t a n t r o l e on t h e hydrogen e v o l u t i o n mechanism.

The d e t e r m i n a t i o n o f t h e hydrogen e v o l u t i o n mechanism on Fe-Cr-Ni-Mo s t a i n l e s s a l l o y e l e c t r o d e s i s o f more t h a n an academic i n t e r e s t owing t o t h e p r a c t i c a l i m p o r t a n c e o f t h i s r e a c t i o n w i t h r e s p e c t t o c o r r o s i o n b e h a v i o u r and hydrogen e m b r i t t l e m e n t o f t h e a l l o y . The hydrogen e v o l u t i o n r e a c t i o n (HER) i s t h e r a t e - d e t e r m i n i n g s t e p (RDS) i n t h e c o r r o s i o n o f many a c t i v e a l l o y s i n a c i d , The a b s o r p t i o n o f hydrogen by t h e metal may r e s u l t f r o m e i t h e r c o r r o s i o n process o r c a t h o d i c p r o t e c t i o n . There w i l l be i n - c r e a s i n g needs i n s t r u c t u r a l m a t e r i a l s capable o f b e i n g used i n environments w h i c h c o u l d i n t r o d u c e hydrogen i n t o t h e m e t a l .

S t a i n l e s s i r o n and n i c k e l - b a s e a1 l o y s a r e c o r r o s i o n r e s i s t a n t because o f t h e forma- t i o n o f a t h i n s u r f a c e phase c a l l e d t h e p a s s i v e f i l m . B u t these a l l o y s a r e s u s c e p t i - b l e t o f a i l u r e i n environments which promote t h e e n t r y o f hydrogen i n t o t h e m e t a l /1, ?/. A r s e n i c compounds a r e known t o i n c r e a s e g r e a t l y t h e hydrogen amount e n t e r i n g i n t o t h e c a t h o d i c a l l y p o l a r i z e d m e t a l .

The purpose o f t h i s i n v e s t i g a t i o n i s t o p o i n t o u t t h a t any m o d i f i c a t i o n o f t h e metal s u r f a c e c o u l d a f f e c t t h e HER mechanism and hence t h e r a t e o f hydrogen e n t r y . S u r f a c e f i l m e f f e c t s on HER a r e p r e s e n t e d i n t h e case o f t h e A.I.S.I. 304 s t a i n l e s s s t e e l and two commercial Ni-base a l l o y s : H a s t e l l o y B-2 and C-276 a l l o y s .

I

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THEORETICAL BACKGROUND

I n a c i d s o l u t i o n s , hydrogen i s e l e c t r o c h e m i c a l l y e v o l v e d a t m e t a l i n t e r f a c e s by o n l y two b a s i c p a t h s which b o t h i n v o l v e a d s o r p t i o n o f hydrogen atoms on t h e metal s u r f a - ces. Adsorbed hydrogen atoms a r e produced by t h e e l e c t r o c h e m i c a l r e d u c t i o n o f hydra- t e d hydrogen i o n s :

~ ~ 0+ ' e + H ads.

+

H20

The d i s c h a r g e o f p r o t o n s can be f o l l o w e d by e i t h e r t h e chemical c o m b i n a t i o n of adsorbed hydrogen atoms t o f o r m hydrogen molecules :

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984571

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JOURNAL DE PHYSIQUE

2 H ads. + H 2 o r by t h e electrochemical desorption :

~ ~ 0 '

+

H ads.

+

e- + H z

which a s s o c i a t e s a s i n g l e adsorbed hydrogen atom with a solvated hydrogen ion from the s o l u t i o n t o produce hydrogen gas i n two successive one e l e c t r o n s t e p s .

Within each path, e i t h e r of t h e consecutive s t e p s can be r a t e determining. I t i s a l s o assumed t h a t t h e adsorption of atomic hydrogen on t h e e l e c t r o d e s u r f a c e follows a Langmuir o r a Temkin-type isotherm. In t h e l a t t e r c a s e , t h e desorption of atomic hydrogen t o give molecular hydrogen may be a c t i v a t e d o r not.

The simultaneous determination of t h e t r a n s f e r c o e f f i c i e n t a and t h e ? - e x t i o n o r d e r n i s an adequate c r i t e r i o n t o d i s t i n g u i s h between t h e H E R mechanistic p o s s i b i l i t i e s .

I1 i s o f t e n not p o s s i b l e t o make any decision about t h e mechanism by considering only a o r n

.

The experimental value of a was derived from t h e Tafel s l o p e

( 2 , 3 RT/ a F ) given by t h e p o l a r i z a t i o n curve E = log i of t h e e l e c t r o d e ( f i g . 1 ) . The r e a c t i o n o r d e r was derived from log i versus pH p l o t s a t c o n s t a n t p o t e n t i a l , assuming t h a t t h e p o t e n t i a l d i f f e r e n c e accross t h e Helmholtz double l a y e r i s independent of pH.

log i (~/cm')

-

3 r

- 6 L I ^I I I

I

-

ZOO 0 E(rnV/N.H.E.)

Fig.1

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Cathodic p o l a r i z a t i o n curves r e l a t i v e t o Hastelloy B 2 a l l o y i n deaerated H2S04 s o l u t i o n f o r d i f f e r e n t pH : p o l a r i z a t i o n towards negative p o t e n t i a l a f t e r 24 hours immersion time / 7 / .

The t h e o r e t i c a l values of t h e Tafel slope and t h e electrochemical r e a c t i o n o r d e r with r e s p e c t t o t h e hydrogen ions have been c a l c u l a t e d f o r t h e p o s s i b l e r a t e d e t e r - mining s t e p s and t h e d i f f e r e n t r a t e s

e

of e l e c t r o d e s u r f a c e coverage w i t h atomic hydrogen ( t a b l e 1 ) . The comparison between t h e t h e o r e t i c a l and t h e experimental values o f t h e above k i n e t i c s parameters allows some p r e d i c t i o n about t h e HER mechanism /3/.

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Tafel slope (mV/decade)

Table 1

-

Tabulation, i n a c i d s o l u t i o n s a t 30°C, of t h e predicted values (assuming

a =

^{y =}0,5) of t h e Tafel s l o p e s and r e a c t i o n orders f o r p o s s i b l e mechanisms of hydrogen evolution when t h e slow s t e p i s r e s p e c t i v e l y t h e discharge, t h e electrochemical desorption o r t h e chemical recombination.

Electrochemical r e a c t i o n o r d e r

RDS

Discharge Electrochemical desorption

The mechanism of t h e HER i s recognized a s a c e n t r a l s t e p i n t h e hydrogen absorption reaction (HAR). The HAR competes f o r t h e same population of H ads. a s t h e H E R . Hydro gen atoms d i f f u s e t o microcracks o r i n t e r n a l c a v i t i e s and t h e r e i n form molecular hydrogen. I f t h e pressure of t h i s hydrogen exceeds t h e spreading p r e s s u r e o f a microcrack, then the embrittlement process occurs. I t has been shown t h a t t h e f u g a c i t y of H Z i n t h e metal can be r e l a t e d t o t h e H Z overpotential by a r e l a t i o n s h i p which i s c h a r a c t e r i s t i c of t h e HER mechanism / 4 / . The present work i s concerned with

t h e e f f e c t s of environment and metallurgical v a r i a b l e s on t h e HER mechanism, and thus upon t h e r a t e of hydrogen a b s o r p t i o n .

I 1

-

EXPERIMENTAL

The nominal composition of t h e s t u d i e d a l l o y s i s i n d i c a t e d i n t h e t a b l e 2 . Samples, c u t o f f from a v a i l a b l e i n d u s t r i a l s h e e t s , a r e annealed and quenched i n water a t room temperature. The a u s t e n i te-martensi t e phase transformation o f t h e A.1 .S. I . 304 s t a i n l e s s s t e e l i s produced by a 50 % p l a s t i c deformation a t

-

196'C /5/.

Isotherm

Langmui r Temki n

Chemical

I

^C/2 ⁼³⁰

desorption

Alloy Fe N i C r Mo W Mn C Si

A.I.S.I. 304 Bal 9 18 1 , 7 0,06 0,5

Hastelloy B-2 2,O Bal 1 , o 26,5 - 1,0 0,02 0 , 1

Isotherm

Langmuir

.

^Temkin

C/Zy=6O C / Z = 30.

Hastelloy C-276 5,O Bal 16 ,O 16,O 3,5 1 , 0 0,02 0,08

e ⁺0

C/B =120.

C / l $ =40

Table 2

-

Nominal chemical composition (wt. %) o f t e s t e d a l l o y s

8 + 0

1 2

C = 2 , 3 RT/F

0 + 1

-

C/t3 =I20 Act.

-

C/O +y=60

e + 1

-

1

I I

'No a c t .

-

C/ltg=40.

Act.

-

l+y=1,5

I

^O ^IPy=l

I

'No a c t .

-

2

8 , y: symmetry f a c t o r s of the t r a n s f e r and desorption b a r r i e r s

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C5-474 JOURNAL DE PHYSIQUE

The t h r e e compartiment electrochemical c e l l i s made o f Pyrex g l a s s . The platinum a u x i l i a r y e l e c t r o d e i s separated from the working e l e c t r o d e by a g l a s s - f r i t t o avoid contamination by oxygen generated a t t h e platinum s u r f a c e . P o t e n t i a l s a r e measured using a Luggin probe whose t i p i s positionned about 1 mm away from t h e e l e c t r o d e s u r f a c e and provided e s s e n t i a l l y s o l u t i o n r e s i s t a n c e f r e e data ( i . e . i r ohmic drops a r e l e s s than 1 mV) a t c u r r e n t d e n s i t i e s under 1 m ~ / c m ~ .

The s o l u t i o n s a r e prepared from a n a l y t i c a l grade reagents and double d i s t i 1 le d water. The acid s o l u t i o n s having d i f f e r e n t pH values a r e obtained by adding s u i t a b l e amounts of s o l i d sodium hydroxide t o 1 N s u l f u r i c a c i d s o l u t i o n . High p u r i t y n i t r o - gen i s bubbled through t h e a c i d s o l u t i o n p r i o r and during a p o l a r i z a t i o n run t o remove dissolved oxygen. The t e m ~ e r a t u r e i s maintained a t 25 o r 30 f 1°C.

Specimens a r e e i t h e r s u r f a c e ground on 600 g r i t S i c paper ( f o r B-2 and C-276 a l l o y s ) o r e l e c t r o l y t i c a l l y polished ( f o r A.I.S.I. 304). Current d e n s i t i e s a r e c a l c u l a t e d on t h e b a s i s of t h e exposed geometrical s u r f a c e a r e a ( 1 cm2).

I11 - RESULTS

Electrochemical parameters a r e summarized i n t a b l e 3. Potentiodynamic (450 mV/h) p o l a r i z a t i o n i s conducted a f t e r t h e steady s t a t e has been reached. The p o t e n t i a l sweep i s made from t h e f r e e p o t e n t i a l towards negative d i r e c t i o n /6-D/.

Temperature('C) Tafel slope n

Austenite s . s . (A.I.S.I. 304) 25 = 120 = 1

Marteniste s . s . ( A . I . S . I . 304) 25 = 120 = 1

Table 3 - Cathodic p o l a r i z a t i o n parameters /6-8/ f o r HER i n deaerated H2S04 solu- t i o n s ( 1 < pH < 2,5)

For Hastelloy a l l o y s , t h e chemical recombination mechanism f o r t h e H E R , according t o the Temkin adsorption isotherm w i t h a c t i v a t e d hydrogen desorption, i s c o n s i s t e n t w i t h t h e experimental data / 6 , 7 / . For the s t a i n l e s s s t e e l , t h e experimental above parameters a r e independent of t h e m i c r o s t r u c t u r e and agree w i t h t h e t h e o r e t i c a l values e i t h e r when t h e RDS i s t h e slow discharge (O-tO) o r when t h e RDS i s t h e electrochemical desorption ( e + I ) . However, t h e probable R.D.S. i s t h e electrochemical desorption r e a c t i o n i n agreement with t h e f a c t t h a t f o r t r a n s i t i o n metals t h e RDS is t h e slow removal of hydrogen absorbed on t h e s u r f a c e /3/.

A phenomenological i n t e r p r e t a t i o n o f the e x i s t e n c e of d i f f e r e n t mechanisms f o r t h e H E R depending on t h e nominal a l l o y composition i s based on t h e s p e c i f i c corrosion r e s i s t a n c e of Haste1 lay a l l o y s /6,7/ and t h e i r a b i l i t y t o form a p r o t e c t i v e film /9,10/. We have i n v e s t i g a t e d by Glow Discharge Spectroscopy /11/ t h e s u r f a c e films formed on B-2 and C-276 a l l o y s during d i s s o l u t i o n i n a deaerated H2S04 s o l u t i o n (pH = 1 ) . A s i g n i f i c a n t enrichment with molybdenum (8-2) or chromium (C-276) i s observed i n t h e passive f i l m . In t n e contrary t o Hastelloy C-276 a l l o y which spontaneous passivates i n t h e s e conditions /6/, t h e A.I.b.1. 304 s t a i n l e s s s t e e l e x h i b i t s an a c t i v e - p a s s i v e t r a n s i t i o n .

The e f f e c t of As203 on t h e k i n e t i c s of t h e HER i s t o modify t h e mechanism, from a slow electrochemical desorption t o a slow chemical combination, w i t h t h e consequent changes i n t h e cathodic parameters ( t a b l e 4 ) .

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A1 1 oy M i c r o s t r u c t u r e T a f e l s l o p e n A u s t e n i t e ( f a c e - c e n t e r e d c u b i c )

-

³⁰

-

²

M a r t e n s i t e (body-centered c u b i c ) = 60

-

¹

T a b l e 4

-

C a t h o d i c p o l a r i z a t i o n parameters a t 25°C f o r t h e HER on t h e c o r r o d i n g A.I.S.I. 304 s t a i n l e s s s t e e l i n deaerated H2S04 s o l u t i o n s ( 1 < pH < 2,5) c o n t a i n i n g 13 mg/l As203

C o r r o d i n g s t a i n l e s s s t e e l m i c r o s t r u c t u r e s behave i n two d i s t i n c t ways w i t h r e s p e c t t o t h e HER. An a c t i v a t e d Temkin i s o t h e r m i s v a l i d f o r m a r t e n s i t e whereas i t i s n o t p o s s i b l e t o d i s t i n g u i s h between t h e Langmuir and Temkin i s o t h e r m s f o r a u s t e n i t e ^{/ 8 / .} D i r e c t d e t e r m i n a t i o n o f t h e e l e c t r o d e coverage by e l e c t r o c h e m i c a l t r a n s i e n t s t u d i e s i s d i f f i c u l t because o t t h e i n t e r f e r e n c e o f m e t a l d i s s o l u t i o n and o x i d e s f o r m a t i o n . When As203 i s added t o t h e H2S04 s o l u t i o n , GDS and ESCA r e s u l t s seem show t h a t e l e m e n t a l a r s e n i c i s formed on t h e f r e e c o r r o d i n g s u r f a c e w i t h a c o n s e q u e n t l y i n h i - b i t e d d i s s o l u t i o n . The HER t a k e s p l a c e a t p o t e n t i a l s 400/500 mV more n e g a t i v e t h a n c o r r o s i o n p o t e n t i a l /8/.

The HER mechanism i s n o t m o d i f i e d ( T a f e l s l o p e

-

120 mV/decade, n = 1 ) when p o l a r i - z a t i o n measurements were r u n f r o m a c a t h o d i c p o t e n t i a l t o t h e r e s t p o t e n t i a l / 8 / . D u r i n g a c a t h o d i c hydrogen-charging a t

-

850 mV/S .C.E. which induces s u p e r f i c i a l c r a c k i n g f o r a u s t e n i t e , d e p o s i t i o n o f elemental a r s e n i c i n c r e a s e s hydrogen absorp- t i o n presumably by h i n d e r i n g t h e e l e c t r o c h e m i c a l d e s o r p t i o n . The r o l e o f p o t e n t i a l appears fundamental f o r t h e e x p l a n a t i o n o f t h e mechanism whereby a r s e n i c can enhance hydrogen e n t r y i n t o s t a i n l e s s s t e e l

.

CONCLUSION

Discharge o f hydrogen occurs by d i f f e r e n t mechanisms on a c t i v e s t a i n 1 ess s t e e l and p a s s i v e Ni-base a l l o y s . D e p o s i t o f elemental a r s e n i c

-

which promotes t h e e n t r y o f hydrogen i n t o t h e s t a i n l e s s s t e e l d u r i n g c a t h o d i c p o l a r i z a t i o n

-

changes t h e hydrogen e v o l u t i o n mechanism o n l y when t h e s t e e l s u r f a c e i s correded.

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1. HABASHI M., GALLAND J., MGm. S c i . Rev. Met. 79 (1982) 311 2. LUNARSKA-BOROWIECKA E., FIORE N.F., M@t. Trans. 12 A (1981) 1 0 1 3. PERALDO BICELLI L., ROMAGNANI C., J . Chim. P h y s . 7 2 (1975) 975 4. BOCKRIS J. O ' M . , SUBRAMANYAN P.K., E l e c t r o c h i m . Acta 16 (1971) 2169 5. BRICOUT J.P., MISSIAEN R., MORIAMEZ C., An. Chim. 7 ( m 8 2 ) 5

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10. BELLINA J.J., KARGOL J.A., FIORE N.F., S c F i p t a Met. 15 (1981) 211

11. BERNERON R., CHARBONNIER J.C., NAMDAR-IRAN1 R., MANEC J., C o r r o s i o n S c i .

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(1980) 899.