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Corrosion Science, Vol. 19, lap. 923 to 936Pergamon Press Ltd. 1979. Printed in Great Bmain.

T H E C O R R O S I O N A N D P A S SI V A T IO N O F I R O N I N T H EP R E S E N C E O F H A L I D E I ON S IN A Q U E O U S S O L U T I O N *M. A. C. DE CASTRO'~ and B. E. WILDE

U.S. Steel Corporation, Research Laboratory, Monroeville, PA 15146, U.S.A.Abstract--The anodic dissolution behaviour of iron in halide solutions has been studied with bothstationary and rotating electrodes. With stationary electrodes active dissolution kinetics are observed,whereas with rotation a pronounced active/passive transition occurs. A distinct pitting potential(Ec) was noted in each solution, the value of E, increasing in the order I> Br>C I>F . Halide ionconcentration and electrode velocity did not have any effect on the value of E~, indicating that thekinetics of pit init iation are independent of mass-transfer effects.During anodic dissolution at potentials more r.egative than E~, an inhibiting effect was noted,the degree of which depended on the atomic radius of the anion. A model is suggested which involvesthree electrode reactions of iron with the electrolyte: (1) Active dissolution involving the well-knownFeOH + (ads) rate-determining step. (2) Above the passivation potential, increased reaction of themetal surface with hydroxyl ions causes passivation due to the enhanced access of OH- to the surfaceand accelerated removal of solvated protons caused by rotation and a thinning of the diffusion layer.(3) At the pitt ing potential , direct reaction of the metal with electro-adsorbed halide ions producespit initiation and growth by a complex ion formation reaction not possible at lower electrodepotentials.

I N T R O D U C T I O NTHE PASSIVATION of iro n in aq ueo us med ia has bee n inve stig ated e xtensi vely ove r anum ber of years, 1-e and in general i t is agreed that passivation occurs because of theforma t ion of a hydrated oxide f ilm at the metal-electrolyte interface. The breakdownof passivity to produc e pits has also been studied extensively in aqueous ha logensolutions, s although the bulk of the data pertains to chloride-c ontaining media, s-l

The purpos e of the present work was to study the anodic dissolution kinetics ofiron in solutions of LiF, LiC1, LiBr and LiI under well defined hydrodyn amic condi-t ions at the interphase, since previous studies 5 have shown that passivation can bemarkedly influenced by such factors in chloride media.

E X P E R I M E N T A L M E T H O DThe iron used in this investigation had the chemical composition shown in Table 3. Cylindricalspecimens 3.3 cm diameter by 3.5 cm long were machined from bar stock which had been annealedat II00C in hydrogen. The surface of each specimen was polished by abrasion down to a 600 gritsilicon carbide finish. The specimen was mounted on the rotation shaft by using a threaded rod similarto the one used by Wilde and Williams previously, u The interface between the metal and the Teflongasket was painted with an alkyd resin (Glyptal) to avoid unwanted crevice corrosion. ~ The base ofthe cylinder was also painted to mask off all but the vertical surfaces to allow constant velocity con-ditions to be maintained.Immediately prior to use, each electrode was degreased in an ultrasonic cleaner containing adetergent, and was washed with distilled water and pickled in 3M HX:I: for 60 s followed by a waterrinse. The electrode was finally placed in a nitrogen-saturated test solution, and was allowed to achieve

a steady-state rest potential, E~o~, (usually 30-20 rain).*Manuscript received 28 February 3979.tVisiting Professor, Federal University of Rio de Janeiro, Coppe, Rio de Janeiro, Brazil.:[:X denotes the appropriate halogen acid.923

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924 M. A. C. DE CASTRO and B. E. WILDE

C

o~C

(Nz

o;E d

0

~. > .

Z

o

0

~.~

i i o,.1o

m d

~ 8d

d

~ .0

0

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T h e c o r r o s i o n a n d p a s s i v a t i o n o f i r o n 9 25P o l a r i z a t i o n c u r v e s w i t h a n d w i t h o u t v e l oc i ty w e re d e t e r m i n e d b y u s i n g a 2 5 m V s t e p -b y - s te pp r o c e d u r e w i t h 3 r a i n in t e r va l s . A W e n k i n g T S I p o t e n t i o s t a t w a s u s e d t o c o n t r o l t h e e l e c t r o d ep o t e n t i a l a n d a H o n e y w e l l E l e c t r o n i c 19 5 s t r i p - c h a r t r e c o r d e r w a s u s e d t o r e c o r d t h e a p p l i e d c u r r e n t ,/,p p. A l l p o t e n t i a l s w e r e m e a s u r e d w i t h r e s p e c t to a s a t u r a t e d c a l o m e l e l e c t r o d e ( S C E ) .A l l s o l u t i o n s w e r e p r e p a r e d f r o m r e a g e n t - g r a d e c h e m i c a l s d i l u t e d w i t h d o u b l e - d i s t i l l e d , d e -

m i n e r a l i z e d w a t e r w i t h a s p e c i fi c r e s i s t a n c e o f 1 4 M r 2 c m - 1. T h e e x p e r i m e n t a l g l a s s w a r e , r e f e r e n c ee l e c tr o d e , a n d L u g g i n - H a b e r p r o b e u s e d w e r e t h e s a m e a s t h o s e r e p o r t e d e l s ew h e re , n A l l e x p er i -m e n t s w e r e c o n d u c t e d a t 2 5 4 - I C .L i g h t a n d e l e c t r o n o p ti c a l e v a l u a t io n s w e r e c o n d u c t e d w i t h a L e i t z m e t a l l o g r a p h a n d a n E T E Cs c a n n i n g e l e c t ro n m i c r o s c o p e ( S E M ) , a n d a n e l e c t ro n m i c r o p r o b e w a s u s ed t o a n a l y s e t h e p r o d u c t s o fd i s s o l u t i o n i n s i d e g r o w i n g p i t s a f t e r r e m o v a l f r o m t h e e l e c t r o l y t e .

E X P E R I M E N T A L R E S U L T SChloride and bromide solutions

T h e 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 i ro n i n 1 M L i B r a t p H 8 is s h o w n i n F ig . 1 f o r ar o t a t i n g e l e c t r o d e a t 1 60 0 rp m . I n c l u d e d f o r c o m p a r i s o n a r e d a t a p r e v i o u s ly r e p o r t e d sf o r i r o n in 1 M L i C I a t p H 8 . A r t a c t i v e -t o - p a s s i v e t r a n s i t i o n o c c u r r e d a t - - 6 5 0 m V( S C E ) , a s i n t h e c a s e o f L i C I , b u t t h e c r i t i c a l a n o d i c c u r r e n t d e n s i t y , i c , a n d t h ep a s s i v e c u r r e n t d e n s i t y , i p , w e r e l o w e r f o r t h e b r o m i d e s o l u t i o n . T h e i n i t i a t i o n o fp i ts o c c u r r e d a t a m o r e p o s i t iv e p o t e n t i a l in b r o m i d e s o l u t i o n t h a n w i t h c h l o r i d es o l u t i o n , t h e p i t t i n g p o t e n t i a l s , E c , b e i n g - - 5 2 5 a n d - - 5 5 0 m V r e s p e c t iv e l y . I n s p e c -t i o n o f F i g . 1 a l s o r e v e a l s t h a t B r 7 i o n a c t s a s a n a n o d i c i n h i b i t o r a t a l l v a l u e s o fa p p l ie d p o t e n t ia l , E a p p , w h e n c o m p a r e d w i t h t h e d i s s o lu t io n r a t e o b s e r v e d i n a ne q u i v a l e n t c o n c e n t r a t i o n o f c h l o r id e .

E q u i v a l e n t d a t a a r e s h o w n i n F i g . 2 f o r t h e a b s e n c e o f r o t a t i o n . W h e r e a s L i C Ip r o d u c e d a r t a c t i v a t i o n - c o n t r o l l e d d i s s o l u t i o n c u r v e ( i . e . n o a c t i v e - t o - p a s s i v e t r a n s i -t i o n ) , a n a c t i v e - to - p a s s i v e t r a n s i t i o n w a s o b t a i n e d i n L i B r , w i t h a p r i m a r y p a s s i v a t i o n

l a

EJl= -

i==o

on ,-l . -

IL l

FI G . I .

- 4 5 0 ~ i

- 5 0 0PURE IRON 25C

-550 ROTATI ON RATE -- 1600 rpm LiC I-1 M pH = 8

- 6 0 0 L i B r - I M p H = 8

- 6 5 0

-700

- 7 5 0

I I1 0 - 6 1 0 " S 1 0 - 4 1 0 - 3

APPLIED CURR ENT DENSITY, A/cm 2A n o d i c p o l a r i z a t i o n c u r v e s fo r i r o n i n li t h i u m c h l o r id e a n d l i t h i u m b r o m i d es o l u t i o n s .

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926 M . A . C . DE CASTRO and B. E. WILDE

t- 5 0 0

- 5 6 0- 6 0 0

o I. ~ - 6 5 ou J - 7 0 0 I

- 7 5 0 I

1 0 - 6 1 0 " 5 1 0 " 4 0 - 3A P P L IE D C U R R E N T D E N S IT Y , A / c m

FIo. 2. The influence of zero rotat ion on the anodi c dissolution kinetics of iron inlithium bromide and lithium chloride solutions.

p o t e n t i a l , Ep p, a n d E c, t h e s a m e a s t h o s e o b s e r v e d u n d e r r o t a t i n g c o n d i t i o n s . T h u st h e ' m a s s t r a n s f e r e f f e c t ' d e s c r i b e d p r e v i o u s l y e i s d i f f e r e n t i n t h e c a s e o f b r o m i d es o l u ti o n s , s u c h t h a t p a s s i v a t io n c a n o c c u r w i t h o u t r o t a t i o n . I t i s o f i n te r e s t t o n o t et h e b r e a k i n t h e c u r v e f o r L i C I a t - - 5 7 5 m V , w h e r e p i t t i n g c o r r o s i o n o c c u r r e d a th i g h e r p o t e n t i a ls .

W i t h i n c r e a s i n g c h l o r i d e i o n c o n c e n t r a t i o n , a d e c r e a s e i n d i s s o l u t i o n k i n e t i c sw a s n o t e d e a t a ll p o t e n t i a ls , ; h o w e v e r , i n c r e a si n g b r o m i d e i o n c o n c e n t r a t i o n i n cr e a s e dt h e a n o d i c d i s s o l u t i o n r a t e , a s s h o w n i n F i g . 3 . T h e g r e a t e s t e f fe c t w a s n o t e d i n t h ev a l u e o f Ep p, w h i c h i n c r e as e d f r o m - - 6 5 0 m V f o r 1 M L i B r to - - 6 0 0 m V i n 4 M L i B r .

A m a r k e d i n fl u e nc e o f p H o n t h e a n o d i c d i s s o l u ti o n k i n e ti c s w a s n o t e d a t t h e 1 ML i B r c o n c e n t r a t i o n l ev e l. D a t a o b t a i n e d u n d e r r o t a t i n g c o n d i t i o n s a t 1 6 0 0 r p m a r es h o w n i n F i g. 4 a n d e q u i v a l e n t d a t a f o r s t a t i o n a r y c o n d i t i o n s a r e s h o w n i n F i g. 5 .W i t h r o t a t i o n , d e c r e a s e o f p H f r o m 6 to 2 p r o g r e s s i v e l y d e s t r o y s p a s s iv i t y , u l t i m a t e l yr e s u l t i n g i n a c t i v a t i o n - c o n t r o l l e d k i n e t i c s a t p H 2 . U n d e r s t a t i o n a r y c o n d i t i o n s , n op a s s i v a t io n w a s o b s e r v e d , a n d a s y s t e m a t i c d e c r e a s e i n T a f e l s l o p e w a s n o t e d w i t hd e c r e a s i n g p H ( F i g . 5 ). I t i s o f i n te r e s t t o n o t e t h a t a t a n y p o t e n t i a l , t h e la pp i n c r e a se sw i t h i n c r e a s i n g p H , a s o b s e r v e d b y o t h e r w o r ke r s l~ , a s w h o s u g g e s t t h a t t h i s b e h a v i o u rc a n b e e x p l a i n e d b y a d i s s o l u t i o n m e c h a n i s m d i r e c t l y i n v o l v i n g a d s o r b e d w a t e rm o l e c u l e s .Fluoride solutions

E x p e r i m e n t s c o n d u c t e d i n L i F w e r e l i m i t e d t o a m a x i m u m c o n c e n t r a t i o n o f0 . 1 M L i F a t p H 8 b e c a u se o f so l u b il it y fa c t o rs . T h e r e s u lt s o f p o l a r i z a t i o n e x p e r i-m e n t s a r e s h o w n i n F ig . 6 . W i t h a s t a t i o n a r y e l e c t r o d e , n o a c t i v e -t o - p a s s i v e t r a n s i t i o nw a s n o t e d , w h e r e a s p a s s i v it y w a s i n d u c e d b y r o t a t i o n , a s w i t h L iC 1 . T h e i p f o r i r o n

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The corrosion and passivation of iron 927

> "E..iE - 500,.J

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T h e c o r r o s i o n a n d p a s s i v a t i o n o f i r o n 9 29

u}>E.. i,,I -zI, -o

O(I-uJ

-400 I

-450

-500

-550

-600

-650

-700

F i o . 7 .

i !

CpH=6

I I10-5 10"6 10"5 10 "4

APPLIED CURR ENT DENSIT Y, A/cm 2T h e i n f l u en c e o f e l ec t r o d e r o t a t i o n r a t e o n t h e a n o d i c d i s s o l u t i o n k i n e t i c s o fi r o n i n l i t h i u m f lu o r i d e s o l u t i o n a t p H = 6 .

-100-150

8 -2ooE -250.3< -300I- -, , Z , - 3 5 0F-~ -400

- 4 5 00n-~- -500,,"I, -E50

- 6 0 0

-850-700

Fxo. 8.

= i#

PURE IRON 2 5 ~ C f ' ~L i I -1M pH = 8IO1.ATI~ 1600 rpmRATE 0 rpm

I I10"6 10-5 10 4 10-3

APPLIED CURR ENT DENSIT Y, A/cm 2T h e i n f l u e n c e o f e le c t ro d e r o t a t i o n o n t h e a n o d i c d i s s o l u t i o n k i n e t ic s o f i r o ni n l i t h i u m i o d id e a t p H = 8 .

by a sharp increase in i a p p a t - - 500 mV and -- 400 inV. Rota tion a t 1600 rev min -1evidenced no pass ivation until -- 350 mV where a sharp decrease in lapp was noted.The general shapes of the curves in LiI are quite different from those of the curves

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930 M . A . C . DE CASTRO an d B. E. WILDE

observed in CI-, Br- and F- solutions, perhaps because the solution contained anunavoidable 6.6 mg 1-1 LilO4 contamination. At Eapp negative to Epp, the iapp wastime dependent, as shown in Fig. 9, and the current increased for three minutes thendecreased over a long period of time, similar to the behaviour observed for iron inLisPO434

FIG. 9.

>. -p.z

t,-zPt.(DJ3gJ

10-5 m

PURE IRON 250CCONSTANT POTENTIALEapp = -600 mVscEL i I -1 M pH = 8

10-6 I I I I3 6 9 12

TIME, minutesThe variation o f applied current density with time at --600 mV (SCE) in lithium iodide solution at pH = 8.

D I S C U S S I O NThe anodic polarization behaviour of iron in LiBr and LiF is very similar to thatreported elsewhere e in LiCl solutions. The pronounced influence of rotat ion on the

dissolution kinetics can be explained by considering the effect of turbulence at theelectrode-electrolyte interface on the mass transport of reaction product away fromthe surface, is Passivation by hydrated oxide formation is favoured by rotationbecause of the removal of protons from the diffusion layer adjacent to the surface,and there is good agreement between thermodynamic data for oxides and hydroxidesof iron at various pH and the passivation potential observed under rotationalcontrol, ze In addition, it has been shown that in LiCI solutions the anodic dissolutionkinetics obey a simple Mueller type model which considers passivation to be a resultof OH- adsorption, a and it is presumed that the same mechanism is operative in LiBrand LiF solutions.

Schwabe x7 has discussed the inhibiting effects of halide ions on the anodic dissolu-tion kinetics, and has shown that the 'inhibition capacity' is a function of the atomicradius of the anion, e.g. I- > Br- > CI-. The results shown in Fig. 1 for Br- and CI-agree with Schwabe ~ in that Br- decreases the anodic dissolution rate over thatobserved at an equivalent concentration of CI- and Eapp. Unfortunately, equivalentdata could not be obtained by using LiF and LiI because of solubility restrictions and

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j

)k. 1~ J

FXG. 10. Scan ning electron migrograph of pits grown in (a) 0.1M L iF, pH 8 withoutrotation. Mag 600 x . (b) 1M LiBr, pH 6 with out rotation. Mag 300 .

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,,/i

. - ~ ..-'~ "

"t:~ ' - -

FIG. I 1. Scan ning electron microg raph s of pits grow n in (a) 4M LiBr, pH 8; (b) 0.1MLiF pH 8, durin g rotation at 1600 rev/m in -1, arrow indicates direction of rotation.Mag 300 x.

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t~%,

~ / r~o

.

~. .

IS P I Fe FeFI~. 12. (a) Mac rogr aph of pi ts gro wn in IM LiI, pH 8, at 1600 rev/min -~, arrowindicates d i rec t ion of rotat ion. Mag 300 . . (b) X-ray analys i s o f corros io n produ cts

in pit, e.g. Fel., .

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The corrosion and passivation of iron 935the presence of oxid izing impuri t ies , respect ively . I t i s of in teres t to note that theef fec ts o f ha l ide ion on the p i t t ing po ten t ia l fo l low the same o rder as found fo rinhib i t ion of anod ic d issolu t ion; e .g . Br - > C1- > F- .

In p rev ious wo rk 6 conduc ted in LiCI , i t was repor ted tha t E c was independen tof velocity . In the prese nt s tudy, i t was aga in foun d that a l tho ugh Ec varied with thehalo gen anion, i t was indepe nden t of veloci ty for a g iven anion. As before, i t wasa l so n o ted tha t p i t in i t ia t ion and g rowth occurred on s ta t ionary specimens in theact iva t ion-co ntro l led region of d issolu t ion (not necessari ly on a passive surface) .These observa t ions can be exp la ined by the concep t o f ha lide ion adso rp t ion on non-pass ivated port ions on the electrode surface to form 'c lus ters ' , as described by o therwo rke rsY . 19 These clus ters can o ccur pre ferent ia l ly on grains that have low Mil lerindices paral le l to the surface? As the electrode potent ia l i s made more posi t ive, acer ta in va lue i s reached a t wh ich a new d i s so lu t ion reac t ion can occur (one tha td i rec t ly invo lves the reac t ion o f the adso rbed an ion and the i ron subst rate ) , topro duc e a p i t nucleus at . E c The doc um ente d va riat ion of E~ with an ion species dependson the a tom ic rad ii and the s t reng th o f the adso rbed bond , wh ich increases in theo rder F - > C1- > Br- > I - .

Al though the p i t in i t ia t ion p rocess i s independen t o f in te r fac ia l hydrodynam ics ,the g row th o f p it s i s marked ly a f fec ted by ve loc i ty . F igu re I0 shows SE M pho to -g raphs o f p it s g rown in LiBr and LiF a t ze ro ve loc ity , where d i sc re te round p i t s a reform ed, which pene trate the metal to a s ignif icant depth . Figures 1 la and b s~_ow pi tsgrow n under the influence of ro ta t ion at 1600 rpm, with the d irect ion of surfacemotion indicated . In th is case, shal low pi ts are formed, wi th a ' s t reamer ' markt ra il ing a f te r the p i t where co r ros ion p rod uc t s f rom ins ide the p i t a re th rown o u t bycentr i fuga l force. A s imilar beha viou r was no ted in LiI so lu t ions , as shown in Fig . 12a long wi th an X -ray ana lysi s o f the p ro duc t s o f co r ros ion ins ide a p i t ( so lid FeI ) .

These observat ions suggest that in a pract ical s i tuat ion , such as a pump impel lerfo r example , a l though p i t in i t i a t ion may occur , p ropaga t ion may be kep t to theto lerable level by the hydrodynamic effects on the so lu t ions ins ide the p i ts .

C O N C L U S I O N SOn the basi s o f the da ta p resen ted the fo l lowing conc lus ions can be d raw n regard -

ing the anodic d issolu t ion of i ron in hal ide media.1. In the absence of ro tat io n , act iva t ion-con tro l led anodic d issolu t ions occurs .Wi th ro ta t ion , a p r onou nced ac t ive- to -pass ive t rans i tion occurred , a long wi th adis t inct p i t t ing potent ia l .2 . Th e observed p i t t ing potent ia ls increased with increas ing halide-ion size, wi thI - > Br - > CI- > F- .

3. The p i tt ing po ten t ia l was independen t o f ha l ide ion concen t ra t ion and e lec t rodeve loc i ty , ind ica t ing tha t hydrodynamic fac to rs a re no t impor tan t in the p i tin i t ia t ion process . Pi t growth, however, was markedly influenced by ro tat ion .

4 . A model based on two compet i t ive anod ic d is so lu tion reac t ions i s p ropo sed toexplain the effect of veloci ty on pass ivat ion , fo l lowed by a p i t in i t ia t ion react ionbe tween the meta l and adso rbed ha l ide ions a t po ten t ia l s above the p i t t ingpote,atial.

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9 36 M . A . C . D E C AS TR Oand B . E . Wi t .DER E F E R E N C E S

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