University of Windsor University of Windsor
Scholarship at UWindsor
Scholarship at UWindsor
Electronic Theses and Dissertations Theses, Dissertations, and Major Papers
1-1-1967
A re-examination of the kinetics of dissolution of tin in
A re-examination of the kinetics of dissolution of tin in
hydrochloric acid.
hydrochloric acid.
David S.P. Poa University of Windsor
Follow this and additional works at: https://scholar.uwindsor.ca/etd
Recommended Citation Recommended Citation
Poa, David S.P., "A re-examination of the kinetics of dissolution of tin in hydrochloric acid." (1967). Electronic Theses and Dissertations. 6492.
https://scholar.uwindsor.ca/etd/6492
A RE-EXAMINATION OF THE KINETICS OF DISSOLUTION OF TIN IN HYDROCHLORIC ACID
A T h e s is
S u b m itte d t o t h e F a c u l t y o f G ra d u a te S t u d ie s th r o u g h t h e D ep artm en t o f C hem ical E n g in e e r in g i n P a r t i a l F u l f i l m e n t
o f t h e R e q u ire m e n ts f o r t h e D egree o f M a s te r o f A p p lie d S c ie n c e a t th e
U n i v e r s i t y o f W indsor
fcy
D avid S .P . Poa
W in d so r, O n ta r io
1967
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f ) V T ' b O ' * I
APPROVED BY:
ABSTRACT
A kinetic study of the dissolution of tin in hydrochloric acid
solutions in the presence and absence of oxygen has been made.
It has been found that the tin dissolution process in aerated
hydrochloric acid solutions appears to occur through three simultaneous
reactions, hydrogen evolution, oxygen depolarization, and an autocatalytic
reaction.
Over the range of conditions studied the dissolution rate may
be expressed by the empirical equation:
— [Bn] - 5.31 X 10" 7 £ [HCl]°[v]0 *98 e" ^ W ~
+ 2.88 X 10“ 5 ^ [HC1]°[v]°*9 8[Pq I1 / 2 e" \ t
2
l TAl1 / 2 0 n l/p l/p ^ - ^ 0
+ 1.25 X 10 [HC1] [v] [P T ' [Sn] ' ' e" RT
2
in which the third term (autocatalytic) applies only after an elapsed
time of 30 minutes.
Low temperature coefficients, and significant effects of
peripheral velocity and turbulence indicate that diffusions! processes
are involved in the controlling step. The dependence of the dissolution
rate on the square root of the oxygen partial, pressure is indicative of
a dissociative adsorption of oxygen on the metal surface.
ACKWOV/TjEDGMENTS
The author vishcs to express his sincere gratitude to
Dr. A.W. Gnyp for his able guidance, constructive criticisms, and
encouragement throughout this work.
Thanks are expressed to Mr. Halil Parlar for machining the
tin samples and assistance in setting up the apparatus.
The financial assistance offered by the National Research
Council has been appreciated..
TABLE OF CONTENTS
Page
ABSTRACT iii
ACKNOWLEDGMENTS iv
T/lBLE OF CONTENTS v
LIST OF TABLES v i i
LIST OF FIGURES v i i i
Cliapter
I. INTRODUCTION 1
II. LITERATURE REVIEW 2
A. Kinetic Studies on Metal Dissolution 2
B. General Review of Studies on Tin Corrosion 2
C. Hydrodynamic Factors in the Dissolution of Metals 5
D. The Analysis of Tin 6
III. EXPERIMENTAL WORK 8
A. Materials 8
B. Apparatus 8
C. Procedure 10
IV. GENERAL THEORY OP’ METAL DISSOLUTION 12
V. EXPERIMENTAL RESULTS All) DISCUSSION 22
A. General Behavior of Tin Dissolution in 1101 Solutions 22
B. Rate Dependence on Tin Ion Concentration 22
C. Rate Dependence on Peripheral Velocity 2.5
D. Rate Dependence on Temperature 29
E. Rate Dependence* on Surface Area and Corroding Solution
Volume 3l
F. Effect of Hydrochloric Acid Concentration 3k
G. Rate Dependence on Oxygen Concentration 3®
H. The Effect of Hydrodynamic. Factors kO
I. Mechanism of Tin Dissolution in Hydrochloric Acid h2
J. Empirical Equation for Tin Dissolution in Hydrochloric
Acid If 5
VI. CONCLUSIONS k9
Page
APPENDIX I. Polarographic Determination of Tin Concentration 51
APPENDIX II. Data of Tin Dissolution 53
APPENDIX III. Nomenclature TO
K3PEKENCES ■ 71
VITA AUCTORIS 73
vi
Table
1.
II.
I I I .
IV.
LIST OF TABLES
Page
Effect of Hydrochloric Acid Concentration on Dissolution
Rate ' 38
Evaluation of Velocity Constant k° for Hydrogen Evolution h6
Evaluation of Velocity Constant k^ for Oxygen Depolarization 1*7
Evaluation of Velocity Constant k° for autocatalytic
Reaction 5 1*7
LIST OF FIGURES
1 . A rran g em en t o f A p p a ra tu s
2 . A T w o -d im e n sio n a l M odel o f P o l i s h e d S u r f a c e s on an A tom ic S c a le
3 . D i s s o l u t i o n o f M e ta l i n A c id ic M edia
4 . D i s s o l u t i o n o f T in i n H y d r o c h lo r ic A cid
5. H a l f - o r d e r D ependence f o r A u t o c a t a l y t i c R e a c tio n
6A. D i s s o l u t i o n a s F u n c tio n o f P e r i p h e r a l V e l o c i t y (h y d ro g e n e v o l u t i o n )
6B. D i s s o l u t i o n a s F u n c tio n o f P e r i p h e r a l V e lo c i ty (o x y g en d e p o l a r i z a t i o n )
7. D i s s o l u t i o n a s F u n c tio n o f P e r i p h e r a l V e l o c i t y ( a u t o c a t a l y t i c r e a c t i o n )
8. D i s s o l u t i o n a s F u n c tio n o f T e m p e ra tu re (H^ e v o l u t i o n )
9 . D i s s o l u t i o n a s F u n c tio n o f T e m p e ra tu re ( oxygen d e p o l a r i z a t i o n )
10 . D i s s o l u t i o n a s F u n c tio n o f T e m p e ra tu re ( a u t o c a t a l y t i c r e a c t i o n )
11 . A c t i v a t i o n E n erg y a s F u n c tio n o f Sample D ia m e te r ( a u t o c a t a l y t i c r e a c t i o n )
12 . D i s s o l u t i o n a s F u n c tio n o f S u r f a c e Area, (Hg e v o l u t i o n and oxygen d e p o l a r i z a t i o n )
1 3. D i s s o l u t i o n a s F u n c tio n o f A/V
(Hg e v o l u t i o n and oxygen d e p o l a r i z a t i o n )
1 4 . D i s s o l u t i o n a s F u n c tio n o f S u r f a c e A rea ( a u t o c a t a l y t i c r e a c t i o n )
1 5 . D i s s o l u t i o n a s F u n c tio n o f A/V ( a u t o c a t a l y t i c r e a c t i o n )
l 6A. E f f e c t o f Oxygen C o n c e n t r a ti o n on Oxygen D e p o l a r i z a t i o n
v i i i
P age
9
13
21
23
2h
26
27
28
30
31
32
33
35
36
37
37
39
Figure Page
l 6B, E f f e c t o f Oxygen C o n c e n t r a ti o n on A u t o c a t a l y t i c R e a c tio n 39 t
1 7 . E f f e c t o f H ydrodynam ic F a c t o r s on H ydrogen E v o l u tio n 111
18. E f f e c t o f H ydrodynam ic F a c t o r s on Oxygen D e p o l a r i z a t i o n Ul
1 9 . E f f e c t o f H ydrodynam ic F a c t o r s on A u t o c a t a l y t i c R e a c tio n Ul
2 0 . C a l i b r a t i o n C urve f o r S a r g e n t M odel XV P o la r o g r a p h 52
CHAPTER I
INTRODUCTION
R e s e a rc h and d e v e lo p m e n t i n t h e f i e l d o f c o r r o s i o n s c ie n c e
h av e "been s t i m u l a t e d b y p ro b le m s r e l a t e d t o a d v a n c e s i n te c h n o lo g y , an d
f a c i l i t a t e d b y p a r a l l e l a d v a n c e s i n o t h e r f i e l d s o f s c i e n c e . D e s p ite
t h e p r o g r e s s w h ich h a s b e e n m ade, h o w e v e r, o u r a p p r o a c h e s t o t h e
p r e v e n t i o n o f c o r r o s i o n i n p r a c t i c e a r e e m p i r i c a l . We s t i l l do n o t h a v e
p r i n c i p l e s w h ich w i l l a llo w u s t o f o r e c a s t t h e b e h a v i o r o f a g iv e n
m a .te r ia l i n a g iv e n s i t u a t i o n . Our i n a b i l i t y t o d e a l w ith c o r r o s i o n on
a more r a t i o n a l b a s i s ste m s l a r g e l y fro m o u r g r e a t ig n o r a n c e o f t h e
d e t a i l s o f t h e m echanism s in v o lv e d .
S e v e r a l i n v e s t i g a t i o n s h a v e b e e n done on t h e m echanism an d
'8 22 21 2h
k i n e t i c s o f t h e d i s s o l u t i o n o f c o p p e r * ’ , t i n an d t i t a n i u m . I t
h a s b e e n shown t h a t o v e r a w id e ra n g e o f c o n d i t i o n s t h e d i s s o l u t i o n o f
t h e s e m e ta ls i n a i r - s a t u r a t e d a c i d i c m e d ia i s a u t o c a t a l y t i c .
The p u rp o s e o f t h e p r e s e n t s tu d y was t w o - f o l d : ( l ) t o a t te m p t
t o d e te r m in e t h e r a t e - d e t e r m i n i n g s t e p and make a f u r t h e r s tu d y o f t h e
m echanism and k i n e t i c s o f t h e d i s s o l u t i o n o f t i n i n h y d r o c h l o r i c a c i d
s o l u t i o n s w ith t h e hope o f p r o v i d i n g a more r e a s o n a b l e d e s c r i p t i o n
c l a r i f y i n g t h e way b y w h ich t h i s m e ta l i s d e s t r o y e d , (2) t o s tu d y t h e
r o l e o f h y d ro d y n am ic f a c t o r s n e a r t h e m e ta l s u r f a c e on t h e d i s s o l u t i o n
p r o c e s s .
2k T h is p r o j e c t was b a s e d p a r t l y on t h e f u n d a m e n ta l w ork o f L u i
who s t u d i e d t h e d i s s o l u t i o n o f t i n i n h y d r o c h l o r i c a c i d s o l u t i o n s .
1
CHAPTER I I
LITERATURE REVIEW
A. K i n e t i c S t u d ie s on M e ta l D i s s o l u t i o n
22 2 V
Lu an d G raydon , s t u d i e d t h e k i n e t i c s a n d m echanism o f
c o p p e r d i s s o l u t i o n i n a q u e o u s ammonium h y d r o x id e and aq u eo u s s u l f u r i c
29
a c i d s o l u t i o n s . Weeks a n d H i l l s i n v e s t i g a t e d t h e i n i t i a l c o r r o s i o n
g
r a t e o f c o p p e r i n 1IC1 s o l u t i o n s . T h e i r w ork was e x te n d e d b y Gnyp .
L a t e r , s y s t e m a t i c r e s e a r c h e s on t h e d i s s o l u t i o n k i n e t i c s o f b r a s s w ere
dene b y K a g e ts u a n d Graydon'*"*' a n d B um bulis and G ray d o n ^, a n d t i t a n i u m b y
k
B c d n e r . The abov e i n v e s t i g a t o r s s t u d i e d t h e r a t e o f m e ta l d i s s o l u t i o n
i n a e r a t e d s o l u t i o n s a s a f u n c t i o n o f t e m p e r a t u r e , oxygen p a r t i a l
p r e s s u r e , r o t a t i o n a l s p e e d , sam ple s u r f a c e a r e a , c o r r o d in g s o l u t i o n
volum e an d a c i d c o n c e n t r a t i o n . O ver a w ide ra n g e o f c o n d i t i o n s , a l l o f
th em fo u n d an a u t o c a t a l y t i c e f f e c t , w i t h d i s s o l u t i o n r a t e i n c r e a s i n g
w ith i n c r e a s i n g m e ta l io n c o n c e n t r a t i o n i n s o l u t i o n .
B. G e n e ra l Review o f S t u d i e s on T in C o r r o s io n
As e a r l y a s 1 8 1 3, t h e d i s s o l u t i o n o f t i n i n a c i d s o l u t i o n s was
2 3
exam in ed b y B e r z e l i u s . A c c o rd in g t o h i s r e p o r t , t i n d i s s o l v e s in
h y d r o c h l o r i c a n d s u l f u r i c a c i d s o l u t i o n s i n t h e s ta n n o u s fo rm w ith t h e
e v o l u t i o n o f h y d ro g e n g a s .
7
E v ans r e p o r t e d t h a t t i n , w h ich s t a n d s im m e d ia te ly b e lo w h y d ro g e n
i n t h e t a b l e o f n o rm a l p o t e n t i a l s , l i b e r a t e s h y d ro g e n fro m d i l u t e HC1 o n ly
v e r y s lo w ly . H ere i t i s t h e h ig h o v e r p o t e n t i a l o f h y d ro g e n e v o l u t i o n
on a t i n s u r f a c e w h ich i s r e s p o n s i b l e f o r t h e s l u g g i s h r e a c t i o n .
3 0
W hitman and R u s s e l s t u d i e d t h e d i f f e r e n c e i n t h e r a t e o f
c o r r o s i o n o f t i n i n m o d e r a te ly c o n c e n t r a t e d a c i d s a t room te m p e r a t u r e i n
t h e p r e s e n c e an d a b s e n c e o f o x y g en . They showed t h a t , i n m o st c a s e s ,
o x i d i z i n g a g e n ts a c t a s d e p o l a r i z e r s , an d i f ad d ed t o a c i d s , w i l l i n c r e a s e
t h e a t t a c k on t i n . I n t h e a b s e n c e o f a i r o r o t h e r o x i d i z i n g a g e n t s , t i n
was v e r y r e s i s t a n t t o d i l u t e a c i d s . T h is i s b e c a u s e t i n h a s a h ig h
h y d ro g e n o v e r p o t e n t i a l , an d i t q u i c k l y becom es p o l a r i z e d b y h y d ro g e n
w h ich p r e v e n t s t h e flo w o f c u r r e n t t h a t acc o m p an ies c o r r o s i o n .
I k
Kohman and S an b o rn r e p o r t e d t h a t , i n a i r - f r e e s o l u t i o n s , t i n
i s d e p o l a r i z e d b y i n c r e a s i n g t h e te m p e r a tu r e o f th e s o l u t i o n , an d h y d ro g e n
e v o l u t i o n o c c u r s . The e f f e c t o f te m p e r a t u r e on t h e c o r r o s i o n o f t i n i n
12
a c i d s o l u t i o n s was a l s o i n v e s t i g a t e d b y K h itr a v and S h o ta lo v a . The
r a t e o f t i n c o r r o s i o n i n 1 -7 M s o l u t i o n s o f h y d r o c h l o r i c a c i d a n d s u l f u r i c
a c i d was o b s e r v e d t o i n c r e a s e w ith i n c r e a s i n g t e m p e r a t u r e . I n t h e 0-80°C
ra n g e t h e r a t e o b ey ed t h e v a n ’t H o ff an d A r r h e n iu s Law s. The r a t e o f
d i f f u s i o n a l s o i n c r e a s e d w ith i n c r e a s e i n te m p e r a tu r e b u t n o t a s r a p i d l y
a s t h e c o r r o s i o n r a t e . The i n c r e a s e o f c o r r o s i o n r a t e w ith i n c r e a s i n g
te m p e r a tu r e i s a s s o c i a t e d w ith a f a l l i n h y d ro g e n o v e r p o t e n t i a l , d e c r e a s e
i n p o l a r i z a t i o n , d e c r e a s e in v i s c o s i t y o f t h e s o l u t i o n and d e s t r u c t i o n
o f p r o t e c t i v e f i l m s .
An i n s t r u c t i v e c a s e o f l o c a l i z e d c o r r o s i o n o f t i n , i n v e s t i g a t e d
b y H o a r ^ , showed t h a t t h e s a l t s o f t i n a r e s t r o n g l y h y d r o ly z e d , an d i n
a n e u t r a l s o l u t i o n a n o d ic a t t a c k a t a weak s p o t on t h e a i r fo rm ed f i l m
c o v e r in g t h e m e ta l w i l l n o t c a u s e t i n c a t i o n s t o p a s s i n t o t h e l i q u i d ,
b u t r a t h e r c a u s e an i n c r e a s e i n t h e f i l m - t h i c k n e s s b y d e p o s i t i o n o f o x id e
and h y d r o x id e . A f t e r a c e r t a i n tim e t h e a c c u m u la te d a c i d i t y a t t h e s e
p o i n t s a p p a r e n t l y becam e s u f f i c i e n t f o r t h e f o r m a tio n o f s o l u b l e s ta n n o u s
i o n s , w h ich r u i n t h e f i l m so t h a t b re a k -d o w n o c c u r r e d w ith t h e f o r m a t io n
5
o f t h e b l a c k s p o t s . B r i t t o n and M ic h a e l s t u d i e d t h e l o c a l c o r r o s i o n o f
t i n i n c h l o r i d e s o l u t i o n s . T h e i r r e s u l t s showed t h a t t h e tim e e l a p s i n g
b e f o r e l o c a l c o r r o s i o n b e g in s i s a f f e c t e d b y s u r f a c e c o n d i t i o n s . L o c a l
c o r r o s i o n i s i n i t i a t e d and a c c e l e r a t e d b y c r e v i c e s r e s u l t i n g fro m s u r f a c e
d e f e c t s o r b y c o n t a c t o f t h e m e ta l w ith a n o t h e r s u r f a c e . The b la c k n e s s
o f t h e s p o t s , a c c o r d in g t o B r i t t o n , i s p r o b a b ly due t o t h e a b s e n c e o f
r e f l e c t i o n fro m t h e l o c a l l y ro u g h e n e d s u r f a c e .
9
Hagymas a n d Q u in ti n s t u d i e d t h e c o r r o s i o n o f t i n i n s u l f u r i c
a c i d . I n 0 .1 t o 1 . 0 M s o l u t i o n s t h e h y d ro g e n o v e r v o lt a g e on t h e t i n
l b e l e c t r o d e d i d n o t v a r y w i t h a c i d c o n c e n t r a t i o n . Kahman and S an b o rn
a l s o r e p o r t e d t h a t t h e r e was no a p p a r e n t i n f l u e n c e o f a c i d c o n c e n t r a t i o n
on t h e c o r r o s i o n o f t i n i n a i r - f r e e s o l u t i o n s .
25
M arsh ak o v , U g a i a n d V ig d o ro v ic h r e p o r t e d t h a t i n a c i d i c m ed ia
t h e c o r r o s i o n o f m a g n e s iu m -tin a l l o y s i s u n if o r m i n c h a r a c t e r . The
sp ecim en s u r f a c e becom es c o a te d w ith a g r a y f i l m w h ich d o e s n o t a d h e re
f i r m l y t o t h e m e ta l. C hem ical a n a l y s i s showed i t t o b e a lm o s t p u re
t i n .
P r o b a b ly t h e m ost e x t e n s i v e and s y s t e m a tic s tu d y on th e
211
-d i s s o l u t i o n r a t e o f t i n was -done b y L u i . He r e p o r t e d t h a t o v e r a w ide
ra n g e o f c o n d i t i o n s t h e d i s s o l u t i o n o f t i n i n h y d r o c h l o r i c a c i d s o l u t i o n s
p r o c e e d s i n tw o a u t o c a t a l y t i c s t a g e s w ith t h e r a t e d u r in g e a c h s t a g e b e i n g
d e p e n d e n t on t h e s q u a r e r o o t o f t h e s t a n n i c io n c o n c e n t r a t i o n i n t h e
q H ydrodynam ic F a c t o r s i n t h e D i s s o l u t i o n o f M e ta ls
S tu d ie s on t h e r a t e s o f h e te r o g e n e o u s p r o c e s s e s i n l i q u i d s
b e g a n w ith t h e s i m p l e s t p ro b le m i n d i f f u s i o n k i n e t i c s , n am ely t h e
d i s s o l u t i o n o f s o l i d s i n l i q u i d s . Upon a n a l y s i s , t h e e x p e r i m e n t a l d a t a
27
c o l l e c t e d b y S h ch u k o rev 1 y i e l d e d t h e f o ll o w i n g e m p i r i c a l e q u a t io n f o r
d i s s o l u t i o n :
Q = k (C - C ) S
v s o '
w h ere Q i s t h e am ount o f m a t e r i a l d i s s o l v e d p e r u n i t t i m e , S i s t h e
s u r f a c e a r e a o f t h e d i s s o l v i n g b o d y , C i s t h e c o n c e n t r a t i o n o f a s
s a t u r a t e d s o l u t i o n , C i s t h e c o n c e n t r a t i o n o f t h e b u l k s o l u t i o n a t a ' o
g iv e n i n s t a n t , an d k i s a p r o p o r t i o n a l c o n s t a n t .
26
F u r t h e r s t u d i e s , p r i m a r i l y b y N e m s t , showed t h a t k i s
p r o p o r t i o n a l t o D, t h e d i f f u s i o n c o e f f i c i e n t o f t h e s u b s ta n c e i n t h e
l i q u i d . Thus t h e e x p r e s s i o n f o r Q may b e w r i t t e n i n t h e fo rm :
D (C - C ) S Q = — £ ---2l _
6
I t h a s b e e n fo u n d t h a t u n d e r o r d i n a r y m ix in g c o n d i t i o n s , t h e
-2 _![.
q u a n t i t y 6 h a s a m a g n itu d e o f 10 t o 10 c m ., w h ich i s e x tr e m e ly s m a ll
com pared t o t h e d im e n s io n o f t h e u s u a l r e a c t i o n v e s s e l s . T h is l e d N e m s t
t o assum e t h a t t h e 6 r e p r e s e n t s t h e t h i c k n e s s o f t h e l a y e r a c r o s s w h ich
t h e d i f f u s i o n o c c u r s i n a m oving l i q u i d . A c c o rd in g t o t h e N e r n s t T h e o ry ,
t h e r e i s a t h i n l a y e r o f s t a t i c l i q u i d im m e d ia te ly a d j a c e n t t o t h e
s u r f a c e o f t h e s o l i d bo d y , a l a y e r th r o u g h w h ich d i f f u s i o n o f t h e
r e a c t i n g m o le c u le s t a k e s p l a c e . Beyond t h i s l a y e r , t h e c o n c e n t r a t i o n i n
t h e b u lk o f t h e s o l u t i o n i s c o n s t a n t b e c a u s e o f t h e l i q u i d m o tio n . r ne
sym bol 5 i s te rm e d t h e t h i c k n e s s o f t h e N e rn s t d i f f u s i o n la y e r . E x p e r im e n ta l
d e t e r m i n a t i o n s h a v e shown t h a t ’ 5, a s a f u n c t i o n o f v e l o c i t y , h a s t h e fo rm :
‘ ■ 4
-V
w here v i s t h e v e l o c i t y o f t h e m oving l i q u i d . The v a l u e o f n v a r i e s fro m
l / 2 t o 1 . I n g e n e r a l , t h e r a t e o f d i f f u s i o n - c o n t r o l l e d r e a c t i o n s can h e
l a r g e l y i n c r e a s e d b y a g i t a t i n g t h e l i q u i d w ith s t i r r e r s o r r o t a t i n g t h e
s o l i d b o d y o f d i s s o l v i n g s u b s t a n c e s b e c a u s e t h e d i f f u s i o n c o e f f i c i e n t i n
a m o ving , t u r b u l e n t medium i s much l a r g e r th a n t h a t i n a s t a t i c medium.
17
L e v ic h was t h e f i r s t t o f o r m u la te t h e d i f f u s i o n f a c t o r f o r a
r o t a t i n g sp ecim en i n s o l u t i o n . He showed t h a t t h e d i f f u s i o n a l f l u x o f
m ass i n a t u r b u l e n t l i q u i d may b e r e p r e s e n t e d b y an e q u a t io n i n t h e fo rm :
J = (D + h u r t ' g f
= (D + P*v")'j£
w here I i s t h e s c a l e o f t h e e d d i e s an d v i s t h e p e r i p h e r a l v e l o c i t y .
The b a r above t h e I v i n d i c a t e s t h e a v e r a g e v a lu e * p i s a c o n s t a n t . The
l a r g e v a l u e o f D e n s u r e s a v i r t u a l l y c o n s t a n t c o n c e n t r a t i o n o f t h e \
s o l u t i o n down t o v e r y s m a ll d i s t a n c e fro m t h e r e a c t i o n s u r f a c e .
D. Hie A n a ly s is o f T in
The a n a l y s i s o f t i n b y t h e p o la r o g r a p h ic m ethod h a s b e e n
18 19 20
w e l l d e v e lo p e d b y L in g a n e } . The r e d u c t i o n o f s ta n n o u s t i n t o
t h e m e t a l l i c t i n p r o d u c e s d e f i n e d w aves fro m H i HC1, w ith a h a lf - w a v e
p o t e n t i a l o f -O.U7V v s S .C .E . when 0 .0 1 p e r c e n t g e l a t i n i s p r e s e n t a s a
s u p p r e s s o r . C h lo r o s ta n n a te com plex io n a l s o p r o d u c e s a w e l l d e v e lo p e d
d o u b le t w ave. The f i r s t wave r e s u l t s fro m t h e r e d u c t i o n o f t h e c h l o r o
7
t o c o m p le te r e d u c t i o n t o t h e m e ta l. A s u p p o r tin g e l e c t r o l y t e c o n s i s t i n g
o f UM NHjjCl, 1M HC1, a n d 0 .0 0 5 p e r c e n t g e l a t i n was u s e d a s t h e maximum
s u p p r e s s o r . The h a lf - w a v e p o t e n t i a l o f t h e f i r s t wave i s - 0 .2 5 v an d
t h a t o f t h e se c o n d i s -0.^>2v v s S .C .E . i n t h e p r e v io u s d e s c r i b e d
c o n d i t i o n s . The f i r s t wave i s n o t f u l l y d e v e lo p e d b e f o r e t h e s e c o n d
s t a g e o f r e d u c t i o n b e g i n s . M easurem ent o f t h e se c o n d wave f o r r o u t i n e
a n a l y s i s i s recom mended b y L in g a n e .
CHAPTER I I I
EXPERIMENTAL WORK
A. M a t e r i a l s
The A n a la r g ra d e t i n "bars u s e d i n t h i s s tu d y w ere s u p p l i e d b y
B r i t i s h Drug House L td . The p u r i t y o f m e ta l was 99*92 p e r c e n t t i n .
The e x a c t a n a l y s i s a c c o r d in g t o t h e m a n u f a c tu r e r showed 0 .0 1 p e r c e n t
l e a d , 0 .0 0 2 5 p e r c e n t c o p p e r , 0 .0 0 2 p e r c e n t b is m u th , 0 .0 0 2 p e r c e n t
i r o n , 0 .0 k p e r c e n t t o t a l f o r e i g n m e t a l s , 0 ,0 0 0 1 p e r c e n t a r s e n i c , an d
0 .0 2 5 p e r c e n t a n tim o n y .
The t i n b a r s , o r i g i n a l l y 1 .0 2 cm. i n d i a m e te r , w ere m e lte d and
rem o u ld ed i n t o 1 . 9 1 an d 2.5^ cm. d ia m e te r b a r s u n d e r a r g o n g a s i n a
vacuum f u r n a c e . C y lin d e r s o f 0 .9 1 5 t o 2 .^ 2 cm. i n d i a m e t e r , O.^B t o
I . 2 7 cm. i n l e n g t h , w ith a c o n c e n t r i c h o le t o f i t o n to a r o t a t i n g s h a f t
w ere m ach in ed fro m t h e s e rem o u ld ed t i n b a r s .
H y d r o c h lo r ic a c i d an d a l l o t h e r r e a g e n t s u s e d w ere o f a n a l y t i c a l
g r a d e . D i l u t e HC1 s o l u t i o n s w ere made w ith d i s t i l l e d w a te r .
B. A p p a ra tu s
The a p p a r a tu s u s e d f o r t h i s i n v e s t i g a t i o n i s shown i n F ig u r e 1 .
The r e a c t i o n c e l l c o n s i s t e d o f a s p e c i a l l y d e s ig n e d j a c k e t e d
v e s s e l f a b r i c a t e d fro m tw o P y re x b e a k e r s . W ater was f o r c e d th r o u g h t h e
s h e l l by means o f a pump fro m a c o n s t a n t te m p e r a tu r e b a t h . The t o p o f
t h e r e a c t i o n v e s s e l was c o v e re d w ith a P l e x i g l a s p l a t e h o ld i n g up t o
t h r e e r a d i a l l y m ounted b a f f l e p l a t e s d e s ig n e d t o p r e v e n t v o r t e x f o r m a tio n
R e p ro d u c e d w ith pe rm is sio n of th e c op yr ig ht o w n e r. F u rt h e r re p ro d u ct io n p ro h ib ite d without p e rm is si o n . PULLEYS
PLEXIGLAS SLEEVE SELF ALIGNING BEARINGS
STAINLESS STEEL SHAFT
GAS INLET
WATER EX IT 1 5 0 0 m l BEAKER
1 0 0 0 m l BEAKER 1 OF 3 BAFFLES
TIN CYLINDER
PLEXIGLAS CAP
WATER INLET
FIGURE 1 . ARRANGEMENT OF APPARATUS
10
when t h e s h a f t was r o t a t e d a t h ig h s p e e d s .
The c y l i n d r i c a l t i n sa m p le s w ere r o t a t e d on a s t a i n l e s s s t e e l
s h a f t . The e n d s o f t h e t i n c y l i n d e r s w ere p r o t e c t e d fro m t h e c o r r o d in g
medium b y P l e x i g l a s s l e e v e s an d a c a p sc re w e d t i g h t l y a t t h e e n d . The
s h a f t was d r i v e n b y a Type 7 HM H oover vacuum c l e a n e r m o to r whose sp e e d
was c o n t r o l l e d b y a a u t o t r a n s f o r m e r a n d a c o n s t a n t v o l t a g e r e g u l a t o r .
S h a f t s p e e d s w ere m e a su re d b y means o f a Type 1 5 3 1 -A S t r o b o ta c p ro d u c e d
b y t h e G e n e r a l R ad io C o r p o r a tio n .
C. P ro c e d u re
A t y p i c a l e x p e rim e n t was r u n b y p o u r in g a m e a su re d volum e o f
HG1 s o l u t i o n i n t o t h e r e a c t i o n v e s s e l an d f l u s h i n g t h e l i q u i d w ith
a p p r o p r i a t e g a s f o r a p p r o x im a te ly 20 m in u te s b e f o r e s t a r t - u p . D u rin g
t h e t e s t p e r i o d t h e h y d r o c h l o r i c a c i d s o l u t i o n was k e p t s a t u r a t e d w ith
a i r , n i t r o g e n , o r o x y g en . L o s s e s o f h y d r o c h l o r i c a c i d w ere m in im iz e d
b y p a s s in g t h e g a s e s th r o u g h a s e r i e s o f w a s h in g b o t t l e s c o n t a i n i n g HC1
s o l u t i o n o f t h e same c o n c e n t r a t i o n a s i n t h e r e a c t i o n v e s s e l . A l l w ash
b o t t l e s w ere k e p t a t t h e same te m p e r a t u r e a s t h e c o r r o d in g s o l u t i o n .
The t i n c y l i n d e r s , p o l i s h e d m a n u a lly t o 3 /0 em ery p a p e r
sm o o th n e ss ( a v e r a g e s u r f a c e ro u g h n e s s l e s s th a n 20 m i c r o i n c h e s ) , w ere
w ash ed i n i t i a l l y w ith d i s t i l l e d w a te r a n d d r i e d w ith f i l t e r p a p e r , th e n
w ashed w ith a c e to n e f o r d e g r e a s i n g , an d f i n a l l y rew ash ed w ith d i s t i l l e d
w a te r . A f t e r e a c h r u n , t h e t i n c y l i n d e r s w ere w ashed an d d r i e d . A
ch e c k on t h e m a t e r i a l b a la n c e was m a in ta in e d b y w e ig h in g t h e c l e a n d r y
t i n c y l i n d e r s b e f o r e an d a f t e r e a c h r u n .
An a p p r o p r i a t e l y s i z e d sam ple o f t h e c o r r o d in g s o l u t i o n was
ti m e . F o r e v e r y sam ple o f c o r r o d in g s o l u t i o n w ith d ra w n , a n e q u a l volum e
o f HC1 s o l u t i o n o f t h e same c o n c e n t r a t i o n was ad d ed t o t h e v e s s e l t o
e l i m i n a t e s o l u t i o n volum e change d u r in g t h e c o r r o s i o n r u n .
CHAPTER IV
GENERAL THEORY OF METAL DISSOLUTION
The d i s s o l u t i o n o f m e ta ls i n a c i d i c m edia i s a n e l e c t r o c h e m i c a l
p r o c e s s . H ow ever, t h e e l e c t r o c h e m i c a l p r o c e s s e s a r e g r e a t l y c o m p lic a te d
b y t h e c h e m ic a l i n t e r a c t i o n s o f t h e s u r f a c e atom s o f t h e m e ta l w i t h t h e
co m ponents o f t h e s o l u t i o n .
I n d e a e r a t e d a c i d s o l u t i o n s , t h e b a s i c d i s s o l u t i o n p r o c e s s o f a n y
m e ta l M, w h ic h d i s p l a c e s h y d ro g e n fro m a c i d s o l u t i o n , c a n b e r e p r e s e n t e d
b y t h e f o ll o w i n g a n o d ic an d c a t h o d ic r e a c t i o n s :
( i ) A node:
M -*■ M++ + 2 e
A ssum ing f o r m a t io n o f d i v a l e n t m e t a l l i c i o n s .
( i i ) C ath o d e:
( i i a ) The d i s c h a r g e o f h y d ro g e n i o n s , g e n e r a l l y
r e p r e s e n t e d "by
H+ + e = H
( P e rh a p s more a c c u r a t e l y w r i t t e n a s : H 0+ + e = H + H^O)
( i i b ) The f o r m a t io n o f m o le c u la r h y d ro g e n fro m a to m ic
h y d ro g e n , e i t h e r b y
H + H = Hg
o r b y
H + H+ + e = H2
(The l a s t r e a c t i o n i s p e r h a p s more a c c u r a t e l y w r i t t e n a s
H+ + H30+ + e = H2 + H20 )
The ab o v e e q u a t io n s a r e o v e r s i m p l i f i e d n e t r e s u l t s o f t h e
13
d i s s o l u t i o n p r o c e s s o f m e ta ls i n a c i d i c s o l u t i o n s . The d e t a i l s o f t h e
m echanism s in v o lv e d s h o u ld h e c o n s id e r e d i f we i n t e n d t o d e a l w ith
c o r r o s i o n p ro b le m s on a more r a t i o n a l b a s i s .
We c u s to m a r ily t r e a t a m e ta l a s a hom ogeneous co n tin u u m . Now
on a n a to m ic s c a l e a m e ta l i s d e c i d e d l y n o t c o n tin u o u s , an d i t i s on
e x a c t l y t h i s s c a l e t h a t t h e i n d i v i d u a l a c t s o f c o r r o s i o n o c c u r , b e c a u s e
c o r r o s i o n i s e s s e n t i a l l y a p r o c e s s i n w h ich t h e atom s o f a m e ta l a r e
rem oved one a t a tim e t h e r e f o r e i n o r d e r t o r e a c h o u r g o a l o f u n d e r s t a n d
in g an d c o n t r o l l i n g c o r r o s i o n we n e e d g r e a t l y i n c r e a s e d k now ledge o f t h e
d e t a i l e d a to m ic m echanism s in v o lv e d .
F i r s t l y i n c o r r o s i o n s t u d i e s we w i l l b e c o n c e rn e d w ith s u r f a c e s
p o l i s h e d b y d i f f e r e n t p r o c e d u r e s . Such s u r f a c e s w i l l n o t b e u n if o r m b u t
w i.ll c o n t a i n many d i f f e r e n t ty p e s o f s i t e s w ith d i f f e r e n t c h e m ic a l o r
p h y s i c a l p r o p e r t i e s . A tw o - d im e n s io n a l m odel o f su ch s u r f a c e s on a n
a to m ic s c a l e i s shown i n F ig u r e 2 .
- I f “
“ If I f
M J J M M M M M M M -I I I I I I I I I M M M M M M M M M
-/
/
/
/
/
/
/
/
/
FIGURE 2 .
Some s u r f a c e ato m s may b e h e l d b y one b o n d , o t h e r s b y tw o , t h r e e ,
a n d so o n . T h ere may t h u s b e many d i f f e r e n t d e g r e e s o f i n t e r a c t i o n s w ith
a m e ta l s u r f a c e . The m ost a c t i v e p o i n t s w i l l b e o c c u p ie d f i r s t an d w ith
t h e l a r g e s t h e a t o f a d s o r p t i o n . A ls o t h e s u r f a c e e n e rg y o f a m e ta l may
I k
v a r y fro m one l o c a l i t y t o a n o t h e r b e c a u s e o f t h e e x p o s u re o f v a r i o u s
c r y s t a l p l a n e s .
s u r f a c e d e f e c t s w h ich may r e p r e s e n t p l a c e s w here t h e atom s a r e m o st
l o o s e l y c o r r e l a t e d t o t h e m e ta l m a t r i x (s u c h a s ^ i n F ig u r e 2 ) . B eca u se
o f t h e i r u n s a t i s f i e d b i n d i n g e n e r g y c o n d i t i o n s , i t i s e a s i e r f o r them
t o a t t r a c t a n d h o ld com ponents fro m t h e s o l u t i o n . T hese s i t e s a l s o
r e p r e s e n t p l a c e s w here t h e atom s a r e d i s a r r a y e d an d t h e r e f o r e e s c a p e
e a s i l y i n t o t h e s o l u t i o n . F o r e x a m p le , t h e c o r r o s i o n o f a m e ta l c r y s t a l
c o n t a i n i n g d i s l o c a t i o n s o f t e n r e s u l t s i n t h e f o r m a tio n o f e t c h p i t s
w h ere th e d i s l o c a t i o n s m e et t h e s u r f a c e .
f o ll o w i n g e le m e n ta r y s t a g e s :
1 . F i r s t , when a s t r o n g a c i d su c h a s h y d r o c h l o r i c a c i d d i s s o l v e i n w a t e r ,
we h av e
The b u l k s o l u t i o n , t h e r e f o r e , c o n s i s t s o f w a t e r , t h e h y d ro g e n i o n s , and
t h e c h l o r i d e i o n s . I n g e n e r a l , t h e c o n t a c t b e tw e e n tw o n o n - m is c ib le
p h a s e s i s acco m p an ied b y a n i n c r e a s e d c o n c e n t r a t i o n o f t h e f l u i d p h a s e
c l o s e t o t h e i n t e r f a c e . T h is te n d e n c y l e a d s t o t h e f o r m a t io n o f a
" d i f f u s i o n l a y e r " . T h e r e f o r e , when a m e ta l sam ple i s p la c e d i n t h e a c i d i c
s o l u t i o n , t h e h y d ro g e n io n s w i l l d i f f u s e th r o u g h t h i s l a y e r , fro m t h e
b u l k s o l u t i o n t o t h e m e t a l - s o l u t i o n i n t e r f a c e
The p o i n t s w here c o r r o s i o n s t a r t s a r e g e n e r a l l y d e te r m in e d b y
The d i s s o l u t i o n p r o c e s s c a n now b e d e s c r i b e d i n te rm s o f t h e
HC1 + HgO ■> h3o+ + C l
4*
15
2 . The a d s o r p t i o n o f h y d ro g e n io n s on t h e m e ta l s u r f a c e
Hi + ¥ *■ ¥ " Hs
w h ere d e n o te s a h y d ro g e n io n a d s o r b e d on an a c t i v e s i t e on t h e m e ta l
s u r f a c e :
3. D is c h a rg e o f a d s o r b e d h y d ro g e n io n s b y v i r t u e o f e l e c t r o n t r a n s f e r
th r o u g h t h e m e ta l:
(3a ) E l e c t r o n s t r a n s f e r r e d fro m a n e ig h b o u r in g v a c a n t s i t e on
t h e s u r f a c e
H+ H+ H H
• 1 1 ■*"*■ 1
(3b ) E l e c t r o n s t r a n s f e r r e d d i r e c t l y fro m t h e s u r f a c e atom s on
w h ich a d s o r p t i o n o c c u re d
V-E+
-F o rm a tio n o f m o le c u la r h y d ro g e n fro m a to m ic h y d ro g e n , b y one o f t h e
f o ll o w i n g p o s s i b l e m odes;
(lta)
H H BL
1 ++ 1 ++ 1
(ifb) 2( ^+-Hg) + ^-H2
s
(U c) The s o - c a l l e d " e l e c t r o c h e m i c a l " o r io n atom r e a c t i o n
¥ + -hs + <
-s
5. Formation o f metallic hydride through a series of successive stages
o f adsorption and discharge
M-H + H. -*• H -M-H
* s 1 s * i
H+
+ + + + 1 s +
H -M-H + H. -*■ H -M-H
s * s 1 s * s
16
+ g - ¥ -H2 + '
S
H+ ' *
H j + « - k- h3 + v T *
J s
6. A b s o r p tio n o f h y d ro g e n ato m s fro m t h e m e ta l s u r f a c e i n t o t h e l a t t i c e
s t r u c t u r e * F o r some m e t a l s , t h e i n c r e a s e d c o n c e n t r a t i o n o f s u r f a c e
h y d ro g e n f a v o r s e n t r a n c e o f h y d ro g e n atom s i n t o t h e l a t t i c e s t r u c t u r e
o f t h e m e ta l.
P
¥
- M - M - M - M - M M M M
-I I I I -V I I TT I I
- M - M - M - M - M M M M
-7. The re m o v a l o f m o le c u la r h y d ro g e n fro m t h e m e ta l s u r f a c e b y one o r
m ore o f t h e f o ll o w i n g p o s s i b i l i t i e s :
(7a ) D i s s o l u t i o n o f m o le c u la r h y d ro g e n i n t o t h e s o l u t i o n a t
t h e m e t a l - s o l u t i o n i n t e r f a c e
¥ “H2 ¥ + h2
s i , d i s s o l v e d
(7b ) A b s o r p tio n o f m o le c u la r h y d ro g e n fro m t h e m e ta l s u r f a c e
i n t o t h e l a t t i c e s t r u c t u r e o f t h e m e ta l
¥ “H2 S
- M - M - M - M - - M - A Tr M M
-I I I I . * I I h ^ I I
- M - M - M - M - M M T4 M
-( 7 c ) F o rm a tio n o f h y d ro g e n g a s b u b b le s on t h e m e ta l s u r f a c e
an d t h e i r s u b s e q u e n t d e ta c h m e n t fro m t h e m e ta l s u r f a c e
w here M - ( H ) r e p r e s e n t s a h y d ro g e n g a s b u b b le a t t a c h e d on t h e m e ta l
2 n
s u r f a c e :
8. D e s o r p tio n o f m e ta l io n s fro m t h e m e ta l s u r f a c e
M++ t M*+
s i
9 . D i f f u s i o n o f t h e m e ta l io n s fro m t h e m e t a l - s o l u t i o n i n t e r f a c e t o t h e
b u l k s o l u t i o n
Mi + "*■ Mb+
I n a e r a t e d s o l u t i o n s , tw o o t h e r r e a c t i o n s c a n o c c u r t o g e t h e r
w i t h h y d ro g e n e v o l u t i o n , n a m e ly , t h e oxygen d e p o l a r i z a t i o n a n d a u t o -
c a t a l y t i c r e a c t i o n s .
s t a g e s a r e i n good a g re e m e n t w ith e x p e r i m e n t a l d a t a f o r t h e r e a c t i o n o f
o xygen d e p o l a r i z a t i o n :
( i ) F o rm a tio n o f a s e m iv a le n t oxygen io n
I n v e s t i g a t o r s ' ^ h a v e shown t h a t t h e f o l l o w i n g s u c c e s s iv e
°2
+
e
0,2( i i ) F o rm a tio n o f p e r h y d r o x y l r a d i c a l
0 " + H+ - H02
( i i i ) F o rm a tio n o f p e r h y d r o x y l io n
HC>2 + + e e ->
( i v ) F o rm a tio n o f h y d ro g e n p e r o x id e
HO" + H+
( v ) R e d u c tio n o f h y d ro g e n p e r o x id e w ith f o r m a t io n o f h y d r o x y l
io n s H20 2 + 2 e 2 OH
18
Many o f t h e s e e le m e n ta r y s t a g e s a r e , to d a y , no l o n g e r g u e s s e s
h u t a r e e x p e r i m e n t a l l y p ro v e n f a c t s . The d e t a i l e d a to m ic m echanism s o f
t h i s p r o c e s s may h e r e p r e s e n t e d h y t h e f o ll o w i n g e le m e n ta r y s t a g e s :
1 0
.. D i f f u s i o n o f oxygen fro m t h e h u l k s o l u t i o n t o t h e m e t a l - s o l u t i o ni n t e r f a c e
% ”2
-w here 0 d e n o te s a n oxygen m o le c u le i n t h e h u l k s o l u t i o n 0 d e n o te s i
a n oxygen m o le c u le a t t h e m e t a l - s o l u t i o n i n t e r f a c e .
1 1
. A d s o r p tio n o f oxygen on t h e a c t i v e s i t e s on th e m etal, s u r f a c e ; t h e r ea r e tw o t y p e s o f a d s o r p t i o n p o s s i b l e f o r ox y g en :
( l l a ) M o le c u la r a d s o r p t i o n
V + o2 j ^ - o 2
i s
( l l h ) D i s s o c i a t i v e a d s o r p t i o n
2 g + 0 t 2 ( g - 0 8 )
12
. F o rm a tio n o f s e m iv a le n t oxygen(1 2 a )
^ - ° P 2 V <* 2
- m - A - m - m - - m - A - m - m
-( 1 2 b )
$ - 0 l
£-0
I£+ -0~ M+ -0
"- M - M - M - M - M M M M
-1 3
. F o rm a tio n o f p e r h y d r o x y l r a d i c a l o r h y d r o x y l r a d i c a l<13a> l + - ° 2 + » - < * / - H ° 2 + # s
s s
(
13
^ ) M+ -0
_ + M-H+ * M+ -HO + MT* s * s ^ s
i h . F o rm a tio n o f p e r h y d r o x y l io n h y v i r t u e o f e l e c t r o n t r a n s f e r
SJ+ -H0 -
15
. F o rm a tio n o f h y d ro g e n p e r o x id e b y one o f t h e f o ll o w i n g p o s s i b i l i t i e s<15a) U ^H O - + * B-h 2 0 2 + U++
S S
( l 5 b )
2
( / - H O ) - %-E2 °2 + Cs
1 6
. R e d u c tio n o f h y d ro g e n p e r o x id e w ith t h e f o r m a tio n o f h y d r o x y l i o n s :T h is s t e p i s i t s e l f a com plex p r o c e s s a n d can b e f u r t h e r b r o k e n down i n t o
t h e f o ll o w i n g s e q u e n c e :
( l 6 a ) R e d u c tio n o f h y d ro g e n p e r o x id e t o h y d r o x y l io n an d
h y d r o x y l r a d i c a l
»-H202 » ¥±HDS +
OH-S
( l 6 b ) R e d u c tio n o f h y d r o x y l r a d i c a l t o h y d r o x y l io n
/ ho -f / + + o h”
1 7 . D e s o r p tio n o f t h e m e ta l io n s fro m t h e m e ta l s u r f a c e
..++ -*■ ,.++
M <■ M.
* s 1
1 8 . D i f f u s io n o f t h e m e ta l io n s fro m t h e m e t a l - s o l u t i o n i n t e r f a c e t o
t h e b u l k s o l u t i o n
F o r m e ta ls t h a t can e x i s t i n two o r more o x i d a t i o n s t a t e s i n
a c i d i c m e d ia , an a u t o c a t a l y t i c r e a c t i o n s h o u ld b e c o n s id e r e d a s a
p o s s i b i l i t y . The f o ll o w i n g r e a c t i o n scheme i s s u g g e s te d a s a p o s s i b l e
m echanism f o r t h i s p r o c e s s when t h e io n o f h i g h e r o x i d a t i o n s t a t e i s
q u a d r i v a l e n t o f t h e ty p e M :
19
. An o x i d a t i o n - r e d u c t i o n p r o c e s s i n t h e b u l k o f t h e s o l u t i o n , a t t h em e t a l - s o l u t i o n i n t e r f a c e , o r a t t h e m e ta l s u r f a c e
( 1 9 a ) < + +
\
*
2 K
-
+
\
\
„++++
+ H„
0
,2 2
.
1
+ + + +
+ ^-H 20 2 + 2 $
s s
20
. An e q u i l i b r i u m b e tw e e n m e ta l io n s o f h i g h e r a n d lo w e r o x i d a t i o ns t a t e s b y v i r t u e o f e l e c t r o n t r a n s f e r , e i t h e r a t t h e m e ta l s u r f a c e o r
a t t h e m e t a l - s o l u t i o n i n t e r f a c e s
On t h e b a s i s o f t h e above d i s c u s s i o n t h e f o l l o w i n g a p p ro a c h
s h o u ld b e a c c e p t a b l e :
I f i t i s n o t c o m p lic a te d b y o t h e r f a c t o r s , a t y p i c a l c o r r o d in g
s o l u t i o n c o n c e n t r a t i o n - t i m e p l o t f o r t h e d i s s o l u t i o n o f a m e ta l c a p a b le
o f e x i s t i n g i n tw o o r more o x i d a t i o n s t a t e s i n a e r a t e d s o l u t i o n s , w i l l
h a v e t h e sh ap e o f c u rv e C i n F ig u r e 3. I t s h o u ld b e p o s s i b l e t o d i v i d e
t h e o v e r a l l e f f e c t i n t o t h r e e s e p a r a t e s im u lta n e o u s c o r r o s i o n p r o c e s s e s .
The p r o p o s e d m echanism s o f m e ta l d i s s o l u t i o n i n a c i d i c s o l u t i o n s , a s
r e p r e s e n t e d b y t h e e le m e n ta r y s t a g e s 1 t o 1 8 , s u g g e s t t h a t t h e r a t e s o f
h y d ro g e n e v o l u t i o n a n d oxygen d e p o l a r i z a t i o n r e a c t i o n s a r e in d e p e n d e n t
o f t h e m e ta l io n c o n c e n t r a t i o n i n t h e s o l u t i o n ( z e r o o r d e r d e p e n d e n c y ).
T h e r e f o r e , t h e c o r r o d in g s o l u t i o n c o n c e n t r a t i o n - t i m e p l o t s f o r t h e s e
tw o r e a c t i o n s s h o u ld b e s t r a i g h t l i n e s a s shown i n F ig u r e
3
* I n t h i sd ia g ra m , t h e a r e a u n d e r s t r a i g h t l i n e A r e p r e s e n t t h e d i s s o l u t i o n e f f e c t (2 0 a )
- M - M - M - M - M - - M M M M M
-(2 0 b )
M.,++++
2
M.,++21
o f h y d ro g e n e v o l u t i o n . The a r e a b e tw e e n s t r a i g h t l i n e s A an d B i n d i c a t e s
t h e d i s s o l u t i o n e f f e c t o f oxygen d e p o l a r i z a t i o n . I t s h o u ld "be n o t i c e d
t h a t t h e s t r a i g h t l i n e B i s a s y m p to tic t o t h e i n i t i a l s t a g e o f c u rv e C
w h ere t h e a u t o c a t a l y t i c r e a c t i o n i s n e g l i g i b l e . The r e g i o n b e tw e e n c u rv e
C a n d s t r a i g h t l i n e B r e p r e s e n t s t h e e f f e c t o f t h e a u t o c a t a l y t i c r e a c t i o n .
g H
-P
£
a 0 •H
t
1
V
c o o
3
-p
o
a u t o c a t a l y s i s .
oxygen
d e p o l a r i z a t i o n
^ e v o l u t i o n
TIME
FIGURE 3. DISSOLUTION OF METAL IN ACIDIC MEDIA
CHAPTER V
EXPERIMENTAL RESULTS AND DISCUSSION
A. G e n e ra l B e h a v io r o f T in D i s s o l u t i o n i n H y d r o c h lo r ic A c id S o lu ti o n s
. The a n a l y s i s o f t h e e x p e r i m e n t a l d a t a o b ta in e d i n t h i s s tu d y
i s b a s e d on t h e a p p ro a c h w h ich was d is c u s s e d p r e v i o u s l y i n C h a p te r IV .
A t y p i c a l m odel o f t h i s a n a l y s i s i s i l l u s t r a t e d i n F ig u r e H w here c u rv e
A shows t h e r a t e d a t a f o r t h e d i s s o l u t i o n o f t i n i n d e a r e a t e d ( n i t r o g e n
g a s s a t u r a t e d ) h y d r o c h l o r i c a c i d s o l u t i o n , a c o n d i t i o n u n d e r w h ich o n ly
h y d ro g e n e v o l u t i o n can o c c u r . C urve C shows t h e r a t e d a t a o b ta in e d
u n d e r t h e same c o n d i t i o n s a s th o s e o f c u rv e A e x c e p t t h a t t h e a c i d
s o l u t i o n was a i r s a t u r a t e d .
O b v io u s ly , c u rv e A co n fo rm s t o a z e r o - o r d e r p l o t o f m e ta l io n
c o n c e n t r a t i o n v s tim e^ H ere t h e e x p o n e n t i a l c o n c e n t r a t i o n - t i m e p l o t o f
c u rv e C i s a c o n f ir m a ti o n o f a n a u t o c a t a l y t i c p r o c e s s . C urve B i s a
s t r a i g h t l i n e p l o t t e d a s y m p t o t i c a l l y t o c u rv e C a t i t s i n i t i a l s t a g e
w here a u t o c a t a l y s i s i s n e g l i g i b l e . T h is i s e s s e n t i a l l y an e x t e n s i o n o f
t h e i n i t i a l c o r r o s i o n r a t e i n t h e p r e s e n c e o f ox yg en. The t o t a l a r e a
u n d e r c u rv e C i s d iv i d e d b y s t r a i g h t l i n e s A and B i n t o t h r e e r e g i o n s ,
e a c h o f w h ich c o r r e s p o n d s t o a s p e c i f i c e f f e c t on t h e o v e r a l l d i s s o l u t i o n
r e a c t i o n a s shown i n F ig u r e
B. R a te D ependence on T in Io n C o n c e n tr a tio n
As shown i n F ig u r e 5 , t h e c u rv e d d e v i a t i o n fro m t h e l i n e a r
z e r o - o r d e r i n i t i a l r a t e p l o t , w h ich i s r e p r e s e n t e d b y t h e d i f f e r e n c e i n
m e ta l io n c o n c e n t r a t i o n b e tw e e n c u r v e s B an d C a s shown i n F ig u r e U, can
23
o u S H ra o H
§
o H X
£
18
16
34,100
c m ./m in .500
m l.1
M HC1
3 .6 6 s q . cm.
d = 0 .9 1 cm.
14
A [S n ] = H e v o l u t i o n e f f e c t 12
A [S n ] = oxygen d e p o l a r i z a t i o n / ^ [ S n ]
e f f e c t /
A [S n ] = a u t o c a t a l y t i c / " /
^ e f f e c t / /
10
8
6
2
A [S n ]
0
0 20
40
60 80 100TIME - m in u te s
FIGUKE
4
. DISSOLUTION OF TIN IN HC1
120
140
(A
[S
n
])
a
u
to
c
a
ta
ly
s
is
X
10
'
(
*
°
W
i
i
t
r
e
)
2k
k
3 ^ ,1 0 0 c m ./m in .
500
m l.'1
M HC1
3 .6 6 s q . cm.
AIR
3
d = 1 ,2 2 cm
d = 1 .0 7 cm
2
1
0
ko 60 80
0 20 100 120
TIME - m in u te s
25
l / 2
b e r e a d i l y c o r r e l a t e d b y a h a l f - o r d e r p l o t . A l l (A[ S n ] ) ' v s tim e p l o t s
g iv e s t r a i g h t l i n e s . T h is i s an i n d i c a t i o n t h a t t h e a u t o c a t a l y t i c p r o c e s s
shows h a l f - o r d e r d ep en d e n ce on m e ta l c o n c e n t r a t i o n . From F ig u r e i t
c a n b e o b s e rv e d t h a t t h e a u t o c a t a l y t i c r e a c t i o n becom es im p o r ta n t o n ly
a f t e r a n e l a p s e d tim e o f 30 m in u te s .
C„ R a te D ependence on P e r i p h e r a l V e l o c i t y
The e f f e c t o f p e r i p h e r a l v e l o c i t y on t h e d i s s o l u t i o n o f t i n
was s t u d i e d i n b o th a i r - an d n i t r o g e n - s a t u r a t e d
1
M HC1
s o l u t i o n s f o rr o t a t i o n a l s p e e d s r a n g in g fro m 1 ,0 0 0 t o 1 2 ,0 0 0 rpm (
2,840
-34
,100
c m ./m in .),I t was fo u n d t h a t t h e r e i s a c o n s i d e r a b l e e f f e c t o f p e r i p h e r a l v e l o c i t y
on t i n d i s s o l u t i o n .
F ig u r e s 6A an d 6B i l l u s t r a t e t h a t k ^ , t h e r a t e c o n s t a n t f o r
t h e h y d ro g e n e v o l u t i o n r e a c t i o n , a n d k ^ , t h e oxygen d e p o l a r i z a t i o n
r e a c t i o n r a t e c o n s t a n t a r e b o t h p r o p o r t i o n a l t o p e r i p h e r a l v e l o c i t y
r a i s e d t o t h e O .98 p o w er. The e f f e c t o f p e r i p h e r a l v e l o c i t y on k ^ , t h e
r a t e c o n s t a n t f o r t h e a u t o c a t a l y t i c p r o c e s s , a s shown i n F ig u r e
7
> c a n "ber e p r e s e n t e d b y a n e q u a t i o n o f t h e fo rm :
k3
”k3
W fw h e re : n - 0 .3 0 f o r
2,840
< v < 2 1 ,2 3 0 c m ./m in .n = 0 .9 0 f o r 2 1 ,2 3 0 < v <
34,100
c m ./m in .I t i s n o t a p p a r e n t why t h e v e l o c i t y e f f e c t shows tw o d i s t i n c t
24
r e g io n s f o r t h e r a t e c o n s t a n t o f t h e a u t o c a t a l y t i c p r o c e s s . L u ir e p o r t e d a somewhat s i m i l a r b e h a v i o r w h ich h e r e l a t e d t o h y d ro g e n b u b b le
f o r m a t io n a t v a r i e s s p e e d s :
" T in e v o lv e s - h y d ro g e n g a s fro m h y d r o c h l o r i c a c i d , b u t i t was
im p o s s ib le t o d e t e c t a n y g a s b u b b le th e m e ta l s u r f a c e i n
1
M HC1
. ByI
7 7 7
t O
UNIVERSITY OF WINDSOR LIBRARY
X
1
0
?
,
(
m
o
le
s
/l
it
r
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)
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in
.
26
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500 m l.
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3 .6 6 s q . cm.
AIR
d = 0 .9 1 cm.
10
5
2
1
6
10 20 3050
v X 10~3 _ c m ./m in .
27
20
500
m l.1
M HC1
3 .6 6 sq.. cm.
AIR
cL = 0 .9 1 cm.
10
•H
*\
OJ
6
10 30v X
10_3
- c m ./m in .FIGURE 6B. DISSOLUTION AS FUNCTION OF PERIPHERAL VELOCITY
28
K'NX
OvO *
ro
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O
lf\
OJ HA
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T. , “p e( M m /
8 3P ) ' o x x cx
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7.
D
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29
c o r r o d in g sp e c im e n s i n
6
M HC1
i t was e a s y t o o b s e rv e g a s b u b b le f o r m a t io n .R o ta tio n o f t h e sam ple a t s p e e d s b e lo w
3
>00
° rpm d id n o t rem ove a l l o ft h e l a r g e b u b b l e s . O ver t h e ra n g e
3>000
t o11,000
rpm , t h e l a r g e v i s i b l eb u b b le s w ere rem oved fro m t h e s u r f a c e q u i t e r e a d i l y b u t v e r y s m a ll o n e s
w ere s t i l l a t t a c h e d t o t h e m e ta l s u r f a c e . Above
11,000
rpm no b u b b le sw ere o b s e r v e d on t h e m e ta l s u r f a c e .
A lth o u g h su c h b e h a v i o r c a n n o t b e o b s e r v e d i n
1
M HC1
s o l u t i o n s ,i t i s n o t u n r e a s o n a b le t o e x p e c t some co m p a ra b le e l i m i n a t i o n o f h y d ro g e n
b u b b le s fro m c o r r o d in g s u r f a c e s . A t low v e l o c i t y , t h e c o r r o d in g s u r f a c e
2k i s p r o t e c t e d b y t h e h y d ro g e n g a s b u b b l e s . "
I n t h e p r e s e n t w ork no s t u d i e s w ere done on h y d ro g e n b u b b le
f o r m a t io n .
D. R a te D ependence on T e m p e ra tu re
The r a t e c o n s t a n t d e p e n d e n c e on te m p e r a tu r e w as s t u d i e d o v e r
t h e ra n g e
25
t o k^°C u n d e r n i t r o g e n , a i r , a n d oxygen s a t u r a t i o n . Ther e s u l t s a r e i l l u s t r a t e d b y t h e A r r h e n i u s ' a c t i v a t i o n e n e r g y p l o t s shown
i n F ig u r e s
8
t o10
.The te m p e r a t u r e v a r i a t i o n s c o r r e s p o n d t o a p p a r e n t a c t i v a t i o n
e n e r g i e s o f 3 .0 6 k c a l p e r m ole f o r t h e h y d ro g e n e v o l u t i o n r e a c t i o n end
3 .6 6
k c a l p e r m ole f o r t h e oxygen d e p o l a r i z a t i o n . T hese low v a l u e s1
13
s u g g e s t t h a t b o t h r e a c t i o n s a r e u n d e r c o m p le te d i f f u s i o n a l c o n t r o l 3 .The a c t i v a t i o n e n e r g y f o r t h e a u t o c a t a l y t i c p r o c e s s i s o f t h e o r d e r o f
5
.5
^ k c a l p e r m o le, a v a l u e somewhat l a r g e r th a n th e 3 -^ k c a l p e r m olet o b e e x p e c te d f o r a s im p le d i f f u s i o n p r o c e s s .
The a c t i v a t i o n e n e r g i e s f o r t h e a u t o c a t a l y t i c p r o c e s s , a s
i l l u s t r a t e d i n F ig u r e
11
, w ere e s s e n t i a l l y c o n s t a n t f o r v a r y i n g sam uleLO
G
k
,
•)
30
500 ml.
1 M HC1
3 . 6 6 sq. cm.
NITROGEN
-
5^9
d = 0 .9 1 cm.
-
6.1
- 6 .3
- 6 . 7
3 . 0
3. 1
3 .2
3-3
3-5
3. 7
( l / T ) X
10
3 - l / K °LO
G
k
31
500 m l.
1
M HC1
3 .6 6 s q . cm.
AIR
d = 0 .9 1 cm. -
5 .0
OJ
lit-, 200 cm ./m i
-
5.6
3-2
3-3
3-7
( l / T ) X
10
3 - l / K °FIGURE 9 . DISSOLUTION AS FUNCTION OF TEMPERATURE
LO
G
k
.
32
-2.7
500 m l.
1 M HC1
3 .6 6 s q . cm.
AIR
3 ^ ,1 0 0 cm ./m i -
2.9
2. h i cm,
I . 8 3 cm,
1 .3 7
cm-
3
.3
'1 .0 7 cm
-
3.5
-
3.7
3 .0
3.1
3*2
3.3
3 .5
3.7
( l / T ) X 1 0 3 - i /k°
A
C
T
IV
A
T
IO
N
E
N
E
R
G
Y
,
K
C
A
L
/G
M
.
M
O
L
.
33
10.0
3 ^ ,1 0 0 c m ./m in .
500
m l.1
M HC1
3 .6 6 s q . cm.
AIR
9 .0
8.0
0 -0
5 .0
3 .0
0 .5
1 . 01 .5
2.02 .5
DIAMETER - cm.
FIGURE
1 1
. ACTIVATION ENERGY AS FUNCTION OF SAMPLE DIAMETER( a u t o c a t a l y t i c r e a c t i o n )
3k
d ia m e te r s a n d r o t a t i o n a l s p e e d s . c o r r e s p o n d in g t o c o n s t a n t p e r i p h e r a l
v e l o c i t y .
E . R ate D ependence on Sample S u r f a c e A re a and C o rro d in g S o l u t i o n Volume
The e f f e c t o f v a r y in g t h e s o l u t i o n volum e an d sam ple s u r f a c e
2k o
a r e a was i n v e s t i g a t e d h y L u i a t 30 C i n a i r s a t u r a t e d
1
M HC1
s o l u t i o n s .A r e - e x a m in a t io n o f h i s d a t a shows t h a t f o r c h a n g e s i n t h e s u r f a c e a r e a
fro m
0.91
t o7-32
c m ., an d i n s o l u t i o n volum e fro m kOO t o600
m l t h er a t e c o n s t a n t s k ^ an d k g a r e d i r e c t l y p r o p o r t i o n a l t o A/V, a s shown i n
F ig u r e s
12
a n d 1 3 .F ig u r e s I k and 15 i l l u s t r a t e t h a t t h e r a t e c o n s t a n t f o r t h e
l / 2
a u t o c a t a l y t i c r e a c t i o n , k ^ , i s p r o p o r t i o n a l t o [A] ' /V r a t i o .
F , E f f e c t o f H y d r o c h lo r ic A c id C o n c e n t r a ti o n
The e f f e c t o f h y d r o c h l o r i c a c i d c o n c e n t r a t i o n was s t u d i e d h y
2k
L u i who r o t a t e d t h e sa m p le s i n a e r a t e d a n d d e a e r a t e d s o l u t i o n s o v e r
t h e c o n c e n t r a t i o n ra n g e
0 .1 0
t o4 .1 0
M HC1
. A n a ly s is o f h i s d a t aa c c o r d in g t o t h e scheme s u g g e s te d i n t h i s p r o j e c t a l s o shows t h a t t h e
d i s s o l u t i o n r a t e o f t i n i s e s s e n t i a l l y in d e p e n d e n t o f a c i d c o n c e n t r a t i o n .
T a b le
1
i n d i c a t e s t h a t t h e r e i s no a p p a r e n t t r e n d f o r a n d k ^ o v e rt h e ra n g e o f c o n c e n t r a t i o n s t u d i e d . S in c e t h e q u a n t i t y (k l + k g ) a p p e a r s
t o h e a l s o in d e p e n d e n t o f a c i d c o n c e n t r a t i o n , t h i s m eans t h a t k ^ a lo n e i s
in d e p e n d e n t o f a c i d c o n c e n t r a t i o n .
D i s s o l u t i o n r a t e s in d e p e n d e n t o f a c i d c o n c e n t r a t i o n w ere
6
23
r e p o r t e d h y o t h e r i n v e s t i g a t o r s ’ . A t p r e s e n t t h e r e a r e no v a l i d
