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9 Corrosion Inhibition and Inhibitors
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RUDOLF H. HAUSLER Gordon Lab, Inc., 925 Patton Rd., P.O. Box 605, Great Bend, KS 67580
An Educational Lecture or Paper on Corrosion I n h i b i t i o n could e a s i l y develop into an ambitious undertaking if it were intended to review the vast literature concerned with the chemistry of Corrosion Inhibi t o r s and the multitude of mechanisms proposed as explanations of t h e i r a c t i o n . The past approaches aimed at understanding Corrosion I n h i b i t i o n have been many, ranging from phenomenological screening of chemical compounds in a given environment to detailed e l e c t r o chemical adsorption studies. Let me state that the ultimate purpose of i n h i b i t o r studies ought to be the development of p r e d i c t i v e criteria for i n h i b i t o r e f f e c tiveness rather than the mere explanation of e x p e r i mental Results or speculation about possible mechanisms. While such p r e d i c t i v e criteria have been developed in a few cases it appears that most mechanistic studies have merely contributed to speculative guide l i n e s h e l p f u l in the unraveling process of i n h i b i t o r a c t i o n , but have been far from successful in providing a complete understanding of p r a c t i c a l i n h i b i t i o n phenomena l e t alone p r e d i c t i v e criteria for more e f f e c t i v e molecules. The reason for t h i s state of a f f a i r s may be seen in past emphasis on surface phenomenological studies which attempted to model the metal surface as an array of surface atoms with some valences saturated by subsurface metal atoms and other valences saturated by ions or molecules making up the environment. This model led to the d e s c r i p t i o n of the interface in terms of the Helmholz and Guy-Chapman double layer t h e o r i e s , and i n h i b i t o r s were v i s u a l i z e d as i n t e r f e r i n g with the double layer structure through adsorption on the surface atoms of the metal, thereby a l t e r i n g the e l e c t r o chemical reaction rates which are governed by the energetics of the double l a y e r . While t h i s model has been
0-8412-0471-3/79/47-089-262$13.85/0 © 1979 American Chemical Society
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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9.
HAUSLER
Corrosion
Inhibition
263
q u i t e s u c c e s s f u l in e x p l a i n i n g changes o f e l e c t r o c h e m i c a l r e a c t i o n r a t e s as a consequence o f changes o c c u r r i n g in the c o m p o s i t i o n o f the e l e c t r o l y t e i t has not been s u c c e s s f u l in d e s c r i b i n g t y p i c a l p r a c t i c a l i n h i b i t i o n phenomena. A r a t h e r l a r g e body o f e v i d e n c e has been accumul a t e d o v e r the p a s t 10 t o 15 y e a r s i n d i c a t i n g t h a t the c h e m i s t r y o f the i n t e r p h a s e between the m e t a l per se and the homogeneous b u l k o f the e l e c t r o l y t e may determine the k i n e t i c s o f t h e c o r r o s i o n p r o c e s s . This i n t e r phase must be v i s u a l i z e d as b e i n g d i s t i n c t l y d i f f e r e n t in c o m p o s i t i o n from e i t h e r the m e t a l or the e l e c t r o l y t e and e x t e n d i n g from m i c r o s c o p i c t o perhaps even macroscopic dimensions. As a consequence the o v e r a l l k i n e t i c s o f the c o r r o s i o n p r o c e s s are d e t e r m i n e d by a comp l e x i n t e r p l a y between the r e a c t i o n r a t e s at the two i n t e r f a c e s made up by the b o u n d a r i e s o f the i n t e r phase and t r a n s p o r t phenomena in the i n t e r p h a s e i t s e l f . I f the c o r r o s i o n p r o c e s s is modeled in t h i s manner, then the c o r r o s i o n i n h i b i t i o n phenomena must be seen as : a) the i n t e r a c t i o n o f a c h e m i c a l substance w i t h the o u t e r s u r f a c e o f the i n t e r p h a s e ; b) an i n t e r a c t i o n w i t h the i n t e r p h a s e i t s e l f t h e r e b y changing i t s c h e m i c a l n a t u r e ; c) or the f o r m a t i o n o f a new interphase. The f o l l o w i n g d i s c u s s i o n w i l l t h e r e f o r e make but a weak attempt t o r e v i e w p a s t approaches t o i n h i b i t i o n theory. R a t h e r , emphasis w i l l be p l a c e d on the i n t e r phase c o n c e p t . I t is hoped t h a t t h e r e a d e r w i l l t h u s be o f f e r e d a s h o r t i n t r o d u c t i o n t o c o r r o s i o n i n h i b i t i o n as w e l l as b e i n g s t i m u l a t e d t o f u r t h e r i n v e s t i g a t i v e e f f o r t s in t h e f i e l d o f c o r r o s i o n i n h i b i t i o n . The
Corrosion
Mechanism
C o r r o s i o n i n h i b i t i o n is g e n e r a l l y d e s c r i b e d as the i n t e r f e r e n c e o f a substance f o r e i g n t o the c o r r o s i v e medium w i t h the c o r r o s i o n r e a c t i o n o r r e a c t i o n s , and i t is v i s u a l i z e d t h a t such i n t e r f e r e n c e t a k e s p l a c e t h r o u g h the a d s o r p t i o n o f the i n h i b i t o r on the m e t a l surface. While t h i s concept has been s u c c e s s f u l in many i d e a l i z e d s i t u a t i o n s , such as the c o r r o s i o n o f i r o n in h y d r o c h l o r i c a c i d CD i t s a p p l i c a t i o n has been f a r t o o g e n e r a l and too p r o l i f i c in o r d e r t o advance the u n d e r s t a n d i n g o f c o r r o s i o n i n h i b i t i o n in any s i g n ificant way. I t may t h e r e f o r e be h e l p f u l t o d i s c u s s b r i e f l y a k i n e t i c model o f the c o r r o s i o n p r o c e s s i t s e l f and t h e n d e r i v e from i t t h e v a r i o u s ways in which i n h i b i t i o n can
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
CORROSION C H E M I S T R Y
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264
take place. T h i s appears a l l the more n e c e s s a r y as c o r r o s i o n a f t e r more t h a n 5 0 y e a r s o f i n t e n s i v e r e s e a r c h is s t i l l c o n s i d e r e d in many t e x t b o o k s t o be a p u r e l y chemical process, while only s p e c i a l l i m i t i n g cases such as g a l v a n i c c o r r o s i o n and perhaps p i t t i n g are r e c o g n i z e d as e l e c t r o c h e m i c a l in n a t u r e . Let us emphasize from the b e g i n n i n g t h a t a l l c o r r o s i o n (the o x i d a t i v e c o n v e r s i o n o f a m e t a l t o i t s meta l i o n s ) is e l e c t r o c h e m i c a l in n a t u r e . This implies the e x i s t e n c e o f s i m u l t a n e o u s a n o d i c and c a t h o d i c c u r r e n t s o f e q u a l magnitude a c r o s s the i n t e r f a c e o f the metal. I t is by no means n e c e s s a r y ( a l t h o u g h somet i m e s u s e f u l ) , t o p o s t u l a t e permanent l o c a l i z e d anodes and cathodes as a m i c r o s c o p i c concept w i t h f i x e d space c o o r d i n a t e s in o r d e r t o d e v e l o p a k i n e t i c model a p p l i c a b l e t o the r a t e s o f the c o r r o s i o n p r o c e s s e s . The c o r r o s i o n r e a c t i o n s , t h a t is,the a n o d i c o x i d a t i o n o f the m e t a l and t h e c a t h o d i c d e p o l a r i z a t i o n by an o x i d a n t , may indeed t a k e p l a c e w i t h s t a t i s t i c a l d i s t r i b u t i o n in time and space on the s u r f a c e o f the m e t a l . The p r o o f o f t h i s t h e s i s was g i v e n by Wagner and Traud (JL) in 1938 by means o f t h e d i s c u s s i o n o f the l i m i t i n g case o f z i n c amalgam d i s s o l u t i o n . L i q u i d z i n c amalgam must be c o n s i d e r e d a c o m p l e t e l y homogeneous phase where i t is i m p o s s i b l e t o d e f i n e , in a r i g o r o u s thermodynamic sense, l o c a l g a l v a n i c elements on the s u r f a c e . The d i s s o l u t i o n o f z i n c amalgam in d i l u t e h y d r o c h l o r i c a c i d c o u l d t h e r e f o r e proceed by a c h e m i c a l mechanism as shown in e q u a t i o n s 1 and 2: Zn + 2H+ {Zn H21
2
**{zn H }
2 +
(1)
2
+
— -
Zn
+ 2
+ H
2
(2)
T h i s model v i s u a l i z e s t h a t z i n c atoms on the s u r f a c e o f the amalgam form a t r a n s i t i o n complex w i t h two p r o t o n s which s u b s e q u e n t l y d i s s o c i a t e s i n t o z i n c i o n s and hydrogen. I f t h i s mechanism were t o p r e v a i l , t h e r a t e o f hydrogen e v o l u t i o n ( o r o x i d a t i o n o f z i n c ) would have t o depend on the c o n c e n t r a t i o n o f t h e t r a n s i t i o n complex i f t h e second r e a c t i o n were r a t e d e t e r m i n i n g , or on the p r o d u c t o f the c o n c e n t r a t i o n s o f z i n c and the p r o t o n s i f the f i r s t r e a c t i o n were r a t e d e t e r m i n i n g . Wagner and Traud, however, have shown t h a t the r a t e o f the hydrogen e v o l u t i o n on a mercury s u r f a c e is i n d e pendent o f s m a l l c o n c e n t r a t i o n s o f z i n c and can t h e r e f o r e be determined e l e c t r o c h e m i c a l l y on a pure mercury electrode. Conversely, the o x i d a t i o n o f z i n c from z i n c amalgam is e s s e n t i a l l y independent o f the pH and can be measured at h i g h pH w i t h o u t i n t e r f e r i n g hydrogen e v o lution. One can t h e r e f o r e determine t h e s e two r a t e s
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
9.
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Corrosion
Inhibition
265
over a wide p o t e n t i a l range and in the case o f the hydrogen e v o l u t i o n f o r d i f f e r e n t p H s . S i n c e in the " c o r r o s i o n " o f z i n c amalgam the o x i d a t i o n o f the z i n c and the e v o l u t i o n o f hydrogen have t o p r o c e e d at the same r a t e in o r d e r t o p r e s e r v e the laws o f e l e c t r o n e u t r a l i t y in the system, one can now use the a b o v e . d e t e r mined r a t e c h a r a c t e r i s t i c s f o r the two r e a c t i o n s in o r der t o p r e d i c t the r e s t p o t e n t i a l o f c o r r o d i n g z i n c amalgam, and t o determine i t s c o r r o s i o n r a t e . The r e s u l t can t h e n be v e r i f i e d i n d e p e n d e n t l y by d e t e r m i n i n g the r a t e o f hydrogen e v o l u t i o n from z i n c amalgam v o l u m e t r i c a l l y and by a l s o a n a l y z i n g f o r z i n c in the r e s u l t i n g solution. These experiments have i n d e e d been s u c c e s s f u l and are c o n s i d e r e d s u f f i c i e n t p r o o f f o r the p o s t u l a t e o f the e l e c t r o c h e m i c a l mechanism f o r the d i s s o l u t i o n o f z i n c amalgam in d i l u t e h y d r o c h l o r i c a c i d , as shown by e q u a t i o n s 3 and 4, where the a n o d i c and c a t h o d i c r e a c t i o n s p r o c e e d e s s e n t i a l l y independent from each o t h e r . For a more d e t a i l e d d i s c u s s i o n o f the d i s s o l u t i o n o f z i n c amalgam the r e a d e r is r e f e r r e d t o the o r i g i n a l l i t e r a t u r e o r a d i s c u s s i o n by H. Kaesche ( 3 ) .
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f
Zn
* Zn +
2H —*H
2
+ 2
+ 2e
- 2e
(3) (4)
f h i s v e r i f i c a t i o n o f the e l e c t r o c h e m i c a l n a t u r e o f the c o r r o s i o n p r o c e s s a l s o l e a d s t o the r e a l i z a t i o n o f f o u r d i s t i n c t and s e p a r a t e b a s i c p r o c e s s e s i n v o l v e d in c o r r o s i o n , which are l i n k e d t o g e t h e r m e r e l y by the n e c e s s i t y o f p r e s e r v i n g e l e c t r o n e u t r a l i t y in the o v e r a l l reaction. These are a) the a n o d i c or o x i d a t i v e p r o c e s s ; b) the c a t h o d i c o r r e d u c t i v e p r o c e s s ; c) the e l e c t r o n i c charge t r a n s f e r p r o c e s s in b o t h d i r e c t i o n s a c r o s s the i n t e r f a c e ; d) the i o n i c charge t r a n s f e r p r o c e s s which is r e q u i r e d t o m a i n t a i n e l e c t r o n e u t r a l i t y on the e l e c t r o l y t e s i d e due t o the d i s a p p e a r a n c e o f i o n i c charges in the c a t h o d i c p r o c e s s and the f o r m a t i o n o f i o n i c charges in the a n o d i c p r o cess . One o f t h e s e f o u r r e a c t i o n s is u s u a l l y the s l o w e s t , and r a t e d e t e r m i n i n g f o r the o v e r a l l p r o c e s s . Corrosion i n h i b i t i o n t a k e s advantage o f t h i s c o m p l e x i t y o f r e a c t i o n s by a t t e m p t i n g t o i n t e r f e r e w i t h any o f them i n d i v i d u a l l y or j o i n t l y . Thus a c o r r o s i o n i n h i b i t o r may f u r t h e r slow the r a t e o f the s l o w e s t r e a c t i o n or may b r i n g about a r a t e l i m i t a t i o n o f one or the o t h e r o f the r e m a i n i n g t h r e e p r o c e s s e s . While i t is g e n e r a l l y known t h a t c o r r o s i o n i n h i b i t o r s may a f f e c t the an-
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
CORROSION C H E M I S T R Y
266
o d i c o r the c a t h o d i c r e a c t i o n , i t is l e s s w e l l known t h a t i n h i b i t i v e means may a f f e c t e i t h e r the e l e c t r o l y t i c c o n d u c t i o n or even the e l e c t r o n i c c o n d u c t i o n p r o cess .
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Systematic C l a s s i f i c a t i o n
Of C o r r o s i o n
Inhibitors
The s e v e r a l thousand s u b s t a n c e s which have in the p a s t been o b s e r v e d t o have c o r r o s i o n i n h i b i t i v e p r o p e r t i e s have o f t e n been c l a s s i f i e d in v a r i o u s ways. The s t a r t i n g p o i n t f o r such c l a s s i f i c a t i o n is the p o i n t o f i n t e r f e r e n c e w i t h the above s k e t c h e d c o r r o s i o n mechanism e i t h e r in a p h e n o m e n o l o g i c a l or in a m e c h a n i s t i c way. A simple system f o r c l a s s i f i c a t i o n , which w i l l be d i s c u s s e d in more d e t a i l l a t e r , is based on whether the i n h i b i t o r i n t e r f e r e s w i t h the a n o d i c o r c a t h o d i c reaction. Thus i n h i b i t o r s are c l a s s i f i e d as a n o d i c or cathodic i n h i b i t o r s . However, t h i s d i s t i n c t i o n was shown t o be too s i m p l i s t i c and a more complex c l a s s i f i c a t i o n was worked out by H. F i s c h e r (JjJ on the b a s i s o f where, i n s t e a d o f how, in the complex i n t e r p h a s e o f a m e t a l - e l e c t r o l y t e system the i n h i b i t o r i n t e r f e r e s w i t h the c o r r o s i o n r e a c t i o n s . The m e t a l - e l e c t r o l y t e i n t e r phase can be v i s u a l i z e d as c o n s i s t i n g o f (a) the i n t e r f a c e per se, and (b) an e l e c t r o l y t e l a y e r i n t e r p o s e d between the i n t e r f a c e and the b u l k o f the e l e c t r o l y t e . On t h i s b a s i s F i s h e r d i s t i n g u i s h e d as shown in T a b l e 1, between " I n t e r f a c e I n h i b i t i o n " and " E l e c t r o l y t e L a y e r Inhibition." I n t e r f a c e I n h i b i t i o n comes about by s u b s t a n c e s which p o s i t i o n themselves immediately at the s u r f a c e o f the m e t a l and d e c r e a s e the r a t e s o f p h y s i c a l , chemi c a l o r e l e c t r o c h e m i c a l p r o c e s s e s in the c o r r o s i o n mechanism. Such p r o c e s s e s may be: a) the charge t r a n s f e r per se; b) the i n t e r r u p t i o n o f the c r y s t a l l a t t i c e ; c) the p a r t i a l s t e p s i n v o l v e d in the a n o d i c o r c a t h o d i c r e a c t i o n s , t h a t is c h e m i c a l r e a c t i o n s e i t h e r p r e c e d i n g or f o l l o w i n g the charge t r a n s fer reactions. D i s c u s s i n g i n t e r f a c e i n h i b i t i o n one f i n d s t h a t the pure s q u e e z i n g out e f f e c t ( s a l t i n g out e f f e c t ) , which may c o n c e n t r a t e i n h i b i t i n g n e u t r a l m o l e c u l e s at the m e t a l e l e c t r o l y t e i n t e r f a c e , w i l l be r a t h e r r a r e . However, i t is p o s s i b l e t h a t the a c t i v i t y o f i o n s or m o l e c u l e s t a k i n g p l a c e in the c o r r o s i o n r e a c t i o n is dec r e a s e d simply by the a c c u m u l a t i o n o f n e u t r a l m o l e c u l e s in the v i c i n i t y o f the m e t a l s u r f a c e . Such s u b s t a n c e s c o u l d be a l c o h o l , water s o l u b l e i n e r t s o l i d s in gene r a l , or i n e r t i o n s .
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
9.
HAUSLER
Corrosion
267
Inhibition
Table I
(A)
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Causes o f I n t e r f a c e - I n h i b i t i o n and I n h i b i t ion
Electrolyte-Layer-
1.
Interface-Inhibition. 1.1 caused by the s q u e e z i n g out effect. 1.2 caused by a d s o r p t i o n . 1.3 caused by e l e c t r o s o r p t i o n . 1.4 caused by coverage w i t h a p o l y m o l e c u l a r o r polymerous l a y e r .
2.
Electrolyte-Layer-Inhibition. 2.1 m e c h a n i c a l E. caused by 2.11 c o l l o i d s o r s u s p e n s i o n s . 2.12 v i s c o u s s o l u t i o n s . 2.13 pores in p o l y m o l e c u l a r or polymerous l a y ers . 2.2 c h e m i c a l E. caused by s u b s t a n c e s r e a c t i n g w i t h components o f homogeneous p a r t i a l r e a c t i o n s o f the e l e c t r o d e r e a c t i o n . 2.3 e l e c t r o c h e m i c a l E. caused by changes o f t h e p o t e n t i a l in the d i f f u s e double l a y e r dependent on a coverage o f the i n t e r f a c e w i t h i o n s
M o s t l y however, t h e s e and o t h e r m o l e c u l e s as w e l l as i o n s or i o n p a i r s coming from the e l e c t r o l y t e may be adsorbed on the m e t a l l i c ( o r s e m i c o n d u c t i v e ) p a r t o f the i n t e r f a c e . D i s r e g a r d i n g the s p e c i a l case o f the p o t e n t i a l o f z e r o charge at the m e t a l l i c s u r f a c e , the conductor w i l l u s u a l l y c a r r y a p o s i t i v e or negative charge which may f a v o r i t s coverage w i t h charged o r p o l a r s u b s t a n c e s o c c u r i n g near t h e i n t e r f a c e . I t is v e r y d i f f i c u l t , however, t o d i s t i n g u i s h between pure a d s o r p t i o n o f n e u t r a l m o l e c u l e s on the s u r f a c e o f t h e c o r r o d i n g m e t a l by means o f Van d e r Waals or a chemi c a l f o r c e s , and e l e c t r o s o r p t i o n which t a k e s p l a c e by p o t e n t i a l dependent e l e c t r o s t a t i c f o r c e s . O f t e n the two mechanims o p e r a t e in c o n j u n c t i o n . I t has been shown f o r i n s t a n c e , t h a t o r g a n i c amines become much more e f f e c t i v e c o r r o s i o n i n h i b i t o r s in a c i d medium i f a h a l i d e is p r e s e n t . H a l i d e i o n s , p a r t i c u l a r l y i o d i d e , a r e s t r o n g l y adsorbed on m e t a l s u r f a c e s thus f o r m i n g a n e g a t i v e l y charged l a y e r on the m e t a l s u r f a c e onto which p r o t o n a t e d o r g a n i c amines adsorb s u b s e q u e n t l y , with a r e s u l t i n g i n h i b i t i o n of corrosion reactions. F i n a l l y , one can t a l k about an i n t e r f a c e e f f e c t which is caused by coverage o f the m e t a l s u r f a c e w i t h a polymerous l a y e r . Some a u t h o r s t h i n k t h a t c e r t a i n
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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i n h i b i t o r s such as the a c e t y l e n i c ones form a polymer l a y e r on the s u r f a c e o f the m e t a l . In o t h e r words, the monomeric i n h i b i t o r m o l e c u l e s undergo e i t h e r c h e m i c a l or e l e c t r o c h e m i c a l r e a c t i o n on the m e t a l s u r f a c e such t h a t a p o l y m e r i c two d i m e n s i o n a l system is formed which t i g h t l y adheres t o the m e t a l s u r f a c e and i n t e r f e r e s w i t h the r a t e s o f the c o r r o s i o n r e a c t i o n s . A l l o f the above i n h i b i t i o n phenomena are v i s u a l i z e d as t a k i n g p l a c e immediately on the m e t a l s u r f a c e o f the c o r r o d i n g specimen. A p p a r e n t l y , t h e r e are ways t h a t one can a f f e c t c o r r o s i o n r e a c t i o n s by i n t e r f e r i n g w i t h p r o c e s s e s which are not on t h e s u r f a c e , but t h o s e in the v i c i n i t y of the s u r f a c e , namely in the e l e c t r o l y t e l a y e r c l o s e s t t o the s u r f a c e . Electrolyte layer i n h i b i t i o n may h i n d e r the f o l l o w i n g p a r t i a l s t e p s o f electrode reactions: 1) T r a n s p o r t of components o f the e l e c t r o d e r e a c t i o n s t o o r from the i n t e r f a c e ; 2 ) P a r t i a l steps o f the homogeneous c h e m i c a l r e a c t i o n s w i t h i n the e l e c t r o l y t e l a y e r . This can be a c c o m p l i s h e d by: a) p u r e l y m e c h a n i c a l o b s t a c l e s assembled in the e l e c t r o l y t e l a y e r (mechanical e l e c t r o l y t e layer i n h i b i t i o n ) ; b) by c h e m i c a l r e a c t i o n s o f components o f the e l e c t r o d e r e a c t i o n s w i t h s u b s t a n c e s assemb l e d in the e l e c t r o l y t e l a y e r ( c h e m i c a l electrolyte layer inhibition); c) by e l e c t r o c h e m i c a l e f f e c t s such as a change o f the z e t a p c t e n t i a l in the d i f f u s e p a r t o f the double l a y e r which c o n t r o l s the m i g r a t i o n o f components o f e l e c t r o d e r e a c t i o n s . T h i s is c a l l e d the e l e c t r o c h e m i c a l e l e c t r o lyte layer inhibition. For more d e t a i l e d d i s c u s s i o n o f the v a r i o u s phen o m e n o l o g i c a l i n h b i t i o n mechanism the r e a d e r is r e f e r r e d t o the o r i g i n a l l i t e r a t u r e . (4_) I t is o b v i o u s now t h a t c o r r o s i o n i n h i b i t i o n is not a simple phenomena, and a d e c i s i o n may have t o be made whether i t may be more u s e f u l t o d i s c u s s c o r r o s i o n inh i b i t i o n a l o n g m e c h a n i s t i c l i n e s or by means o f a t a b u l a t i o n o f c h e m i c a l compounds which have been found e f f e c t i v e under s p e c i f i c c i r c u m s t a n c e s . However, the d i g e s t i o n o f e x h a u s t i v e t a b l e s o f c h e m i c a l compounds and t h e i r a p p l i c a t i o n as c o r r o s i o n i n h i b i t o r s as p r e s e n t e d in C. Nathan's book (JL) l e a v e s the r e a d e r as d i s s a t i s f i e d and p u z z l e d as do e l a b o r a t e c l a s s i f i c a t i o n s y s tems o f c o r r o s i o n i n h i b i t o r s which i n v a r i a b l y l a c k exp e r i m e n t a l s u b s t a n t i a t i o n and have at t h i s stage l i t t l e p r e d i c t i v e value.
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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As a consequence i t is judged t o be more u s e f u l f o r t h e n o v i c e in t h e f i e l d o f c o r r o s i o n i n h i b i t i o n t o f a m i l i a r i z e h i m s e l f w i t h some o f t h e fundamental in v e s t i g a t i v e means and s u b s e q u e n t l y be c o n f r o n t e d w i t h some s p e c i a l cases o f c o r r o s i o n i n h i b i t i o n which may o r may not l e n d themselves t o g e n e r a l i z a t i o n .
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Experimental Observation of Corrosion I n h i b i t i o n The study o f c o r r o s i o n p r o c e s s e s in r e c e n t y e a r s has been almost e n t i r e l y based on t h e so c a l l e d Evans diagram. C o n s i d e r i n g how o f t e n t h i s diagram has been misused o r m i s r e p r e s e n t e d suggests t h a t a b r i e f d i s c u s s i o n may a g a i n be in o r d e r at t h e r i s k o f b e i n g r e p e t i v e or t r i v i a l . The Evans diagram ( 1 ) is a g r a p h i c a l p r e s e n t a t i o n in s e m i l o g a r i t h m i c c o o r d i n a t e s o f t h e a n o d i c and c a t h o d ic r e a c t i o n r a t e s e x p r e s s e d as p a r t i a l c u r r e n t s de pendent on p o t e n t i a l . The b a s i s f o r t h e Evans diagram is t h e c o r r o s i o n model d i s c u s s e d above: Me
^Me
n +
+ ne
(5)
n
Ox + n e — ^ R e d -
(6)
These p a r t i a l r e a c t i o n s proceed w i t h e q u a l r e a c t i o n r a t e s when t h e m e t a l is f r e e l y c o r r o d i n g . I n o r der t o e x p r e s s t h e r e a c t i o n r a t e s in terms o f a c u r r e n t , t h e c o n v e r s i o n p e r time is m u l t i p l i e d by t h e Faraday c o n s t a n t a c c o r d i n g t o t h e f o l l o w i n g e q u a t i o n : J
(7)
= Q' · F · η
Where J e q u a l s c u r r e n t , η number o f e l e c t r o n s t r a n s f e r r e d p e r molecule, F e q u a l s Faraday c o n s t a n t and Q" is the r a t e o f t h e r e a c t i o n e x p r e s s e d in terms o f moles per u n i t t i m e . I f t h e above e q u a t i o n is d i v i d e d by t h e s u r f a c e a r e a t h e c u r r e n t d e n s i t y can be e x p r e s s e d as follows : J
- _ .
=
0
—
F.n
·
,
—
(8)
The p a r t i a l a n o d i c ( i ) and c a t h o d i c ( i ) c u r r e n t d e n s i t i e s a r e then e x p r e s s e d as e x p o n e n t i a l f u n c t i o n s of the o v e r - p o t e n t i a l ( n n ) which is t h e d i f f e r e n c e between t h e o p e r a t i n g p o t e n t i a l (ε) and t h e e q u i l i b r i u m p o t e n t i a l ( E E ) of the p a r t i c u l a r p a r t i a l r e a c t i o n . These r e l a t i o n s h i p s a r e shown g r a p h i c a l l y and e x p l i c i t l y in F i g . 1 . Thus by d e f i n i n g t h e a n o d i c and c a t h o d i c c u r r e n t p o t e n t i a l r e l a t i o n s h i p s f o r a c o r r o d i n g system a
a
a
Q
c
c
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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and by p l o t t i n g them as shown in a p o t e n t i a l vs l o g i diagram, one q u i c k l y f i n d s the p o i n t where the two l i n e s i n t e r c e p t , which is the p o t e n t i a l at which a n o d i c and c a t h o d i c r e a c t i o n r a t e s are e q u a l . The c u r r e n t at t h i s p o i n t becomes c o r r o s i o n c u r r e n t o r c o r r o s i o n r a t e w h i l e the c o r r e s p o n d i n g p o t e n t i a l is the c o r r o s i o n potential . I t must be r e a l i z e d t h a t in a c o r r o d i n g system the p a r t i a l c u r r e n t s cannot be d i r e c t l y o b s e r v e d . If a c o r r o d i n g p i e c e o f m e t a l is made an e l e c t r o d e in an e l e c t r o c h e m i c a l c e l l the c o r r o s i o n r e a c t i o n s a l r e a d y p r o ceed w i t h a g i v e n r a t e . The m e t a l w i l l t h e r e f o r e a s sume t h e c o r r o s i o n p o t e n t i a l ( e c o r r ) and the system is in a s t e a d y s t a t e c o n d i t i o n as opposed t o an e q u i l i brium s t a t e . No c u r r e n t w i l l f l o w in the e x t e r n a l c i r c u i t at t h i s p o i n t . I f now by means o f an e x t e r n a l c i r c u i t the p o t e n t i a l o f the c o r r o d i n g m e t a l is moved in the a n o d i c o r c a t h o d i c d i r e c t i o n , the a n o d i c o r c a t h o d i c c u r r e n t w i l l f l o w in the e x t e r n a l c i r c u i t ( H ^ i e x t ) . T h i s o b s e r v e d e x t e r n a l c u r r e n t is the e q u i v a l e n t o f t h e a l g e b r a i c sum o f the a n o d i c and c a t h o d i c p a r t i a l c u r r e n t s and can t h e r e f o r e be p r e d i c t e d from the i n d i v i d u a l c h a r a c t e r i s t i c s as shown in F i g . l . I f the o p e r a t i n g p o t e n t i a l is s u f f i c i e n t l y n e g a t i v e o f the c o r r o s i o n p o t e n t i a l , the c u r r e n t - p o t e n t i a l b e h a v i o r o f the e x t e r n a l c u r r e n t w i l l resemble the p a r t i a l c a t h o d i c c h a r a c t e r i s t i c because under t h e s e c o n d i t i o n s the ano d i c c u r r e n t has become e x t r e m e l y s m a l l . The p o t e n t i a l r e g i o n s where e i t h e r o f t h e p a r t i a l r e a c t i o n s is negl i g i b l y s m a l l w i t h r e s p e c t t o the o t h e r are c a l l e d the T a f e l r e g i o n s . The c u r r e n t p o t e n t i a l b e h a v i o r in the T a f e l r e g i o n s o f an a c t i v e l y c o r r o s i n g p i e c e o f m e t a l are o f t e n used t o draw c o n c l u s i o n s w i t h r e s p e c t t o the mechanism o f t h e p a r t i a l c o r r o s i o n r e a c t i o n s . I t is t h e r e f o r e in p r i n c i p a l p o s s i b l e t o e s t a b l i s h the Evans diagram from fundamental knowledge o f the i n d i v i d u a l r e a c t i o n s o r e x p e r i m e n t a l l y by s t u d y i n g the p o l a r i z a t i o n b e h a v i o r o f a c o r r o d i n g p i e c e o f m e t a l in a g i v e n environment. F o r more d e t a i l e d d e r i v a t i o n o f e l e c t r o c h e m i c a l r a t e e q u a t i o n s see CD. I t is i m p o r t a n t t o remember t h a t some assumptions have been made in the d e r i v a t i o n o f F i g . l . F i r s t , the e q u a t i o n s g i v e n t h e r e a r e a p p l i c a b l e o n l y i f the e l e c t r o n t r a n s f e r is the r a t e d e t e r m i n i n g step in the part i a l c o r r o s i o n r e a c t i o n s . T h i s is i m p o r t a n t w i t h r e s p e c t t o t h e c a l c u l a t i o n o f the T a f e l s l o p e (RT/anF) o r the i n t e r p r e t a t i o n o f an e x p e r i m e n t a l one. I t is f u r t h e r assumed t h a t d u r i n g the p o l a r i z a t i o n o f the t e s t e l e c t r o d e ( c o r r o d i n g p i e c e o f m e t a l ) the c o m p o s i t i o n of the s o l u t i o n in t h e v i c i n i t y o f the e l e c t r o d e remains
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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constant. T h i s is o f t e n not the case s i n c e the consumption o f an o x i d a n t (oxygen o r p r o t o n s ) d u r i n g c a t h odic p o l a r i z a t i o n r a p i d l y leads to d i f f u s i o n l i m i t a t i o n ( d i f f u s i o n o v e r - p o t e n t i a l ) w h i l e secondary r e a c t i o n s d u r i n g p o l a r i z a t i o n in the a n o d i c d i r e c t i o n o f t e n l e a d t o p r e c i p i t a t i o n o f m e t a l h y d r o x i d e s and c o n s e q u e n t l y p a s s i v a t i o n phenomena. In s p i t e o f such e x p e r i m e n t a l d i f f i c u l t i e s the Evans diagram has been e x t r e m e l y u s e f u l in d e t e r m i n ing c e r t a i n c h a r a c t e r i s t i c s of i n h i b i t o r s . In a c i d sol u t i o n s f o r example, the e l e c t r o d e r e a c t i o n s f o l l o w a b e h a v i o r which is q u i t e p r e d i c t a b l e on the b a s i s o f the e l e c t r o n t r a n s f e r b e i n g the r a t e d e t e r m i n i n g s t e p . Thus Hackerman (JJ and Nobe (JL) have s t u d i e d such s y s tems e x t e n s i v e l y and found t h a t the e f f e c t i v e n e s s o f c e r t a i n amine i n h i b i t o r s can be e x p l a i n e d by t h e i r adsorption behavior. The a d s o r p t i o n i s o t h e r m s in t u r n proved t o be p r e d i c t a b l e on the b a s i s o f m o l e c u l a r s t r u c t u r e and c o n f i g u r a t i o n . In n e u t r a l or a l k a l i n e s o l u t i o n , or s o l u t i o n s o f low c o n d u c t i v i t y , however, one f i n d s v e r y s m a l l T a f a e l r e g i o n s , or the T a f a e l r e g i o n s are e s s e n t i a l l y n o n e x i s tent. The r e a s o n s f o r such b e h a v i o r are many: a) S t e r n (JL) f o r example has shown t h a t the c a t h o d i c p o l a r i z a t i o n c u r v e s f o r i r o n c o r r o d i n g in oxygen f r e e sodium c h l o r i d e s o l u t i o n show hydrogen i o n d i f f u s i o n l i m i t a t i o n at a c u r r e n t o f 1 0 " a/cm at a pH o f 2. Thus in such systems above a pH 1.5 e s s e n t i a l l y no T a f e l r e g i o n is o b s e r v e d due t o a d i f f u s i o n o v e r - p o tential. b) R e s i s t a n c e s or o v e r - p o t e n t i a l s o t h e r than t h o s e d e t e r m i n i n g the r a t e o f the e l e c t r o n t r a n s f e r r e a c t i o n are o f t e n i n c l u d e d in the c u r r e n t p o t e n t i a l measurement. The most f r e q u e n t s i t u a t i o n is the i n c l u s i o n o f an ohmic r e s i s t a n c e which o c c u r s between the worki n g e l e c t r o d e and the r e f e r e n c e e l e c t r o d e . Such ohmic e l e c t r o l y t e r e s i s t a n c e s can e a s i l y be determined by p o s i t i o n i n g the r e f e r e n c e e l e c t r o d e at v a r y i n g d i s t a n c e s from the working e l e c t r o d e , or by v a r i o u s mathematic a l p r o c e d u r e s i n c l u d i n g a u t o m a t i c i R - d r o p compensating electronic devices. M a n s f e l d has r e c e n t l y d i s c u s s e d the e f f e c t o f such r e s i s t a n c e on c u r r e n t p o t e n t i a l c u r v e s (10). A more s e r i o u s r e s i s t a n c e o f t e n i n f l u e n c i n g the p o t e n t i a l d e t e r m i n a t i o n in p o l a r i z a t i o n s t u d i e s is caused by the f o r m a t i o n o f . a c o r r o s i o n p r o d u c t l a y e r on the s u r f a c e o f the c o r r o d i n g t e s t e l e c t r o d e . This s i t u a t i o n is c o n s i d e r a b l y more complex because, f i r s t , t h i s a d d i t i o n a l r e s i s t a n c e is most l i k e l y not ohmic in n a t u r e due t o the s e m i c o n d u c t i n g p r o p e r t i e s o f the c o r r o s i o n p r o d u c t l a y e r and second, a n o d i c and c a t h o d i c 4
2
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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CORROSION
CHEMISTRY
r e a c t i o n s do not n e c e s s a r i l y t a k e p l a c e at the same l o cation. Thus the a n o d i c r e a c t i o n can be v i s u a l i z e d as t a k i n g p l a c e at the m e t a l - s c a l e i n t e r f a c e w h i l e t h e c a t h o d i c r e a c t i o n may t a k e p l a c e at t h e s c a l e - e l e c t r o l y t e i n t e r f a c e . The r e s u l t a n t p o l a r i z a t i o n diagram is shown in F i g . 2 . Note t h a t not o n l y the e x t e r n a l c u r r e n t is s u b j e c t t o an iR drop (the case most o f t e n d i s c u s s e d ) but the p a r t i a l c u r r e n t s are a l s o s u b j e c t t o a d i s t o r t i o n caused by t h e a d d i t i o n a l r e s i s t a n c e . It must f u r t h e r be remembered t h a t the r e s i t a n c e may be d i f f e r e n t in magnitude f o r the c a t h o d i c and a n o d i c p a r t i a l currents. T h i s s i t u a t i o n was d i s c u s s e d by H a u s l e r ( 1 1 ) in some d e t a i l . Effect
Of
I n h i b i t o r s On
P o l a r i z a t i o n Behavior
One o f the most i n t e r e s t i n g and perhaps most d i f f i c u l t t o u n d e r s t a n d phenomena in c o r r o s i o n i n h i b i t i o n is in f a c t t h a t i n h i b i t o r s do not a f f e c t a n o d i c and c a t h o d i c r e a c t i o n s t o the same degree. Thus one f i n d s , as shown in F i g . 3 t h a t in some i n s t a n c e s the c a t h o d i c r e a c t i o n r a t e is r e d u c e d w h i l e t h e a n o d i c r e a c t i o n r a t e remains the same or v i c e v e r s a . As can r e a d i l y be u n d e r s t o o d from t h e diagram in F i g . 3 , t h i s d i s t i n c t i o n can be made on the b a s i s o f a s h i f t in the c o r r o s i o n p o t e n t i a l and i f more d e t a i l e d i n f o r m a t i o n is r e q u i r e d from a complete study o f the p o l a r i z a t i o n curves. A b r i e f remark maybe in o r d e r here w i t h r e s p e c t t o the t e r m i n o l o g y used in such c a s e s , as i t is o f t e n c o n f u s i n g : C a t h o d i c i n h i b i t i o n r e s u l t s in a s h i f t o f t h e c o r r o s i o n p o t e n t i a l in the a n o d i c or n e g a t i v e d i r e c t i o n w h i l e a n o d i c i n h i b i t i o n r e s u l t s in a p o t e n t i a l s h i f t in the c a t h o d i c or more p o s i t i v e d i r e c t i o n . I t has been shown in t h e l i t e r a t u r e many t i m e s t h a t some systems a r e c l a s s i c a l l y behaved as w i l l be d i s c u s s e d in more d e t a i l e d below, w h i l e o t h e r s may show b o t h e f f e c t s , t h a t is, a c o m b i n a t i o n o f a n o d i c and c a t h o d i c i n h i b i t i o n in v a r y i n g d e g r e e s . Therefore, w h i l e the c o r r o s i o n p o t e n t i a l may g i v e a p r e l i m i n a r y i n d i c a t i o n o f the i n h i b i t o r mechanism, p o l a r i z a t i o n c u r v e s s h o u l d be r e c o r d e d f o r more complete assessment o f i t s mode o f a c t i o n . T h i s is a l l the more important s i n c e a r e d u c t i o n o f the a n o d i c r e a c t i o n r a t e , f o r ins t a n c e , c o u p l e d w i t h an a c c e l e r a t i o n o f the c a t h o d i c r e a c t i o n r a t e may r e s u l t in a l a r g e c a t h o d i c p o t e n t i a l s h i f t w i t h no change in t h e o v e r a l l c o r r o s i o n r a t e . C o n v e r s e l y , i f b o t h a n o d i c an c a t h o d i c r e a c t i o n r a t e s a r e r e d u c e d t o the same e x t e n t as shown in Fig.M-» no s h i f t in t h e c o r r o s i o n p o t e n t i a l is o b s e r v e d w h i l e the i n h i b i t i o n e f f e c t is the l a r g e s t p o s s i b l e . T a b l e 2 i l -
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
9. HAUSLER
Corrosion
273
Inhibition
— '«t an F
7Q
= l
a°
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V
6
e x
'
^RT
V
a
E
(l-a)n F \_i =-i -exp(-
-
c
c
R
T
%
)
V '- ' c e E
c
log i
Wt
=
'ext
=
Σΐ ,Ι α
0
an F 'corr L Pl" RT
-(l-«)n F c
Figure 1. Anodic and cathodic current-potential system
/-T^a
,
RT ^corr'J
^corr " E X P < K
ex
rehtionships in a corroding
+ blog i + i R Q
Q
// // // ./
// /
^ ^ 7
V
?
\^77 — ,0
9
•• ext = !
η^
= α
a
= a + blog i
c
= a'+ b'log i
Q
c
' + b'log i + i R c
c
Wr
W ^ P ^ Î ^ o f r ^ a ^ ' ^ p f ^ ^ H o r r " ^
Figure 2.
Effect of electrolyte resistance
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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CORROSION CHEMISTRY
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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HAUSLER Corrosion Inhibition
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
CORROSION
276
CHEMISTRY
Table I I WEIGHT LOSS MEASUREMENTS OF CARBON STEEL IN 10% SULFUR IC ACID AS A FUNCTION OF TYPE AND CONCENTRATION OF IN HIBITOR AT 2 5 C. Inhibitor
C o n c e n t r a t i o n Weight l o s s C o r r o s i o n of I n h i b i t o r mg/2 5cm . Potential (mol/1) day mV v s . H 2
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2
Blank 0 Aniline 0.0063 Diethylaniline ^ 0.0063 p-Phenylene diamine 0.0 06 3 3-Naphthyl-amine sat.sol. Phenyl-α-naphthyl amine s a t . s o l . 0.0063 Pyridine 0.0063 Quinoline 0. 063 Quinoline 0.0063 ^a-Naphthoquinoline 0.063 a-Naphthoquinoline 0063 β-Naphthoquinoline 063 β-Naphthoquinoline 0063 2,4,-Dimethylquinoline 063 2,4,-Dimethylquinoline , 0063 2,6-Dimethylquinoline , 063 2,6-Dimethylquinoline E t h y l q u i n o l i n i u m bromide0.0063 Acridine sat.sol. A c r i d i n e orange sat.sol. Acridine red sat.sol. Acriflavine sat.sol. Water s o l u b l e P e t r o l e u m sat.sol. sulfonate sat.sol Sulfonated o i l sat.sol. Commercial I n h i b i t o r A sat.sol. Β C sat.sol. D sat.sol.
956 943 513 886 374 938 863 370 122 78 59 73 58 321 178 175 64 63 404 24 61 112
235 233 209 225 210 2 34 232 201 181 177 172 178 172 192 176 192 170 184 227 201 184 196
170 23 23 44 85 51
212 227 227 216 218 223
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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9.
HAUSLER
Corrosion
Inhibition
277
lustrâtes t h e s e p o i n t s in a s u r v e y o f p o t e n t i a l s h i f t s and c o r r o s i o n r a t e s f o r v a r i o u s i n h i b i t o r s on carbon s t e e l in s u l f u r i c a c i d . Such r e l a t i v e l y simple e l e c t r o c h e m i c a l t e c h n i q u e s t h e r e f o r e can be c o n s i d e r e d u s e f u l t o o l s f o r the r a p i d assessment o f i n h i b i t o r a c t i v i t y and f o r the purpose o f o b t a i n i n g p r e l i m i n a r y inf o r m a t i o n on a p o s s i b l e i n h i b i t i o n mechanism. Howe v e r , p o l a r i z a t i o n measurements, e i t h e r in the form o f " L i n e a r p o l a r i z a t i o n measurements" o r T a f e l - s l o p e det e r m i n a t i o n s are f r a u g h t w i t h p e r i l ( c . f . H a u s l e r 11). Even i f a system is r e l a t i v l y w e l l behaved, the i n f o r m a t i o n a l v a l u e o b t a i n e d from p o l a r i z a t i o n curves is r e l a t i v e l y s m a l l and s h o u l d be combined w i t h more ext e n s i v e d e t e r m i n a t i o n o f the e l e c t r o d e k i n e t i c parameters, a d s o r p t i o n s t u d i e s and mass t r a n s f e r s t u d i e s in o r d e r t o e l u c i d a t e the mechanism o f a p a r t i c u l a r c o r r o s i o n i n h i b i t o r o r c l a s s o f compounds e x h i b i t i n g i n h i b i t i v e e f f e c t s . In s h o r t o n l y a complete d e s c r i p t i o n o f the i n t e r p h a s e c h e m i s t r y w i l l e v e n t u a l l y l e a d t o the p r e d i c t i v e c r i t e r i a f o r i n h i b i t o r b e h a v i o r . Very few such i n v e s t i g a t i o n s have been c a r r i e d out in the p a s t . I t appears t h a t i n v e s t i g a t o r s were m o s t l y s a t i s f i e d w i t h d e m o n s t r a t i n g the i n h i b i t o r y e f f e c t o f c h e m i c a l s u b s t a n c e s and s u b s e q u e n t l y f o r c i n g t h o s e s u b s t a n c e s i n t o one o r the o t h e r s i m p l e mechani s t i c concepts. The few i n v e s t i g a t i o n s which have p i n p o i n t e d v a s t l y more complex b e h a v i o r , have m o s t l y been o v e r l o o k e d . In the i n t e r e s t o f s t i m u l a t i n g more r e s e a r c h in the f i e l d o f i n h i b i t o r chemistry, t h i s r e view s h a l l s t r e s s the more e x o t i c i n v e s t i g a t i o n s . In p a r t i c u l a r i t w i l l be s u g g e s t e d t h a t c h e m i c a l r e a c t i o n s o f i n h i b i t o r s o c c u r i n g in the i n t e r p h a s e between c o r r o s i o n p r o d u c t s and the i n h i b i t o r are a more g e n e r a l and w i d e s p r e a d phenomenon t h a n has g e n e r a l l y been bel i e v e d in the p a s t . I n h i b i t i o n By T h i o u r e a And
Quinoline
Derivatives
A l a r g e number o f i n v e s t i g a t i o n s in a c i d media have l e d t o the c o n c l u s i o n t h a t the i n h i b i t i o n e f f e c t caused by r e l a t i v e l y s m a l l and s i m p l e m o l e c u l e s is due t o t h e i r a d s o r p t i o n on the m e t a l s u r f a c e . Compounds o f t h i s n a t u r e u s u a l l y c o n t a i n s u l f u r and n i t r o g e n , o r are o f the groups o f h i g h e r a l k y l - a l c o h o l s and f a t t y a c i d s . T y p i c a l compounds t o be d i s c u s s e d h e r e in more d e t a i l are q u i n o l i n e and t h i o u r e a d e r i v a t i v e s . F i g . 5 shows a comparison of the e f f e c t i v e n e s s o f s e v e r a l such compounds determined by means o f weight l o s s measurements on carbon s t e e l in 5% s u l f u r i c a c i d at 40° C. as a f u n c t i o n o f the i n h i b i t o r c o n c e n t r a t i o n . A cur-
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
10
10
10
Inhibitor Concn (mol/l)
-4 10
10
Springer-Verlag
0.25
0.50
0.75
1.00
2
Jf
ure 5. Weight loss of 0.1%C-steel in 5% aqueous H SO at 40°C as a function of concen tration of various quinoline and thiourea inhibitors (3)
10
L
Ethyl-thiourea m-tolyl- thiourea o- S p-tolylthiourea ~"
Methyl-thioure»
Thiourea
Quinoline 2,6-Dimethyl-quinoline N-ethyl-quinoline - & β-naphthoquinoline
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9.
HAUSLER
Corrosion
Inhibition
279
s o r y e x a m i n a t i o n o f F i g . 5 might l e a d t o t h e c o n c l u s i o n t h a t the r e l a t i o n s h i p s o f c o r r o s i o n r a t e v s . i n h i b i t o r c o n c e n t r a t i o n are b a s i c a l l y q u i t e s i m i l a r f o r t h e two c l a s s e s o f compounds and d i f f e r o n l y q u a n t i t a t i v e l y w i t h r e s p e c t t o the e f f i c i e n c y . I t w i l l be shown how e v e r , t h a t t h e r e are indeed p r o f o u n d d i f f e r e n c e s be tween the two c l a s s e s o f i n h i b i t o r s . Hoar and H o l i d a y (12) found r e l a t i v e l y simple c o n d i t i o n s as a r e s u l t o f t h e i r i n v e s t i g a t i o n o f the c o r r o s i o n i n h i b i t i o n o f i r o n in 5% s u l f u r i c a c i d by 2 , 6 - d i m e t h y l q u i n o l i n e . These a u t h o r s determined f i r s t the e x t e r n a l c u r r e n t - p o t e n t i a l c u r v e s in the u n i h i b i t e d s o l u t i o n and t h e n the a n o d i c p a r t i a l c u r r e n t - p o t e n t i a l c u r v e s at d i f f e r e n t i n h i b i t o r c o n c e n t r a t i o n s and the q u a n t i t y o f d i s s o l v e d i r o n in t h e e l e c t r o l y t e at c e r t a i n e l e c t r o d e p o t e n t i a l s . The d i f f e r e n c e between t h e a n o d i c e x t e r n a l c u r r e n t and the i n d e p e n d e n t l y determined a n o d i c p a r t i a l c u r r e n t ( d i s s o l v e d Fe) is the c a t h o d i c p a r t i a l c u r r e n t d e n s i t y . The r e s u l t s as o b t a i n e d by Hoar and H o l i d a y are shown in F i g . 6 . The dashed curve r e p r e s e n t s the e x t e r n a l p o l a r i z a t i o n b e h a v i o r in the absence o f i n h i b i t o r and the b l a c k l i n e s are the T a f e l s l o p e s f o r the a n o d i c p a r t i a l c u r r e n t d e n s i t y (the m e t a l d i s s o l u t i o n ) f o r different i n h i b i t o r concentrations. The c a t h o d i c p a r t i a l c u r r e n t d e n s i t y (hydrogen e v e o l u t i o n ) is found f o r a l l v a l u e s o f the i n h i b i t o r c o n c e n t r a t i o n s in the shad ed a r e a . T h e r e f o r e , i t is o b v i o u s t h a t the i n h i b i t o r in t h i s case a c t s e x c l u s i v e l y by r e d u c i n g the a n o d i c r e a c t i o n r a t e but not the c a t h o d i c one. Kaesche and Hackerman (13) have i n v e s t i g a t e d the i n h i b i t i o n o f s e v e r a l a l i p h a t i c and a r o m a t i c amines on pure i r o n c o r r o d i n g in IN hydrochloric acid. These a u t h o r s o b s e r v e d in t h i r t e e n out o f f o u r t e e n cases t h a t the i n h i b i t i o n was b o t h a n o d i c and c a t h o d i c , a l b e i t predominantly anodic. The e x c e p t i o n was methylamine which a c t e d o n l y c a t h o d i c a l l y . In the case o f the c o r r o s i o n i n h i b i t i o n on pure i r o n by β-naphthoquinoline in sodium s u l f a t e / s u l f u r i c a c i d s o l u t i o n one o b s e r v e s a simple p a r a l l e l s h i f t o f the a n o d i c and c a t h o d i c T a f e l l i n e s towards s m a l l e r v a l u e s o f c u r r e n t d e n s i t y . Here the e f f e c t is almost s y m e t r i c a l , i n d i c a t i n g t h a t t h i s i n h i b i t o r a c t s t o t h e same e x t e n t upon a n o d i c and c a t h o d i c r e a c t i o n r a t e s . T h e r e f o r e , the e f f e c t o f 3 - n a p h t h o q u i n o l i n e can be e x p l a i n e d on t h e b a s i s t h a t i t s a d s o r p t i o n b l o c k s a f r a c t i o n θ o f the m e t a l s u r f a c e f o r a l l electrode r e a c t i o n s . I f equation 9 describes the e x t e r n a l p o l a r i z a t i o n b e h a v i o r in terms o f a f u n c t i o n o f the p a r t i a l c u r r e n t p o t e n t i a l r e l a t i o n s h i p f o r the a n o d i c and c a t h o d i c r e a c t i o n s in the u s u a l terms:
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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280
CORROSION
-024
CHEMISTRY
-022 -0.20 -0.18 -0.16 -0.14
Potential vs. nH (V) 2
Springer-Verlag Figure 6.
Current-potential diagram for 0.1 % C-steel in 5% aqueous Η ^0^ at 40°C for various concentrations of 2,6-dimethylquinoline (3)
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
2
9.
HAUSLER
( i
Corrosion
281
Inhibition
ext o=iiI o- ' P[ /(Ban>o]- cath>o>
)
e
£
e
( i
n
e
*Pi- < cath>o! e /
B
9
where: i° = exchange c u r r e n t d e n s i t y ε = electrode potential B = anodic T a f e l slope cath cathodic T a f e l slope t h e n e q u a t i o n 10 d e s c r i b e s the p o l a r i z a t i o n i n h i b i t e d e l e c t r o d e s in the same terms: a n
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B
=
( i e x t >I=(i£ >I n
e x
ε
behavior
B
P I ' < a n > i H i c a t h >I· (
^-{^^cath^f I f the i n h i b i t o r does not β-naphthoquinoline), (B
of
an>o = < B
a n
)
affect
(B
i ;
c
a
t
h
the T a f e l
)
= (B
0
c a
slopes
th)l
1
0
)
(see
the i n h i b i t e d a n o d i c c u r r e n t d e n s i t y is a simple f r a c t i o n o f the u n i n h i b i t e d a n o d i c c u r r e n t d e n s i t y as shown in e q u a t i o n 12. ( i °an ) !± = ( ian° ) ο (1-Θ) a n
and
f o r the
(12)
Λ
cathodic
partial
current
I = ο
( 1
"
θ )
( 1 3 )
I t f o l l o w s t h a t the i n h i b i t e d e x t e r n a l c u r r e n t is a s i m i l a r f r a c t i o n o f the u n i n h i b i t e d e x t e r n a l c u r r e n t as i n d i c a t e d in e q u a t i o n 14 ( i
e x t > I = o
J " (igat>o'
(l-9 ) exp|-e/(B 2
c a t h
) | 0
(16)
S i n c e d i m e t h y l q u i n o l i n e is o n l y a m a r g i n a l c o r r o s i o n i n h i b i t o r i t is p r o b a b l y o n l y m a r g i n a l l y o r weekly ad sorbed. One can f u r t h e r s p e c u l a t e t h a t the a d s o r p t i o n o c c u r s o n l y on the s i t e s S-, , t h a t is the l a t t i c e d i s s o l u t i o n s i t e s which w i l l f a v o r the a d s o r p t i o n o f f o r e i g n m o l e c u l e s from an e n e r g e t i c p o i n t o f view. T h e r e f o r e , 9 w i l l be a p p r o x i m a t l y z e r o . E q u a t i o n 16 then reduces t o e q u a t i o n 17: 2
(i
ext>I = (iaVo
( 1
θ
- 1
}
e x
PI
e 1 (B
an>ci "
'igathV
expj-e/(B
c a t h
) j 0
(17)
By p r o p e r adjustment o f the parameter 9-j_ the curves in F i g . 6 can then be c a l c u l a t e d . S i m i l a r c a l c u l a t i o n s have been c a r r i e d out f o r the i n h i b i t i o n of i r o n c o r r o s i o n in h y d r o c h l o r i c a c i d by V i c t o r i a b l u e which, a c c o r d i n g t o E l z e and F i s c h e r (14) does not a f f e c t the T a f e l s l o p e s , but reduces both the a n o d i c and c a t h o d i c p a r t i a l r e a c t i o n s t o a d i f f e r e n t degree. In t h i s p a r t i c u l a r case θ]_ and 9 have t o be a d j u s t e d by way o f a t r i a l and e r r o r c a l c u l a t i o n . T h i s simple model o f the i n h i b i t o r a c t i o n which is based e s s e n t i a l l y on the p o t e n t i a l independence o f the i n h i b i t o r a d s o r p t i o n is, however, o f t e n not ap plicable. Kaesche (15) i n d i c a t e s t h a t the c o r r o s i o n i n h i b i t i o n o f pure i r o n in s u l f u r i c o r p e r c h l o r i c a c i d by p h e n y l - t h i o u r e a s t r o n g l y a f f e c t s the s l o p e s o f the p o l a r i z a t i o n c u r v e s , l e a v i n g the c o r r o s i o n p o t e n t i a l s almost unchanged F i g . 7 . In f a c t , the p o l a r i z a t i o n curves f o r the i n h i b i t e d s i t u a t i o n do not e x h i b i t r e a l T a f e l behavior. This behavior f i n d s a p a r t i a l explan a t i o n in the f a c t t h a t the mechanism o f the hydrogen e v o l u t i o n appears t o be changed in the p r e s e n c e o f 2
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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p h e n y l t h i o u r e a . S p e c i f i c a l l y , i t appears the 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 o f hydrogen is g r e a t l y h i n d e r e d . T h i s mechanism may p o s s i b l y f i n d c o n f i r m a t i o n in the i n d e p e n d e n t l y o b s e r v e d f a c t t h a t in the p r e s e n c e of p h e n y l t h i o u r e a c o n s i d e r a b l y more hydrogen d i f f u s e s i n t o the m e t a l . However, the i n h i b i t i o n mechanism o f p h e n y l t h i o u r e a is p r o b a b l y c o n s i d e r a b l y more c o m p l i c a t e d t h a n t h a t , as can be seen from the v e r y complex r e l a t i o n s h i p o f the c o r r o s i o n p o t e n t i a l w i t h the i n h i b i t o r con centration. T h i s is shown in F i g . 8 , in comparison with a s i m i l a r r e l a t i o n s h i p f o r g-naphthoquinoline. Hoar (16) found t h a t in the c o r r o s i o n i n h i b i t i o n o f i r o n in h y d r o c h l o r i c a c i d by β-naphthoquinoline, the c o r r o s i o n p o t e n t i a l i n c r e a s e s m o n o t o n i c a l l y w i t h in c r e a s i n g i n h i b i t o r c o n c e n t r a t i o n , w h i l e in the case of o - t o l y l t h i o u r e a one o b s e r v e s f i r s t a d e c r e a s e of the c o r r o s i o n p o t e n t i a l f o l l o w e d by an i n c r e a s e at h i g h e r i n h i b i t o r concentrations. A s i m i l a r predominant in h i b i t i o n o f the c a t h o d i c p a r t i a l r e a c t i o n at s m a l l in h i b i t o r c o n c e n t r a t i o n s is e x h i b i t e d a l s o by phenylt h i o u r e a a c c o r d i n g t o Kaesche. F u r t h e r m o r e , in the s e r i e s o f the t h i o u r e a d e r i v a t i v e s one o f t e n f i n d s c o r r o s i o n a c c e l e r a t i o n at s m a l l c o n c e n t r a t i o n s , as f o r in s t a n c e in the case o f p h e n y l t h i o u r e a at c o n c e n t r a t i o n s o f 10moles per l i t e r . T h i s appears t o be due t o a s m a l l c a t h o d i c d e c o m p o s i t i o n of t h i o u r e a and i t s de r i v a t i v e s in the course of which hydrogen s u l f i d e is formed. As is w e l l known, hydrogen s u l f i d e tends t o a c c e l e r a t e c o r r o s i o n , in p a r t i c u l a r the a n o d i c p a r t i a l r e a c t i o n o f d i s s o l u t i o n o f i r o n , which has been demon s t r a t e d i n d e p e n d e n t l y by o t h e r a u t h o r s (17). I t is g e n e r a l l y assumed t h a t i o n s which can a c c e l e r a t e e i t h e r o r both p a r t i a l r e a c t i o n s in a c o r r o s i o n p r o c e s s are c a p a b l e of b e i n g adsorbed on the i r o n s u r face. Thus i t is known t h a t hydrogen s u l f i d e i o n s which a c c e l e r a t e both p a r t i a l r e a c t i o n s o f a c i d c o r r o s i o n ( a l t h o u g h p r e d o m i n a n t l y the a n o d i c o n e ) , and f o r m i c a c i d m o l e c u l e s which c a t a l y z e the c a t h o d i c p a r t i a l r e a c t i o n but i n h i b i t the a n o d i c one, as w e l l as commercial i n h i b i t o r s which r e d u c e both p a r t i a l r e a c t i o n s , are in f a c t adsorbed on the i r o n s u r f a c e . As a consequence the mere f a c t t h a t a d s o r p t i o n t a k e s p l a c e cannot be used t o p r e d i c t an e x p e c t e d change in c o r r o s i o n r a t e as i t is a l s o known t h a t h a l i d e i o n s c a t a l i z e the a n o d i c d i s s o l u t i o n o f i n d i u m , w h i l e hydroxy1 a d s o r p t i o n c a t a l y z e s the a n o d i c d i s s o l u t i o n of i r o n . F u r t h e r m o r e , i t is a l s o known t h a t c e r t a i n i o n s can act e i t h e r as a c a t a l y s t o r an i n h i b i t o r when a d s o r b ed on the m e t a l s u r f a c e depending on the type of m e t a l considered. K o l o t y r k i n (IB) o b s e r v e d t h a t the a d s o r p -
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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4000
-0.45
-0.40 -035 -030 -0.25
Electrode Potential (V) Springer—Verlag Figure 7. Poforization of carbonyl iron in IN NaClO^/HClO^ at pH 2, 25°C deaerated, with and without 7 · 10~ mol/L phenylthiourea (3) 5
Ο /3-Naphthoqulnollne Δ o-tolyl-thlourea
Springer-Verlag Figure 8. Weight loss and rest potential of mild steelin10% H SO as a function of the concentration of β-naphthoquinoline and o-tolylthiourea (3) z
h
Inhibitor Concn (10 mol/l)
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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t i o n o f i o d i n e i o n s c o n s i s t e n t l y i n c r e a s e s the hydrogen o v e r - v o l t a g e on s i l v e r but d e c r e a s e s i t on mercury. On l e a d one o b s e r v e s t h a t s m a l l amounts o f adsorbed i o d i n e i n c r e a s e the hydrogen o v e r - p o t e n t i a l , l a r g e r amounts, however, d e c r e a s e i t . In the case o f the hydrogen evo l u t i o n i t is p r o b a b l y the h e a t o f a d s o r p t i o n o f the atomic hydrogen on a p a r t i c u l a r m e t a l which c o n t r o l s the a c c e l e r a t i o n or i n h i b i t i o n o f t h i s r e a c t i o n upon a d s o r p t i o n o f a f o r e i g n i o n . With r e s p e c t t o the k i n e t i c s o f the a n o d i c d i s s o l u t i o n o f m e t a l , however, the i m p o r t a n t parameter is most l i k e l y the s t r e n g t h o f the complex f o r m a t i o n between s u r f a c e m e t a l atoms and the adsorbed p a r t i c l e . I f the complex f o r m a t i o n is weak, t h e r e f o r e h a r d l y a f f e c t i n g the bonding f o r c e s h o l d i n g the s u r f a c e atoms in the m e t a l l a t t i c e , one would e x p e c t i n h i b i t i o n t h r o u g h simple b l o c k i n g o f the dissolution sites. Conversely, i f strong l i g a n d forces a c t between the s u r f a c e m e t a l i o n s and the adsorbed p a r t i c l e , one would e x p e c t c a t a l y s i s o f the m e t a l d i s solution. A l o n g t h e s e l i n e s one a l s o has t o c o n s i d e r the p o s s i b i l i t y t h a t one k i n d o f adsorbed p a r t i c l e can be d i s p l a c e d by a d i f f e r e n t k i n d . One would t h e r e f o r e suggest t h a t i n c r e a s i n g the c o n c e n t r a t i o n o f t h i o u r e a d e r i v a t i v e s e v e n t u a l l y leads to c o r r o s i o n i n h i b i t i o n because of c o m p e t i t i v e a d s o r p t i o n w i t h the hydrogen s u l f i d e i o n which causes a c c e l e r a t i o n o f c o r r o s i o n . A c c e l e r a t i o n of c o r r o s i o n has been demonstrated w i t h s t r o n g complexing agents such as EDTA s a l t s . Howe v e r , as EDTA is s u b s t i t u t e d w i t h l o n g e r a l k y l c h a i n s , the c a t a l y t i c e f f e c t is g r a d u a l l y l o s t and i n h i b i t i o n is o b t a i n e d . T h i s s t r o n g l y s u g g e s t s t h a t the a d s o r p t i o n o f the i n h i b i t o r p a r t i c l e on the m e t a l s u r f a c e , a p u r e l y i n t e r f a c i a l phenomenon, is not the predominant f e a t u r e t o be s t u d i e d , but t h a t in f a c t the c h e m i s t r y in the i n t e r p h a s e , in p a r t i c u l a r , the f o r m a t i o n o f c o r r o s i o n p r o d u c t s and c o r r o s i o n p r o d u c t l a y e r s has t o be g i v e n more c o n s i d e r a t i o n . A case in p o i n t is a study made by Ross (JJï) on the d i s s o l u t i o n o f i r o n in 0.5 m o l a r s u l f u r i c a c i d in the p r e s e n c e o f t h i o u r e a at 40° C. The r e s u l t s o f t h i s s t u d y , which was conducted as a f u n c t i o n o f the f l o w r a t e , are shown in F i g . 9 . I t appears t h a t the u n i n h i b i t e d d i s s o l u t i o n o f i r o n f o l l o w s e x p e c t e d mass t r a n s f e r b e h a v i o r b o t h in the l a m i n a r and t u r b u l e n t r e g i o n s . However, at two i n h i b i t o r c o n c e n t r a t i o n s marked d e v i a t i o n s from the e x p e c t e d mass t r a n s f e r b e h a v i o r are observed. Ross attempted t o e x p l a i n t h e s e r e s u l t s on the b a s i s t h a t d i f f e r e n t i n h i b i t o r c o n c e n t r a t i o n s a f f e c t the a n o d i c and c a t h o d i c p o l a r i z a t i o n in d i f f e r e n t ways, t a k i n g a l s o i n t o c o n s i d e r a t i o n t h a t at s m a l l
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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t h i o u r e a c o n c e n t r a t i o n s hydrogen s u l f i d e is formed on the s u r f a c e o f the m e t a l c a t a l i z i n g the a n o d i c r e a c t i o n , t h e r e b y a c h i e v i n g a c o r r o s i o n r a t e which is h i g h e r t h a n the b l a n k c o r r o s i o n r a t e . However, R o s s s e x p l a n a t i o n l e a v e s many ends u n t i e d . One has t o exp l a i n f i r s t o f a l l the mass t r a n s f e r b e h a v i o r f o r the u n i n h i b i t e d d i s s o l u t i o n o f i r o n in s u l f u r i c a c i d . At a p r o t o n c o n c e n t r a t i o n o f one normal and at c o r r o s i o n r a t e s as measured, p r o t o n d i f f u s i o n cannot be r a t e controlling. Second,the i r o n d i f f u s i o n away from the meta l s u r f a c e is not e x p e c t e d t o be r a t e l i m i t i n g , s i n c e i r o n can in f a c t d i f f u s e away from the s u r f a c e as f a s t as i t is formed, a f a c t t h a t has been e x p e r i m e n t a l l y v e r i f i e d by t h i s a u t h o r (see b e l o w ) . One must t h e n assume t h a t the o b s e r v e d mass t r a n s f e r b e h a v i o r is caused by a secondary r e a c t i o n most l i k e l y the d i s s o l u t i o n of a c o r r o s i o n p r o d u c t . I t is known t h a t c e r t a i n i r o n s u l f a t e s are q u i t e i n s o l u b l e in w a t e r , such as the monohydrate o f the f e r r o u s s u l f a t e , and f e r r i c sulfate. In R o s s s experiment oxygen was present s i n c e the a u t h o r does not mention any p r e c a u t i o n s f o r k e e p i n g oxygen out o f the e x p e r i m e n t a l system. At low t h i o u r e a c o n c e n t r a t i o n s ( 2 1 0 ~ m o l a r ) i t is o b s e r v e d t h a t the c o r r o s i o n r a t e v a r i e s approximatel y w i t h the 1/5 power o f the f l o w r a t e . This author has o b s e r v e d t h a t , in hydrogen s u l f i d e s a t u r a t e d s o l u t i o n s at 70° C in the p r e s e n c e o f s m a l l amounts o f ammonium c h l o r i d e * the c a t h o d i c p a r t i a l r e a c t i o n f o r i r o n c o r r o s i o n v a r i e s w i t h the l / 6 t h power o f the f l o w rate. T h i s maybe a c o i n c i d e n c e ; however, s i n c e the f o r m a t i o n o f hydrogen s u l f i d e and i r o n s u l f i d e on c o r r o d i n g i r o n s u r f a c e s in a c i d t h i o u r e a c o n t a i n i n g media has been o b s e r v e d i n d e p e n d e n t l y by o t h e r a u t h o r s , i t is q u i t e l i k e l y t h a t the abnormal mass t r a n s f e r beh a v i o r o b s e r v e d by Ross f o r the s m a l l t h i o u r e a concent r a t i o n is in f a c t caused by an i r o n s u l f i d e l a y e r on the s u r f a c e o f the m e t a l . At h i g h e r t h i o u r e a concent r a t i o n s t h e r e may w e l l be c o m p e t i t i o n between the t h i o u r e a m o l e c u l e and the hydrogen s u l f i d e in the comp l e x i n g r e a c t i o n w i t h i r o n . I t s h o u l d be n o t e d , t h a t the next h i g h e r t h i o u r e a c o n c e n t r a t i o n is t h r e e o r d e r s o f magnitude l a r g e r but a c h i e v e s o n l y a m a r g i n a l inhibition effect. One would t h e r e f o r e assume t h a t t h i o u r e a is in c o m p e t i t i o n w i t h s u l f a t e or water f o r l i g a n d p o s i t i o n s around the i r o n . S i n c e t h i o u r e a is a n e u t r a l molecule, i t is q u i t e u n d e r s t a n d a b l e t h a t i t w i l l reduce the d i s s o l u t i o n r a t e o f i r o n s u l f a t e merely by b l o c k i n g a c c e s s o f water m o l e c u l e s t o the i r o n s u l fate surface. A complete e x p l a n a t i o n o f Ross's s c a n t y d a t a is c o m p l i c a t e d by the p r e s e n c e of oxygen which
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1
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w i l l i n t e r f e r e w i t h the c o r r o s i o n and complexing r e a c t i o n s on the s u r f a c e o f the m e t a l . Nevertheless, this p a r t i c u l a r i n v e s t i g a t i o n is o f g r e a t v a l u e in t h a t i t p o i n t s out the importance o f s t u d y i n g i n h i b i t o r e f f e c t i v e n e s s under dynamic flow c o n d i t i o n s , and in under l i n i n g f u r t h e r the importance o f the c h e m i s t r y o c c u r r i n g in a phase immediately a d j a c e n t t o the m e t a l s u r f a c e but d i s t i n c t l y d i f f e r e n t from the b u l k of the c o r r o s i v e medium. Downloaded by UNIV OF MISSOURI COLUMBIA on September 24, 2013 | http://pubs.acs.org Publication Date: January 31, 1979 | doi: 10.1021/bk-1979-0089.ch009
Acetylenic Inhibitors A c e t y l e n i c compounds have l o n g been known as e f f e c t i v e - c o r r o s i o n i n h i b i t o r s in h y d r o c h l o r i c a c i d . T e d e s c h i and coworkers (20,21) have p r e s e n t e d o v e r the p a s t 10 y e a r s e x t e n s i v e i n v e s t i g a t i o n s o f t h e s e com pounds. T h e i r r e s u l t s are summarized in T a b l e s 3, 4 and 5. Some o f the b a s i c f e a t u r e s of t h e s e compari sons are the p o s i t i o n o f the h y d r o x y l group and the po s i t i o n o f the t r i p l e bond. Thus i t appears t h a t the hy d r o x y l group has t o be in α-position t o the a c e t y l e n i c f u n c t i o n and optimum e f f i c i e n c y is o b t a i n e d i f the a c e t y l e n i c f u n c t i o n is t e r m i n a l . These two e f f e c t s can be e x p l a i n e d on the b a s i s o f the tautomerism shown in e q u a t i o n 18. (18)
T h i s f o r m a l i s m i m p l i e s t h a t the p o l a r i z a t i o n o f the t r i p l e bond can be s t a b i l i z e d f i r s t by a n o n c l a s s i c a l carbonium i o n and f u r t h e r by an α-keto-double bond c o n f i r g u r a t i o n which is known t o complex s t r o n g l y w i t h t r a n s i t i o n metal ions. I t is n o t e d t h a t the h y d r o x y l group has t o be l o c a t e d not o n l y in α-position but on a secondary carbon atom f o r s t r o n g c o r r o s i o n i n h i b i t i o n to r e s u l t . T h i s is t o be e x p e c t e d , s i n c e p r o t o n s are much more apt t o form n o n c l a s s i c a l carbonium i o n s t h a n m e t h y l groups. However, i f the t e r m i n a l p r o t o n on the t r i p l e bond is s u b s t i t u t e d w i t h a s t r o n g e l e c t r o p h i l i c group,the p o l a r i z a t i o n o f the t r i p l e bond becomes s t r o n g enough t o i n v o l v e the above i n d i c a t e d tauto merism. One can f u r t h e r r a t i o n a l i z e t h a t a compound w i t h a n o n - t e r m i n a l t r i p l e bond e x p e r i e n c e s some s t e r i c h i n d r a n c e in the f o r m a t i o n of complexes w i t h t r a n s i t i o n metal ions. Thus, the e f f e c t i n d i c a t e d in T a b l e 3 which T e d e s c h i a s c r i b e d t o s t e r i c h i n d r a n c e is r a t h e r a k i n e t i c e f f e c t c o n c e r n i n g the i n t r a - m o l e c u l a r s h i f t s
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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Table I I I Decreasing 1. L o c a t i o n o f T r i p l e R
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2
c
5
R]_
Rl
^
£
ÔH
2
3
= CH ^C H 3
h
OH
R]_ = H or CH R
Bond:
x
r 2
Inhibition
R L
1 1
"
A_
c = c
OH "
OH
_ =c_c_ c
"
2. S t e r i c h i n d r a n c e : H H H — 6 — C = C H > CH —6—C=CH>> OH ÔH 3
R ~2
OH
CH CH —Ç—C=CH 3
3
OH
3. A l k y l Chain Length: H
R — C H > CH CH ^ o f T r i p l e Bond and H y d r o x y l F u n c t i o n :
R2: C H n > Ο ι + Η > C H , 5
9
3
y
2
5
3
3
4. P o s i t i o n
CH — C H — C H — C H — C = C H 3
2
2
CH — C H — C = C — C H — C H — 0 H 3
2
2
2
OH INHIBITOR
NO INHIBITOR
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
290
CORROSION
CHEMISTRY
T a b l e IV Type C h a i n Formula
OH
Length C o r r o s i o n Rate
OH—CH —C===CH
1°
3
CH OH—CH—C=CH I CH
3°
4
CH —CH —CH —CH—C=CH
2°
6
3°
6
2
0. 026
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q
>1.6
3
3
2
2
0. 002
OH
CHq CH —CH—CH —Ç—C=CH CHo OH 3
2
>1.6
Table V Formula ÇH lo 3
Temperature
C o n c e n t r a t i o n C o r r o s i o n Rate
3
C-
200
0.2
6H
175
0.3
0.221 >1.8
>1.8
H
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CORROSION CHEMISTRY
Table 10 also shows a comparison between differ ent propylenediamine derivatives with decreasing mole cular weight. A tallow propylenediamine (C^g) which has been proposed as a process corrosion inhibitor in the literature many times, is a very efficient materi al. As the molecular weight decreases due to a re duction of the alkyl chain length to 15 C-atoms and 11 C-atoms the corrosion inhibiting tendency decreases gradually. This would have been expected because the water solubility of these compounds increases with de creasing alkyl chain length. Yet another way to vary the formation constant of the surface complex is to change the number of amine groups in the molecule. One of the f i r s t process corrosion inhibitors proposed in the literature was oleylamine. As has been shown in (2J.) , oleylamine is quite effective in the range of 3-4 ppm. The tallow propylenediamine derivative which has about the same molecular weight i s , however, at least ten times as active as oleylamine. This concept represents a new theory for the ac tion of process corrosion inhibitors. In contrast to the old filming amine theory the new one is based on a specific adsorption mechanism involving an ion ex change step. It can correctly predict the decrease of protection efficiency with increasing pH. It also ex plains structural differences between inhibitors such as increased efficiency of propylenediamine deriva tives over primary amines and decreased efficiency with decreasing molecular weight of the amine group compound. Literature Cited 1. Annand, R.R., Hurd, R.M., Hackerman, N., J. Electrochem. Soc, 112, 138, (1965). Parsons, R., 3rd European Symposium on Corr. Inhib., Ferrara, Italy, 1970, Proceedings p. 3 (1971). 2. Wagner, C., Traud, W., Z. Electrochemie 44 391 (1938). 3. Kaesche, Η., Die Korrosion der Metalle, Springer Verlag, Berlin 1966, p. 68. 4. Fischer, Η., 3rd European Symposium on Corr. Inhib., Ferrara, Italy, 1970 Proceedings, p. 16(1971). 5. Nathan, C., Corrosion Inhibitors, National Associ ation of Corrosion Engineers, Houston, Texas, 1973 , cf. Riggs, O.L. p. 7. 6. Vetter, Κ., Electrochemical Kinetics, Academic Press, New York, London, 1967.
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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9. HAUSLER
Corrosion Inhibition
319
7. Hackerman, N., Corrosion/74, National Meeting of the NACE, Chicago, 111., paper 73, (1974). 8. Nobe, Κ., Hussein, Ν., Corrosion/73, Annual Meet ing NACE, paper #26 (1973). 9. Stern, M., J. Electrochem. Soc. 102 609 (1955). 10. Mansfeld, F., Corrosion, 32 (4) 143 (1976). 11. Hausler, R.,., Corrosion, 33 (4) 117 (1977). 12. Hoar, T.P., Holliday, R.D. J. Appl. Chem. 3 502 (1953). 13. Kaesche, Η., Hackerman, N., J. Electrochem. Soc. 105, 4, (1958). 14. Elze, J., Fischer, H., J. Electrochem. Soc. 99, 259 (1952). 15. Kaesche, H., Z. Electrochemie 63, 492 (1959). 016. Hoar, T.P., Pittsburgh Intern. Conf. Surface Re actions, p. 127, Pittsburgh Corrosion Pub lications Company (1948). 17. Bartonicek, R., Proc. 3rd Internat'l Congr. on Mettallic Corrosion, Moscow, 1966 Vol. 1 p. 119 (1969). 18. Kolotyrkin, Y.,., Trans. Faraday Soc. 55, 455 (1959). 19. Ross, T.K., Jones, D.H., J. Applied Chem. 12, 317 (1962). 20. Tedeschi, R., Proceedings of the 25th Conference, NACE, March, 1969, p. 173, (1970). 21. Tedeschi, R., National Conference NACE, Annaheim Calif. 1973, paper #24. 22. Tedeschi, R., Corrosion, 31 (4) 130, (1975). 23. Poling, G.W., J. Electrochem. Soc. Vol. 114, 1209 (1967). 24. Hausler, R.H., Corrosion, 33 (4) 119 (1977). 25. Gardner, G., Corrosion Inhibitors, C. Nathan editor, National Assoc. Corrosion Engineers, Houston, Texas (1973). 26. Parsons, R., Advances in Electrochemistry and Electrochem. Eng. Vol. 1, ed. Delahay p. 65 Interscience, New York, (1961). 27. Delahay, P., Double Layer and Electrode Kinetics, Wiley, New York, (1965). 28. Bockris, M., Modern Electrochemistry, Plenum Press (1972). 29. Ranney, M.W., Corrosion Inhibitors, Manufacture and Technology, Noyes Data Corporation, Park Ridge, N.J. (1976). 30. Bregman, J.I., Corrosion Inhibitors, The Mac Millan Co., New York, N.Y., p. 198 (1963). 31. Nathan, C., Corrosion Inhibitors, National Assoc. Corr. Engineers, Houston, Texas, 1973 cf.p.42
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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CORROSION CHEMISTRY
32. L a r r i s o n , K.L., Use of P o l a r i z a t i o n Data In the F i e l d Evaluation of Oil Well Corrosion Treatment, paper presented at the NACE South Central Region, Conference, Oct. 18, 1966. 33. Frank, W.J., The Oil and Gas Journal May 31, p. 63 (1976). 34. Hausler, R.H., Proceedings 3rd. European Symposium Corr. I n h i b i t o r s , Ferrara, I t a l y , (19 70) p.399 (1971). 35. Sardisco, T.B., Corrosion 19 354 (1963), Corrosion 21 69 (1965). 36. Hausler, R.H., Materials Performance, 13, ( 9 ) , 16 (1974). 37. Hausler, R.H., e t . a l . , Corrosion/74, Annual NACE Meeting, Chicago, Ill., paper #123 (1974). 38. Hausler, R.H., NACE Annual Meeting, Philadelphia P.A., paper #63 (1970). 39. Simkovich, G., e t . a l . paper 12-69, 34th Midyear Meeting API, Div. Refining, Chicago, Ill. May 12, (1969). 40. Bennet, C.O., Meyers, J.E., Momentum, Heat and Mass Transfer, McGraw Hill Publishing Co.,(1975). RECEIVED September 1, 1978.
In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
