Corrosion Chemistry - American Chemical Society

9 Corrosion Inhibition and Inhibitors

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

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.


9.

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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


CORROSION C H E M I S T R Y


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


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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 ) .


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-



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 .


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 .


9.

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Corrosion

267

Inhibition

Table I

(A)


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



268


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.


9.

Corrosion

HAUSLER

269

Inhibition

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 .


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



270


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



9.

HAUSLER

Corrosion

271

Inhibition

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



272

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 -


9. HAUSLER

Corrosion

273

Inhibition

— '«t an F

7Q

= l

a°


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



CORROSION CHEMISTRY



HAUSLER Corrosion Inhibition


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


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



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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-



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-tolylthiourea o- S p-tolylthiourea ~"

Methyl-thioure»

Thiourea

Quinoline 2,6-Dimethyl-quinoline N-ethyl-quinoline - & β-naphthoquinoline



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:



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)


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


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



9.

HAUSLER

Corrosion

Inhibition

283

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 -


CORROSION CHEMISTRY

284


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)



9.

HAUSLER

Corrosion

Inhibition

285

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



286 CORROSION


CHEMISTRY

9.

HAUSLER

Corrosion

Inhibition

287

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


1

1

e

1


CORROSION

288

CHEMISTRY

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


Corrosion

9. H A U S L E R

289

Inhibition

Table I I I Decreasing 1. L o c a t i o n o f T r i p l e R


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


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


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

i t was shown t h a t the "protectivèness" o f the r a t e c o n t r o l l i n g i r o n s u l f i d e f i l m i n c r e a s e s w i t h i n c r e a s i n g pH and d e c r e a s e s w i t h i n c r e a s i n g oxygen c o n c e n t r a t i o n . However, the concent r a t i o n o f d i s s o l v e d s u l f i d i c s u l f u r has e s s e n t i a l l y no e f f e c t on i r o n s u l f i d e f i l m i n the r e g i o n o f 1500 t o 25,000 ppm (1Z). A most s u r p r i s i n g b e h a v i o r was found when the pol a r i z a t i o n c u r r e n t o f a c o r r o d i n g specimen was o b s e r v e d as a f u n c t i o n o f flow r a t e . Fig.20 shows the r e s u l t s which were o b t a i n e d . 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 c u r r e n t s i n m i l l i a m p s a t p o t e n t i a l s of +_ 50 m i l l i v o l t s from the c o r r o s i o n p o t e n t i a l are p l o t t e d a g a i n s t f l o w rate. In s e v e r a l runs r e p r o d u c i b l e b e h a v i o r was observed. The c a t h o d i c c u r r e n t v a r i e s w i t h flow r a t e t o the -g- t h power, w h i l e the a n o d i c c u r r e n t v a r i e s w i t h f l o w r a t e t o the ^_ t h power. T h i s r e l a t i o n s h i p h o l d s t r u e o v e r more t h a n one decade o f f l o w r a t e s . It is v e r y d i f f i c u l t t o e x p l a i n such r e s u l t s i n terms o f c o n v e n t i o n a l mass t r a n s f e r l i m i t a t i o n s . I t i s w e l l known t h a t the d i f f u s i o n l i m i t e d c u r r e n t v a r i e s app r o x i m a t e l y p r o p o r t i o n a l l y t o the f l o w r a t e i n the t u r b u l e n t r e g i o n and w i t h the square r o o t o f f l o w r a t e i n the l a m i n a r r e g i o n . I f on the o t h e r hand the p o l a r i z a t i o n c u r r e n t was t r a n s p o r t l i m i t e d a c r o s s the c o r r o s i o n


HAUSLER

Corrosion

Inhibition

Polarization Current at Ε - E

C Q | T

= ± 5 0 mV

2nd Run


1st Run

2nd Run Ο

Anodic

Δ

Cathodic J~V 0.05

J ~V

0.1

0.2

0.4

0.6 0.8 1.0

FLOWRATE (gal/min) Figure 20.

Iron corroding in H S containing medium 2

Table VIII

METAL

LIQUID

SCALE

Corrosion: Fe — Fe + 2e

H S - FeJ + 2® + FeS + H 2

2

2H = H + 2® +

2

{H S + 1/2 0 = H 0 + S + 2®} =

2

2

2

2e + 2® - € Fe + FeJ - € ++

Solid State Transfer a(Fep) ~~dx d(©) dx

Liquid Tronsfer dC + ti

D + u

1

°2

δ" s



312

CORROSION

CHEMISTRY

p r o d u c t l a y e r , n o dependency on f l o w r a t e s h o u l d be found. I t i s t h e r e f o r e n e c e s s a r y t o p o s t u l a t e a more com p l e x mechanism, such as i s suggested i n T a b l e 8. Since the c o r r o d i n g m e t a l i s covered by a c o r r o s i o n p r o d u c t l a y e r , the f o l l o w i n g r e a c t i o n s have t o t a k e p l a c e a t the m e t a l / s c a l e i n t e r f a c e : a. o x i d a t i o n o f i r o n ; b. the consumption o f e l e c t r o n h o l e s by e l e c t r o n s ( o r p o s s i b l y d i s c h a r g e o f p r o t o n s w i t h subsequent s o l u t i o n o f h y d r o gen i n the m e t a l ) ; c. the combination o f i r o n i o n v a c a n c i e s w i t h newly formed i r o n i o n s . The l a t t e r two p r o c e s s e s take p l a c e w i t h the r e l e a s e o f energy. These r e a c t i o n s n e c e s s i t a t e a c o n c e n t r a t i o n g r a d i e n t through the c o r r o s i o n p r o d u c t l a y e r o f i r o n i o n v a c a n c i e s and electron holes. ( I t i s u n d e r s t o o d t h a t the mechanism c o u l d be w r i t t e n i n terms o f i r o n i o n s moving i n t e r s t i t i a l l y , i n which case the s c a l e would be an e l e c t r o n conductor. T h i s mechanism seems l e s s l i k e l y how ever). S i n c e the c o r r o s i o n p r o c e s s t a k e s p l a c e w i t h f o r m a t i o n o f i r o n s u l f i d e the f o l l o w i n g r e a c t i o n s have t o take p l a c e a t the s c a l e l i q u i d i n t e r f a c e : a. f o r mation o f i r o n s u l f i d e and i r o n i o n v a c a n c i e s ; b. d i s charge of p r o t o n s t o form hydrogen and e l e c t r o n h o l e s ; c. i f oxygen i s p r e s e n t , the e l e c t r o n h o l e s can be formed by r e d u c t i o n o f oxygen. The f o r m a t i o n o f hy drogen from p r o t o n s a l s o n e c e s s i t a t e s a p r o t o n con c e n t r a t i o n g r a d i e n t i n the l i q u i d phase ( i f oxygen i s p r e s e n t , the c o r r e s p o n d i n g oxygen c o n c e n t r a t i o n g r a d i e n t would have t o e x i s t as w e l l ) . The r e a c t i o n s t a k i n g p l a c e a t the s c a l e l i q u i d i n t e r p h a s e can be v i s u a l i z e d as e q u i l i b r i u m r e a c t i o n s . T h i s means t h a t the s u l f i d e c o n c e n t r a t i o n i n f l u e n c e s the i r o n i o n vacancy c o n c e n t r a t i o n i n the s c a l e , and the p r o t o n concen t r a t i o n i n the l i q u i d a f f e c t s the e l e c t r o n h o l e con c e n t r a t i o n i n the s c a l e . S i n c e the n a t u r e of the s c a l e determines the r a t e o f the s o l i d s t a t e t r a n s f e r phenomena, and the f l o w r a t e determines the l i q u i d boundary l a y e r c o n c e n t r a t i o n g r a d i e n t , hence the l i q u i d - s o l i d i n t e r f a c i a l p r o t o n , H S and oxygen concen t r a t i o n s , i t f o l l o w s t h a t l i q u i d and s o l i d s t a t e mass and charge t r a n s f e r a r e l i n k e d t o g e t h e r by the c h e m i c a l e q u i l i b r i a e s t a b l i s h e d a t the s c a l e - l i q u i d i n t e r f a c e . An attempt was made t o f o r m u l a t e t h e s e r e l a t i o n s h i p s a n a l y t i c a l l y , i n o r d e r t o c o n f i r m the dependence o f c o r r o s i o n r a t e on f l o w r a t e observed i n F i g . 2 0 . Thus e q u a t i o n 23 r e s t a t e s the f o r m a t i o n o f i r o n i o n v a c a n c i e s and e l e c t r o n h o l e s a t the s c a l e / l i q u i d i n t e r face. HS £l-*Fe£ + 2 Φ + FeS + H (23) 2

v

2

2


9.

HAUSLER

Corrosion

Κ 2-**2H

2H+

6

2(FeS)


313

Inhibition

(24)

+ Fe"

= (H*) +

(25)

(Φ)

I t i s a l s o assumed, f o l l o w i n g Simkovich (2JL), t h a t p r o t o n s can d i s s o l v e i n t e r s t i t i a l l y i n the i r o n s u l f i d e l a t t i c e ( e q u a t i o n 24). The r e q u i r e m e n t f o r e l e c t r o n e u t r a l i t y i n the i r o n s u l f i d e l a t t i c e l e a d s t o e q u a t i o n 25. Assuming the r a t i o o f hydrogen p a r t i a l p r e s s u r e t o h y d r o g e n s u l f i d e p a r t i a l p r e s s u r e t o be c o n s t a n t , e q u a t i o n s 2 3 through 2 5 can be r e a r r a n g e d t o : 2

Κ

Kr

Ί

1

+

(H+)

(26)

(Θ)* and (27)

(Φ)' A + Β

+

(H )

i n d i c a t i n g t h e cube o f t h e e l e c t r o n h o l e c o n c e n t a t i o n t o be i n v e r s e l y p r o p o r t i o n a l t o t h e p r o t o n c o n c e n tration. A c c o r d i n g t o B e n n e t t and Meyers (40.) t h e mass f l u x a c c r o s s t h e l i q u i d boundary l a y e r can be f o r m u l a t ed as f o l l o w s : 1/3 Re}- (Sc) K"3 ( CH N H+ cath (28) = K (C^+-C +) (Urn) * 0

H +

4

W

P

c )

H

Ο

where (%+-C +) = P r o t o n g r a d i e n t i n l i q u i d boundary layer Re^ = Reynolds number Sc = S c h m i t t number Urn = Average f l o w v e l o c i t y K4 = combined c o n s t a n t s S i n c e i n the system i n v e s t i g a t e d o n l y the l i n e a r v e l o c i t y changes, a l l o t h e r v a r i a b l e s and c o n s t a n t s are lumped t o g e t h e r i n . Assuming now t h a t the e l e c t r o n i c c o n d u c t i v i t y o f the s c a l e i s the l i m i t i n g f a c t o r r a t h e r than the i o n i c one, the c a t h o d i c c u r r e n t can be s e t t o be p r o p o r t i o n a l t o the e l e c t r o n h o l e concen t r a t i o n times the e l e c t r o n h o l e m o b i l i t y : H

L

(29)

cath

S u b s t i t u t i n g e q u a t i o n 27

into equation

29


314


( i

and

cath catn

) 3

=*Φ — : — Φ A+B(H )

f i n a l l y with equation (i )3 cath " Q l

28 K

;

cu


(

3

0

)

+

· U 1/2 m " 4

0

t a

oç^2 ^" ^

n s

m

(31) B

icath

I t i s d i f f i c u l t t o estimate a l l the constants i n q u a t i o n 31 i n o r d e r t o a r r i v e a t a q u a n t i t a t i v e e v a l u a tion. However, s i n c e t h e c u r r e n t i n c r e a s e s w i t h i n c r e a s i n g mean v e l o c i t y , i t might be s a i d t h e e f f e c t s n e a r l y c a n c e l i n t h e denominator o f e q u a t i o n 3 1 . T h i s would mean t h a t i n d e e d t h e c a t h o d i c c u r r e n t becomes a l most p r o p o r t i o n a l t o t h e mean v e l o c i t y w i t h t h e 1/6 power. Even though t h i s argument i s n o t s t r i c t l y q u a n t i t a t i v e , i t s t i l l suggests a reasonable e x p l a n t i o n f o r t h e r a t h e r u n u s u a l mass t r a n s f e r e f f e c t o b s e r v e d i n F i g . 20. The above r a t i o n a l e a p p l i e s o n l y t o t h e c a t h o d i c c u r r e n t , however, a s i m i l a r d e r i v a t i o n can be used t o c a l c u l a t e t h e a n o d i c c u r r e n t dependence on flow r a t e . I n h i b i t o r I n t e r a c t i o n With The I r o n S u l f i d e S u r f a c e Since protons are discharged i n the c o r r o s i o n process, and s i n c e t h e s e p r o t o n s a r e f u r n i s h e d from t h e c o r r o s i v e medium, t h e r e must e x i s t some mechanism f o r p r o t o n a d s o r p t i o n on t h e i r o n s u l f i d e s u r f a c e . It is r e a s o n a b l e t o assume t h a t on t h e i r o n s u l f i d e s u r f a c e t h e r e a r e s u l f i d e g r o u p s , t h e v a l e n c e s o f which a r e not t o t a l l y n e u t r a l i z e d by i r o n m o l e c u l e s . These p a r t i a l l y n e u t r a l i z e d s u l f i d e c e n t e r s can adsorb p r o tons. This leads to the p o s t u l a t i o n of a protonated i r o n s u l f i d e s u r f a c e group: - F e S — H . This surface group f a c i l i t a t e s t h e d i s c h a r g e o f hydrogen a c c o r d i n g to equation 32. 2 JFeS - H ( - 2Θ z~*z2 JFe-S~| + H

2

(32)

where : Φ = e l e c t r o n h o l e ; p o s i t i v e charge In o r d e r t o stop t h e c o r r o s i o n p r o c e s s t h i s r e a c t i o n has t o be impeded. T h i s can be a c h i e v e d i f t h e p r o t o n adsorbed on t h e s u r f a c e c o n t a i n i n g t h e i r o n s u l f i d e s p e c i e s i s r e p l a c e d by a c a t i o n , t h e r e d u c t i o n o f which i s n o t as e a s i l y a c c o m p l i s h e d as t h e r e d u c t i o n of t h e p r o t o n . I t i s s u g g e s t e d t h a t t h e a l k y l ammonium


9.

HAUSLER

Corrosion

315

Inhibition

i o n can d i s p l a c e the p r o t o n v i a an i o n exchange mechanism d e s c r i b e d i n équation 3 3 , the e q u i l i b r i u m cons t a n t o f t h i s r e a c t i o n b e i n g d e s i g n a t e d as K^: JFeS-Hf +


where : > FeS-RNH^f

RNH^

^=^5JFeS-RNH | + H

(33)

+

3

= surface

complex

I t s h o u l d be noted t h a t t h i s i s not a simple ads o r p t i o n o f the a l k y l ammonium i o n on the s u r f a c e of the i r o n s u l f i d e but can be c o n s i d e r e d as an i o n i c r e a c t i o n i n which a p r o t o n i s b e i n g s e t f r e e . I t i s now r e a s o n a b l e t o c o n s i d e r the f a c t o r s which may i n f l u e n c e the c o n c e n t r a t i o n o f t h e s u r f a c e complex formed from i r o n s u l f i d e and the a l k y l ammonium i o n . Going t h r o u g h a simple s e r i e s o f e q u i l i b r i u m c a l c u l a t i o n s , i t i s found t h a t the c o n c e n t r a t i o n o f the s u r f a c e comp l e x e q u a l s the p r o d u c t o f a s e r i e s o f e q u i l i b r i u m cons t a n t s times the p r o t o n c o n c e n t r a t i o n o f the medium, as shown i n e q u a t i o n 34. (JFeS-RNH |) 3

=

K

4

· K

±

· K

+

2

· (H )

· C

N

' C

s

(34)

where : Κ K

l

κ

2

+

(H )

c

N

formation constant d i s s o c i a t i o n constant of surface d i s s o c i a t i o n c o n s t a n t o f amine proton concentration c o n c e n t r a t i o n of amine c o n c e n t r a t i o n of s u r f a c e - s u l f i d e JFe-S"|

+

FeS-H

JFe-SH|

The two main f a c t o r s which a f f e c t the concent r a t i o n o f the s u r f a c e complex are i t s f o r m a t i o n cons t a n t K4 and the pH. The l a t t e r has been shown t o be true experimentally (Table 9). C h e m i s t r y Of C o r r o s i o n I n h i b i t o r s . There are two q u e s t i o n s which a r i s e w i t h r e s p e c t t o . These are 1. How can the f o r m a t i o n c o n s t a n t o f the s u r f a c e comp l e x be i n c r e a s e d ? 2. Why i s n ' t the a d s o r p t i o n p r o c e s s r e v e r s i b l e ? That i s t o say, why do s m a l l c o n c e n t r a t i o n s o f amine (0.25 ppm) l e a d t o l a r g e s u r f a c e coverage and consequently large i n h i b i t o r e f f i c i e n c y ? A l t h o u g h water m o l e c u l e s do not appear i n Equat i o n 34, i t i s n e v e r t h e l e s s r e a s o n a b l e t o assume t h a t


316


T a b l e IX

Concentration

Inhibitor 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

Armand Kathy

pH 4.2-4. 5 2.5 2.5-3

Norman Dora

i n ppm f o r 90-95% P r o t e c t i o n

Talbot

3-4 0.3-0. 5

pH 6-6.5

pH 7-7.2

10-15

20

10

20

7-10

20

10-15

20

P e t e r E.

0.2

n. a.

Conny E.

0.25-0. 5

2

5

Teddy E.

0.2-0. 3

2

2-3

n. a.

the i r o n s u l f i d e s u r f a c e i s c o v e r e d w i t h a l a y e r o r more o f water m o l e c u l e s , because t h e p r o t o n adsorbed on t h e .surface i r o n s u l f i d e has a tendency t o d i s s o c i a t e and, t h e r e f o r e , needs t o be s o l v a t e d . The f r e e a l k y l ammonium i o n and i t s c o u n t e r i o n , e.g.., a b i s u l f i d e i o n , a r e both s o l v a t e d by water. In the adsorpt i o n s t e p an e l e c t r o n i c a l l y n e u t r a l s u r f a c e complex i s formed. Thus, a p r o t o n s o l v a t e d by water and i t s c o u n t e r i o n , t h e b i s u l f i d e i o n , a r e s e t f r e e . The e s s e n t i a l l y n e u t r a l s u r f a c e complex, does n o t r e q u i r e s o l v a t i o n and because o f i t s h y d r o p h o b i c n a t u r e w i l l have a tendency t o d i s p l a c e water m o l e c u l e s from t h e iron s u l f i d e surface. Since the desorption process would a g a i n r e q u i r e s o l v a t i o n o f t h e i r o n s u l f i d e s u r f a c e , i t can no l o n g e r t a k e p l a c e because water molec u l e s a r e permanently d i s p l a c e d from t h e s u r f a c e by the h y d r o p h o b i c n a t u r e o f t h e s u r f a c e complex. As a consequence i t can be p r e d i c t e d t h a t an i n h i b i t o r w i t h a h i g h e r l i p o p h i l i c c h a r a c t e r w i l l be more permanently adsorbed. I t i s known t h a t amines become more l i p o p h i l i c , or water i n s o l u b l e , w i t h i n c r e a s i n g a l k y l c h a i n l e n g t h . Thus water s o l u b l e amines such as m o r p h o l i n e ( T a b l e l O ) show no i n h i b i t i o n e f f i c i e n c y a t t h e s e s m a l l concent r a t i o n s because t h e i r a d s o r p t i o n i s as easy as t h e i r desorption.



Cll

Alkyl_

0 0

5 10

0 81 97

0.5 2 5

30 100

0.5 1

0

56

97

99

100

0.125

0.25

0.5

0.75

Morpholine Concen P r o t e c tion tration % ppm

ft/sec

0.062

Propylenediamine Concen P r o t e c tion tration % ppm

7

2

HS

Protec tion %

1 5

C Alkyl^ Propylenediamine Concen P r o t e c tion tration % ppm

Toluene-Water, S a t u r a t e d w i t h 2 00 ppm C I " 70°C Carbon S t e e l , l i n e a r v e l o c i t y

Concen tration ppm

Tallow Propylenediamine

Conditions :

E f f e c t i v e n e s s of Various A l k y l Propylenediamines and Water S o l u b l e Amines as P r o c e s s C o r r o s i o n I n h i b i t o r s


χ

H

ο*

-α

CO h-*

δ*

g-

ο

ο

ε

cl

>


318

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.



9. HAUSLER

Corrosion Inhibition

319

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Corrosion Chemistry - American Chemical Society

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