Polymeric Materials for Corrosion Control - American Chemical Society

2 Alternating Current Impedance and Underfilm Darkening Studies on Acidic Water-Based Anticorrosive Paints

Downloaded by UNIV OF ARIZONA on June 8, 2017 | http://pubs.acs.org Publication Date: October 14, 1986 | doi: 10.1021/bk-1986-0322.ch002

P. J. Moreland and J. C. Padget Imperial Chemical Industries PLC, Mond Division, Technical Department, P.O. Box 8, The Heath, Runcorn, Cheshire, England

The protective properties and interface reactions on mild steel substrates of an acidically formulated (pH5) water-borne paint based upon a chlorine containing vinyl acrylic copolymer have been examined using a variety of techniques. Traditional electrochemical polarisation curves as well as ac impedance studies were used to investigate the corrosion process in "wet" formulations associated with the occurrence or absence of "flash rusting". Investigation of an underfilm darkening phenomenon observed upon exposure testing of some similarly formulated coatings and associated with excellent long term protective performance are also presented. An arrest in a corrosion process, after some period involving insignificant metal loss was observed, evidenced the formation of a protective interface film. The characterisation of the film showed that its properties were in accord with the recognised protective performance of the coating system. An a c r y l a t e m o d i f i e d v i n y l c h l o r i d e - v i n y l i d e n e c h l o r i d e l a t e x copolymer ( H a l o f l e x 202) has been developed i n our l a b o r a t o r y (1,2) s p e c i f i c a l l y f o r t h e p r e p a r a t i o n o f water-borne a n t i - c o r r o s i v e primer paints. This c a r e f u l l y designed copolymer, hereafter r e f e r r e d t o as a c h l o r i n e - c o n t a i n i n g v i n y l a c r y l i c copolymer, e x h i b i t s a low water vapour p e r m e a b i l i t y (detached f i l m ) o f a p p r o x i m a t e l y 100 f o l d l e s s than t h a t o f t y p i c a l a c r y l i c l a t e x polymers intended f o r t h e p r e p a r a t i o n o f a n t i - c o r r o s i v e p r i m e r s , and when f o r m u l a t e d i n t o paint i s capable o f g i v i n g e x c e l l e n t a n t i - c o r r o s i v e performance on smooth and b l a s t e d s t e e l . C h l o r o p o l y m e r l a t i c e s d i f f e r from o t h e r l a t i c e s i n t h a t they undergo a d e h y d r o c h l o r i n a t i o n r e a c t i o n a t a l k a l i n e pH; t h e h i g h e r the pH t h e h i g h e r t h e r a t e o f d e h y d r o c h l o r i n a t i o n . Thus when such a l a t e x i s formulated i n t o a p a i n t a t t h e t y p i c a l p a i n t pH (7-9) 0097-6156/86/0322-0018S06.00/ 0 © 1986 American Chemical Society

Dickie and Floyd; Polymeric Materials for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


2.

MORELAND AND

PADGET

Acidic Water-Based Anticorrosive Paints

19

t h e r e i s a downward d r i f t i n pH and an i n c r e a s e i n c h l o r i d e i o n concentration i n the aqueous phase. A l t h o u g h the r a t e of d e h y d r o c h l o r i n a t i o n can be reduced by r e d u c i n g the c h l o r i n e c o n t e n t o f the polymer, i n our e x p e r i e n c e t h i s r e d u c t i o n i s a t the expense o f both the b a r r i e r p r o p e r t i e s and the a n t i - c o r r o s i v e performance. We have shown t h a t the r a t e o f d e h y d r o c h l o r i n a t i o n o f the h i g h c h l o r i n e content copolymer i s n e g l i g i b l y s m a l l at pH ^ 4.5. A c c o r d i n g l y a c i d i c p a i n t f o r m u l a t i o n s were developed (1,3) (see A p p e n d i x ) , which e x h i b i t e d v e r y l i t t l e change i n pH or c h l o r i d e i o n concentrations during storage. Such a c i d i c paint fomulations require the dispersed components (ie polymer and pigment p a r t i c l e s ) t o be s t r o n g l y s t e r i c a l l y s t a b i l i s e d i f they are t o remain c o l l o i d s t a b l e . Ethylene oxide-propylene-ethylene oxide b l o c k copolymers were found t o be p a r t i c u l a r l y s u i t a b l e as they i n c r e a s e d the r a t e and e x t e n t o f l a t e x p a r t i c l e c o a l e s c e n c e due t o a s u r f a c e p l a s t i c i z a t i o n effect(£), w i t h o u t downgrading the b a r r i e r p r o p e r t i e s or a n t i - c o r r o s i v e performance. We r e c e n t l y r e p o r t e d an ac impedance study on t h i s system (H). A l t h o u g h a c i d i c p a i n t f o r m u l a t i o n s based on the c h l o r i n e - c o n t a i n i n g vinyl acrylic l a t e x copolymer give excellent anti-corrosive performance, they do e x h i b i t two u n u s u a l f e a t u r e s not p r e s e n t i n the c o r r e s p o n d i n g a l k a l i n e f o r m u l a t i o n s : a)

The e x t e n t o f f l a s h r u s t i n g on g r i t b l a s t e d m i l d s t e e l d u r i n g t h e "wet" f i l m c o n d i t i o n p r o g r e s s i v e l y decreases with d e c r e a s i n g pH, b e i n g e x t e n s i v e a t pH ^7 and b e i n g very s l i g h t a t pH ' 10 ohm on 10 cm , but the apparent f i l m r e s i s t a n c e decreased upon exposure and e x h i b i t e d a minimum between 9 and 23 days exposure. The f i l m ' s b e h a v i o u r can be m o d e l l e d as a Randies e q u i v a l e n t RC c i r c u i t i n which i o n i c f i l m r e s i s t a n c e R d e c r e a s e d and f i l m c a p a c i t a n c e C i n c r e a s e d w i t h exposure time up t o 9 days. The depressed semi c i r c u l a r b e h a v i o u r at 4 and 9 days i n d i c a t e d a d i s p e r s i o n i n the time c o n s t a n t RC f o r the f i l m . Between 9 and 23 days i t i s e v i d e n t t h a t the f i l m r e s i s t a n c e i n c r e a s e d and c o n t i n u e d t o do so up t o the 51 day measurement. S i m i l a r b e h a v i o u r was o b t a i n e d f o r t h i s t y p e o f c o a t i n g under c o n s t a n t immersion c o n d i t i o n s ( F i g u r e 8) though the r e c o v e r y t o h i g h e r impedance at 51 days was not as marked. In a l l c a s e s , even at minimum impedance v a l u e s the f i l m r e s i s t a n c e was h i g h at > 5 χ 1 0 ohms on u n i t a r e a and compared f a v o u r a b l y w i t h the r e s i s t a n c e c r i t e r i o n f o r a p r o t e c t i v e c o a t i n g ( 9 ) . 8

2

7




M O R E L A N D A N D PADGET

F i g u r e 4.

F o i l r e s i s t a n c e changes-constant immersion 3% N a C l .


26

P O L Y M E R I C M A T E R I A L S FOR CORROSION

CONTROL


16Γ4

10

0

20

30

40

5

0

Time, days

F i g u r e 5.

F o i l r e s i s t a n c e changes-alternate

immersion i n 3% NaCl.

0 13

5.2

"» • 0

10

F i g u r e 6.

20

Time, days

cT

30

Mean c o r r o s i o n r a t e - t i m e

40

curves.


J 50

M O R E L A N D A N D PADGET


27


2.

9 Days

23 Days

51 Days

10 cm

F i g u r e 8.

Nyquist p l o t s - c h l o r i n e containing v i n y l c o a t i n g c o n s t a n t l y immersed i n 3% NaCl.

acrylic


Area

P O L Y M E R I C M A T E R I A L S FOR CORROSION C O N T R O L

28


The i n i t i a l decrease i n i o n i c f i l m r e s i s t a n c e and i n c r e a s e i n c a p a c i t a n c e can be a s s o c i a t e d w i t h e i t h e r NaCl e l e c t r o l y t e o r water e n t r y i n t o t h e f i l m . From ER measurements t h i s p e r i o d i s a s s o c i a t e d w i t h a m e t a l l o s s p r o c e s s a t t h e s u b s t r a t e s u r f a c e . However, between 9 t o 23 days t h e i o n i c f i l m r e s i s t a n c e i n c r e a s e s , which i s a s s o c i a t e d w i t h an a r r e s t i n m e t a l l o s s a t t h e s u b s t r a t e s u r f a c e i n ER measurements. I t a p p e a r s , t h e r e f o r e , t h a t w i t h t h e knowledge o f an underfilm darkening phenomenon occurring at the substrate/coating i n t e r f a c e , a f i l m of a protective ( i e passive or h i g h i o n i c r e s i s t a n c e ) n a t u r e i s produced d u r i n g exposure. As shown i n F i g u r e 9 t h e impedance o f t h e a c r y l i c coating i m m e d i a t e l y showed low v a l u e s which d i d n o t i n c r e a s e . The c o a t i n g showed marked r u s t i n g and e x f o l i a t i o n . C h l o r i n a t e d rubber c o a t i n g s maintained a high impedance similar to that of the c h l o r i n e - c o n t a i n i n g v i n y l - a c r y l i c c o a t i n g s though t h e development o f a p i n h o l e a f t e r l o n g exposure l e d t o a lower impedance as shown i n F i g u r e 9. The t h r e e c o a t i n g systems were a l s o exposed t o h o t s a l t s p r a y . I n t h i s c a s e , i t appeared t h a t t h e minimum impedance o f t h e c h l o r i n e - c o n t a i n i n g v i n y l - a c r y l i c coating occurred w i t h i n the f i r s t 5 hours exposure and t h e r e a f t e r t h e impedance remained h i g h (>10 ohms). T h i s b e h a v i o u r i s p r o b a b l y due t o f a s t e n t r y o f e l e c t r o l y t e and/or water i n t o t h e f i l m under t h e more a g g r e s s i v e c o n d i t i o n s t o form an i n t e r f a c e f i l m . As i n p r e v i o u s experiments t h e a c r y l i c c o a t i n g had low impedance (10 ohm) unless a pinhole developed. 7

8

Acrylic

2E9

2E8

2E7 10 cnr Area

F i g u r e 9.

N y q u i s t - a c r y l i c and c h l o r i n a t e d rubber a l t e r n a t i v e l y immersed i n 3% NaCl.

coatings


2.

MORELAND AND

PADGET

29


Chemical Characterisation. Chemical characterisation of the u n d e r f i l m d a r k e n i n g beneath c h l o r i n e - c o n t a i n i n g v i n y l - a c r y l i c f i l m s was sought u s i n g a v a r i e t y o f t e c h n i q u e s on the s u b s t r a t e s u r f a c e as w e l l as the b a c k s i d e o f s t r i p p e d f i l m s . The s u b s t r a t e s were g r i t b l a s t e d and p l a i n m i l d s t e e l Q p a n e l s exposed up t o 98 days i n hot s a l t spray and examined w i t h i n hours o f r e m o v a l . XRD i d e n t i f i e d Fe 0 p l u s p a i n t c o n s t i t u e n t s o f BaSO , T i 0 and Zn (?0 ) t o g e t h e r w i t h a l e s s i d e n t i f i a b l e major phase (7.7A, 2.68A, 2.36A). T h i s phase i s now b e l i e v e d t o be a member o f a c l a s s o f compounds r e f e r r e d t o as the p y r o a u r i t e group. These compounds have the g e n e r a l f o r m u l a : 3

3

li

Μ


h

u

2

2

χ

R

y

(0H)

2 x + 3 y

_

2 z

(A*-) .2H 0 Z

2

where M i s a d i v a l e n t c a t i o n , R i s a t r i v a l e n t c a t i o n and A i s an a n i o n , commonly C 0 ^ ~ , but can be OH" C I " and p o s s i b l y o t h e r s ( 1 0 ) . The members of t h i s group form hexaganol p l a t e type c r y s t a l s of w h i c h p y r o a u r i t e has t h e f o r m u l a Mg F e (0H) (CO)^ 3 H 0. The l a y e r s t r u c t u r e of such compounds a l l o w s the accommodation o f a v a r i e t y of a n i o n s and cations (1_0) and the l a r g e number of h y d r o x y l groups may p r o v i d e a b u f f e r i n g c a p a c i t y . T h i s b u f f e r i n g c a p a c i t y has been r e c o g n i s e d as a p r o b a b l e i n f l u e n t i a l f a c t o r i n the p r o t e c t i v e p r o p e r t y afforded by s i m i l a r h y d r o t a l c i t e type f i l m s on aluminium i n sea water ( JJ_). The b u f f e r i n g p r o p e r t i e s c o u l d a l s o c o n s t r a i n l o c a l i s e d a t t a c k and promote l a t e r a l movement o f the c o r r o s i o n p r o c e s s and f i l m f o r m a t i o n . 2

6

2

1 6

2

SEM s t u d i e s s u p p o r t e d the above i n o b s e r v a t i o n o f p l a t e l e t type c r y s t a l s c o n t a i n i n g Fe and CI (by EDAX). ESCA r e v e a l e d little d e t a i l but SIMS i d e n t i f i e d a number of hydroxy and_oxychloride s p e c i e s i n c l u d i n g Fe(0H)+, Fe0+, FeO-, FeOCl- and F e C l ^ t o s u p p o r t the presence of c h l o r i d e i n the p y r o a u r i t e type f i l m . LIMA i n d i c a t e d a number Fe 0 + peaks w i t h χ as y as h i g h as 2 or 3 χ y whereas χ and y are g e n e r a l l y 2 f o r FeOOH. Conclusions F l a s h r u s t i n g e x h i b i t e d i n n e u t r a l t o a l k a l i n e water borne formulations appears t o occur t h r o u g h a l o c a l i s e d corrosion p r o c e s s p r o b a b l y i n v o l v i n g g r i t " a c t i v i t y " p r e s e n t from b l a s t i n g , e i t h e r d i r e c t l y or i n d i r e c t l y , i n an e l e c t r o c h e m i c a l p r o c e s s . At such pH the r a p i d o x i d a t i o n o f f e r r o u s t o f e r r i c i o n produces intense l o c a l p r e c i p i t a t i o n of f e r r i c hydroxide evidenced as f l a s h - r u s t s p o t s . The p r o c e s s can be e l i m i n a t e d by f o r m u l a t i n g at a lower pH, eg pH 4.5 which g i v e s r i s e t o a u n i f o r m c o r r o s i o n p r o c e s s at the s u b s t r a t e s u r f a c e . I t has been shown t h a t a c h l o r i n e c o n t a i n i n g v i n y l a c r y l i c c o a t i n g can be s a t i s f a c t o r i l y f o r m u l a t e d at t h i s pH. Under t h i s c o n d i t i o n , o x i d a t i o n t o f e r r i c i o n w i t h subsequent p r e c i p i t a t i o n does not o c c u r and hence f l a s h r u s t i n g i s not o b s e r v e d . I t appears t h a t under a g g r e s s i v e c o r r o s i v e c o n d i t i o n s e l e c t r o l y t e may enter the f i l m and s t i m u l a t e c o r r o s i o n but the c h l o r i n e


30

POLYMERIC M A T E R I A L S FOR CORROSION C O N T R O L

c o n t a i n i n g v i n y l a c r y l i c c o a t i n g q u i c k l y promotes f o r m a t i o n o f a p r o t e c t i v e f i l m , as e v i d e n t by ER, ac impedance and i r o n p i c k u p measurments, w i t h i n s i g n i f i c a n t o v e r a l l m e t a l l o s s . Indeed, ac impedance measurements on t y p i c a l s u b s t r a t e s u r f a c e s i n d i c a t e d f i l m r e s i s t a n c e s t o remain v e r y h i g h even d u r i n g f o r m a t i o n o f t h e interface film. The l o n g term protective f i e l d performance behaviour o f such c o a t i n g r e f l e c t s the p r o t e c t i v e c h a r a c t e r o f t h i s system. However, water borne systems f o r m u l a t e d a t pH 7-9 eg c o n v e n t i o n a l a c r y l i c s a r e both capable o f p r o d u c i n g f l a s h r u s t i n g d u r i n g c o a t i n g and i n c a p a b l e o f p r o d u c i n g i n - s i t u p r o t e c t i v e f i l m s i n the presence o f c o r r o s i v e e n v i r o n m e n t s .


Literature Cited 1 2 3 4 5 6 7 8 9 10 11

Burgess, A J; Caldwell, D; Padget, J C.; JOCCA, 1981, 64, 175. US Patent 4,341,679. European Patent 0,035,316. Padget, J C.; Moreland, Ρ J; J Coating Technol, 1983, 55, 39. Piens, M; Verbist, R; In "Corrosion Control by Organic Coatings", NACE, Houston, Texas, 1980; p 163. Dravnieks, A; Cataldi, H A; Corrosion, 1954, 10, 224. Sykes, J; Lewis, G; unpublished work at Oxford University. Minegishi, T; Asaki, Z; Higuchi, B; Konds, Y; Met Trans B, 1983, 14B, 17. Bacon, R C; Smith J J; Rugg, F M; Ind Eng Chem, (1948), 40, 161. Taylor, H F W; Mineralogical Mag, 1973, 39, 377. Austing, C E; Pritchard, A M; Wilkins, Ν J M; Desalination, 1973, 12, 251.

Appendix Primer Latex

Formulation

Based on C h l o r i n e C o n t a i n i n g V i n y l Acrylic

Ingredient V i n y l A c r y l i c Copolymer ( H a l o f l e x 202)

% w/w

59.7

N o n - i o n i c b l o c k copolymer 3.1 surfactant

Ingredient

% w/w

M i c r o n i s e d Z i n c Phosphate

5.8

Micronised barytes Titanium dioxide

H y d r o x y - p r o p y l methyl cellulose

0.2

Water

De-foamer

0.2

pH

Butyl glycol

2.0

Pigment volume concentration

15.6 2.6 10.8

5

20

The name Haloflex is a trademark, the p r o p e r t y o f I m p e r i a l Chemical I n d u s t r i e s PLC. RECEIVED March 5, 1986


Polymeric Materials for Corrosion Control - American Chemical Society

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