Effects of Corrosive Environments on the Locus and Mechanism of

17

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 4, 2016 | http://pubs.acs.org Publication Date: October 14, 1986 | doi: 10.1021/bk-1986-0322.ch017

Effects of Corrosive Environments on the Locus and Mechanism of Failure of Adhesive Joints J. W. Holubka, W. Chun, A. R. Krause, and J. Shyu Polymer Science Department, Ford Motor Company, Dearborn, MI 48121

The effects of corrosive environments on the locus and mechanism of failure of adhesive bonds has been investigated. Adhesive chemistry resistant to degradation and swelling by water and corrosion products form the most durable adhesive bonds. The nature of the chemistry at the adherend/adhesive interface plays an important role in determining the durability of an adhesive bond in a corrosive environment. The durability of adhesive bonds to metal (steel) adherends is greatly influenced by the surface chemistry of the metal. For adhesive systems that employ adhesive primers for the metal adherend prior to bonding, the durability of the adhesive bond is largely controlled by the corrosion resistance of the primer. Surface analysis of adhesive bonds which failed after exposure to corrosive environments suggests that the degradation of interfacial layer is one of the principal modes of bond failure. The extent of adhesive bond failure under corrosive environments is greatly accelerated when cyclic mechanical stresses are imposed on the adhesive bond during exposure. Three to four orders of magnitude reduction in fatigue life of adhesive bonds is observed for bonds exposed to environment prior to fatigue testing. A l t h o u g h numerous s t u d i e s (1-3) have d e s c r i b e d work aimed a t e s t a b l i s h i n g c r i t e r i a f o r the d u r a b i l i t y o f a d h e s i v e j o i n t s , a thorough u n d e r s t a n d i n g o f e f f e c t s o f the c h e m i c a l and m e c h a n i c a l p r o p e r t i e s , on the d u r a b i l i t y o f a d h e s i v e bonds i s l a c k i n g . More s p e c i f i c a l l y , the e f f e c t s o f s u r f a c e p r e p a r a t i o n and dynamic l o a d i n g , e s p e c i a l l y under e n v i r o n m e n t a l s e r v i c e c o n d i t i o n s , has n o t been e x p l o r e d i n d e t a i l f o r automotive s t r u c t u r e s . I n t h i s paper, a d e s c r i p t i o n o f the e f f e c t s o f environment on the d u r a b i l i t y o f a d h e s i v e bonds i s p r e s e n t e d . P a r t i c u l a r a t t e n t i o n i s g i v e n t o 0097-6156/86/0322-0194$06.00/0 © 1986 American Chemical Society

In Polymeric Materials for Corrosion Control; Dickie, Ray A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

17.

HOLUBKAETAL.

Locus and Mechanism of Failure of Adhesive Joints

195

c o r r e l a t i n g the e f f e c t s o f a d h e s i v e c h e m i s t r y w i t h d u r a b i l i t y o f the bond. The e f f e c t s o f environment, w i t h and w i t h o u t l o a d i n g o f the bond a r e a l s o d e s c r i b e d .


Experimental M a t e r i a l s . The a d h e s i v e s and p r i m e r s u s e d i n t h i s s t u d y were model and commercial m a t e r i a l s t h a t were c u r e d a c c o r d i n g t o c o n d i t i o n s a p p r o p r i a t e f o r the s p e c i f i c a d h e s i v e c h e m i s t r y . A d h e s i v e s A and Β were c o n v e n t i o n a l epoxy/Versamid and e p o x y / d i c y a n d i a m i d e a d h e s i v e s , r e s p e c t i v e l y . A d h e s i v e s C and D were commercial u r e t h a n e and epoxy/polyamide a d h e s i v e s , r e s p e c t i v e l y . A d h e s i v e Ε was a conven t i o n a l t w o - p a r t epoxy/amidoamine a d h e s i v e . A d h e s i v e F was a v i n y l p l a s t i s o l a d h e s i v e . The a d h e s i v e p r i m e r s used i n t h i s s t u d y were a u r e t h a n e c r o s s l i n k e d epoxy e l e c t r o c o a t p r i m e r and s p r a y p r i m e r s b a s e d on t a l l o i l m o d i f i e d epoxy e s t e r , and p o l y e s t e r p o l y o l / i s o c y a n a t e c h e m i s t r y . D i c y a n d i a m i d e was o b t a i n e d from A l d r i c h C h e m i c a l Company. Epon 828 was o b t a i n e d from S h e l l C h e m i c a l Company. Genamid 250 and V e r s a m i d 115 were o b t a i n e d from H e n k e l Company. The s t e e l s u b s t r a t e s used were c o l d r o l l e d s t e e l and B o n d e r i t e 40 phosphated s t e e l from P a r k e r . The composite was s h e e t m o l d i n g compound from R o c k w e l l . A d h e s i v e Bonding Technique. S t a n d a r d p r o c e d u r e s f o r p r e p a r i n g a d h e s i v e bond specimens were used. The composite was i n i t i a l l y sanded w i t h 240 g r i t emery paper and t h e n t h o r o u g h l y r i n s e d w i t h methylene c h l o r i d e . S t e e l s u b s t r a t e s were r i n s e d t h o r o u g h l y w i t h methylene c h l o r i d e . Bonds were p r e p a r e d as one i n c h o v e r l a p shear specimens; bond t h i c k n e s s was 0.76 mm. Bond t h i c k n e s s was d e f i n e d u s i n g 1.5 mm l o n g w i r e s o f t h e a p p r o p r i a t e t h i c k n e s s . X-Rav P h o t o e l e c t r o n S p e c t r o s c o p y . S p e c t r a were o b t a i n e d u s i n g a Leybold-Heraeus LHS-10 S p e c t r o m e t e r u s i n g a Mg Κα anode. A l l b i n d i n g e n e r g i e s f o r the o b t a i n e d s p e c t r a were r e f e r e n c e d t o the h y d r o c a r b o n a l k a n e l i n e a t 285.0 eV. F a t i g u e T e s t i n g . F a t i g u e t e s t i n g was c o n d u c t e d u s i n g an MTS c l o s e d l o o p e l e c t r o h y d r a u l i c t e s t machine t h a t was o p e r a t e d i n l o a d c o n t r o l , t e n s i o n - t e n s i o n mode. Loads i n t h i s e x p e r i m e n t ranged from a minimum o f 10% o f the maximum l o a d t o maximum l o a d s o f 200-700 psi. C o r r o s i o n T e s t i n g . S a l t s p r a y t e s t i n g (ASTM-B-117-62,64) was used t o d e t e r m i n e d u r a b i l i t y o f a d h e s i v e bond i n c o r r o s i v e environment. Lap s h e a r samples were exposed t o s a l t s p r a y f o r 14 days and t h e n i m m e d i a t e l y t e s t e d f o r l a p shear s t r e n g t h . R e s u l t s and D i s c u s s i o n Locus and Mechanism o f A d h e s i o n F a i l u r e d u r i n g C o r r o s i o n E f f e c t s o f A d h e s i v e C h e m i s t r y . I n p r e v i o u s s t u d i e s (4-6_) on t h e c o r r o s i o n i n d u c e d a d h e s i o n l o s s o f c o a t i n g s from s t e e l s u r f a c e s , a p r i m a r y mechanism f o r c o a t i n g d e a d h e s i o n was polymer d e g r a d a t i o n a t the c o a t i n g / m e t a l i n t e r f a c e by c o r r o s i o n r e a c t i o n s t h a t g e n e r a t e hydroxide i o n :


POLYMERIC MATERIALS FOR CORROSION CONTROL

196

Fe -> F e +

2e" + H 0+ 1/2 0


2

2

2

+ 2e~

- 20H'

The e x t e n t o f c o a t i n g a d h e s i o n f a i l u r e was found t o be dependent upon t h e r e s i s t a n c e o f t h e polymer i n t h e c o a t i n g t o h y d r o l y s i s by c o r r o s i o n generated hydroxide. I n t h i s s t u d y , s i m i l a r t r e n d s have been o b s e r v e d f o r a d h e s i v e s . T a b l e I shows t h e r e s u l t s o f s a l t s p r a y c o r r o s i o n on a s e r i e s o f bonds between c o l d r o l l e d s t e e l adherends and a d h e s i v e s o f v a r y i n g c h e m i s t r y . The r e s u l t s show t h a t t h e r e i s a d i r e c t c o r r e l a t i o n between t h e c h e m i s t r y o f t h e a d h e s i v e polymer and t h e d u r a b i l i t y o f t h e s e r i e s o f a d h e s i v e bonds s t u d i e d . The l o c u s o f a d h e s i o n f a i l u r e a l s o appears t o be r e l a t e d t o t h e type of adhesive chemistry. I n t h i s study, a d h e s i v e s b a s e d on polymers h a v i n g a wide range o f h y d r o l y s i s r e s i s t a n c e were examined. A d h e s i v e s b a s e d on h y d r o l y s i s r e s i s t a n t c h e m i s t r y ( i . e . , a d h e s i v e s D and E) show a h i g h r e t e n t i o n o f i n i t i a l p r o p e r t i e s a f t e r exposure to an a g g r e s s i v e c o r r o s i o n environment and t h e f a i l u r e o c c u r s c o h e s i v e l y w i t h i n the adhesive. The c u r e r e a c t i o n s o f t h e s e a d h e s i v e s i n v o l v e t h e f o r m a t i o n o f h y d r o l y s i s r e s i s t a n t carbonn i t r o g e n bonds i n r e a c t i o n s i n v o l v i n g t h e f r e e N-H f u n c t i o n a l i t y o f the V e r s a m i d o r Genamid h a r d e n e r w i t h t h e o x i r a n e f u n c t i o n a l i t y o f the epoxy r e s i n t h a t i s p r e s e n t i n t h e a d h e s i v e f o r m u l a t i o n :

0 R

1

- NH

2

R' - NH

2

R - C - NH^

+

0

/ \ C H - CH - C H 2

OH

2

- R" - C H

,,

2

2

OH

I I R ' -N' -CH -CH-CH -R -CH -CH-CH -N-R R - C - NH ^NH ^R - Ν - Ν - R' X

2

/ \ - CH - C H

2

,

2

2

ο II - C - R

I n c o n t r a s t , a d h e s i v e s b a s e d on h y d r o l y s i s prone c h e m i s t r y ( i . e . , a d h e s i v e s C, F and G where u r e t h a n e and e s t e r l i n k a g e s a r e formed d u r i n g c r o s s l i n k i n g ) a r e degraded by c o r r o s i o n g e n e r a t e d h y d r o x i d e and s u b s e q u e n t l y show s i g n i f i c a n t r e d u c t i o n s i n a d h e s i v e bond durability. S a p o n f i c a t i o n r e a c t i o n s are the p r i n c i p a l degradation p r o c e s s e s f o r t h e p o l y e s t e r and p o l y u r e t h a n e b a s e d a d h e s i v e s . E v i d e n c e from i n t e r f a c i a l a n a l y s i s o f s u r f a c e s g e n e r a t e d as a r e s u l t o f a d h e s i v e bond f a i l u r e , which we w i l l see l a t e r i n t h i s report, indicates that degradative processes consistent with h y d r o l y s i s r e a c t i o n s i n v o l v i n g c o r r o s i o n generated hydroxide i o n c o n t r i b u t e s i g n i f i c a n t l y t o bond f a i l u r e . These p r o c e s s e s r e s u l t i n the f o r m a t i o n o f h y d r o p h i l i c i o n i c and p o l a r p r o d u c t s a t t h e i n t e r f a c e between t h e a d h e s i v e and t h e m e t a l s u b s t r a t e :

0

0

Il R-O-C-R' + OH'

H -> ROH + "O-C-R'

0 II R-O-C-NH-R' + 20H"

-

ROH + CO3- + NH

2

- R'


17.


HOLUBKA ET AL.

Table I.


Adhesive

Adhesive

Structure/Property Studies S t r e n g t h (MPa) Initial S a l t Spray (14 Days)

Chemistry

Locus o f Failure

A

Epoxy/Dicy

11.,96

5.,21

Cohesive

Β

Epoxy/Dicy

16..24

8.,23

Cohesive

C

Urethane

8..04

3.,69

Adhesive

D

Epoxy/Polyamide

9,.62

7..55

90% C o h e s i v e 10% A d h e s i v e

Ε

Epoxy/Amidoamine

7..93

7,.55

Cohesive

F

Vinyl

6,.27

0,.69

Adhesive

G

Acrylic

8 .96

1 .03

Adhesive

Plastisol

The p r e s e n c e o f t h e s e i o n i c m a t e r i a l s a t the i n t e r f a c e , as w e l l as the c o r r e s p o n d i n g r e d u c t i o n o f polymer c r o s s l i n k d e n s i t y a s s o c i a t e d w i t h t h i s d e g r a d a t i v e p r o c e s s , l i k e l y c o n t r i b u t e t o the o b s e r v e d r e d u c t i o n i n bond d u r a b i l i t y . In a s p e c i f i c example o f a d h e s i v e bonds between c o l d r o l l e d s t e e l and SMC adherends ( T a b l e I I ) an a d h e s i v e b a s e d on h y d r o l y s i s r e s i s t a n t epoxy c h e m i s t r y ( i . e . , a d h e s i v e E) was compared w i t h an a d h e s i v e b a s e d on h y d r o l y s i s prone u r e t h a n e c h e m i s t r y ( i . e . , a d h e s i v e C) i n composite t o c o l d r o l l e d s t e e l bonds. A f t e r corro s i o n t e s t i n g , a s i g n i f i c a n t d i f f e r e n c e i n both r e t e n t i o n of i n i t i a l bond s t r e n g t h and l o c u s o f f a i l u r e was o b s e r v e d . F o r bonds p r e p a r e d w i t h a d h e s i v e E, l i t t l e i f any r e d u c t i o n o f the i n i t i a l bond s t r e n g t h was o b s e r v e d a f t e r c o r r o s i o n t e s t i n g . The l o c u s o f f a i l u r e f o r b o t h the t e s t e d and u n t e s t e d bonds was l a r g e l y i n the T a b l e I I . Composite t o M e t a l Bonding: Urethane v s . Epoxy A d h e s i v e s

Test

Urethane Failure Strength (MPa)

Urethane Strength Failure (MPa) 4.97

Fiber

Tear

Adhesion Loss to S t e e l

4.48

Fiber

Tear

Fiber

4.87

Fiber

Tear

Unexposed SMC/CRS

5.23

Fiber

Corrosion Exposed SMC/CRS

3.21

Untested SMC/SMC

5.36

Tear

Tear


197


198


SMC adherend ( f i b e r t e a r out o b s e r v e d ) ; o n l y about 10% o f the f a i l u r e was a t t h e a d h e s i v e - m e t a l i n t e r f a c e . The l o c u s f a i l u r e was t y p i c a l o f bonds i n v o l v i n g composite m a t e r i a l s . Bond s t r e n g t h v a l u e s f o r these c o m p o s i t e / m e t a l bonds compared f a v o r a b l y w i t h s i m i l a r bonds i n v o l v i n g c o m p o s i t e / c o m p o s i t e bonds where t h e e f f e c t s of c o r r o s i o n r e a c t i o n s are not present. F o r bonds p r e p a r e d w i t h a d h e s i v e C ( h y d r o l y s i s prone m a t e r i a l ) , n e a r l y a 40% r e d u c t i o n i n bond s t r e n g t h was o b s e r v e d and t h e f a i l u r e was e n t i r e l y a t the m e t a l - a d h e s i v e i n t e r f a c e . The o b s e r v e d r e s u l t s w i t h composite m e t a l bonds i n c o r r o s i o n a r e c o n s i s t e n t w i t h a c o r r o s i o n i n d u c e d degradat i o n o f t h e a d h e s i v e a t the m e t a l - a d h e s i v e i n t e r f a c e t h a t reduces the o v e r a l l s t r e n g t h o f t h e a d h e s i v e below t h e c o h e s i v e s t r e n g t h o f the composite (hence, the change i n l o c u s o f bond f a i l u r e ) . The i n t e r f a c i a l a d h e s i v e bond s u r f a c e s g e n e r a t e d as a r e s u l t o f c o r r o s i o n i n d u c e d f a i l u r e ( f o r a d h e s i v e s C and E) have been examined using x-ray photoelectron spectroscopy. The r e s u l t s o f these s t u d i e s a r e shown i n T a b l e I I I and F i g u r e s 1 and 2. Changes i n T a b l e I I I . X-Ray P h o t o e l e c t r o n S p e c t r o s c o p y o f Composite t o M e t a l Bond F a i l u r e i n C o r r o s i o n Sample Ç

P e r c e n t Atomic C o m p o s i t i o n 0 Ν Fe

N/0

Adhesive C Unexposed

80.2

17.3

2.5

--

.145

Adhesive C After Corrosion Polymer I n t e r f a c e

73.0

24.9

2.1

--

.084

Adhesive C After Corrosion Metal Interface

51.7

42.5

1.7

4.1

.040

Adhesive Ε Unexposed

78.5

15.4

6.1

--

.396

Adhesive Ε After Corrosion Polymer I n t e r f a c e

82.0

12.6

5.4

--

.428

Adhesive Ε After Corrosion Metal Interface

49.9

45.4

--

4.7

0

e l e m e n t a l c o m p o s i t i o n were o b s e r v e d w i t h t h e u r e t h a n e b a s e d adhesive a f t e r c o r r o s i o n induced adhesion f a i l u r e . The most n o t a b l e change o b s e r v e d i s i n the N/0 r a t i o . There i s a marked r e d u c t i o n o f n i t r o g e n c o n c e n t r a t i o n ( i . e . , N/0 r a t i o f o r u n t e s t e d a d h e s i v e C s u r f a c e o f 0.145 reduced t o N/0 r a t i o o f 0.084 f o r i n t e r f a c i a l a d h e s i v e s u r f a c e a f t e r c o r r o s i o n ) . The lower n i t r o g e n c o n c e n t r a t i o n on t h e i n t e r f a c i a l a d h e s i v e C s u r f a c e a f t e r c o r r o s i o n i s c o n s i s t e n t w i t h a d e g r a d a t i o n o f polymer w i t h l o s s o f n i t r o g e n - c o n t a i n i n g



17.


HOLUBKA ET AL.

291.0

2890

287.0

2850

BINDING ENERGY, eV

F i g u r e 1. C I s X-Ray P h o t o e l e c t r o n S p e c t r a (XPS) o f i n t e r f a c i a l s u r f a c e s o f a d h e s i v e C showing (a) C I s XPS spectrum o f u n t e s t e d A d h e s i v e C s u r f a c e h a v i n g peaks at 285.0 eV, 285.8 eV, 286.8 eV, and 289.3 eV; (b) C I s XPS s p e c trum o f i n t e r f a c i a l A d h e s i v e C polymer s u r f a c e a f t e r c o r r o s i o n showing peaks i d e n t i c a l t o ( a ) ; (c) C I s XPS spectrum o f i n t e r f a c i a l A d h e s i v e C metal s u r f a c e a f t e r c o r r o s i o n showing com ponents a t 285.0 eV, 286.1 eV, 287.3 eV, and 288.9 eV; (d) C I s XPS spectrum o f c o l d r o l l e d s t e e l s t a n d a r d showing peaks a t 285.0 eV, 286.5 eV, and 288.7 eV.

287 0

285.0

BINDING ENERGY, eV

F i g u r e 2. C I s XPS o f i n t e r f a c i a l s u r f a c e s o f A d h e s i v e Ε showing ( a ) C I s XPS spectrum o f u n t e s t e d A d h e s i v e Ε s u r f a c e h a v i n g peaks at 285.0 eV, 285.7 eV, 286.7 eV, and 287.9 eV; (b) C I s XPS s p e c trum o f i n t e r f a c i a l A d h e s i v e Ε polymer s u r f a c e a f t e r c o r r o s i o n showing peaks i d e n t i c a l t o ( a ) ; ( c ) C I s XPS spectrum o f i n t e r f a c i a l Adhesive Ε metal surface a f t e r c o r r o s i o n showing com ponents a t 285.0 eV, 286.1 eV, 287.3 eV, and 288.9 eV; (d) C I s XPS spectrum o f c o l d r o l l e d s t e e l s t a n d a r d showing peaks a t 285.0 eV, 286.5 eV, and 288.7 eV.


199


200


s p e c i e s d u r i n g c o r r o s i o n . The presence o f n i t r o g e n on the i n t e r f a c i a l m e t a l s u r f a c e i s c o n s i s t e n t w i t h the presence o f polymer r e s i d u e on t h a t s i d e o f the i n t e r f a c e . The h i g h r e s o l u t i o n C I s XPS s p e c t r a f o r a d h e s i v e C ( F i g u r e 1) show t h a t the i n t e r f a c i a l s u r f a c e s g e n e r a t e d as a r e s u l t o f bond f a i l u r e ( F i g u r e s IB and 1C) d i f f e r from the u n t e s t e d a d h e s i v e . The o b s e r v e d changes i n the s p e c t r a are c o n s i s t e n t w i t h a h y d r o l y s i s p r o c e s s t h a t c l e a v e s a w a t e r s o l u b l e a m i n e - c o n t a i n i n g component from the c r o s s l i n k e d network o f a d h e s i v e C. I n c o n t r a s t , the d e l a m i n a t e d p o r t i o n o f the a d h e s i v e Ε bond t h a t f a i l e d a d h e s i v e l y shows o n l y minor changes i n c o m p o s i t i o n (Table I I I ) . N/0 r a t i o s remain e s s e n t i a l l y unchanged f o r b o t h u n t e s t e d a d h e s i v e Ε s u r f a c e and the i n t e r f a c i a l a d h e s i v e s u r f a c e g e n e r a t e d as a r e s u l t o f bond f a i l u r e . Bond f a i l u r e i s l i k e l y t o r e s u l t from a s i m p l e d i s p l a c e m e n t o f the a d h e s i v e from the s t e e l adherend by water o r by a s i m p l e p e e l p r o c e s s d u r i n g the bond f a i l u r e t h a t o c c u r s w i t h o u t polymer d e g r a d a t i o n . H i g h r e s o l u t i o n C I s s p e c t r a f o r a d h e s i v e Ε ( F i g u r e 2) show n e a r l y i d e n t i c a l s p e c t r a f o r b o t h u n t e s t e d and t e s t e d s u r f a c e s , s u g g e s t i n g the l a c k o f s i g n i f i c a n t polymer d e g r a d a t i o n i n t h i s c o r r o s i o n t e s t . The absence o f n i t r o g e n on the i n t e r f a c i a l m e t a l s u r f a c e as w e l l as a C I s n e a r l y i d e n t i c a l t o s t e e l s t a n d a r d f u r t h e r i n d i c a t e s t h a t bond f a i l u r e o c c u r r e d w i t h o u t polymer d e g r a d a t i o n . The E f f e c t o f A d h e s i v e P r i m e r s . I n p r a c t i c e , a d h e s i v e bonds i n v o l v i n g m e t a l adherends o f t e n use p r i m e r s as p r e t r e a t m e n t s o f the m e t a l s u r f a c e p r i o r t o bonding. Table IV shows the d u r a b i l i t y o f c o m p o s i t e - m e t a l bonds p r e p a r e d w i t h a d h e s i v e C o v e r a s e r i e s o f p r i m e r s ( o f v a r y i n g c o r r o s i o n r e s i s t a n c e ) i n 240 hour s a l t s p r a y t e s t . The r e s u l t s i n d i c a t e t h a t the performance o f bonds i s d i r e c t l y r e l a t e d t o the c o r r o s i o n r e s i s t a n c e o f the p r i m e r used t o p r e p a r e the adherend s u r f a c e . I n g e n e r a l , the a d h e s i o n o f the p r i m e r t o the s t e e l adherend, r a t h e r t h a n the a d h e s i v e c h e m i s t r y , T a b l e IV. E f f e c t o f P r i m e r C h e m i s t r y on Bond S t r e n g t h o f SMC/Primed S t e e l Epoxy A d h e s i v e Bonds Primer Chemistry

Primer Adhesion Loss i n SS*(mnO

Epoxy Ecoat

0

Epoxy E s t e r

Bond S t r e n g t h (MPa) Initial A f t e r SS

Failure

6.27

5.86

Fiber Tear

2-3

6.59

5.29

Fiber Tear + Primer

Urethane

4-5

6.36

2.73

Primer

Epoxy

Urethane

Effects of Corrosive Environments on the Locus and Mechanism of

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