Organic Corrosion Inhibitors to Improve the Durability of Adhesion

22 Organic Corrosion Inhibitors to Improve the Durability of Adhesion Between Aluminum and Polymeric Coatings 1

Downloaded by MONASH UNIV on November 26, 2014 | http://pubs.acs.org Publication Date: October 14, 1986 | doi: 10.1021/bk-1986-0322.ch022

L. J. Matienzo , D. K. Shaffer, W. C. Moshier, and G. D. Davis Martin Marietta Laboratories, 1450 South Rolling Road, Baltimore, M D 21227

The application of selected organophosphonates and organosilanes onto pre-treated aluminum surfaces improves their environmental corrosion resistance and bond durability with an external polymeric coating. Ionizable phosphonates, such as n i t r i l o t r i s methylene phosphonic acid (NTMP), adsorbed at monolayer concentrations, are effective inhibitors against hydration and are compatible with a nitrile-modified epoxy adhesive material. Aqueous solutions of selected organosilane compounds containing reactive side chains (e.g., epoxy, mercapto) render protection against both hydration and localized corrosion, and provide good adhesive bond durability with both n i t r i l e -modified and polyamide (primer) epoxy resin systems. Wedge test results suggest that the curing process (e.g., percent crosslinking) of the epoxy-polyamide primer system is not affected by the addition of organosilanes, but may be affected by NTMP. The results of substrate surface characterization, adsorption behavior of applied films, and evaluation of candidate inhibitors by chemical, mechanical, and electrochemical test methods are presented. Mechanisms to explain the observed behavior of the various phosphonate and silane polymer systems are discussed. The e n v i r o n m e n t a l d u r a b i l i t y o f a d h e s i v e l y - b o n d e d aluminum s t r u c t u r e s i s o f prime i m p o r t a n c e i n the a i r c r a f t i n d u s t r y * Whether the a d h e s i v e f u n c t i o n i s s t r u c t u r a l o r p r o t e c t i v e , proper p r e t r e a t m e n t of the aluminum p r i o r t o epoxy bonding remains e s s e n t i a l f o r d e v e l o p i n g h i g h bond s t r e n g t h s . ( 1 - 8 ) The i n c e n t i v e t o e l i m i n a t e e n v i r o n m e n t a l l y u n d e s i r a b l e m a t e r i a l s , such as chromâtes, has l e d t o t h e c o n s i d e r a t i o n o f o r g a n i c i n h i b i t o r compounds as a n t i c o r r o s i o n a d d i t i v e s . 7

Current address: IBM Corporation, Systems Technology Division, Endicott, NY 13760 0097-6156/ 86/0322-0234$06.00/0 © 1986 American Chemical Society

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

MATIENZO ETAL.

Durability of Adhesion Between Aluminum and Coatings


The o v e r a l l performance o f a polymer-metal bond system i s a f f e c t e d by: 1.

S u r f a c e roughness of the s u b s t r a t e , needed t o p r o v i d e good m e c h a n i c a l i n t e r l o c k i n g w i t h t h e polymer ( 6 - 8 ) ;

2.

Long-term s t a b i l i t y of the AI2O3 i n a humid environment ( 9 - 1 2 ) ;

3.

S e l e c t i o n o f c o r r o s i o n i n h i b i t o r s which slow down the t r a n s f o r m a t i o n of AI2O3 i n t o A100H ( 1 2 - 1 4 ) ;

4.

E f f e c t of a s e l e c t e d c o r r o s i o n i n h i b i t o r on t h e c u r i n g of t h e p o l y m e r i c c o a t i n g .

T y p i c a l l y , the f i r s t t h r e e e f f e c t s , which d e a l w i t h m e t a l polymer and m e t a l - i n h i b i t o r i n t e r a c t i o n s , a r e s t u d i e d w i t h respect t o corrosion c o n t r o l processes. I n t h i s paper, we have a l s o examined the r e l a t i o n s h i p between t h e i n h i b i t o r compounds and the p o l y m e r i c top coat from both a c h e m i c a l and a p h y s i c a l point of view. The r e s u l t s o f our i n v e s t i g a t i o n s on metal-polymer systems t r e a t e d w i t h s e l e c t e d i n h i b i t o r s a r e p r e s e n t e d . U s i n g etched A l s u b s t r a t e s , o r g a n o s i l o x a n e and organophosphonate compounds a r e e v a l u a t e d w i t h r e s p e c t t o t h e i r r e s i s t a n c e t o e n v i r o n m e n t a l d e g r a d a t i o n and o v e r a l l bond d u r a b i l i t y . I n a d d i t i o n t o t h e i r i o n i z a b l e (RCf ) m o i e t i e s f o r bonding t o t h e m e t a l o x i d e s u r f a c e , many of these compounds c o n t a i n f u n c t i o n a l groups (X) designed t o couple c h e m i c a l l y w i t h a p p l i e d p o l y m e r i c epoxy systems. T h i s scheme i s i l l u s t r a t e d i n Equation ( 1 ) . CORROSION-RESISTANCE FUNCTION OF INHIBITOR

Oxide surface

Inhibitor Phosphonates: R=H,Me,Et n=2 Silanes: R=Me, Et n=3

S t u d i e s f o r the organophosphonates i n c l u d e t h e e f f e c t s o f s o l u t i o n pH on t h e i r a d s o r p t i o n onto t h e a n o d i z e d aluminum s u b s t r a t e s . Mechanisms a r e d i s c u s s e d f o r the r e s p e c t i v e i n t e r a c t i o n s o f t h e i o n i z a b l e phosphonate and n e u t r a l s i l a n e compounds w i t h two d i f f e r e n t p o l y m e r i c epoxy systems.


236

POLYMERIC MATERIALS FOR CORROSION CONTROL

Experimental

S u b s t r a t e C h a r a c t e r i z a t i o n . Test coupons and panels of 7075T6 aluminum, an a l l o y used e x t e n s i v e l y f o r a i r c r a f t s t r u c t u r e s , were degreased i n a commercial a l k a l i n e c l e a n i n g s o l u t i o n and r i n s e d i n d i s t i l l e d , d e i o n i z e d w a t e r . The samples were then s u b j e c t e d t o e i t h e r a s t a n d a r d F o r e s t P r o d u c t s L a b o r a t o r i e s (FPL) treatment (1) o r t o a s u l f u r i c a c i d a n o d i z a t i o n (SAA) process (10% H S 0 , v/v; 15V; 20 m i n ) , two methods used f o r s u r f a c e p r e p a r a t i o n of a i r c r a f t s t r u c t u r a l components. The m e t a l s u r f a c e s were examined by s c a n n i n g t r a n s m i s s i o n e l e c t r o n microscopy (STEM) i n the SEM mode and by 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 (XPS).


2

4

Epoxy Systems Two c o n v e n t i o n a l epoxy-based p r o d u c t s were used f o r these studies: 1.

A n i t r i l e - m o d i f i e d epoxy s t r u c t u r a l (American Cyanamid FM 123-2)

CH

3

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0

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3

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2.

adhesive

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An unpigmented epoxy-polyamide top coat EPON 1001-T75 epoxy and Versamid 115 amidopolyamine).

(Shell

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3 V

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ρ

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Versamid 115 (Amine value = 230-246)

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EPON 100Ό-Τ75 (Epoxide eq. wt. = 450-550)

The FM 123-2, s u p p l i e d as a supported f i l m on a k n i t t e d m i c r o f i l a m e n t n y l o n c a r r i e r , was a p p l i e d d i r e c t l y t o one p r e t r e a t e d (SAA o r FPL) A l p a n e l (15 cm χ 15 cm χ 0.3 cm). A second p a n e l was then p r e s s e d onto the exposed a d h e s i v e s i d e and t h e "sandwich" s t r u c t u r e was s u b s e q u e n t l y cured a t 120°C under 40 p s i f o r 1 h r . A 1:1:1 m i x t u r e of the epoxy-polyamide f o r m u l a t i o n (EPON r e s i n : Versamid c u r i n g agent : t h i n n e r , MIL-T-81772) was sprayed onto prepared 7075-T6 specimens t o a t h i c k n e s s ( d r y ) o f 0.015 - 0.023 cm.


22.

MATIENZO ET AL.


I n h i b i t o r Coverage


The FPL- o r SAA-prepared coupons and panels were immersed f o r 30 min i n an aqueous ( o r a q u e o u s / a l c o h o l i c ) i n h i b i t o r s o l u t i o n at room t e m p e r a t u r e , f o l l o w e d by r i n s i n g i n d i s t i l l e d , d e i o n i z e d water and f o r c e d a i r d r y i n g . Coverage l e v e l s of phosphonate and s i l a n e i n h i b i t o r s were determined by XPS from the s u r f a c e c o n c e n t r a t i o n r a t i o s of t h e i r c h a r a c t e r i s t i c elements, P/Al or S i / A l , r e s p e c t i v e l y . Selected organosilanes were a p p l i e d t o the m e t a l s u r f a c e by s p r a y i n g , a f t e r d i s s o l u t i o n (0.1 - 0.5%, v/v) i n t h e EPON-Versamid primer formulation. Corrosion

Testing

Hydration. The t r e a t e d aluminum specimens were p l a c e d i n a Blue M H u m i d i t y chamber m a i n t a i n e d a t 65°C and 95% r e l a t i v e h u m i d i t y f o r s p e c i f i e d time p e r i o d s , removed and then d r i e d . Wedge T e s t . The adhesive bond d u r a b i l i t i e s of the i n h i b i t o r t r e a t e d 7075-T6 s u r f a c e s were e v a l u a t e d by wedge t e s t s (ASTM D-3762) on bonded specimens u s i n g the FM 123-2 epoxy a d h e s i v e t o s i m u l a t e t h e epoxy p r i m e r . The specimens were p l a c e d i n a h u m i d i t y chamber a t 65°C and 95% r e l a t i v e h u m i d i t y and removed a t s p e c i f i e d time i n t e r v a l s t o r e c o r d t h e c r a c k t i p l o c a t i o n s ; a f t e r each e x a m i n a t i o n , they were r e t u r n e d t o t h e h u m i d i t y chamber. E l e c t r o c h e m i s t r y . The e l e c t r o c h e m i c a l b e h a v i o r of the t r e a t e d specimens was a n a l y z e d by a n o d i c a l l y p o l a r i z i n g the specimens w i t h a PAR Model 273 p o t e n t i o s t a t / g a l v a n o s t a t t o assess t h e a b i l i t y of t h e i n h i b i t o r s t o promote p a s s i v a t i o n i n c h l o r i d e c o n t a i n i n g e l e c t r o l y t e s . The A l s u r f a c e s were p o l i s h e d t o a 4000 g r i t f i n i s h w i t h S i C paper, degreased w i t h a s o l v e n t , and washed i n an a l k a l i n e d e t e r g e n t s o l u t i o n . Each sample t o be p o l a r i z e d was then p l a c e d f o r 30 min i n deaerated e l e c t r o l y t i c s o l u t i o n s c o n t a i n i n g c h l o r i d e (0.002 Ν KC1) as t h e a g g r e s s i v e i o n s p e c i e s and 0.1 Ν Na2S0^ t o m i n i m i z e the impedance of t h e e l e c t r o l y t e . I n h i b i t o r s were added t o the s o l u t i o n i n c o n c e n t r a t i o n s known t o p r o v i d e the optimum s u r f a c e coverage. (15) Each sample was scanned a n o d i c a l l y from the e q u i l i b r i u m c o r r o s i o n p o t e n t i a l a t a r a t e of 0.5 mV/s i n order t o determine t h e p r o p e n s i t y of the a l l o y t o p i t i n a s p e c i f i c electrolytic solution.

Results S u b s t r a t e C h a r a c t e r i z a t i o n . Venables e t a l . ( 7 ) have d e s c r i b e d t h e FPL o x i d e morphology u s i n g STEM i n t h e SEM mode. F i g u r e 1 i s an i s o m e t r i c r e p r e s e n t a t i o n of the FPL s u r f a c e . I n c o n t r a s t , the SAA process produces a much t h i c k e r o x i d e l a y e r , i s o m e t r i c a l l y r e p r e s e n t e d i n F i g . 2. Q u a n t i t a t i v e XPS a n a l y s i s i n d i c a t e s t h a t a c o n s t a n t


238



ο

! F i g u r e 1. S t r u c t u r e of a F o r e s t P r o d u c t s prepared aluminum s u r f a c e ( 7 ) .

L a b o r a t o r i e s (FPL)

F i g u r e 2. S t r u c t u r e of a s u l f u r i c a c i d - a n o d i z e d surface.

(SAA) aluminum

c o m p o s i t i o n ( 3 0 % A l , 60% 0, 3.5% S) i s o b t a i n e d a f t e r f i v e minutes of a n o d i z a t i o n . The s u r f a c e l a y e r i s A ^ C S O ^ ^ on t o p of the ΑΙ^Οβ m a t r i x . I n h i b i t o r Coverage. The a d s o r p t i o n b e h a v i o r of phosphonate compounds, i n c l u d i n g those shown i n F i g . 3, on FPL-prepared aluminum s u r f a c e s , has been r e p o r t e d . (11,12,14) The r e s p e c t i v e c o n c e n t r a t i o n s o f Ρ o r S i and A l were measured by XPS. F i g u r e 4 shows t h e c o r r e l a t i o n of Ρ coverage on SAA s u r f a c e s f o l l o w i n g treatment i n NTMP s o l u t i o n over a c o n c e n t r a t i o n range of 10 t o 10 ppm. U s i n g a c o n s t a n t immersion time o f 30 m i n u t e s , t h e a d s o r p t i o n maximum observed was a t t r i b u t e d t o a m u l t i l a y e r b u i l d u p of t h e i n h i b i t o r compound i n t h e pH 2-4 r e g i o n . ( 1 5 )


22.

MATIENZO ET AL.



The r e l a t i o n s h i p between the l e v e l of NTMP coverage and the s o l u t i o n pH has been r e p o r t e d p r e v i o u s l y . ( 1 5 ) E s s e n t i a l l y , i f the NTMP s o l u t i o n c o n c e n t r a t i o n i s m a i n t a i n e d at 100 ppm, an a d s o r p t i o n maximum i s observed between pH 2-4. The s u r f a c e coverages f o r t h r e e o r g a n o s i l a n e s u s i n g d i f f e r e n t s o l u t i o n a d s o r p t i o n c o n d i t i o n s , i n c l u d i n g type o f s o l v e n t system, s o l u t i o n pH, and t o t a l immersion t i m e , a r e shown i n Table I . The r e s u l t s i n d i c a t e t h a t t h e e x t e n t t o w h i c h these m a t e r i a l s a r e adsorbed onto the m e t a l s u r f a c e depends p r i m a r i l y upon t h e aqueous c o m p o s i t i o n and pH of t h e i n h i b i t o r s o l u t i o n . Both f a c t o r s i n f l u e n c e t h e h y d r o l y s i s of s i l i c o n a l k o x y groups t o s i l a n o l s , which a r e the m o i e t i e s t h a t a c t u a l l y bond t o the m e t a l s u r f a c e .

Phosphonates (pH 3)

NTMP

0 DAB Ρ OEt

Silanes P-810

P-113

Q G-6720

1-7840

M-8500

A-800

F i g u r e 3.

3

HN ?

Si(0Me)

3

Chemical s t r u c t u r e s of c o r r o s i o n i n h i b i t o r compound



240


10

10

1

2

10

3

10

10

4

5

fNTMP](ppm)

Figure 4· NTMP coverage as a function of concentration on SAAprepared 7075-T6 aluminum surface»

TABLE I Silane Adsorption onto FPL 7075-T6 Surfaces Silane

Concentration (ppm)

Solvent System

-ËL

Immersion Time

Si/Al

P-810

1000

W (water)

4

40 min

0.040

P-810

1000

W (water)

7

40 rain

0.070

P-810

1000

M/W (methanol-water,

4

40 min

0.076

P-810

1000

W

4 hr

0.100

G-6720 G-6720 G-6720 G-6720

1000 1000 1000 1000

A-800 A-800 A-800

1000 1000 1000

1:1)

w w M/W

4 .

W

4 7 4 4

40 40 40 4

min min min hr

0.148 0.083 0.127 0.158

W W M/W

4 7 4

40 min 40 min 40 min

0.364 0.460 0.330

Hydration Resistance. V i s u a l examination of a series of i n h i b i t o r - t r e a t e d FPL-prepared 7075-T6 A l coupons exposed to high humidity conditions f o r specified time i n t e r v a l s indicated good short-term hydration resistance for several phosphonate and silane compounds (no v i s i b l e discoloration)· The most e f f e c t i v e silane compound tested contained the mercapto (-SH) functional group.


22.

MATIENZO ET AL.



A d h e s i v e Bond D u r a b i l i t y SAA S u r f a c e * The c o r r o s i o n r e s i s t a n c e and a d h e s i v e c o u p l i n g c a p a b i l i t i e s of t h e i n h i b i t o r s were e v a l u a t e d by the wedge t e s t . R e s u l t s o b t a i n e d u s i n g SAA-prepared s u r f a c e s ( F i g . 5) i n d i c a t e d t h a t two i n h i b i t o r t r e a t m e n t s (100 ppm NTMP and 5000 ppm e p o x y s i l a n e ) p r o v i d e d systems which outperformed those t r e a t e d w i t h h i g h e r c o n c e n t r a t i o n s of NTMP and by FPL a l o n e , but were a p p a r e n t l y no more e f f e c t i v e than t h e SAA c o n t r o l w i t h r e s p e c t t o o v e r a l l a d h e s i v e bond d u r a b i l i t y . However, XPS a n a l y s i s ( F i g . 6 ) , i n c o n j u n c t i o n w i t h SEM e x a m i n a t i o n of the f a i l e d debonded s i d e s , i d e n t i f i e d the t r u e modes of f a i l u r e . The SAA c o n t r o l ( h y d r a t e d o x i d e on both s i d e s under SEM; h i g h A l and 0 l e v e l s on both s i d e s ) f a i l e d w i t h i n the o x i d e . E x a m i n a t i o n o f t h e specimen t r e a t e d w i t h m u l t i l a y e r - f o r m i n g 5000 ppm NTMP s o l u t i o n ( d i s t i n c t "metal" and " a d h e s i v e " s i d e s under SEM; h i g h A l and 0, low C l e v e l s on " m e t a l " s i d e ; h i g h C, low A l and 0 l e v e l s on " a d h e s i v e " s i d e ) i n d i c a t e d t h a t t h e f a i l u r e o c c u r r e d between the metal and t h e a d h e s i v e ( i . e . , adhesive f a i l u r e ) . A l t h o u g h d i s t i n c t "metal" and " a d h e s i v e " s i d e s were apparent upon v i s u a l e x a m i n a t i o n of t h e debonded s u r f a c e s t r e a t e d w i t h 100 ppm NTMP, SEM a n a l y s i s showed t h e presence of an adhesive l a y e r on t h e "metal" s i d e . XPS a n a l y s i s i n d i c a t e d low A l and 0 and i d e n t i c a l h i g h C l e v e l s on both debonded s i d e s , c o n f i r m i n g a f a i l u r e w i t h i n t h e a d h e s i v e l a y e r (cohes i v e f a i l u r e ) , i . e . , t h e best p o s s i b l e performance i n a g i v e n adherend-adhesive system. T h i s r e s u l t i s s i m i l a r t o t h a t o b t a i n e d u s i n g a 2024 A l a l l o y prepared by the p h o s p h o r i c a c i d - a n o d i z a t i o n (PAA) process (16) and i n d i c a t e s t h e importance of monolayer NTMP coverage f o r good bond d u r a b i l i t y (Fig. 4 ) . FPL S u r f a c e . A second wedge t e s t was performed t o e v a l u a t e s i x s i l a n e s and NTMP u s i n g t h e t h i n n e r , more s e n s i t i v e FPL o x i d e ( F i g . 7 ) . The r e s u l t s i n d i c a t e d t h a t f o u r s i l a n e s performed b e t t e r than t h e FPL c o n t r o l but n o t as w e l l as NTMP; one s i l a n e ( a n aminopropyl d e r i v a t i v e ) performed v e r y p o o r l y w i t h r e s p e c t t o t h e c o n t r o l ; and one s i l a n e (M-8500, m e r c a p t o p r o p y l d e r i v a t i v e ) outperformed a l l o f t h e o t h e r i n h i b i t o r systems, i n c l u d i n g NTMP and o r g a n o s i l a n e s c o n t a i n i n g m e t h y l , p h e n y l , i s o c y a n a t e , and epoxide s i d e c h a i n s . Subsequent XPS a n a l y s i s of t h e adsorbed m e r c a p t o s i l a n e i n h i b i t o r showed h i g h c o n c e n t r a t i o n s of S i and S near t h e s u r f a c e , w i t h c o r r e s p o n d i n g low A l , t h e S b e i n g p r i m a r i l y i n the reduced " i n h i b i t o r " ( i . e . , R-SH) form r e l a t i v e t o s u l f a t e (-SO^ ) w h i c h r e s u l t e d from t h e FPL p r e t r e a t m e n t . P r i m e r Epoxy v s . N i t r i l e - M o d i f i e d Epoxy. The c o m p a t i b i l i t y of the epoxy-polyamide primer w i t h t h e n i t r i l e - m o d i f i e d epoxy a d h e s i v e f a c s i m i l e and t h e aluminum o x i d e s u r f a c e was a l s o e v a l u a t e d by the wedge t e s t , s i n c e e a r l i e r t e s t s u s i n g t h e p r i m e r as t h e adhesive had f a i l e d i m m e d i a t e l y . As shown i n F i g . 8, t h e a d d i t i o n of t h e primer d i r e c t l y t o t h e prepared



242


F i g u r e 5. Wedge t e s t r e s u l t s f o r i n h i b i t o r - t r e a t e d SAA 7075-T6 aluminum specimens.

WEDGE TEST BOND FAILURE ANALYSIS

2(H

ι 1 - SAA 2 = SAA + NTMP (5000 ppm I 3 = SAA + NTMP (100 ppm)

1 Metal Side I

I Adhesive side

F i g u r e 6. XPS s u r f a c e a n a l y s i s r e s u l t s f o r debonded SAA 7075-T6 aluminum specimens.



22. MATIENZO ET AL.

FPL WEDGE 1ES1 RESULIS


Ε

l

Τ

I

I

ο

I

lo

I

10

I

100

1000

TIME (hr)

F i g u r e 7. Wedge t e s t r e s u l t s f o r i n h i b i t o r - t r e a t e d FPL 7075-T6 aluminum specimens.

7

FPL + Primer (Pig.) .FPL- Primer NTMP

PL+ Primer (Unpig.) M-8500

3.0 -

0

I

1

I

I

10

100

I

1000

TIME (hr)

F i g u r e 8. Wedge t e s t r e s u l t s f o r primed FPL 7075-T6 specimens.


243


244


m e t a l s u r f a c e improved the o v e r a l l d u r a b i l i t y o f the aluminuma d h e s i v e system. The a p p l i c a t i o n of the m e r c a p t o p r o p y l s i l a n e d e r i v a t i v e by p r e a d s o r p t i o n onto the s u b s t r a t e s u r f a c e e f f e c t i v e l y m a i n t a i n e d o r s l i g h t l y enhanced the bond s t r e n g t h between the primer and the FPL o x i d e . I n c o n t r a s t , adsorbed NTMP, which p r e s e r v e s the i n t e g r i t y of the ( n i t r i l e - m o d i f i e d , h i g h - t e m p e r a t u r e cured) a d h e s i v e - m e t a l bond, f a i l e d t o prevent r a p i d d e t e r i o r a t i o n o f the (epoxy-polyamide, room-temperature cured) p r i m e r - m e t a l bond. XPS a n a l y s i s of the debonded specimens showed t h a t t h e unprimed and primed FPL c o n t r o l and m e r c a p t o s i l a n e - t r e a t e d specimens f a i l e d p r i m a r i l y w i t h i n the o x i d e , which r e p r e s e n t e d the weakest l a y e r i n the system. On the o t h e r hand, the NTMPt r e a t e d sample debonded between the o x i d e and the polyamide primer. E l e c t r o c h e m i c a l T e s t i n g . P o t e n t i o d y n a m i c p o l a r i z a t i o n measurements p r o v i d e d a s e n s i t i v e means of e v a l u a t i n g the i n h i b i tors w i t h respect t o environmental ( C l ~ ) c o r r o s i o n protect i o n . The r e s u l t s o b t a i n e d from a n o d i c a l l y p o l a r i z i n g p o l i s h e d 7075-T6 A l samples are p r e s e n t e d i n F i g . 9. For t h e c o n t r o l e l e c t r o l y t e (O.IN N a S 0 , 0.002N KC1, no i n h i b i t o r ) , p i t t i n g was observed almost i m m e d i a t e l y on the s u r f a c e , and the aluminum showed no e v i d e n c e o f p a s s i v a t i o n . The a d d i t i o n of NTMP t o the s o l u t i o n d i d not appear t o p r o t e c t the m e t a l 2

4

F i g u r e 9. E l e c t r o c h e m i c a l t e s t r e s u l t s f o r two d i f f e r e n t i n h i b i t o r s i n O.IN Na S0^ e l e c t r o l y t e w i t h added KC1. 2


22.

MATIENZO ET AL.


s u r f a c e , when compared t o t h e s o l u t i o n s c o n t a i n i n g no i n h i b i t o r s . I n c o n t r a s t , the sample p o l a r i z e d i n t h e e l e c t r o l y t e s o l u t i o n c o n t a i n i n g 1000 ppm m e r c a p t o p r o p y l s i l a n e (M-8500) d i d p a s s i v a t e , and had a d i s t i n c t p i t t i n g p o t e n t i a l w e l l above (more n o b l e than) t h e p o t e n t i a l where p i t t i n g i n i t i a t e s i n NTMP-containing e l e c t r o l y t e s . The A l was p a s s i v a t e d up t o -200 mV (SCE), and t h e a l l o y s u r f a c e remained f r e e from p i t s . Above t h i s p o t e n t i a l , s m a l l p i t s began t o form.


Discussion The c o r r o s i o n r e s i s t a n c e and polymer-bonding c o m p a t i b i l i t i e s of t h e i o n i z a b l e organophosphonates and t h e n e u t r a l organo s i l a n e s are d i r e c t l y r e l a t e d to t h e i r inherent chemical p r o p e r t i e s . S p e c i f i c a l l y , NTMP i n h i b i t s the h y d r a t i o n of AI2O3 and m a i n t a i n s or improves bond d u r a b i l i t y w i t h a n i t r i l e - m o d i f i e d epoxy a d h e s i v e which i s cured a t an e l e v a t e d t e m p e r a t u r e . The m e r c a p t o p r o p y l s i l a n e , i n a d d i t i o n t o these p r o p e r t i e s , i s c o m p a t i b l e w i t h a room t e m p e r a t u r e - c u r e d epoxypolyamide p r i m e r and a l s o e x h i b i t s r e s i s t a n c e t o l o c a l i z e d e n v i r o n m e n t a l c o r r o s i o n . These r e s u l t s , i n c o n j u n c t i o n w i t h the adsorbed i n h i b i t o r f i l m s and t h e m e t a l s u b s t r a t e s u r f a c e s , are s u b s e q u e n t l y d i s c u s s e d . S u r f a c e Morphology. The i n i t i a l i n t e g r i t y of an a d h e s i v e l y bonded system depends on t h e s u r f a c e o x i d e p o r o s i t y and m i c r o s c o p i c roughness f e a t u r e s r e s u l t i n g from e t c h i n g o r a n o d i z a t i o n p r e t r e a t m e n t s . (17) The SAA s u r f a c e c h a r a c t e r i z e d i n t h i s study c o n s i s t s of a t h i c k (9 ym), porous columnar l a y e r which p r o v i d e s e x c e l l e n t c o r r o s i o n r e s i s t a n c e i n b o t h humid and a g g r e s s i v e ( i . e . , C l ~ ) media. I The t h i n n e r FPL o x i d e (7) p r o v i d e s a s u i t a b l e s u b s t r a t e s u r f a c e f o r e v a l u a t i n g the c a n d i d a t e i n h i b i t o r s . I n h i b i t o r Adsorption. There a r e some i n t e r e s t i n g d i f f e r e n c e s between t h e a d s o r p t i o n of i o n i z a b l e (aminophosphate) and n e u t r a l ( o r g a n o s i l a n e ) compounds onto FPL- and SAA-prepared aluminum s u r f a c e s . The a c i d i c NTMP s p e c i e s , c o n s i s t i n g of a q u a t e r n a r y ΞΝ Η and u n p r o t o n a t e d -0*" groups i n s o l u t i o n , ( 1 8 ) e x h i b i t s a pH-dependent m u l t i l a y e r a d s o r p t i o n maximum i n an aqueous s o l u t i o n . A t low s o l u t i o n c o n c e n t r a t i o n s (

2

LMW Amines

(2)

>90°C

solution

Tautomeric forms o f d i c y a n o d i a m i d e The c u r i n g sequence and k i n e t i c s o f t h i s a d h e s i v e system prevent t h e NTMP from i n h i b i t i n g the r e a c t i v e amines u n t i l t h e c u r i n g r e a c t i o n i s w e l l under way. I n c o n t r a s t , t h e epoxypolyamide primer c o n t a i n s f r e e amino groups a t room temperature and may be i n h i b i t e d by t h e e l e c t r o p h i l i c NTMP s p e c i e s p r i o r t o c u r i n g . ( E q u a t i o n 3) ^JTMP E p o x i d e + Polyamine

>

Cured primer

(3)

25°C A s i m i l a r example o f c u r i n g i n h i b i t i o n i n an a c i d i c medium has been observed when moderate amounts o f s a l i c y l i c a c i d were added t o epoxy-amine m a t r i x systems. (26) Such b e h a v i o r may b e r e s p o n s i b l e f o r t h e poor bond s t r e n g t h s observed w i t h the NTMP-containing o x i d e - p r i m e r specimens. I n environments c o n t a i n i n g an a g g r e s s i v e s p e c i e s ( e . g . , C l ~ ) , t h e a n i o n may i n t e r a c t w i t h and become i n c o r p o r a t e d i n t o the NTMPo x i d e m a t r i x , whereby i t can a t t a c k the m e t a l s u r f a c e . The h y d r a t i o n r e s i s t a n c e o f the o r g a n o s i l a n e compounds was r e f l e c t e d by the wedge t e s t performances of our s i l a n e -



22.

MATIENZO ET AL.

Durability of Adhesion Between Aluminum and Coatings 247

t r e a t e d systems. E l e c t r o c h e m i s t r y r e s u l t s f u r t h e r i n d i c a t e d t h a t t h e adsorbed s i l a n e f i l m m a i n t a i n e d the r e s i s t a n c e i n environments c o n t a i n i n g a g g r e s s i v e ( C l ~ ) s p e c i e s and o x i d i z i n g c o n d i t i o n s . I n c o n t r a s t t o NTMP, however, c e r t a i n o r g a n o s i l a n e s a r e c o m p a t i b l e and r e a c t i v e w i t h the epoxy polyamide primer as w e l l as w i t h the n i t r i l e - m o d i f i e d a d h e s i v e , w h i c h , i n most c a s e s , s t r e n g t h e n s the oxide-epoxy bond. T h i s r e s u l t i s presumably due t o a c h e m i c a l c o u p l i n g o f f u n c t i o n a l epoxide o r mercapto s i d e c h a i n s on the o r g a n o s i l a n e w i t h the epoxy c o a t i n g s d u r i n g the c u r i n g p r o c e s s . The a m i n e - c a t a l y z e d mercaptan-epoxide r e a c t i o n ( E q u a t i o n 4) proceeds e x o t h e r m a l l y a t room temperature ( 2 7 , 2 8 ) . The o r d e r of average r e l a t i v e n u c l e o p h i l e - d i s p l a c e m e n t r a t e s ( T a b l e I I ) f u r t h e r suggests t h a t mercaptans r e a c t s i g n i f i c a n t l y f a s t e r than amines and t h a t the a d d i t i o n of the mercaptide (RS~) i o n t o t h e epoxide group i s the r a t e determining step (30).

0 R-SH

R" N

OH

3

+ CH2-CH-CH20R

1

>

R-S-CH -CH-CH OR 2

F

2

(4)

The mercaptan-epoxide c o u p l i n g r e a c t i o n i n d i c a t e s the t r u e b i f u n c t i o n a l n a t u r e of such i n h i b i t o r s . The poor performance of t h e a m i n o s i l a n e compound [expected t o s t r e n g t h e n the m e t a l - a d h e s i v e bond ( 2 0 ) ] i s not c o m p l e t e l y understood a t t h i s t i m e . Using

TABLE I I Average R e l a t i v e N u c l e o p h i l e - D i s p l a c e m e n t

Nucleophile

R e l a t i v e Rate

680,000

C^H^S

470,000

C H S6

Rates^

5

2

s o 2

3

3,000

~

1,000

C H 02

5

400

w

30

(CH ) N 3

3

1

N0 " 3

( ^ D a t a from Ref. ( 2 9 ) .

American Chemical Society Library 1155 for 16th St., N.W. In Polymeric Materials Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986. Washington, D.C. 20036

248


s i m i l a r c o n c e n t r a t i o n s , Walker (31) has shown t h a t some s i l a n e s are e f f e c t i v e a d h e s i o n promoters f o r urethane and epoxy p a i n t s on aluminum and m i l d s t e e l s u r f a c e s and s i g n i f i c a n t l y improve t h e i n i t i a l , wet, and r e c o v e r e d bond s t r e n g t h s .


Conclusions The i o n i c phosphonates l i k e NTMP a r e e f f e c t i v e h y d r a t i o n i n h i b i t o r s because they can form an i n s o l u b l e complex w i t h t h e o x i d e s u r f a c e . They a r e u s e f u l as epoxy adhesive c o u p l e r s i n cases where the adhesive and i t s c u r i n g c y c l e a r e c o m p a t i b l e w i t h t h e adsorbed phosphonate m o l e c u l e . (14) Wedge t e s t r e s u l t s i n d i c a t e t h a t i n two epoxy-aluminum systems s t u d i e d , c e r t a i n o r g a n o s i l a n e s tend t o both i n c r e a s e t h e epoxy-metal bond d u r a b i l i t y and m a i n t a i n h y d r a t i o n r e s i s t a n c e . The r e s u l t s of anodic p o l a r i z a t i o n e x p e r i ments f u r t h e r suggest t h a t these s i l a n e f i l m s a r e e f f e c t i v e against l o c a l i z e d p i t t i n g . Adsorbed NTMP e x h i b i t s a pH-dependent s u r f a c e coverage on anodized aluminum, which i n c l u d e s a r e g i o n c h a r a c t e r i z e d by a m u l t i l a y e r o f hydrogen-bonded phosphonate m o l e c u l e s . These t h i c k l a y e r s are weak and f a i l t o p r o v i d e good bond d u r a b i l i t y i n a humid environment. NTMP monolayers a r e p r o t e c t i v e a g a i n s t h y d r a t i o n and a r e c o m p a t i b l e w i t h a n i t r i l e - m o d i f i e d epoxy a d h e s i v e , but not w i t h an epoxy-polyamide primer t o p c o a t . In c o n t r a s t , h y d r o l y z e d s i l a n e compounds, presumably adsorbed as o l i g o m e r i c f i l m s , c o n f e r c o r r o s i o n r e s i s t a n c e i n both h y d r a t i n g and C I e n v i r o n m e n t s . These i n h i b i t o r s can a l s o couple w i t h a p p l i e d epoxy primer or adhesive f o r m u l a t i o n s t o f u r t h e r p r o t e c t the m e t a l a g a i n s t c o r r o s i o n by s t r e n g t h e n i n g the m e t a l - e p o x i d e bond. The o r g a n o s i l a n e s do n o t appear t o a f f e c t t h e c u r i n g p r o c e s s , e.g., % c r o s s l i n k i n g , of t h e p o l y m e r i c epoxy systems. Acknowledgments The authors w i s h t o thank Drs. John D. Venables and John S. Ahearn f o r t h e i r v a l u a b l e d i s c u s s i o n s i n c o n j u n c t i o n w i t h t h i s work. The support o f t h e N a v a l A i r Development C e n t e r , which sponsored these s t u d i e s under c o n t r a c t N00019-82-C-0439, i s a l s o g r a t e f u l l y acknowledged.

Literature Cited 1. 2. 3. 4. 5. 6.

H.W. Eichner and W.E. Schowalter, Forest Products Laboratory, Madison, WI, Report No. 1813 (1950). G.S. Kabayashi and D.J. Donnelly, Boeing Co., Seattle, WA, Report No. DG-41517 (Feb. 1974). Fokker-VFW, Amsterdam, Process Specification TH 6.785 (August, 1978). W.M. McCracken and R.E. Sanders, SAMPE J., 5, 37 (1969). J . C . McMillian, J . T . Quinlivan, and R.A. Davis, SAMPE, 7, 13 (1976). J.M. Chen, T.S. Sun, J.D. Venables, and R. Hopping, Proc. 22nd National SAMPE Symposium, 25 (April 1977).


22.

MATIENZO ET AL.

7. 8. 9. 10. 11.


12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

25. 26. 27. 28. 29. 30. 31.

Durability of Adhesion Between Aluminum and Coatings 249

J.D. Venables, D.K. McNamara, J.M. Chen, and T.S. Sun, Appl. Surf. S c i . , 3, 88 (1979). W. Brockman in Adhesion Aspects of Polymeric Coatings, K.L. Mittal, ed., 265 (Plenum, New York, 1983). J.D. Venables, D.K. McNamara, J.M. Chen, B.M. Ditchek, T . I . Morgenthaler, T.S. Sun, and R.L. Hopping, Proc. 12th Nat. SAMPE Symposium, Seattle, 909 (October 1980). G.D. Davis and J.D. Venables in Durability of Structural Adhesives, A . J . Kinloch, ed., 43 (Applied Science, Essex, 1983). D.A. Hardwick, J.S. Ahearn, and J.D. Venables, J . Mater. Sci., 19, 223 (1984). J.S. Ahearn, G.D. Davis, T.S. Sun, and J.D. Venables, op. cit. Ref. 8, 281. J.D. Venables, M.E. Tadros, and B.M. Ditchek, U.S. Pat. 4,308,079 (1981). G.D. Davis, J.S. Ahearn, L.J. Matienzo, and J.D. Venables (accepted by J. Mater. Sci.). Matienzo, L. J.; Shaffer, D. K.; Moshier, W. C.; Davis, G. D., J. Mater. Sci. 21, 1601, 1986. D.A. Hardwick, J.S. Ahearn, and J.D. Venables, J . Mater. Sci. (in press). J.D. Venables, J . Mater. S c i . , 19, 2431 (1984). R.P. Carter, M.M. Crutchfield, and R.R. Irani, J . Inorg. Chem., 6, 943 (1967). K.W. Allen, A.K. Hansrani, and W.C. Wake, J. Adhes., 12, 199 (1981). E.P. Plueddemann, Proc. 24th Ann. Tech. Conf., Reinf. Plastics/Composites Div., SPI, Sec. 19-A (1969). K.W. Allen and M.G. Stevens, J . Adhes., 14, 137 (1982). F . I . Belskii, I.B. Goryunova, P.V. Petrovskii, T.Y. Medved, and M.I. Kabachnik, Academia Nauk USSR, 1, 103 (1982). J . C . Bolger, op. cit. Ref. 8, 8. F . J . Boerio, R.G. Dillingham, and R.C. Bozian, 39th Annual Conference, Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, Inc., Session 4-A, 1 (January 1984). A.F. Lewis and R. Saxon in Epoxy Resins: Chemistry and Technology. C.A. May and Y. Tanaka, eds., 413 (Marcel Dekker, New York, 1973). A.M. Ibrahim and J . C . Seferis, Polym. Compos., 6, 47 (1985). K.R. Kramker and A . J . Breslau, Ind. Eng. Chem., 47, 98 (1956). E.H. Sorg and C.A. McBurney, Mod. Plastics, 34, 187 (1956). C.A. Stretweiser, Chem. Rev., 56, 571 (1956). Y. Tanaka and T.F. Mika, op. cit. Ref. 25, 169. P. Walker, J . Oil Colour Chem. Assoc., 65, 436 (1982).

RECEIVED January 27, 1986


Organic Corrosion Inhibitors to Improve the Durability of Adhesion

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