A Study of Corrosion Initiation on Polyimide Coatings

Corrosion Science, Vol. 33, No. 8, pp. 1203-1226, 1992 0010-938X/92 $5.00 + 0.00 Printed in Great Britain. ~ 1992 Pergamon Press Ltd

A STUDY OF C O R R O S I O N INITIATION ON P O L Y I M I D E COATINGS

F. BELLUCCI, L. NICODEMO, T. MONETTA, M. J. KLOPPERS* and R. M. LATANISION*

Department of Materials and Production Engineering, University of Naples, Piazzale Tecchio, 80125 Naples, Italy

*The H. H. Uhlig Corrosion Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A.

Abstract--The protective properties of thin polyimide (PI) films on aluminum and iron metallic substrates were studied using a.c. and d.c. electrochemical techniques. The effects of storage time, film thickness, and the synergistic effect of temperature (80°C) and relative humidity (85 %) on the lifetime of the PI were investigated. The results obtained can be summarized as follows: (i) early failure was observed for the PI/AI samples tested directly after curing, while extended lifetime was observed for samples stored in a desiccator for 2 months before testing, (ii) early failure was also observed for the PI/Fe samples even after storage in a desiccator before testing, (iii) the lifetime increases by increasing the film thickness, and (iv) temperature and relative humidity (RH) dramatically affect failure. A mathematical model is presented to relate coating failure to salt uptake by the PI. By applying this model to the experimental findings, an estimate of the heterogeneities or defects in the PIs is given.

INTRODUCTION

CORROSION p h e n o m e n a affect ing mic ro -e lec t ron ics c i rcui t ry and devices a re the same as those e n c o u n t e r e d with o the r eng inee r ing systems. T h e ma in d i f fe rence is the very low co r ros ion ra te (of the o r d e r of p A cm -2) tha t e l ec t ron ic c i rcui t ry can to l e ra t e be fo re fa i lure . 1

E l e c t r o c h e m i c a l fa i lure of devices was widely r e p o r t e d in the l i t e ra tu re and a t t r i bu t ed to ionic con t aminan t s , the re la t ive humid i ty ( R H ) and the app l i ed e lec t r ic field. 2-5 P ro t ec t i on of meta l l i c c i rcui t ry f rom aggress ive species is o b t a i n e d by the use of p o l y m e r i c ( epox ies , s i l icones and po ly imides ) o r ce ramic encapsu la t ing mate r ia l s . 6

P ro t ec t i on f rom the e n v i r o n m e n t is cri t ical to device p e r f o r m a n c e , since the ingress of mo i s tu re and o t h e r con t aminan t s can resul t in chemica l a t t ack of the chip me ta l l i za t i on o r can a l te r the e lec t r ica l charac te r i s t i cs of the device . l -7 It has been shown 2'7"8 tha t i n t eg ra t ed circui t ( IC) chips may fail dur ing service due to cor ros ion at the p o l y m e r / m e t a l in ter face . Dev ice re l iab i l i ty o f ten d e p e n d s on the ra te of a r r iva l of ionic c o n t a m i n a n t s at the packag ing /me ta l l i c c i rcui t ry in ter face . 1-5 The tech- no logy r e l a t ed to packag ing r a the r than the increas ing chip dens i ty may be the issue l imit ing device p e r f o r m a n c e . 6

The m e c h a n i s m by which an organic coa t ing p ro tec t s a meta l l i c ma te r i a l f rom the e n v i r o n m e n t is a comp lex and not well u n d e r s t o o d process . So rp t ion and t r anspo r t of cha rged ( ions) and u n c h a r g e d (wate r , oxygen) species affect the co r ros ion b e h a v i o r of the po lymer i c /me ta l l i c sys tem. 9-12 M a n y p r o p e r t i e s of p o l y m e r s (p rocessab i l i ty ;

Manuscript received 23 July 1991; in amended form 11 November 1991.

1203

1204 F. BELLUCCI et al.

elec t r ica l p rope r t i e s ; chemica l , t he rma l , mechan ica l and e n v i r o n m e n t a l s tabi l i ty) affect the i r sui tabi l i ty and re l iabi l i ty as p ro t ec t ive o rgan ic coat ings . 13,14

In this p a p e r the use o f P M D A - O D A * PI as an organic low die lec t r ic cons tan t packag ing ma te r i a l (mul t i leve l d ie lec t r ic ) in mic ro-e lec t ron ics was inves t iga ted . PIs (po ly imides ) a re a class o f p o l y m e r s wi th a high glass t rans i t ion t e m p e r a t u r e Tg, (e .g . 325°C for B T D A - O D A * and 400°C for P M D A - O D A ) and an open mo lecu l a r s t ruc ture which m a k e s p e r m e a t i o n of wa te r molecu les and oxygen inevi tab le . 15-21 In add i t ion , the p re sence of c o n t a m i n a n t ions (Na +, C I - ) and gases (SO2, H2S, e tc . ) , in high humid i ty env i ronmen t s , can resul t in the in i t ia t ion of cor ros ion . 1-5 A n under - s tanding of the co r ros ion b e h a v i o r of the PI /meta l l i c sys tem, may the re fo re r ep re sen t the ra te d e t e r m i n i n g s tep in improv ing the re l iab i l i ty of ICs.

M a n y pape r s have add res sed the in ter rac ia l in te rac t ion b e t w e e n PI and meta l s , 22-26 whereas the e n v i r o n m e n t a l d e g r a d a t i o n of the PI /meta l l i c subs t ra te sys tem is less well d o c u m e n t e d , z7 The p u r p o s e of the work desc r ibed he re in is to p rov ide an unde r s t and ing of the p ro t ec t ive p r o p e r t i e s of the PI on the AI and Fe meta l l ic subs t ra tes using a.c. ( i m p e d a n c e spec t roscopy) and d.c . (po la r i za t ion res is tance m e t h o d ) e l ec t rochemica l techniques . The th ickness of the PI inves t iga ted ( P M D A - O D A ) , va r i ed be tween 1 and 3 /~m in the case of the PI/A1 sys tem, and b e t w e e n 5 and 15/~m in the case of the P I /Fe sys tem, respec t ive ly . This p a p e r will address the effect of: (i) s to rage t ime , (ii) film th ickness and (iii) t e m p e r a t u r e and R H on coa t ing p e r f o r m a n c e . In add i t ion , an a t t e m p t will be m a d e to clarify the role of ionic con taminan t s on l i fe t ime and to re la te fa i lure to the ionic diffusion across the PI. A m a t h e m a t i c a l m o d e l is p r e s e n t e d tha t al lows the d i sc r imina t ion b e t w e e n fa i lure due to e lec t ro ly tes p e n e t r a t i o n th rough r e d u c e d pa thways (defects o r he t e ro - genei t ies ) of the coa t ing , and fa i lure due to the in ter fac ia l adhes ion loss.

EXPERIMENTAL METHOD Coating preparation

Commercially available PMDA-ODA was investigated in this paper. PMDA-ODA is the synthetic analogue of Kapton PI. Polyamic acid as PI precursor was supplied by E. I. du Pont de Nemours and Company, as the amic acid solution of poly [N,N'-(p,p'-oxydiphenylene) pyromellitimide]. The polymer film of interest was deposited on a 2.5" (6.35 cm) diameter wafer using the spin coating technique (6000 rpm for 90 s) and then cured. A r-stage cure (135°C for 10 min in air) followed by a final curing step at 400°C for 45 min in nitrogen was adopted.

Samples The metallic substrates for corrosion testing were aluminum produced by electron beam evaporation

of aluminum onto silicon wafers and pure (99.998%) Fe. The aluminum deposition was carried out at a vacuum base pressure of about 10 -8 Torr. The thickness of the deposit was typically 1000 ,~. The silicon/aluminum (Si/AI) wafers and the iron metallic substrate were subsequently spin coated with the PI as described above.

Apparatus The experimental apparatus for the electrochemical investigation consisted of a perspex cylinder

attached to the surface of a coated substrate in order to create a vessel to hold the electrolyte. The coated metal was the working electrode. A saturated calomel electrode (SCE) was used as the reference electrode. All reported potentials are referred to the latter electrode. A platinum foil was used as the counter electrode. Except where otherwise specified, specimens were stored for 2 months in a desiccator

*PMDA-ODA is made from pyromellitic dianhydride (PMDA) and oxydianiline (ODA) while BTDA-ODA is made from benzophenone tetracarboxylic dianhydride (BTDA) and oxydianiline.

Corrosion initiation on polyimide coatings 1205

before testing. Samples tested were those which did not exhibit any pinholes according to the following test.

After storage in a desiccator for 2 months, the samples were assumed as being dry and the film capacitance, G, was determined using the impedance spectroscopy (a.c.) technique (see below) and the apparatus described above. Mercury was used as the electrolyte in order to avoid water and salt contamination of the specimen (exposure to an aqueous salt solution before the start of the test may lead to an initiation of the degradation process, with resultant artificially short measured lifetime). Two different impedance spectra were observed among the samples investigated. The first exhibited resistive components at low frequencies, while the second showed pure capacitive behavior even at 1 mHz. The resistive component observed in the former case was attributed to pinholes or diffuse heterogeneities. The samples that exhibited a resistive component were considered as defective and were discarded.

Methods Electrochemical impedance spectroscopy was performed at the corrosion potential in the case of

aqueous solutions of NaCl (see below), and at zero applied potential when mercury was used as the electrolyte. Experimental tests were carried out using a Schlumberger Solartron 1250 Frequency Response Analyzer connected to a Solartron 1286 potentiostat. The analyser was controlled by a Hewlett- Packard desktop computer. Usually, 10 points were measured for each decade of frequency. The frequency used ranged between 65 kHz and 1 mHz. The amplitude of the superimposed potential was 5 inV.

The polarization resistance method, developed by Stern and Geary 2s from the earlier work of Wagner and Traud, 29 is currently applied to painted metals and is considered, to some extent, a non-destructive test. This test method is, however, destructive (to some extent) in nature because ion movement is forced through the paint film in one direction or another (depending on the polarity of the d.c. applied voltage), causing acceleration or deceleration of corrosion degradation of the painted metal. To reduce this effect, the overpotential applied in the polarization resistance method was l0 mV periodically reversed to balance ion movement in the anodic and cathodic reaction. Since the original relationship only applies in the absence of ohmic potential drop, which is not the case under study, the terminology 'pseudo- polarization resistance' will be used to describe these measurements. The pseudo-polarization resistance of the coated sample was evaluated by measuring the steady-state current after an anodic and cathodic polarization of 10 mV has been applied. The value taken was the average of three measurements.

Air saturated sodium chloride (up to 0.5 M) was the test solution in the cases where an aqueous electrolyte was employed. It was prepared from reagent grade material and distilled water (18 MI) cm resistivity). The temperature was 20 +_ l°C.

The transparent nature of the PI allowed visual evaluation of the delamination area as a function of the immersion time without perturbing the system under study. The delamination area was measured by taking periodically pictures of the specimens and evaluating the number and the area of each corrosion spots observed.

E X P E R I M E N T A L R E S U L T S A N D D I S C U S S I O N

A s u r v e y o f t h e l i t e r a t u r e r e v e a l e d t h a t c o r r o s i o n f a i l u r e o f e l e c t r o n i c m a t e r i a l s

a n d d e v i c e s is a c o m p l e x a n d n o t we l l u n d e r s t o o d m a t t e r , o f t e n d e p e n d e n t o n t h e R H

a n d o n t h e e l e c t r i c a l s t a t e o f t h e e q u i p m e n t . ~-6 N o e x p e r i m e n t a l w o r k , h o w e v e r , h a s

b e e n r e p o r t e d d e s c r i b i n g t h e e l e c t r o c h e m i c a l a n d c o r r o s i o n p r o p e r t i e s o f P I / m e t a l l i c

s u b s t r a t e s d e s p i t e t h e e x t e n s i v e u s e o f P I as d i e l e c t r i c in t h e p a c k a g i n g o f i n t e g r a t e d

c i r cu i t s . 6-s'13 S o m e e x p e r i m e n t a l r e s u l t s d e s c r i b i n g t h e e l e c t r o c h e m i c a l a n d c o r r o -

s i o n b e h a v i o r o f P I /A1 a n d P I / F e s y s t e m s in a e r a t e d N a C I s o l u t i o n as a f u n c t i o n o f

i m m e r s i o n t i m e b y u s i n g t h e i m p e d a n c e s p e c t r o s c o p y a n d t h e p o l a r i z a t i o n r e s i s t a n c e

t e c h n i q u e s a r e d e s c r i b e d b e l o w .

The PI/AI system The effect of storage time. M e t a l l i c s u b s t r a t e s (Si /A1) w e r e i d e n t i c a l l y p r o c e s s e d

a n d s p i n c o a t e d w i t h P I ( 1 . 2 / ~ m t h i c k ) . S o m e s p e c i m e n s w e r e i m m e r s e d in a e r a t e d

1206 F. BELLUCC! et al.

10

E u 8

E J = 0 ,. 6

---: 4 N

_o 2

1.2 ~m PI/AI 0.5 M NaCI, Sample "A"

Od " . . . ,

l d . . . " . . . : : : : ! : : ; : .

3d "":ilh," " h .

8 d ................ "'~.

........................ ,,,.,,. • , . :~

I I I I I I

2 0 2 4 - 1

log o~, rad s

100 PI/'AI NaCI, 1 2 p m 0.5 M Sample

8 0 , . . - " : : : ~ : : : . . . . . . . . . . . . ' a ) Od -

a "" " ' . : : : "

6 0 l d "

_~ . " " . "8d

=m 40 ' "'.' 3d ' • . . . : . . "

G) " ' " " . . . . . . 1 4 d . . " 20 "" " ' ' ""

".'""..:.., .::::i ..................... "'"'" r - O . ' . . , - ' . "-

0 i I ' i I i I

- 2 0 2 4 - 1

log co, rad s

FIG. 1. (a) Bode plot as a funct ion o f immersion time for the 1.2/xm thick PI /AI system (sample A ) in air-saturated 0.5 M NaC1 at room temperature; (b) phase angle plot as a function of immersion time for the 1.2/~m thick PI/AI system (sample A) in air-saturated

0.5 M NaC1 at room temperature.

0.5 M NaCI immediately after curing (sample 'A'), while others were first stored in a desiccator for 2 months before testing (sample 'B').

Figure l(a) and (b) show the Bode and the phase angle plots for sample A as a function of immersion time. The rest potential E0 was also recorded. The E 0 vs time behavior will be analysed in a subsequent section together with the film capacitance, Cf time decay plot.

After 1 day of immersion, the film response departs, at low frequencies, from a purely capacitive behavior ( -1 slope on the Bode plot). The resistive component at low frequencies is associated with the film resistance Rf. The value observed is of the order of 107 ~r~ c m 2 . Failure by corrosion of the metallic substrate has been observed for a great number of cases as soon as Rf falls below 10 7 ~ cm2. 3°

After 8 days of immersion, the Rf falls below the criterion value and, as expected, a more complex impedance spectrum is observed. In this latter case the Bode and phase angle plots show two time constants (rf for the film and rot for the metal), given by:

r f ---- R f C f ; "t'ct = R t C d (1)

where Cf is the film capacitance, Rt is the charge transfer resistance and Cd the double


E O

E o r -

N_

o

10

2 - 2

1.2 ttm PI/AI 0.5 M NaCl, Sample "B"

0 -30d 47d .......

. . . . . . . . . . . . . : : . . . . . "::=.

53d"":;:.:: .. "'--. .......... ":~??:.

60d ............ ii ~:.,.~ ,. "~qh =.

' % .

I I , I

0 2 4 - I

log co, rad s

100

• 80

~ 6o

~ 40

I D

~ 20

I1, 0 - 2

• . . . . . . . . . . . - . . . . . .

47d" .' 53d -..

i " '6o~

- " . .

. ' : . . . . . , . . . . . . . . "

I i I I I I

0 2 4 - 1 log co, rad s

FIG. 2. (a) Bode plot as a function of immersion time for the 1.2/~m thick PI/AI system (sample B) in air-saturated 0.5 M NaCI at room temperature; (b) phase angle plot as a function of immersion time for the 1.2/~m thick PI/AI system (sample B) in air-saturated

0.5 M NaCI at room temperature.

layer capacitance, respectively. The time constant at higher frequencies is due to the film if rf < lrct , which is usually the case. The experimental results reported in Fig. l (b) show that with prolonged immersion time (14 days) diffusional processes within pores in the film were taking place.

Results of sample B are reported in Fig. 2(a) and (b). As can be seen, the Bode and phase angle plots exhibit the same features as those exhibited by sample A but on a different time scale. Pure capacitive behavior is observed for immersion times up to 30 days, followed by the appearance of a resistive component with prolonged immersion while two time constants can be seen at longer immersion times (53 days). Also in this case, a second time constant was observed when the film resistance fell below 107 ~ cm 2. No diffusional processes were observed in this case even after 92 days of immersion. The latter data were omitted for the sake of simplicity.

The impedance response over the lifetime of the film reported in Figs 1 and 2 can be related to the various stages of degradation at the PI/A1 interface. Initially, the film acts as a pure dielectric separating the metallic substrate from the aggressive external environment. This behavior results in purely capacitive behavior (see for example Fig. la after i day immersion and Fig. 2a for immersion times up to 30 days). The second step is water and ion uptake within the film, either homogeneously


FIG. 3.

101

E o

. 10 0

• o 10 "1

f,,

"~ lo-2

10 -a

1.2 p.m PI/AI 0.5 M NaCI Sample "A"

~ le "B"

i l I l i , i ,

100 200 300 400 500

t ime, h D e l a m i n a t i o n a r e a as a f u n c t i o n o f i m m e r s i o n t ime fo r the s a m p l e s A a n d B ,

r e spec t ive ly in a e r a t e d 0.5 M NaC1 s o l u t i o n a t r o o m t e m p e r a t u r e .

through the film or heterogeneously due to defects. At this stage the water and ion uptake can modify the capacitive response leading to either one (see Figs la and 2a up to 3 and 47 days of immersion, respectively) or two time constants (Figs la and 2a after 8 and 53 days, respectively). Two time constants are indicative of the first separation of film and metal substrate properties. With continued immersion, the film resistance decreases due to both water and ionic species that are absorbed into the film. 9 At longer immersion times, the rate of diffusion of species through pores in the film may become comparable with the charge transfer process and a diffusional response may appear (see Fig. 1 after 14 days).

The time of failure was taken as the time required to separate the film and the metal response. On this basis, the sample A exhibits a lifetime ranging between 3 and 8 days, while the lifetime of sample B ranges from 47 to 53 days. There is not a simple explanation for such a macroscopic difference in lifetime between otherwise similar samples. An attempt is given in the following section to explain this result, while a deeper analysis will be presented in the Mathematical model section.

The electrochemical and corrosion behavior described in Figs 1 and 2 indicates that the curing and processing of PI may affect the environmental degradation of the PI/AI system. Residual stress, due to curing, and osmotic stress, arising when the PI is exposed to an aqueous NaC1 solution due to trapped water at the interface, can contribute to a local microcracked area through which ions can easily diffuse to reach the AI interface. Samples stored in a desiccator can release both thermal stress and trapped solvent and water. Water and solvent release may also enhance the adhesion of the PI at the metallic interface leading to an extended lifetime. Even if both samples did not exhibit any pinholes according to the test described in the experimental section, the reliability of the coatings was somewhat different. The more time allowed for sample equilibration in the desiccator, the better the sample performed. The rate of delamination (evaluated as described in the experimental section) and the type of corrosion observed were also different. Sample A corroded more homogeneously whereas sample B showed few localized black corrosion spots. This phenomenon can be clearly seen by the presence of a diffusion tail on the phase angle plot at low frequencies after 14 days of immersion in NaCI (Fig. lb). The higher rate of delaminated area for sample A as shown in Fig. 3, supports the conclusion that corrosion is taking place over the entire surface.


FIG. 4.

1 0 t - i i

2.4 ~ m P I /A I 0 .5 M N a C I

0 d . . E

8 2 5 d .... .. ' : : :~: . . . . . . . . . . .

" - 1 : . >

.E 6 "':::. : : : : : .

N_-- ~3o~ .................... ...... ---: 4 ................ ".i:::::, N " " . - - %..

2 i i i i i i i

2 0 2 4 - 1 log e), tad s

Bode plot as a funct ion o f immersion t ime for a 2.4/~m thick PI /AI system in air-saturated 0.5 M NaCI at room temperature.

The effect of film thickness. The effect of the coating thickness on the coating lifetime was also investigated. Two Si/AI samples were coated with 1.2 and 2.4/~m thick PI, respectively. These samples were processed according to the procedure described above and stored in a desiccator for 2 months before use. The electrochemical behavior exhibited by the 1.2/~m thick coated sample was similar to that reported in Fig. 2 and is omitted for the sake of simplicity. The Bode plot for the 2.4/~m film in aerated 0.5 M NaCI solution at room temperature is shown in Fig. 4.

Departure from a pure capacitive behavior was observed after 25 days of immersion. A second time constant characterizing the onset of an interracial electrochemical process was observed after 130 days. This time was taken as the sample lifetime. The rate of delamination and the distribution of corrosion spots were similar to those observed for sample B. As can be seen from the Bode plot, coating degradation occurs when R e falls below 107 ~ cm 2. This observation is in agreement with the previous results. In the mathematical section an attempt will be given to relate the observed macroscopic difference in the lifetime experimentally observed for the PI/A1 samples to the coating's structure and thickness, and to the interracial adhesion loss.

The effect of temperature and RH. The synergistic effect of the temperature and RH was investigated by carrying out impedance spectra at 80°C and 85% RH. These conditions are the same as those used in one of the accelerated tests adopted to evaluate chip failure. 6 Samples were stored in a desiccator for 2 months prior to usage. Mercury was used as the electrolyte in order to avoid salt contamination. This technique is similar to the experimental technique adopted by Leidheiser. 11 This technique is commonly used to measure the degradation of coatings without exposing the specimen to an aqueous electrolyte during the measurement. The mercury provides electrical contact to the sample surface (through an immersed Pt electrode) but can be easily removed in order to expose the specimen at 80°C and 85% RH.

Results are shown in Fig. 5(a) and (b) in the form of Bode and phase angle plots. After 8 days of exposure at 80°C and 85% RH, the film departs from a pure capacitive behavior and a resistive component at low frequencies (of the order of 107 ~ cm 2)


E 0

E J ¢ O t -

O~ o

10

2 - 2

• ' o ' 1,2 ~m PI/AI 80 C, 85% RH

2d " ' ,

8d ".. • - . . . . " ' " " - . . "" ' . .

~8~i ..... .... " ...... ::....

......... :::!iii::.. ' " ; ; : : : ,

I i I i I i

0 2 4 - 1

log co, raft s

100

8O

~ 60

d ~ 40

0 ~ 20 lU

O .

0 - 2

2d" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . : .

8d ..." ................ . . . . . . - " '

. . . , . " . . . . . . . . , . . ,."

• :'""" ".. ." "18d . . . . . .

I , I i I i

0 2 4 - 1

log co, rad s

Fla. 5. (a) Bode plot as a function of exposure time at 80°C and 85% R H for a 1 .2#m thick PI/A1 system: the impedance measurements were carried out in Hg as the electrolyte; (b) phase angle plot as a function of exposure time at 80°C and 85% R H for a 1.2/~m thick

PI/AI system. The impedance measurements were carried out in Hg as electrolyte.

was observed. After 18 days of exposure, a second time constant was observed. At the same time black corrosion spots were visually detected.

The appearance of the second time constant probably indicates the onset of an electrochemical process at the PI/AI interface. The impedance spectra were carried out using mercury as the electrolyte; C1- ion contamination could not result from the experimental procedure adopted. There are, however, two possible routes for ionic contamination, namely (i) surface contamination due to handling, and (ii) residual impurities coming from the PI itself. The synergistic action of the high temperature and R H enhance ionic diffusion which ultimately leads to early failure of the PI/AI system. It should be also pointed out, however, that the similarites between the Bode plots observed with mercury as the electrolyte and with an air-saturated 0.5 M aqueous solution of NaCI, do not necessarily mean that the same degradation process is occurring in both cases.

The PI/Fe system The corrosion behavior of PI/Fe samples was investigated by using the pseudo-

polarization resistance method and the impedance spectroscopy. Experiments were


F I G . 6 .

1°8 r o E 107 f

• ~ 10 6

n, fl" |

lO s

10 4

i i i

5 /am PI/Fe 0.5 M NaCI

I I I

200 400 600

time, h

800

Polarization resistance Rp as a function of immersion time for the 5 #m thick PI/Fe system in air-saturated 0.5 M NaCl at room temperature.

carried out only on samples stored in a desiccator for 2 months. The Rp measurements were per formed on three different specimens in order to assess reproducibility of data.

Figures 6 and 7 show the Rp as a function of the immersion time for Fe samples coated with 5 and 15 # m PI, respectively. Results reported in these figures clearly indicate the lack of reproducibility. Variation of one or two orders of magnitude in the values of Rp at the same immersion time were observed among otherwise similar samples. The values of Rp and its change with immersion time are commonly used indicators of poor protection according to the criterion reported in the literature. 3° Deeper insight on the corrosion process taking place at the PI/Fe interface can be obtained by the impedance spectroscopy technique, as will be shown below.

As for the PI/A1 system, the experimental findings will be reported in terms of Bode and phase angle plot as a function of immersion time. Figure 8(a) and (b) shows the data for the 5/xm thick PI while the data for the 15/~m samples are reported in

F I G . 7 .

I0 s

10 7

i i i

15 I~m PI/Fe 0.5 M NaCt

E u

E .= 10 6 0

. a.

105

1 0 4 , i i i

0 2 0 0 4 0 0 6 0 0 8 0 0

time, h

Polarization resistance Rp as a function of immersion time for the 15 Ftm thick PI/Fe system in air-saturated 0.5 M NaC1 at room temperature.

1212 F. BELLUCC[ et al.

o

E 0

C

_o

2 -2

, , ,

5 pm PI/Fe 0.5 M NaCI

0d , , . , . . . . . . . . . , . . . .

• ' , . . , . . . . . . . . . . . . . . . . .= : : . . . .

... l h 3d ............... " . " " ' . . . . . . , . . .

"" ' " , , . . . . ? . . . . .

I , I I

0 2 4 - 1

log co, rad s

log Z 1

log Z 2

log Z 3

100

• 80

60

C ~ 40

" 20

0 - 2

, i

5 lam PI/Fe 0 5 M NaCI i

. . . . I . . . . . . . .

Od ." .? ' . ' •

. . . : , i . . " '

lh "

3d ......................... :::ii..:::: . . . . . . . "

. " ."

' " . . , . . : = : : . . . . . . " ' " '

I i I i

0 2 4 - 1

log co, tad s

FIG. 8. (a) Bode plot as a function of immersion time for the 5/~m thick PI/Fe system in air-saturated 0.5 M NaCI at room temperature; (b) phase angle plot as a function of immersion time for the 5 #m thick PI/Fe system in air-saturated 0.5 M NaCI at room

temperature.

Fig. 9(a) and (b). A quick departure from a purely capacitive behavior for the PI/Fe system compared to that of the PI/AI system is observed. Two time constants were clearly observed after 1 and 5 h of immersion for the 5 and 15 #m films, respectively. The observation of a second time constant indicates the onset of a corrosion process occurring at the metallic substrate. The results of Figs 8(b) and 9(b) show that, after 3 days of immersion, a diffusional process within pores in the film is taking place. As in the case of the PI/AI system, the time of failure was taken as the time required to separate the film and the metal response. On this basis, the 5 ktm coated Fe sample exhibits a lifetime of 1 h, while the lifetime for the thicker sample is 5 h. Both lifetimes are of orders of magnitude lower than those exhibited by the coated and stored A1 samples. The metallic substrate, thus, can affect the corrosion behavior of the PI/metallic system in aerated 0.5 M NaC1 solution. As will be shown in a subsequent section, the dielectric properties of PI are not affected by the metallic substrate when exposed to a dry environment.

Dielectric properties of applied PI In this section the dielectric properties of the coating will be discussed as they


E O

E ,,C O

o

, i f

15 lam PI/Fe 0,5 M NaCI

5h

3d

Od

....... :::::iiiiiii?-.... • . . " - , . .

...... .::;:::::.

2 - 2

, I I I

0 2 4

log co, rad s ' l

(I) .= IO} O

" O

C a

0

f -

O .

100

80

60

40

20

0 - 2

15 I.tm PI/Fe 0.5 M NaCI . , . : : ":

. . . i : : z' . . '= . . . " ..

0d . ." '

5h .........-..i:i:i.....'...: ::::'::""

.. " . . i : ) " 3d ".-.--.

I I

0 2 4 6 - 1

log o~, rad s

FIG. 9. (a) Bode plot as a function of immersion time for the 15/~m thick PI/AI system in air-saturated 0.5 M NaCI at room temperature; (b) phase angle plot as a function of immersion time for the 15 #m thick PI/AI system in air-saturated 0.5 M NaCI at room

temperature.

relate to the corrosion of the metallic substrate, to the water and salt uptake and to the film thickness.

Film capacitance and rest potential time decays• The variation of rest potential and the film capacitance as a function of immersion time for the PI/A1 system (samples A and B) and for the PI/Fe system are reported in the form of a double plot in Figs 10 and 11, and in Figs 12 and 13, respectively. The values of Cf were obtained from the high frequency part of the Bode plot. These values were in good agreement with the values as calculated at the frequency of 6.5 kHz where the reactive component of the total impedance approaches the value 1/(2:rfCf). In Figs 10 and 11 are also reported: (i) the pitting potential of pure A1 in neutral aerated 0.5 M NaC1 equal to - 70 0 mV(SCE) (arrow on the right hand scale), 31 and (ii) the time at which black corrosion spots were visually observed (arrow on the upper scale). The latter time is also reported for the PI/Fe system in Figs 12 and 13.

Both PI/AI samples (A and B) exhibited the same capacitance value after immersion in the test solution (3.89 nF cm-2). The rest potential, however, was quite different. Sample A exhibited a potential of - 760 mV whereas sample B exhibited a


s . o . . . . . . . . . . . . 1 0 0 0

I Sample "A" / ~

- - - - a - - - Cf [ "m

E > u

,, o ~ c

O " 4 , 0

. . . . . . . a . . . . . . . . i . . . . . . . i , . -lO0O 3'50"1 100 101 102 O"

t ime , h

FIG. 10. Rest potential, Eo, and film capacitance, Cf, as a function of immersion time for the 1.2 #m thick PI/A1 system (sample A) in air-saturated 0.5 M NaCI solution at room

temperature.

potential of - 2 2 0 mV. The less noble potential is very close to the AI pitting potential. This indicates the presence of reduced pathways for the external salt solution. These pathways could be either holes invisible to the impedance test with the mercury electrolyte described in the experimental section, or may be due to diffuse heterogeneities. The value of Cf, however, indicates that the PI was not yet affected by the corrosion process at the interface. This is because the electrochemical reaction taking place at the metallic interface occurs at low rates as described below.

Pure AI in aerated 0.5 M NaCI solution exhibits a mixed potential equal to -1200 mV(SCE). 31 The AI corrosion rate, even in high NaC1 concentration, is negligible due to the poor catalytic behavior of the A1 for the oxygen cathodic reduction. 31 Significant corrosion rates can only be observed at potential values above the pitting potential [ -700 mV(SCE)]. It can be concluded that - 7 6 0 mV as rest potential may well be compatible with low corrosion rate. After 1 day of immersion, a potential of - 640 mV(SCE) (above the pitting potential) and a film

s,o ;.2.~,~, Pi#,io:~ MNa5', . . . . . ~ . . . . . . t000 Sample "B" I ~

~, 4,5 E • u ,, 0 E C o P I,M O" 4,0

3,5 . . . . . " . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -1000 0 "1 10 0 10 1 10 2 10 3 10 '

time, h

FIG. 11. Rest potential, E0, and film capacitance, Cf, as a function of immersion time for the 1.2 #m thick PI/AI system (sample B) in air-saturated 0.5 M NaCl solution at room

temperature.


1,4

E o 1,2

t , I . P

1,0

0,8

. . . . . . . i . . . . . . . i . . . . . . . . i . . . . . . . . , . . . . .

5 p.m PI/Fe 0.5 M NaCI

\ l

0-2 10-1 10 0 101 102

t, hr

-400

E -600 -

t. o

-800 O-

FIG. 12, Rest potential, E0, and film capacitance, Cf, as a function of immersion time for the 5/~m thick PI/Fe system in air-saturated 0.5 M NaC1 solution at room temperature.

capacitance of 4.19 nF cm-2 were observed. Both trends indicate the occurrence of a degradation process at the AI interface. Even if no visual corrosion spots were observed, the tendency of the PI/AI system is towards early failure.

The observed increase of the rest potential with time may be due to: (i) presence of cathodic sites (PI itself, contaminants on the metallic substrate, aluminum compound formed during the curing of PI), (ii) macroscopic defects originated by microcracks due to unreleased stress, and (iii) A1 surface heterogeneities. For prolonged immersion, the rest potential varied between -700 and -587 mV while the film capacitance reached a maximum value of 4.8 and then decreased to 4.6 nF cm -2. At this stage, black corrosion spots were visually detected.

Figure 11 shows the results of sample B. The time decay of both E 0 and Cf exhibited a less dramatic behavior. A constant Cfwas measured up to 120 h followed by a smooth increase from 120 to 456 h of immersion. A sharp increase was then observed with a maximum value (4.87 nF cm -2) very close to that of sample A. It is remarkable to observe the occurrence of the sharp decrease of E0 [from -480 to -900 mV(SCE)] at roughly the same time where a sharp increase in Cewas observed.

0,46 ........ , ........ , ........ , ........ , . . . . . . . 400 15 urn PI/Fe 0.5 M NaCI

~, 0,44 ~ _ ~ i Cf

E 0,42 - ' ~ \ ~ = o >

~- / ~ -6oo E =. o" o,4o =o

0,38

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . -800 0 ' 3 6 0 2 10 "1 10 o 101 102 10-

l ime, h

FIG. 13. Rest potential, E0, and film capacitance, Cf, as a function of immersion time for the 15/~m thick PI/Fe system in air-saturated 0.5 M NaCI solution at room temperature .


The lowest potential measured in this case [ -900 mV(SCE)] may be ascribed to the absence of impurities at the interface or to the reduced contaminations in the curing process. Oxygen reduction on pure AI is a very slow process. Thus, also in this case cathodic sites were operating. Once the pitting potential is established, the electrochemical degradation is fast and failure (black spots) can be detected.

At longer immersion time, sample B exhibited behavior different than that of sample A. An increase of the rest potential was observed with a final value of - 9 0 mV. This result, in conjunction with the impedance response (absence of a diffusive component in the phase angle plot), can be explained assuming that the cathodic and anodic sites were separated. The anodic sites correspond to the points at which corrosion spots were observed, while the rest of the metallic surface acts as the cathode. Sample A showed a stable rest potential even after corrosion spots were observed. This trend together with the appearance of a diffusive tail (see Fig. 1 at low frequencies) indicates that anodic and cathodic area coincides. The degradation of the PI/AI system seem to be dispersed rather than localized in this case.

The time behavior of the film capacitance exhibited by the PI/Fe systems is similar to that shown by the sample A, as described above. A sharp increase in the film capacitance (from 0.82 to 1.4 nF cm -2) was observed for early immersion followed by a sharp decrease at longer immersion. Also in this case it is remarkable to observe the occurrence of the sharp increase of Cf at the same time where a decrease of the rest potential was observed. Both samples exhibit a continuous decrease of Eo that appears more pronounced for the sample with the thinner coating. A potential of - 62 0 mV(SCE), a value close to the corrosion potential of pure iron in aerated 0.5 M NaCI solution, was observed after 1 h of immersion. This result indicates that the salt solution has penetrated through the coating reaching the metallic interface. At this stage the corrosion process can initiate. The value of Cf, 1.07 nF cm -2, indicates that the PI has been affected by the electrochemical reaction. Even if no visual corrosion spots were observed, the tendency of the PI/Fe system is toward early failure. Black corrosion spots were visually observed after 35 h of immersion.

The time decay of the 15/~m coated Fe sample is less dramatic. Penetration of the electrolyte through holes or heterogeneities occurs at a reduced rate due to the increased thickness and visible corrosion spots were observed after 120 h of immersion in the test solution.

The results described in these figures show a close analogy between reliability and film capacitance. A rise in capacitance preceded visible breakdown: Cf measurements could, thus, be used as a non-destructive test to detect early failure of organic packaging materials on metallic substrate.

Capacitance measurements have also been used to determine the effective lifetime of a coating.32 A rise in film capacitance preceding visible film breakdown in aerated artificial sea-water was reported in the literature and ascribed to a different mode of water uptake in the coating. 32-35 A detailed discussion on the mode of water uptake is described in the subsequent section.

Water uptake. The dielectric properties of a coating varies as water is taken up. The film capacitance increases as a function of the immersion time in NaCl solutions. This increase in capacitance has been associated with water penetration into the coating. 32


FIG. 14.

'?, E

i i C

o"

Film "capacitance'of dr; supp'orted 'Pl

..,,'°

...'""P I / A I ....'"" c =3.24

o

I I

0,50 0,75 - t

l /L, 1.tm

Pl/Fe c =4.57.

j , "

1 ,e/" o"**" ...J ..'"

f / ..,'" z~.'..oO."

0 ~ u,O0 0,25

Film capacitance, Cf, as a function of the reciprocal of the PI film thickness for dry PI/AI and PI/Fe systems at room temperature.

The amount of water taken up by the coating can be calculated from the capacitance values using the following relationship: 33

Xv = 100 log (Cf,t/Cf,o) (2) log 80

where Xv is the volume percentage of water taken up by the film, and Cf,t and Cf,o are the film capacitances at time t and zero, respectively. Equation (2) was derived assuming: (i) a linear relationship between the permittivity of the polymer-water system and those of the pure components, (ii) a random distribution of water, and (iii) a linear relationship between permittivity and capacitance. Reasonable agreement between water uptake determined gravimetrically and calculated using equation (2) was observed for relatively small amounts of water taken up. 34 Poor agreement was observed at higher water content. 35

By applying equation (2) to the data reported in this paper for the 1.2 (sample A and B) and 2.4pm thick PI, an estimate of water uptake equal to 2.7% vol/vol 4 wt% was obtained. This result in conjunction with the values of the rest potential, suggests a different distribution mode of the water in sample A that leads to an early failure of this sample when immersed in the salt solution. The water taken up by the 5 and 15 /xm PI was equal to 4.7% vol/vol (7.0 wt.%). The water taken up by the PI coated on AI is in the range of the values for PI reported in the literature. 17-21 Somewhat higher is the value of the water taken up by the PI coated on Fe. This result may suggest that the additional water taken up by the PI/Fe system can be distributed into the coating (holes) or at the polymer/metallic interface.

The effect of coating thickness. Assuming that the PI behaves as a pure capacitor, the permittivity e can be calculated by the following equation:

1 Cf : CE 0 ~ (3)

where Cf is the film capacitance in nF cm-2 e0 is the absolute permittivity (8.854 × 10 -14 F cm-2), and L is the film thickness in cm. Figure 14 shows the capacitance Cf as a function of 1/L for the dry PI/AI and PI/Fe systems investigated. The experimen-


tal points lie on a straight line that extrapolates to the origin of the axis in agreement with equation (3). The estimated relative permittivity is equal to 3.24 and 4.57 in the case of the PI/A1 and PI/Fe systems, respectively. These values are the range of the data reported in the literature for PI. 7"37 The AI and Fe metallic substrates, therefore, do not affect the PI dielectric behavior at least when the PI is in the dry state.

The effect o f NaCI. An increase of the coating capacitance for prolonged immersion in salt solution has been reported in the literature and attributed to an increase in the water uptake or to a different mode of the water distribution into the film. 32'35 Since this rise preceded visible breakdown, it can be concluded that water uptake and its distribution into the PI is affected by the salt content of the film and the salt transport through the PI. The follwing arguments support this hypothesis.

When a coated metal is immersed in an aqueous salt solution, the first step of the overall degradation process is water and salt uptake. Water and salt uptake in a homogeneous free standing polymeric film of thickness L can be described according to Fick's law by using the dimensionless time variable r = tD/L 2 (D is the diffusion coefficient). 38 The thermodynamic equilibrium between the external and internal (film) solution, occurs for r - 1. Accordingly, the time required to saturate a supported PI film of thickness L (neglecting the effect of the metallic substrate), is given by:

t -> (2L)-----f2 (4) D

The diffusion coefficient D of water and of CI- across PI have been reported in the literature as 2 × 10 -9 cm 2 s - l , 17"20'21 and 1 × 10 -13 cm 2 s - l , 39 respectively. A 1.2/~m thick PI film will be, thus, saturated by the water and the CI- after 29 s and 160 h, respectively. The latter time is of the same order of magnitude as the time at which the rise in the film capacitance was observed (see Fig. 11). Since ions are taken up by a polymeric material with their hydration shell, the latter result suggests that the observed rise in capacitance could be attributed to the ion bound water. It is assumed also in this case that a random distribution of the ion bound water exists in the PI. Thus, equation (2) can still be applied.

Due to the low dielectric constant of the polymer-water-sal t solution (of the order of 10 in this case) 4° water molecules are weakly bound to the ions. At this time, the homogeneous dispersed water in the film may cluster with the water associated to the absorbed ions leading to macroscopic clusters. If such clusters are in contact, they may create paths of reduced ionic resistance through which CI- can easily reach the metallic interface to initiate pitting. Of course, the greater the external NaC1 concentration, the larger the number of columns that can be developed and the shorter the lifetime.

The time to saturate a 2.4 pm thick film with water and NaC1 according to equation (4), is 115 s and 27 days, respectively. Also in this case the lifetime experimentally observed in 0.5 M NaCI (130 days) was greater than the time required to saturate the 2.4 ,um film with NaC1 in agreement with the previous result. In addition, this result supports the idea that the 1.2 and the 2.4 pm thick films can be considered homogeneous as far as the water and the salt uptake are concerned. If heterogeneities are present salt can easily reach the interface through the pores and


early failure is expected. This aspect will be dealt with in the mathematical model section.

The time to saturate a 5 and 15/~m thick film with water and NaCI according to equation (4), is 500 s and 115 days, and 1.2 h and 3 years, respectively. The experimentally short lifetime observed in the case of the PI/Fe system clearly indicates the presence of diffuse heterogeneities or pores through which the salt solution can easily diffuse to reach the metallic interface. Early failure is, thus, expected in this case. An estimate of the pore sizes that can account for the early failure observed in this case will be at tempted in the next section.

Using a reasonable value of the activation energy (46 kJ mole -1) for ionic diffusion in polymers, 41 a 10-fold increase in D is expected if the temperature is increased from 20 to 85°C. From equation (4), this in turn leads to a 10-fold decrease in the saturation time. If salt uptake is the rate determining step in the degradation of the interface, a 10-fold decrease in the lifetime of the device is expected. Experi- mentally, a 5-fold decrease was observed (Fig. 5) in the 85°C test. This correlates well (within a factor of two) with the predicted value. This descrepancy could be, probably, ascribed to both the approximate value of the activation energy used and to a non uniform contaminants distribution into the PI.

Mathemat ical m o d e l In this section an attempt will be made to introduce and to discuss the concept and

the role of pores as related to the coating lifetime. The aim of this section is to relate coating lifetime to coating properties such as porosity, tortuosity factor, C1- ion diffusion and distribution coefficient, and coating thickness. Two approaches will be presented and discussed. In the first one it will be assumed that the solution filling the heterogeneities or pores is not in thermodynamic equilibrium with the electrolyte solution dissolved in the homogeneous part of the film, while in a second approach thermodynamic equilibrium is assumed. Before discussing these models some preliminary concepts will be introduced.

As reported above, coating breakdown is related to the underfilm corrosion process. This process can take place when all the necessary ingredients, i.e. water, oxygen and electrolyte (C1-) are available at the interface. Water, oxygen and ions are taken up into the PI film at a rate depending on their diffusion coefficient, D. Each sorption process is based on the following first and second Fick's law:

J = - D --OC (5) Ox

OC 02C - D - ( 6 )

Ot Ox 2

where J is the solute flux over a distance x, C the concentration and D is either the water, the oxygen or the chloride ion diffusion coefficient.

In order to obtain the amount of penetrant (water, oxygen and chloride ion) taken up by the coating as a function of the time as well as its equilibrium value, equation (6) must be solved with the appropriate boundary conditions. The thermodynamic equilibrium rather than the kinetics of solute uptake, will be assumed in this paper as the parameter that affects interfacial coating breakdown. As already reported, the thermodynamic equilibrium between the external and internal (film)

1220 F. BELLUCCl et al.

F~. 15.

External electrolyte ~i c19 ;4

X

. . . . . . . . ~ ~ ~ m e r i ' :if:: (d) "::ijii!::l~ i

I¢ I

(c) T L

1 N

Schematic of the coating metallic interface showing the different type of pores.

solution, occurs for r -> 1, while the time required to saturate a supported film of thickness L is given by equation (4). Since the C1- diffuse into the PI at the lowest rate compared to those of water and oxygen, then the time required to reach equilibrium between the CI- dissolved into the PI and the CI- of the external electrolyte is the rate determining step for corrosion to occur at the PI/metallic interface. This time is defined as the minimum (coating) lifetime, MLT and is given by:

M L T - (2L)2 (7) D

In effect, the MLT corresponds to the time required for the electrolyte to reach the maximum concentration at the coating/metallic interface. The time at which this interface fails can be no less than the MLT, but it can be very much larger, depending upon the Cl- ion concentration at the coating/metal interface, the kinetics of the interfacial corrosion and delamination rate, for example.

If the coating is homogeneous, then the diffusion coefficient to be inserted into equation (7) is the chloride ion homogeneous diffusion coefficient into the PI fully saturated with water. If the coating is heterogeneous, then the diffusion coefficient to be used in conjunction with equation (7) must take into account: (i) the salt transported into the pores, (ii) the salt transported into the homogeneous part of the coating, and (iii) the salt transport between the volume of solution filling the pores and the homogeneous part of the coating. In each of these cases, equations (5) and (6) must be modified appropriately to account for these additional effects. Before addressing these issues, two extreme cases will be analysed first: (i) a homogeneous film of thickness L fully saturated with water, and (ii) a stagnant water layer of thickness L separating the metallic interface from the environment. The latter case can be considered equivalent to that of a coating fully porous. In the first case D = Ds, f while in the second case D = Ds,w, where Ds, e and Ds,w represent the CI- diffusion coefficient in the homogeneous film and in water, respectively. The minimum lifetime of a coating, therefore, varies between MLTw = (2L)2/Ds,w and MLTf = (2L)2/Ds,f.

The real situation differs from the above two limiting cases. The coating will contain, to some extent, pores of not well defined geometry and structure such as those schematically reported in Fig. 15 as (a), (b), (c), (d) and (e). It should be


appreciated that the MLT for cases (a), (d) and (e), are expected to be of the same order of magnitude since in each case the electrolyte must penetrate the same effective path length through homogeneous film. On the other hand, the ultimate failure of the interface is likely to occur more readily for case (a), which includes the interface between the coating and substrate than for either case (d) or (e) which are removed from the interface. In order to account for the presence of such pores in the mathematical model, the coating porosity 0 can be defined as: 0 = Vp/V T, where Vp and V T are the volume of pores and the nominal volume of the coating, respectively. No pores size distribution will be considered in the present investigation.

In the first approach it is assumed that only pores of the type (b) and (e) exist. In addition it is further assumed that the pores diameter, dp, is large enough to neglect the flux of CI- from the solution filling the pores to the pores wall (homogeneous part of the coating). According to the schematic of Fig. 15, the flux N is assumed to be much less than the flux J. First and second Fick's law describing salt uptake into these pores are still given by equations (5) and (6). The time to saturate the continuous pores is much smaller than the time required to saturate the homogeneous part of the film. In this case the MLT, is given by:

MLT = (2L)2vP (8) DS,W

where Vp is a pore tortuosity factor defined as the ratio between the real path length and the nominal thickness [rp = 1 for (c) type pores and rp > 1 for (b) type pores]. The MLT is, thus, a function of rp and of L 2, and does not depend on the porosity 0.

As second approach, it is assumed that equilibrium exists at any time t between the homogeneous part of the coating and the electrolyte solution filling the pores. The unsteady state processes of interest are, in this case, those occurring into the homogeneous part and into the pores of the film. The solute flux into the homogeneous film, Jr, and into the pores, Jp (first Fick's law), can be written as:

j f = _ D s , f (1 -- O) Off (9) rf OX

0 OC Jp = -Ds,w - - - - (10)

Vp Ox

where rf and rp are the film and pores tortuosity factor, respectively. Due to the thermodynamic equilibrium between the solution filling the pores and the solution homogeneously dissolved into the film, the electrolyte concentration into the film, Cf, is given by: Cf -- KC, where K is the distribution coefficient that is assumed constant with the concentration.

After some manipulations, the second Fick's law describing the uptake of salt into the pores can be written as:

OC _ De q O2C (11) Ot OX 2

where Deq is an equivalent diffusion coefficient given by:


10 3 . . . . . . . . i . . . . - " • • • . . - = - • •

P /AI ......... 0"~0 ............. " 102 ....... . ...... -9 . / "

o "B" ..-" .... . ........ 0=10 ..... .......

.O ' ' ' ' y ' ' " ' " " " " " ' ' J " " " ' -8 101 : ":~A . . . . . . . . . . . . . . . • ............. O=10 .............. ,.~ O ............. . .......... . .......... -7

100 ..........-'" ........ . ........ . ............ 0=10

£, ............... . . ; 1 0 1 ....... " ....... ..e .......... P I / F e

1 0 2 " " .... 0.5 M NaCI

1°3 0 ' . . . . . . . . . . . . . . . .

1 101 10 2

L, pm

FIG. 16. MLT in days as a function of L for different values of 0 for the PI in presence of 0.5 M NaCI. In the same Fig. the experimental lifetime for the PI/A1 (samples A and B) and

the PI/Fe system in air-saturated 0.5 M NaCI at room temperature are also reported.

D~ f K ( 1 - 0) + Dsw ~ • ' r . (12)

Deq = K(1 - . 0 ) + 0

Due to the assumption made on the constancy of K with the concentration, equation (11) is equivalent to equation (6). The MLT is, thus, given by:

M L T - (2L)2 (13) O e q

Equation (13) shows that the MLT depends, in this case, on L 2 and on the equivalent diffusion coefficient, O e q that is depending on the porosity 0, on the distribution coefficient K, on the tortuosity factors, re and I'p, and on the C1- diffusion coefficient in the water and into the homogeneous part of the film. Equation (12) well describes the two limiting cases of 0 ~ 1 and 0 --~ 0 mentioned above. In the former c a s e Deq --~ Ds,w (layer of water of thickness L), in the latter c a s e O e q ~ D s , f (homogeneous film of thickness L).

The MLT theoretical findings obtained above (equations 8 and 13) can be compared with the lifetime experimentally observed for the systems investigated in this paper. In order to compare theory with experiments a criteria for the failure of a coating should be available. Different criteria have been reported in the literature to assess coating breakdown as related to water or salt taken up by the coating for prolonged immersion in NaC1 solutions. 32'3s'36 Two different paint behavior types were described in the literature, which were independent of the method to determine coating lifetime.35'36 The first showed a decrease in lifetime with an increase in NaCI concentration, 35 while the second showed the opposite behavior. 36 The appearance of a second time constant in the Bode (or phase angle) plot was used as the criterion to determine lifetime "in this paper. The second time constant is associated with an electrochemical reaction at the coating/metallic interface. These values for the PI/AI system (samples A and B) and for the PI/Fe system are shown in Fig. 16 as a function of the film thickness on a log-log plot.


The difference in the lifetime experimentally observed in the case of PI/A1 (samples A and B) and PI/Fe systems could be ascribed, according to equation (8), to the presence of macroscopic defects characterized by a difference (by orders of magnitude) of the tortuosity factor. It is reasonable, however, that due to the small values of the thickness L, rp is probably of the order of 1, therefore the experimental lifetime values cannot be ascribed to different tortuosity factors among the samples investigated. At the same time it should be pointed out that pores with d p - L can be considered as large defects. Although this approach fails in describing the experimental lifetime findings, an estimate of rp and of dp were obtained. These estimates will be used to evaluate the MLT values as shown below.

To compare the MLT as calculated by the second approach (equations 12 and 13) with the experimental lifetime, it is reasonable to assume, rf - ~p - 1 as described above. The diffusion and distribution coefficient of CI- across PI have been reported in the literature as 1 × 10-13 cm 2 s - 1 and 0.01 39 while the CI- diffusion coefficient in water is equal to 1.5 × 10 -5 cm2s-1. On these bases, the theoretical MLT values as a function of L are reported in Fig. 16 for different values of the porosity 0. As can be seen, the experimental lifetime data for the sample A (PI/AI system) lie in the vicinity of the MLT values for 0 -- 0, while those of sample B lie well above the MLT values for 0 = 0. This result indicates that both the PI/A1 samples (A and B) can be assumed as homogeneous as far as the CI- ion diffusion across the PI is concerned in spite of the shorter lifetime exhibited by sample A. This result is also consistent with the test carried out with the mercury as the electrolyte that revealed the absence of macroscopic defects. If, on the other hand, heterogeneities such as pores (b) are present in the case of sample A, the order of magnitude of these invisible 'holes' must be very low as will be shown later.

As reported above, the time at which failure is observed can be larger than the MLT depending on the type and on the pores distribution into the PIs. The macroscopic difference in the lifetime observed for samples A and B, could be attributed, therefore, to a greater number of (a) type pores in sample A. Unreleased water or solvent trapped at the interface, may have acted as a source of stress reducing the adhesion between the polymer and the metallic substrate leading to such interfacial defects. In these defects water can easily accumulate, thus providing the medium in which the C1- ions coming from the homogeneous part of the coating can concentrate up to its equilibrium value that is 0.5 M. In the presence of this high CI- ion concentration, A1 corrodes at a high rate leading to the early failure of the PI/A1 system. Although it is not possible at this time to discriminate between the two possible degradation mechanisms [via distributed thin type (b) pores or via interfacial (a) type defects], the early failure observed for sample A can be attributed to a poor interfacial adhesion rather than originated from large coating defects due to processing.

Quite different is the behavior of the PI/Fe system. The experimental lifetime values lie well below the MLT value for 0 = 0. The experimental lifetime well coincide with the MLT values obtained assuming a porosity 0 = 10 - 7 . The presence of (b) and (e) type pores can, thus, account for early failure observed in this case. An estimate of the pores size diameter, dp, can be at tempted on the basis of the following argument. A coating containing n (c) type pores of diameter dp (for 1 cm 2 of nominal coating surface) is characterized by a value of 0 ---- nd 2. SEM carried out on PI of comparable thickness did not reveal any kind of visible defects of the order of less


than 0.1/~m. It can be concluded that a porous film with 0 = 10-7 is compatible with a number of pores n > 10 3. Diffuse heterogeneities rather than macroscopic defects are, therefore, responsible for the corrosion behavior of the PI/Fe system. Such invisible pores formed during the processing of the PI dramatically affect the PI/Fe lifetime.

The theoretical analysis presented above allows the prediction that heterogeneities as small as 0.1 ktm (that are difficult to detect even at SEM) can reduce the MLT of thin film by orders of magnitude. In addition, a method has also been established to distinguish between failure due to defects into the coating [(b) and (c) pores], from failure due to the loss of interfacial adhesion [(a) defects] depending upon their position with respect to the MLT values for 0 = 0. The closer are the experimental lifetime data to the MLT values for 0 = 0, the larger is the metallic surface involved in the interracial corrosion process and the greater is the number of (a) type heterogeneities. As ultimate goal of this analysis, an estimate of the minimum pore size diameter for homogeneity (0 = 0) is also presented.

For 0 ~ 0, i.e. for extremely narrow pores, the mass balance applied to a shell of thickness dx leads to:

OC 32C N.Trdp (14) Sp ~ = SpDs, w Ox ~ --

where N is the flux of solute from the inner of the pore to the solid film, Sp and dp are the pore surface and diameter, respectively. In order to solve equation (14) an expression for the flux N must be used to avoid the simultaneous solution of Fick's second law into the pores and into the homogeneous part of the film. It is assumed that the flux N is expressed by the film penetration theory, i.e. N = KCX/(Ds.f/Jrt), where K is the distribution coefficient. Upon substitution of this expression into equation (14) and after some manipulation, one has:

OC- DswO2C KC ~-~p/~S~'f (15) Ot • Ox 2 ~ Jrt "

By introducing the following dimensionless variable: 7 = C/C=, x* = x/L and r* = tDs,w/L z, equation (15) yields:

0 7 _ 027 4 K L / Dsf Or* O(X*) 2 ~ 7d--pp ~ / D ~ * (16)

Equation (16) differs from Fick's second law. The difference is due to the second term on the right hand side. This term becomes important when the ratio dp/L is such that:

dp 4 K / Ds,r (17)

Equation (17) allows estimation of the order of magnitude of the pores diameter foc which all the solute entering into the pore is adsorbed into the homogeneous film. This case is not of practical interest since the time to saturate this pore will be equal to infinity due to the use of the film penetration theory. It is suitable to estimate the pores size for r* - I and for a 1/tm thick film. On the basis of the diffusion coefficient


values repor ted above, d p = 10 -6 #m. This value is in the range of a tomic distances. The conclusion is drawn that a coat ing can be considered as h o m o g e n e o u s f rom the mass t ranspor t point of view when heterogenei t ies with dp as large as a tomic distances are present . On the basis of this result and on the fact that the lifetime of the sample A lie in the vicinity of the M L T values for 0 = 0, it can be concluded that sample A can be assumed as homogeneous .

It should be pointed out , however , that the validity of the model discussed so far is restricted to the assumptions of: (1) validity of Fick 's law, (ii) equil ibrium be tween the so lu t ion filling the pores and that homogeneous ly dispersed into the film, (iii) constant distr ibution coefficient K with the concent ra t ion , and (iv) absence of fixed charge in the polymeric matrix.

CONCLUSIONS

(1) The lifetime of the PI/A1 system in aera ted 0.5 M NaCl solution increases if the samples are s tored in a desiccator after curing. This process allows the release of both the water and the solvent t rapped in the PI as a result of the curing schedule and the processing of the PI itself.

(2) The lifetime decreases with decreasing film thickness, and increasing temperature . Corros ion at the PI/metall ic interface depends on the ionic ra ther than on the water arrival at the metallic substrate.

(3) The capaci tance of the PI exhibits a rise at a given t ime when immersed in NaCI. This increase was explained assuming clustering be tween the water homogeneously dispersed in the film and the ion-bound water.

(4) The metallic substrate does not affect the dielectric proper t ies of the dry PI for the thin (1.2 ~m) and the thick (15/~m) film investigated.

(5) The role of pores and the concept of M L T is in t roduced and used to explain the lifetime experimental ly observed in the case of the PI /AI and PI/Fe systems.

(6) Interracial failure due to adhesion loss (interfacial defects) can be discriminate f rom failure due to pores present in the coating if use is made of the M L T concept .

Acknowledgements--The authors wish to thank Professor S. D. Senturia of the Microsystem Laboratory Technology of MIT for providing the PI/AI and the PI/Fe samples. The financial support of CNR (Progetto Finalizzato Chimica Fine e Secondaria II) is also gratefully acknowledged.

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A Study of Corrosion Initiation on Polyimide Coatings

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