Indian Journa l of Chcmistry Vol. 41A. April 200] . pp. 807-8 12 --

Dynamics of the influence of redox active conducting polymer on corroding interface

S Sh riram, S Karthikeyan, S Pitchumani * & N G Renganathan

Emerging Concepts and Advanced M ateri als Unit. EEC Di vis ion, Central Electrochemical Research Institule, Karaikudi , India 6]0 00 -

Received 7 Novell/bel' 2002

A slUdy of AIIAl20 3/polyaniline interface in a corrosive medium has been reported. T he system has been used to evolvc an equi va lent circuit model. based on electroanalytical measurements. The model evolved has been used to simulate in ter fac ial parameters such as Cdl • Rei' Q. Rs for different experimental conditions. and these have heeil used to diagonise the nature and typc of protecti ve role the polyaniline coating offers . Synergizat ion of experimelllally observed parametcrs and simulatcd paramcters fro m equi va lent circuit models has been demonstrated as a protocol to eva luate the pro tecti ve nature of polyanilinc coating.

Alumi nium, an ex tensively used metal, is susceptible to instantaneous degradation of its surface when it is in contact with unfriendly electrolytes. To make thi s surface stab le, the primary protection extended is anodization wh ich enhances I the funct ionality of the metal. Though it is greatly proven to be the first effecti ve step, i t has its limitations owing to its porous n~lture even at higher thickness and subsequently low corrosion res istance. Hence conversion coatings upon All A I20 , have been recommended as a standard engineering practice. Various metall ic layers li ke nickel, chromium and cadmium have been grown upon A luminium alloys/ox ides, to further strengthen the surface2

. However, there is an inherent problem in such coatings, in that, aluminium is anodic to most metals and hence a necessaril y pore-free coating becomes mandatory. Alternatively, the use of organic coatings is an acceptable methodology in streng­thening the surface3

.s. The main advantages of such

coatings are: i) they ci rcumvent prob lems encountered due to amphoteric nature of A lc0 3, ii ) there is no question of immersion depos ition, a cause for poor adhesion of metallic layers and iii ) absence of induced stress otherwise possible in metalli c layers. These fac ts have been well demonstrated for AI surface with the use of acrylic, alkyl and vi nyl polyesters6

. T hese types of coat ings provide a barrier protection to the A I surface, wh ile the possil) i!ity of imparting pass ive protection has been recently demonstrated by us 7. In this paper, we attempt to illustrate for the first time,

the changes in the dynamics of corrod ing A I interface through an equivalent circuit model.

Materials and Methods

Dispersions of polyan iline were prepared by suspension polymeri zation techn ique, based on the patented procedure developed by uss. The aluminium substrate was subjected to standard cleaning and surface treatment procedures and anodized potentio­statica lly in an oxalic acid medium at 40 V , adopting standard procedures9

. The polymeric dispersi on was then coated on the anodized fi lm and dried. T he coated panels were subjected to Polarization and elec trochemical impedance spectroscopy measure­ments, using BAS 100B Electrochemical Anal yzer (area of exposure= 1 cm2

) in a three electrode assembly using 3% NaCl as a probe medium, with saturated calomel electrode as the reference.

Results and Discussion

T he corrod ing interface for AI in this investigation has been chosen as 3% NaCI. though A I is known to extensively degrade in alkaline medium. Since a redox acti ve electronic conducting polymer has been chosen a~ the candidate for the protective layers. the use of alkali medium has been avoided, since this tends to deactivate the electronic states o f conducting

I k· h d ' . 10 po ymers, ma Ing t em non-con uctlllg species . The choice of polyaniline as a cand idate arises of its

808 INDIAN J C HEM . SEC. A. APRIL 2002

stature as best understood system in the famil y of conducting polymers5

. Further our own specific process of imparting polyaniline coat ing to anodized aluminium surface has been used in thi s investigat ion. Essenti all y, a dip coat ing process using polyaniline dispers ion has uniformly yielded a 20 )1m film, which is maintained throughout the indi vidual diagnostic measurements and ana lyses .

Po/arizal ion Measurelllents

Figure I shows a set o f potentiodynamic polari­zat ion curves recorded for A luminium, anod ized aluminium and polyaniline coated anodized alumi­nium. It shows that both anodized aluminium and polyaniline coat ing are able to make the basic aluminium surface less corros ive, by shi f ting ECorr to more posi tive values and ICorr to lesser values which has cl earl y demonstrated that the protecti ve mechanism is pass i vati on , in both anodization and polymer coating upon the anodized surface. Further, both ox idati on and reducti on branches of the cu rves are well defined and the redox reactions operative in these cases can be attributed to the known cOlTos ion react ions of the base metal surface and also the pass ivation mechanism of the anodization and polyaniline coating. The role of polyaniline in the protecti ve mechanism illustrated in Scheme I is analogous to Wess ling's model I I of protective ac tion by conducting polymers. Thus the reaction sequence can be represented as fo llows:

Oxidized PAnL\': ( 1

4 e-

4W ~ 2H2

Schcme I

PAni

Thi s clearl y shows that the passi va tion reaction effected by polyaniline redox chemistry is operative as ev inced by its redox ca taly ti c behaviour thereby shifting ECorr of aluminium towards more positive side. The ex trapolation of the Taffel regions of the po lyaniline coated aluminium in the polarization curve y ielded a nobler potential confirming the enobli za ti on process already proved by Wessling.

- 3.5

-4 .8

<t; -6 .2

OJ 0

-7 .6 ] -'

-8 .9

- 10.3 0

>I Bart' AI h , \llOdil,t'lI AI (" PAIII ("Mllt' ll

allodizl'd AJ

-0 .2

- ·h

t ' '.

0 ' ­- .10 l ' f)tI H ll i; tI .V

a . ,

-0 .8

Fig. I- Potentiodynamic po lari zation response of a) unprotec ted. b) anodized. c) polyaniline cOaled anodized A I surfaces

AC Impedance Measurelllen[s The enobli zati on process is pr imaril y due to the

redox chemistry of the conducting polymer at the corroding aluminium interface, whose properti es are dynamicall y vari able based on the nature and type of the protection offered by the over layers on the aluminium metal surface. To illustrate these changes, the ex pression of the protec ti ve role through equivalent circuit models arri ved through AC impedance measurements is cons idered the best option. AC impedance measurements were carried out on both unprotected and protected AI surfaces. Figure 2 shows such a response in the form of Nyquist plots. These plots clearl y demonstrate the protective role of polyaniline coating imparted and thus compliment the results of polarization measurements shown in Fig. I .

The Nyquist plot of unprotected A I interface shown in the inset of Fig. 2 is characteri zed by a semicircle w ith a low va lue of Reh clearl y indicati ve of its corrosive nature. On the other hand. the yquist plot of protected A I interface features two semicircles: the one at the higher frequency reg ion is attributed to the anodized part of the AI interface and the other at lower frequencies is attributed to the conductive polyaniline layer. The latter feature clearly shows the parti cipation of the redox species of polyaniline as is ev ident from the charge transfer reacti on associated with the semicircle observed. The important parameters used to diagnose these changes are the experimentally observed Re, and Cdl va lues, reflect ing the interfacial dy namics. The observed va lues of Re,

and Cdl are as follows: A I: 27 kohms (Re,) , S x 10·R F (ell ), AIIAbO,: 3 Mohms (Re,), IOx10·7 F (ell) I"

A IIA bO:/PA ni : 2S Mohms (Re,), 8x 10·6 F (C,II ).

SHRIR AM el al .: IN FLUENCE OF CONDUCTI G POLYMER ON CORRODING INTERFACE 809

.§ IJ . IJUIJ 0

.! c:

HI. IIUll

'" ~. OCO -

H. llno

- 2. 51J

. . /

27 . 50

• 1-'

Z' (I[-i 'j nhm)

3·' .5U

I l I

Fi g. 2- Nyqu ist plot of the AC impedance responses of I) Pani protected. ii ) unprotected aluminium surface (show n as an inset)

Fi g. 3--Eq ui valent circuit model representing the Pani/AI, 0 3/A I corroding interface

20 . 00 +

10.00

0.00

- 10 . 00

I ___ ~ ____ I -

HI . 08 0 . OC:I 1(:) . 00

+ a

Generation of equivalent circuit lII.ode! To illustrate the utility of these parameters In

understanding of the protecti ve interface, an equi valent circuit model has been evolved using Boukamp Software l 3

. Figure 3 shows the eq ui valent circuit that best describes the experimentall y observed response. The circuit shows two blocks in seri es. afte r the solution resistance (Rs). The first block represents the interface between the corros ive medium and the conducting polyaniline coating, in terms of the capacitance of the polymer film (Co) in parallel with

. ,'" ct.l c .,. .

o

it) 204 It IJ) 048 It c) n h d) 96 b

rP

n

__ --L ___ I_ l

1_

2 0 . 00 :-1(') . (:10 10 .00 :'0 . 00 hO . 00

7.rr. (Mnhtol)

Fig. Supcrimposed nyq ui st responses of pani protected A I/A I,0 3 interface after va ri ous hours of ex posure

810 INDI AN J CHEM. SEC. A. APRIL 2002

Table I- Simulated circuit parameters for various hours of exposure

Exposure Time Rs Rex 106 Q Ccx IO·6 F Qx lO·5 Q. I R] kQ R,x 104 Q C,x IO·7 F (h) Q

24 98.740 1.660 8.420 1.2090 10.054 2.23 12 1.6997

48 58.6 17 1.9 198 7.422 1.2 16 8.001 1.9549 1.11795

72 105.94 2.3375 7.284 1.21 87 3.6 11 1.7709 2.3543

96 78.654 3.9793 5.683 1.262 1.249 1.597 1 2.76 16

Table 2- Simulated parameters governing the performance of Pani/A I20 iA I interface ( derived from Ph ase angle plots)

Exposure Time Breakpoint frequency <Pmax (degrees) Delamination Ratio Water uptake v. (h) (fb) Hz

24 0.2 195

48 0.2238

72 0.2674

96 0.2860

the coating res istance (Re). A more co mplex block, representing the embedded oxide layer upon the bare aluminium surface, fo llo ws it. The latter has been di scussed in several references as the model descri-b· b ' I . II' - 91415 Ing a arn er ayer protectll1g a meta IC surtace' . . In additi on to that the present c ircuit di scusses the effect of the redox active po lymer coating in further pass ivati on of the surface. A constant phase e lement (Q) has been introduced to account for the quas i­di ffu sion phenomenon encountered across the film. R2 corresponds to the pore res istance whi Ie C3 and RJ

charac terize the ox ide/metal inte rface . S ince thi s equi va lent c ircuit model provides an

optio n to understand the interfac ia l process occurring at the aluminium corroding in terface, the data simul ated based o n thi s mode l can be used to further in vestigate the dynami cs of the changes taking place at the in terface. To verify the va lid ity of thi s approach. AC Impedance measure ments of po ly­anili ne protec ted a luminium surface were carri ed out in the same corroding medium (3% NaC I) as a functi o n of exposure with respect to time. Figure 4 shows the superimposed Nyq ui st plo ts fo r di ffe rent d uratio ns of exposure of the coating to the corros ive sod ium chloride soluti on. Thi s ind icates an increase in diameter of the low frequency semicirc le as a function of immersion time, which cor'esponds to an increase

(D) (% by weight)

86.5 0.0703 0.0220

78 .5 0.01 03 0.0288

76.5 0.0036 0.033 1

75.0 0.0025 0.0897

in Rei' T his is quite contrary to the no rmal behav io ur assoc iated with a good no n-conducting barri er coating, which typica ll y shows a semicirc le of decreasi ng di ameter with increase in ex posure time to the corrosive solu ti on 15. The increase in Rei observed here is due to the redox chemi stry of po lyaniline wh ich facilitates strengthening of the pass ive layer at the AI/Po lymer interface, and due to the partia l red uctio n of po lyan ili ne to its less conducting state as ill ustrated in Sche me I.

Application of the equivalent circllil lllodel Further simu latio n o f the ex peri menta lly observed

AC impedance respo nses of polyanil ine coated a lumin ium surface sho ws a good fit a ll th rough the vario us depths of expos ure, confirming the va li dity of the model. The parameters computed from th ese models with respec t to ex posure time are shown in Tab le 1. These data c lear ly how the changes realizable in the dynami cs of the protective mechani sm, which is re fl ected in the ph ase angle va lues . These va lues a lso suggest a poss ibil ity of predi cting the lo ngev ity of the pro tecti on that is poss ibl e with thi s type of redox ac tive conducting po lymer coating. To il lustrate this feat ure, phase angle plots were generated based on the data computed and shown in Table 2. T he same is represented in Fi g. 5.

SHRIR AM et 01 .: INFLUENCE OF CON DUCTING POLYM ER ON CORRODI NG INT ERFACE 81 1

( •. .,M .N. ~ .. ,.. tl , ' _ ..... n lll.

::i rr'l " l ... t In n

1 . n\1

~ . uu

."

a

H " tH .~ It . U" % . 1:''' 1."" i,...,., ,, - 10 ....

.~aSllrerte nl

1. 00- ,Si.l1ulat iOll ;

0.08 -Zindg • 10"'

" '.

us

.n,.. .• ~ I I,..,.._ l1t

%. HH ~ :"'"" l .• tiull

1 . UtJ

n . HH 1 . t'\ri ? ) t h t 'J .. 1U~ 6

. -...... .. ""' ....... .. ~ ~ . ...... I . , '''',''

.. . ~ 1 ....

, ,· ,. . ... 1 .. 1M .... !.

h . \

~ .. , . \

2. ~ 3.88 Zreal • IB' 1

c

.' . I

:I. . tWf :1. un Z .' r..'\ 1 • 10 " (,

d

- -- --.: tt" ~. ~ , \'fd .~ l-V"" • 1"'" t t .. ~ r"" I - l'" (,

Fig. 5-S ill1ul ~ ted versus experimenta l plot ~fter (a) 24 h. (b) 4& h. (c) 72 h and (d) 96 h of exposure

Thi s indicates the presence of two distinct interfaces attri buted to Al20 3 and polyaniline layers as protecti ve measures and is in consonance with the ex perimentally observed features both in polari zation and AC impedance meas urements. Further, the va lues of max imum phase angle (<Pmax) being closer to 90° clearl y illustrates the capacitati ve behav iour of the polyaniline coating, which has already been well proved l 6

. Bes ides, the determinati on of breakpoin t frequency (fb) ' delaminati on ration (D) and water uptake (v) have been determines usi ng the model and the si mulated phase angle plot based on the methodology described earlier l 4

.17 These values

computed as a functi on of ex posure time are shown in Table 2. The importance of these va lues li es in the estimation of the li fe of the pro tecti ve ac ti on of the redox polymer imparted to the con od ing alum inum interface. This has indeed been found poss ible based on both experimentally observed and simul ated interfac ial parameters. Thus the changes occurring at the aluminium interface in the corros ion med iu m has been examined to prov ide a protocol in dec iding not only the nature and type of protec ti on but also its performance with res pect to longev ity.

Conclusion Thus, thi s study indicates that the changes in the

interfacial dynamics of the corroding alumini um interface is quanti fiable. This has been illustrated with the use of a redox acti ve electroni call y conducting polyaniline as a protec ti ve coating, which on polarization and AC impedance analyses has given rise to developing an equi valent model to descri be the changes observed in the interfac ial dynami cs of AIIAI20 3/PAni. This equi valent circuit developed based on the electrochemical measurements has also been used to di agoni se the protecti ve rol e of the coating, thus leading to the unders tanding of the longev ity of the protecti ve measure .

Acknowledgement The authors thank the Director, CEC RI , Karaikudi,

for hi s kind support and permi ss ion to publi sh th is work in thi s special issue.

References Werni ck S. Pinner R & She~sby P G. The sl.lIface treatlllellt alld fi ll ish illg oj alulllilliulII alld its allovs. (ASM Int ern~t i o n~1

Finishi ng Pub. L td). V Edn. Vo l. I & II. 2 Jones D A, Prillciples alld p revelltioll of" corrosioll. II Edn.

(Prenti ce Hall ) 1996.

8 12 INDIAN J CHEM . SEC. A. APRIL 2002

3 Wick s Zeno W Jr . Jones Frank N & Peter Pappas S, Orgallic coal illgs - Sciellce alld teclillologv. Vol. I . (John Wil ey and Sons. In ) 1992.

4 Phull1 M C. Cllrrellt topics ill electrochemistry. 2( 1993) p.1 07 5 Deberry D W. J electrochelll Soc, 132 ( 1985) p.1 022. 6 Uhlig's Corrosioll halldbook. edited by R Winston Rev ie. II

Edition. (ECS Series. John Wiley and Sons. Inc) p7 12 7 Karthikeyan S. Shrirall1 S. Pitchull1ani S. Mohan S, Kanagaraj

D & Raj V. Proc Vol On corrosioll . Covering the 200th

Annual M eet of ECS. San Francisco (2000), p.51 2 R Phani K L N. Pitchull1ani S. Rav ichandran S & Rangaraj an S

K. Illdiwl Patellt 424/DELl93 9 Husler P & Beck F. J appl Electrochelll , 20 ( 1990) p.596.

10 Mac Diarll1id A G, Chi ang J C. Richter A F. SOll1asiri N L 0 & Epstein A J in COlldliClillg polymers. edited by L Al acer (Reidel. Dordrecht. The Netherlands) 1987. p.1 05.

II (a) Wess ling B. Adv Materials. 6( 1994) 226: (b) Mater Corr. 47 ( 1996) 439.

12 Gabriell i C. Identificatioll of electroch emical processes bv frequency response allulysis" (Fall1borough. Hants. UK) 1980.

13 Boukall1p B A. EQUIVCRT. PAS , Equivalent circllit IIsers lIlanual. Uni versity ofTwente ( 1989)

14 Mansfield F. J appl Electrochem. 25( 1995 ) 187.

15 Macdonald 0 O. Transient techlliques iii electrochemistrv' ( Plenum Press, New York) ( 1977)

16 Leinad Gnana Li ssy S, Pirchumani S, Jayakumar K. Mat O em Phys. 76 (2002) 143.

17 Kousik G, Pitchull1ani S & Renganathan N G, Prog org CO(l/. 43 (200 I ) 286.