corrosion protection of mild steel by polypyrrole coatings in acid sulfate solutions

1、Corrosion protection of mild steel by polypyrrole coatings in acid sulfate solutionsPergamon Ekcrrochimica Am. Vol. 42, No. I I. pp. 1685-1691. 0 1997 Elsevier Science Ltd. All rights reserved. Printed in Great Britain 00134686/ 97 1997 PII: soo13-468q%)oo313-1 $17.00 + 0.00 Corrosion protection of

2、mild steel by polypyrrole coatings in acid sulfate solutions N. V. KrstajiC, B. N. Grgur, S. M. JovanoviC and M. V. VojnoviC Faculty of Technology and Metallurgy, University of Belgrade, Karnegieva 4, 11001 Belgrade, Yugoslavia (Received 16 April 1990, in revised form 3 July 1996) Abstract-The inves

3、tigated by electrochemical impedance spectroscopy. Electrodeposition from aqueous solution of pyrrole and oxalic acid at a constant current density of 1 mA cmm2 and yields strongly adherent and smooth polymer layers. The impedance spectra were treated through the nonlinear least squares (NLS) fittin

4、g to estimate the parameters of the proposed equivalent circuit, based on physical model for the corrosion of mild steel protected by polypyrrole films in acid sulfate solutions. 0 1997 Elsevier Science Ltd. All rights reserved. corrosion behaviour of mild steel covered by the electrodeposited polyp

5、yrrole films was of polypyrrole was performed Key words: Corrosion, impedance, polypyrrole, coating, electropolymerization. 1. INTRODUCTION Electronically conducting polymers have been inves- tigated for use as the electrode materials for a number of applications, including 1, 21, electrochromic dis

6、plays 3, electrochemical sensors 4 and electrochemical capacitors 5. One of the most promising of the conducting polymers for its high conductivity, stability and ease of synthesis is polypyrrole. Most of the recent investigations into the electrode mechanism of polypyrrole the reduction/oxidation o

7、f the polymer, specifically the diffusion of the counter contribution of capacitive us Faradaic charge trans- fer during the potential sweep rate of polypyrrole lo, 111. In the last ten years, conduction polymers, mainly based on the polymeriz- ation of heterocycles at inert anodes (Pt, Au, GC), has

8、 been developed. More recently, however, some attempts were reported to transfer this technique to common metals (such as iron) without excessive anodic dissolution of the base electrode 12-l 71. Beck et al., 16 showed that it is possible to obtain strongly adherent, smooth polypyrrole film on iron,

9、 by the electrodeposition from aqueous solutions of pyrrole and oxalic acid. The realization of polypyrrole films on Fe is a first step for the corrosion rechargeable batteries have centred around ions 6-91 and the electrodeposition of protection of iron by conductive polymer coating. DeBerry 171 re

10、ported reducing the corrosion rate of stainless steel with an electroactive polyaniline coating in perchloric acid solution. Polyaniline coating provided a form of anodic protection that significantly corrosion rate due to the polyaniline capable of maintaining the native passive film on the metal.

11、On the other hand, other authors 18, 191 have found that electrochemically provided very little corrosion protection to mild steel surfaces. Wesslling 20 reported that mild steel, stainless steel and copper were all found to passivate in contact with dispersions of doped polyaniline. Passivation was

12、 found to occur by the formation of an oxide layer on mild steel induced by contact with the polyaniline. The intriguing findings of Wesslling 2 l of the high level of corrosion protection polyaniline to mild steel exposed to dilute HCl stimulated us to investigate corrosion protection of mild steel

13、, provided electrochemically deposited diluted acid sulfate solutions. reduced the redox states deposited polyaniline provided by doped the mechanisms of by polypyrrole films in 2. EXPERIMENTAL Polypyrrole films of different thickness on mild steel were obtained by electrodeposition aqueous solution

14、 of 0.1 mol dmw3 pyrrole 0.1 mol dm- oxalic acid at a constant current density from and 1685 N. V. KrstajiC et al. of 1 mA cm-*. The conditions for electrodeposition were: temperature 25°C; purging stirred, stagnant electrolyte. A glass cell with two platinum sheet counter electrodes were emplo

15、yed for the electrocoating process. The current efficiencies, vi, for the electrodeposition were determined using the following equation (cJ16), with Ar, non- II = (2 + y)(hF I + o.hndlMFe) Q(Mm + YMA) (1) where Am is the experimental mass difference between the blank sheet and the coated electrodep

16、ositing current, where iron dissolves, M, the molecular weight of the monomer (pyrrole), MA the molecular weight of the inserted anion, Mre the atomic weight of iron, Q the charge for electrodeposition corresponds to the positive potential plateau, where polypyrrole is formed), and y the degree of i

17、nsertion of anions into polypyrrole film during its electrodepo- sition The thickness (6) of polypyrrole determined using the following equation: sheet, I the rind the induction time of polypyrrole (Q deposit was 6 _ y _ Q(Mm + yMA)ti (2 + Y)FhJ (2) where mtppy corresponds of polypyrrole deposit wit

18、h vi = 100%. The pyrrole was freshly distilled under argon just before use. Electrolytes were prepared with bidistilled water. The working electrodes, mild steel sheets, were first mechanically polished with fine emery papers (2/O, 3/O and 4/O, respectively), polishing alumina of 1 urn (Banner Scien

19、tific Ltd.) on the polishing cloths (Buehler Ltd.). After mechanical polishing the traces of polishing removed from the electrode surface in an ultra-sonic bath for 5 min. the procedure was repeated three times. The exposed surface area during electrodeposition was 8 cm*. The working surface area of

20、 the coated samples in the corrosion experiments was reduced to 1 cm*, using a Teflon holder. The counter electrode used was made of platinum wire, while the reference electrode was a saturated calomel electrode (see). All experiments concerning the corrosion investi- gations presented in this paper

21、 were performed in 0.1 mol dmW3 H2S04 (Merck p.a.) solutions standard three-compartment ture The corrosion behaviour of mild steel covered by the electrodeposited polypyrrole gated by the ac impedance impedance measurements circuit potentials with a PAR 273 potentiostat connected to a PAR controlled

22、 by a computer through a GPBI PCZA interface. The impedance measurements were carried out in the frequency region of 50 mHz to 100 kHz. to the theoretical mass and then with alumina were in a cell at room tempera- films was investi- technique. were carried out at open The ac 5301 lock-in amplifier,

23、Cdl Fig. 1. Equivalent circuit considered for the corrosion of mild steel in acid sulfate solution. RI, electrolyte resistance; cdl, double layer capacitance; ZC, impedance of the anodic metal dissolution; ZH, impedance of the cathodic hydrogen evolution reaction. There were ten frequency points per

24、 decade above 5 Hz. The fast Fourier technique was used in the impedance measurements in the frequency region below 5 Hz. The real (Z) and imaginary (Z) components of the impedance spectra in the complex plane were analyzed nonlinear least squares (NLS) fitting program to estimate the parameters of

25、the equivalent electrical circuit. transformation (FTT) using the 3. RESULTS AND DISCUSSION (a) Physical model for the corrosion of bare mild steel The physical model for the interpretation electrochemical impedance spectra of corroding mild steel surface in sulfuric acid medium was developed under

26、the following assumptions: of the (1) The corrosion reactions of iron in acid sulfate solutions are the anodic metal dissolution Fe -t Fe*+ + 2e log (i /Acm) Fig. 2. Steady-state potentiostatic polarization curves for mild steel in 0.1 mol dm-3 HzS04 obtained after I.5 h of exposure time. Corrosion

27、protection of mild steel 1687 60 0 0 Z/mm2 -400 g.9 -600 . 0 Fig. 3. Nyquist plot for the corrosion of mild steel in 0.1 mol dm-3 H2S04 solution, measured after tea, = 1.5 h at the corrosion potential; (-) transfer function of the system (equation (3). optimum fit of the theoretical I I 1 I 500 loo0

28、 1500 2000 Tie/s. curves and the cathodic hydrogen evolution Fig. 4. Potential-time electrodeposition 0.1 mol dm- 1 mA cm-2. Deposition time (s): IJ, 900; 0, 1200; A, 1800. for on mild steel from 0.1 mol dm-3 the galvanostatic of polypyrrole H&04 and pyrrole at 2H+ + 2e -+ Hz. (4) Both reactions

29、 are assumed to run in parallel (2) In a first approximation, considered as mainly charge-transfer controlled. (3) The anodic metal dissolution and the cathodic hydrogen evolution are described by impedance Zre and Zn2, respectively, the explicit form of which is determined by the charge-transfer (4

30、) The interface of the system is characterized by the double layer capacitance polishing with alumina of 1 pm is well below the level of roughness likely to interpretation of ac results. ditions, shown in Fig. 2. b+lb-I Jcorr = 2.303(b+ + Ib_I)R, both reactions are kinetics. Cdl, as we believe that

31、lead to problems in Figure 3 shows impedance spectra measured at the corrosion potential for mild steel after 1.5 h of exposure time. Optimum fit of the theoretical transfer function of equation (5) is also shown as a full line. The physical model and the experimental data agree very well with respe

32、ct to the shape of the impedance spectra in the Nyquist representation. parameters and calculated value for the corrosion current density are summarized in Table 1. Extrapolation of the cathodic polarization curve in Fig. 2 gives practically (88.10e6 A cm-*). These results demonstrate that the corro

33、sion behaviour of the studied system can be reasonably described by the physical model men- tioned above. Some raising of the corrosion rate of mild steel with time was obtained as a result of roughening of the surface, which could be expected for the corrosion of metals with soluble products. Optim

34、um fit The complete transfer function of the model system is given by the overall admittance Y = Zg ?+ Zi! +jwCdl (5) and is represented in the equivalent circuit of Fig. I. The low frequency limits of the impedances ZF, and ZH2 of the charge transfer controlled iron dissolution and the hydrogen evo

35、lution, respectively, the same value of j, lim, _ 0 Zre = RF and lim, + OZIQ = REQ (6) are of central importance for the determination corrosion rates. In relatively simple corrosion systems characterized only by charge-transfer corrosion processes, the corrosion current density is correlated to the

36、 polarization resistance R, which is defined by, R; = Rg + RHZ. The corrosion current density, _L, can be calculated additional information from potentiostatic ation measurements, under quasi-steady-state of controlled from R, using polariz- con- (b) Physical model for the corrosion of mild steel co

37、vered by polypyrrole films Figure 4 displays the potential us time curves for electrodeposition of polypyrrole onto mild see1 at a constant current density of 1 mA cme2. The induc- tion period, r#nd, is attributed to an active dissolution of the iron at negative potentials for 5-8 min where the pote

38、ntial shifts into the positive direction, as the result of precipitation polypyrrole film is electrodeposited potential plateau. A pronounced of Fe(II)-oxalate at the positive potential and peak Table 1. The fitted parameter values of the equivalent circuit for the corrosion of mild steel in 0.1 mol

39、 dm-3 HzS04 at 25°C and calculated value for jcO, according to equation (7) Rd(f2 cm*) RH,(R cm) RF cm*) C&F cm-2) jEO+ IOW cm-9 2.25 558.4 141.8 124 99 1688 N. V. KrstajiC ef al. Table 2. Current efficiencies tl, (equation (l), and thicknesses (S) (equation (2) for the electrodeposition of

40、 polypyrrole on mild steel at a constant current density of 1 mA cm-2 in 0.1 mol dm-3 pyrrole and 0.1 mol dm- oxalic acid solution e4 Am(g) vi(%) Gm) 900 1200 1800 2400 3000 - 0.0007 - 0.0006 0.0011 0.0022 0.0024 22 22 42 46 41 0.2 0.5 1.6 2.5 2.8 which precedes the positive potential plateau was at

41、tributed to a phase formation of the Fe(I1) oxalate interlayer 16 at the beginning of the electrodeposi- tion of polypyrrole film. The thickness of the coatings was calculated under the assumption that the density of the film is p = 1.5 g cmW3 and using equation shown in Table 2 were calculated with

42、 y = 0.2. Figure 5 shows the corrosion potential US time curves of the electrodes with different film thick- nesses. The starting values of corrosion potentials were about 0.4 V us see and correspond potential of the redox process in polypyrrole film. The corrosion potential shifts to more negative

43、values and the thinner the polypyrrole layer the lower is the time of achievement of the steady-state value. For example, the steady-state corrosion potential value was achieved after about 70 min, for the electrode coated by a 2.8 pm thick polypyrrole steady-state corrosion potential values were ve

44、ry near the corrosion potential of bare mild steel electrode, obtained in the same solution. These results suggest that polypyrrole coating protection of mild steel due to the polypyrrole redox states, in the mentioned corrosion environment. contact with electrolyte, the polypyrrole layers are rapid

45、ly undoped. The process is driven by the anodic dissolution of metal through (2). The data to the film. The cannot provide anodic In the pores of the . Frequency ! Hz Fig. 5. Corrosion electrodes coated by polypyrrole layers. The thicknesses of polypyrrole coatings are marked in the figure. potentia

46、l-time curves for mild steel Fig. 7. Bode plots of a mild steel protected by polypyrrole film (2.5 pm) in 0.1 mol dm- Hz!304 at 25°C at the open circuit (corrosion) potential. Exposure time, 93.5 h. 5 cnq, Rf ,G I1 II -J-4 Rw a) RF b) Fig. 6. Proposed equivalent circuit (a) and physical model (

47、b) for the corrosion of mild steel electrode coated by polypyrrole layer in acid sulfate solutions. Rr, sum of the electrolyte and polypyrrole film ohmic resistances; &, metal-polymer interface capacitance; Rmp, metal-polymer interface resistance; C, the geometric capacitance; Rp, the pore resis

48、tance; C, the pore capacitance; impedance; Cr, the polypyrrole film capacitance and RF, anodic metal dissolution resistance. W, Warburg the coatings. metal is high and the corrosion presented with following: (1) anodic dissolution of iron During this short period, Fe + Fez+ + 2e the corrosion of pro

49、cess can be (8) Corrosion protection of mild steel 1689 moo 1500 1 - 2.25 ll. 2 - 45.5 h. 3 69.5 h. - 4-214h. Z I f&m2 Fig. 8. Typical complex plane plots for the impedance of a 2.5 pm thick polypyrrole film on mild steel in 0. I mol dm- H2S04 solution, after various exposure times. The insert s

50、hows the semi-circle at high frequencies which represents the response of metal-polymer interface. and (2) cathodic accompanied process reduction of polypyrrole ion film by the concurrent undoping (RY+Az-) + 2nye + (R). + nyA*- (8b) where (R), = polypyrrole film; A- = C20: - The UC impedance investi

51、gation of the corrosion processes which take place at a covered electrode/electrolyte after achievement of a stable corrosion potential. Impedance data were analyzed by the equivalent circuit, given in Fig. 6(a) and corresponding physical model, presented in Fig, 6(b). The capacity C, of about 0.2 p

52、F cm-* did not vary significantly with the film thickness and, hence, should be assigned, with a corresponding technique was used in the interface, charge- I“ “. “I 300 “E 200 8 ? i+l 100 0 transfer interface 22. The series resistance, Rf, is the sum of the film resistance and the electrolyte resist

53、ance. Because of the presence of thin channels of electrolyte running through the pores present in the polymer coating, the totally reduced conductive polymer has a finite ionic conductivity, equivalent to a resistance, R, assigned as a pore resistance, which is in parallel with a geometric capacita

54、nce impedance arising from the parts of the metal wetted by the penetrating aqueous solution is represented with resistances which are in parallel capacitance. These are RF, the charge-transfer resistance for the iron dissolution process occurring at the metal/solution interface at the bottom of the

55、 pores and &, the pore capacity arising from the resistance, Rmp, to the metal/polymer C,. The interfacial with a loo0 N j : 9 500 t .,.,.,.,. b) t=45,5 h F cl e 69,5 h “ E ; I loo0 12.6 Hz t=214h 1 500 ? $I 500 0 0 0 500 1000 1500 2000 2500 3000 0 1000 2000 3000 4000 ZlQCll12 zltirn solution af

56、ter various exposure Fig. 9. Nyquist plots of 2.5 pm thick polypyrrole film on mild steel in 0.1 mol dm- H2S04 times. Fitted curves are presented by solid lines. 1690 N. V. Krstajic et al. 0 100 ml so4 400 Time I hours 10 (r , . , , . , 0 loo 200 300 400 Tie I hours Fig. 10. Fitted parameters vs tim

57、e curves of a 2.5 urn thick polypyrrole film on mild steel in 0.1 mol dm-3 HrS04 solution. (a) capacitive components equivalent circuit; (b) resistance components of the same circuit. of the proposed charge separation at the same interface. The C, value on a coated sample should be expected to be ve

58、ry much smaller than the corresponding Cdl value for the bare metal surface in contact solution. This is because of the very much smaller contact area between the metal and an aqueous solution in the film-covered case. The impedance of a corresponding parallel cathodic reaction process can be presen

59、ted by the capacity Cf of polypyrrole film in series with Warburg impedance, since the reduction of the polymer is controlled by the diffusion of dopant ions in the film 23. Figure 7 shown Bode plots of mild steel electrode protected by polypyrrole film thickness of 2.5 urn, (after 93.5 h of immersion time) which confirm the existence of three time constants. In order to shown that the impedance data reflect the corrosion behaviour of coated metal, tests were performed over an extended time period (