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Volume 2 Paper 8

Electro-polymerisation of phenol derivatives on zinc-coated steel

Y. Yoshida, J. Marsh* and J. D. Scantlebury
Corrosion and Protection Centre, UMIST, P.O.Box 88, Manchester M60 1QD, UK
*CAPCIS LTD., CAPCIS House, 1 Echo street, Manchester M1 7DP, UK
E-mail:

Abstract

Electro-polymerisation of phenol derivatives on zinc-coated steel in hydro-alcoholic media has been investigated.  Thin, adherent poly-phenylene oxide (PPO) films form on the zinc surface without treatment (untreated zinc) by anodic oxidation of 2-allylphenol, provided the electrolytic solution contains a large amount of amine.  On the other hand, under the same condition, they do not form on the zinc surface that is subjected to a passivation treatment in alkaline solution containing sulphide ions (S treated zinc) before the electrolysis.  Control of passivation film formation on zinc surfaces is required to obtain good electro-polymerised films by anodic oxidation.

Keywords: Electro-polymerisation, Poly-phenylene oxide film, Zinc-coated steel, Phenol derivative, Amine

Introduction

Chromate materials have been used for painted zinc-coated steel in pre-treatments and/or undercoats to improve its corrosion resistance and paint adhesion.  However, because of environmental concerns, use of harmful materials such as chromate is being controlled and banned.  This has lead to vigorous research for finding alternatives to chromate for improving the product lifetime.

As one of the possible alternatives, the application of thin polymer films as pre-treatments has been examined [1].  The relation of the required properties with the composition of the polymer film such as variety and density of functional groups is essential.  However it has not been fully researched so far because it is difficult and time-consuming work to prepare polymers with various composition changes by conventional methods.  From this viewpoint, electro-polymerisation can be a useful polymer producing process.

Electro-polymerisation is a polymer coating process in which polymerisation of monomeric materials and coating formation by the polymer produced is conducted at the same time on metal electrode surface electrochemically.  One of the biggest benefits of this process is that a wide variety of chemical compositions can be obtained because, unlike polymers, monomeric materials are far easier to dissolve in usual common solvents and this leads to unique films unobtainable by the conventional processes.  In addition, better reproducibility can be obtained because the reaction is electrically controlled.

The application of thin electro-polymerised films on steel as adhesion enhancing pre-treatments has been examined [2].  However, although zinc is one of the most useful materials, little work has been done so far because zinc is so active that it is difficult to obtain polymer films on it, especially by anodic oxidation [3,4].  In this paper, we discuss the preparation of thin electro-polymerised films onto zinc-coated steel and also examine the effect of surface treatment on electrolysis, comparing the zinc without treatment (untreated zinc) and the zinc subjected to the sulphide passivation treatment (S treated zinc) that has been successfully used for electro-polymerisation of pyrrole [4].

Experimental

All chemicals were purchased from Aldrich and used without further purification.  Electro-galvanised steel sheet (thickness: 1.0mm, coating amount: ca. 20g/m2) was used as the substrate.  They were covered by a thick layer of epoxy resin, leaving a 10cm2 electrode surface.  The electrodes were degreased in acetone and then polarised cathodically in 0.1M NaOH aqueous solution at -1.6V vs. a saturated calomel electrode (SCE) for 10 minutes.  For the S treated zinc, after immersion in 0.2M Na2S aqueous solution for 18 hours, electrodes were washed with water and then dried at room temperature.

Electro-polymerisation was conducted in a 1/1 water/ethanol solution containing 0.25M 2-allylphenol and allylamine.  Experimental conditions, including the electrode treatment and the composition of prepared electrolytic solution, are listed on Table 1.  The pH of the solutions was adjusted by addition of sulphuric acid.  The solutions with low amine concentration and those excluding 2-allylphenol were prepared for comparison.

Electrolysis was conducted using cyclic voltammetry and potentiostatic methods after the electrodes were immersed in the electrolytic solutions for 10-20 minutes.  For cyclic voltammetry, the electrodes were polarised anodically from rest potential to 1.4V(SCE) at the speed of 20mV/second and then back to the start potential at the same speed.  This procedure was repeated three times.  For the potentiostatic method, the electrodes were polarised anodically at 1.0V(SCE) for 10 minutes.  After the electrolysis, the electrodes were washed with water and ethanol, dried at room temperature and heated at 150�C for 30 minutes.

The polymer films obtained on electrodes were analysed using Fourier transform infrared absorption spectrophotometry (FT-IR) and scanning electron microscopy (SEM).  IR spectra of the polymer were obtained using Perkin-Elmer 2000 spectrophotometer by the single reflection method.  The morphology of the films were observed using HITACHI-AMRAY scanning electron microscope.

Results and Discussion

Cyclic voltammetry

Figure 1 shows cyclic voltammograms for the untreated zinc-coated steel in the solution with 2-allylphenol of pH11.0 (a), and the solution without 2-allylphenol of pH11.0 (b).  In both graphs, the rest potential is approximately �1.3V(SCE) and the zinc dissolution and the following passivation peak is seen in the first forward sweep.  Comparing (a) with (b), the current increase from about 0.4V(SCE) in (a) corresponds to the oxidation of the 2-allylphenol and the increase is suppressed with cycles.

In the same way, the current increase starts from 0.6-0.7V(SCE) for pH10.2 and pH9.4 (Figure 1(c)), which means that the phenol oxidation potential goes down with increasing pH.  However the current density in the phenol oxidation for pH10.2 and 9.4 is much bigger than that for pH11.0.  This is contradictory to the case of steel and platinum substrates for which the current density increases with raising pH because of the increase of more reactive phenolate ions in the solution [5].  The nature of the zinc oxide passivation film seems to affect the decrease of current density for pH 11.0.  Comparing the three zinc dissolution and passivation peaks, they become bigger and sharper with decreasing pH, whereas zinc hydroxide is most stable around pH9.4.  It is presumed that zinc oxide passivation films become less stable in the pH9.4 and pH10.2 solutions because of the presence of more sulphate ions from pH adjustment, and that less stable passivation films allow more current flow to be available for the phenol oxidation through the oxide films.  However, this requires further investigation.

In the cyclic voltammograms for the S treated zinc-coated steel shown in Figure 2(a)-(c), rest potential is approximately �0.6V(SCE).  This is far more positive than that of untreated zinc.  Also, no zinc dissolution and passivation peak is seen in the first forward sweep.  For the phenol oxidation, almost the same trend can be seen as untreated zinc, although the current density is far smaller.  It seems that, by the sulphide passivation treatment, zinc passivation films become more stable due to the incorporation of zinc sulphide into the films and this restricts the current flow available for the phenol oxidation through the films.

Potentiostatic electrolysis

The current-time transients during anodic oxidation at 1.0V(SCE) over a period of 10 minutes are shown in Figure 3(a) (untreated zinc) and 3(b) (S treated zinc) and the result of the anodic polarisation including the appearance of electrode surface is summarised in Table 2.  For untreated zinc, in all the solutions including 2-allylphenol and high(1.6M) or moderate(0.53M) amounts of allylamine (No.1, 2, 3 and 4), the current density gradually decreases with time, and after the electrolysis polymer films obtained were seen visually on the electrode surfaces and they show water repellent properties which are characteristic of the presence of poly-phenylene oxide coatings.  In addition, these films have been identified as poly-(2-allyl)phenylene oxide by IR spectra shown in Figure 4 [6].

On the other hand, for the solutions excluding the phenol (No.9, 10 and 11), the current density falls during the first minute, then increases gradually, and after the electrolysis no polymer film was seen visually on the electrode surface.  Also, for the low amine concentration solution (No.5), the overall current density is lower than that for higher amine concentration solution, but shows a fluctuation, and no polymer film was seen visually after the electrolysis.

For S treated zinc, the trend is almost the same as untreated zinc although the current density is lower by about one order of magnitude than that for untreated zinc.  However, on the zinc electrolysed in the solutions including 2-allylphenol and high amounts of allylamine (No.6, 7 and 8), no film was seen visually.  In addition, for the pH11.0 solution (No.6) slight current increase is seen after 4 minutes, and for the pH9.4 solution a current fluctuation is seen.

From these results obtained for untreated zinc and S treated zinc, it can be found again that the nature of the zinc passivation film affects the oxidation or polymerisation of the phenol, which causes the current suppression.  It is presumed that the polymer films on the S treated zinc electrodes are too thin to be detected because the zinc passivation films are so stable that they restrict the current flow used for the phenol oxidation.  From the fact that the thickness of the film formed in the solution No.2 is about 1mm (Photo 1), if current efficiency is similar in every solution, the film thickness obtained in the solution No.1 is presumed to be about 0.2mm and that in the solutions No.6 to 8 is much thinner, that is about 0.02-0.04mm.  This thickness seems difficult to observe, especially on a rough zinc-coated steel surface.  Also, the current-time transients for S treated zinc (No.6 to 8) and that for untreated zinc in the solution containing low concentrations of amine (No.5) show fluctuations presumably because the polymer films are so thin that breakdown and healing of the films occurs repeatedly [7].

Effect of amine

From the result of cyclic voltammetry and potentiostatic electrolysis, it is found that zinc oxide (sulphide) passivation films restrict the polymer film formation and the relevant polymer film can be obtained on the zinc surface where the zinc passivation film is not too stable.  From the viewpoint of roll of amine, some kinds of amine compounds, especially allylamine, are reported to strongly absorb on metal surface, control the metal dissolution and contribute to adhesive film formation [5].  In this research, it was found that more amine was required to cover the rougher surface of unpolished zinc-coated steel electrode compared to polished steel.

In addition, some kinds of amine compounds can suppress the oxide film formation and it has been reported that the film formation by electro-polymerisation can be accelerated by addition of suitable amines for the electrode metal [8].  A large amount of amine may reduce the zinc oxide films to a thickness low enough to enable electro-polymerisation.  It is presumed that surface of S treated zinc was covered with a film too stable to be disrupted by the amine.  This restricted the electro-polymerisation of phenol, leading to very thin (or non-existent) polymer films.

Conclusion

To obtain thin electro-polymerised films on zinc as an adhesion enhancing pre-treatment, we have investigated the conditions under which phenol derivatives are electro-polymerised and have concluded that control of oxide passivation film formation on zinc is required for electro-polymerisation.  Sulphide treated surfaces are not suitable for electro-polymerisation because the passivation film is so stable that it restricts polymer film formation.  In addition, compared to steel and platinum, higher concentrations of amine are required for covering the rougher surface of zinc electrode and suppressing the passivation film formation.  In the future, by application of the obtained thin film as pre-treatment on zinc, the effect of the film on adhesion and corrosion resistance will be investigated.

References

[1] W. Funke, Prog. Org. Coat., 28 (1996) 3

[2] J. Marsh, J. D. Scantlebury and S. B. Lyon, J. Appl. Polym. Sci., 59 (1986) 897

[3] G. Mengoli, S. Daolio and M. M. Musiani, J. Appl. Electrochem., 10 (1980) 459

[4] B. Zaid, S. Aeiyach, P. C. Lacaze and H. Takenouti, Electrochimica Acta, Vol. 43, Nos 16-17 (1998) 2331

[5] J. Marsh, Ph.D. thesis, UMIST (1992)

[6] G. Mengoli and M. M. Musiani, Electrochimica Acta, Vol. 31, No.2 (1986) 201

[7] C. Cachet, C. P. DePauli and R. Wiart, Corros. Sci., 25 (1985) 493

[8] M. C. Pham, P. C. Lacaze, P. Mourcel and J. E. Dubois, J. Appl. Electrochem., Vol. 16 (1986) 393

Table 1.  Electrode treatment and electrolytic solution composition

No

Electrode treatment

Solution composition (M)

Solution pH

 

2-allylphenol

Allylamine

1

Untreated

0.25

1.6

11.0

High amine, 1.6M

2

Untreated

0.25

1.6

10.2

 

3

Untreated

0.25

1.6

  9.4

 

4

Untreated

0.25

0.53

10.7

Moderate amine, 0.53M

5

Untreated

0.25

0.12

10.2

Low amine, 0.12M

6

S treated

0.25

1.6

11.0

 

7

S treated

0.25

1.6

10.2

 

8

S treated

0.25

1.6

  9.4

 

9

Untreated

-

1.6

11.0

(Comparison)

10

Untreated

-

1.6

10.2

(Comparison)

11

Untreated

-

1.6

  9.4

(Comparison)

12

S treated

-

1.6

11.0

(Comparison)

13

S treated

-

1.6

10.2

(Comparison)

14

S treated

-

1.6

  9.4

(Comparison)

Table 2.  Result of anodic oxidation at 1.0V(SCE) for 10 minutes

No

Period of current suppression (minute)

Electrode surface after polarisation

Electrode appearance

Water repellent

1

> 10

Thin blue film (interference colour)

Yes

2

> 10

Yellow thick film (adhesion failure)

Yes

3

> 10

Dark yellow thick film

Yes

4

> 10

Thin blue film (interference colour)

Yes

5

> 10 (fluctuating)

No change

No

6

4 (fluctuating)

No change

No

7

> 10

No change

No

8

> 10 (slightly
     fluctuating)

No change

No

9

< 1

Corrosion

No

10

< 1

Slight blackening

No

11

< 1

Blackening and corrosion

No

12

< 1

No change

No

13

< 1

Slight blackening

No

14

< 1 (slightly
    fluctuating)

Slight blackening and corrosion

No


Figure 4   IR spectra of the polymer films obtained from pH 9.4 (A), 10.2 (B) and 11.0 (C) solution with 2-allylphenol and high concentration of allylamine on untreated zinc-coated steel by anodic oxidation at 1.0V(SCE) for 10 minutes.

Photo 1   SEM picture of the polymer film that is cracked and detached from the substrate.  The film was obtained from pH 10.2 solution with 2-allylphenol and high concentration of allylamine on untreated zinc-coated steel by anodic oxidation at 1.0V(SCE) for 10 minutes.