P.Garc�sa (*), L. Ga. Andi�na, F. Casesb , ,R. Lapuentec, E. Morall�nc and J. L. V�zquez c
(a) Departamento de Ingenier�a de la Construcci�n, O.P.,
Inf. Urbana, Universidad de Alicante. Apdo. 99. 03080-Alicante, Spain.
(b)Departamento de Ingenier�a Textil y Papelera, EPS de Alcoy, Universidad Polit�cnica de Valencia. Paseo del Viaducto, 1. 03800-Alcoy, Spain
(c)Departamento de Qu�mica-F�sica. Universidad de Alicante. Apdo. 99. 03080-Alicante, Spain.
(*) Corresponding Author
Postal Address : Apdo. 99. 03080-Alicante, Spain.
Cyclic voltammetry, chronoamperometry and FTIR-ATR techniques have been employed to investigate the phenol electropolymerization on stainless steel electrodes in carbonate aqueous medium and .in solutions obtained by filtering of Calcium Aluminate Cement and Ordinary Portland Cement slurries. The phenol electropolymerization occurs on a passivated surface and leads to adherent and stable polymeric film exhibiting a partial protection against corrosion. This film maintains the aromatic character and contains ether-linked rings.
Keywords: Stainless steel, phenol, coating, calcium aluminate cement, electrochemical methods, reflection spectroscopy, corrosion.
In recent years the convenience of substitution of Carbon Steel(CS) by Stainless Steel(SS) as reinforcing material for concrete has been proposed in various international instances. This change is required based on the superior corrosion behaviour of SS in aggressive environments. However taking into account the high cost of this material it seems an interesting task to enhance as much as possible that behaviour. One of the alternatives for that purpose is the application of organic coatings on the steel surface. Phenol and its substituted derivatives can also be electropolymerized by oxidation in aqueous and non-aqueous solutions giving phenol polymeric films [1-20]. In general, these films are very thin, adherent and present low water mobility and low permeability to different ionic and molecular species. The use of alkaline solutions in the electropolymerization of phenol on stainless steel electrodes can help to obtain stable passive layers that significantly allow the formation of polymeric films on the electrode surface.
This paper responds to a double objective: first to study the formation and further characterisation of a polymeric film onto SS electrodes deposited by electrochemical oxidation of phenol (concentration 0.06 M) added to a water solution of Sodium Carbonate 0.1 M. Secondly the behaviour of the coated electrodes against corrosion has been studied as immersed in solutions obtained by filtering of Calcium Aluminate Cement and Ordinary Portland Cement slurries (pH values 11.4 and 12.6,respectively), including the influence of chloride ions present in the medium.. The characteristics of the polymeric films formed have been studied by cyclic voltammetry, Potentiostatic Current Transients and ATR-FTIR techniques. Scanning Electron Microscopy (SEM) has been also used to study the film morphology and Resistance Polarization Technique has been employed for evaluation of instantaneous corrosion current density.
The test solution was 0.1 M Na2CO3 from Merck p.a. The phenol solution was prepared from Merck p.a. reagents. The cement solution was obtained by filtering out the solid components of a cement slurry prepared at water/cement ratio=2.0. Water was obtained from a Millipore-Milli-Q System with a resistivity near to 18.2 MΩ cm. All potentials are referred to the reversible hydrogen electrode (RHE) immersed in the same test solution.
The electrode material used in this work was stainless steel (SS) (chemical composition in wt %: C ≤ 0.050, Si≤ 0.750, Mn ≤ 2.000, P 0.040, S 0.015, Cr 18-19, Ni 8.5-9). Cylindrical electrodes with 8 mm diameter, were employed.
The SS electrodes were degreased with acetone. Before each electrochemical experiment, these were polished with alumina of 0.05 μ m grade (Buehler) and after that, they were cleaned with ultrapure water in an ultrasonic bath. To remove surface oxides, the electrodes were cathodically polarized in the test solution at a potential of 0.5 V for five minutes. For electropolymerization process, the electrode treated as above was immersed in carbonate+phenol solutions and the polarization programme was started.
The Resistance Polarization (Rp) measurement techniques, developed by Stern et al [21, 22], have been used to evaluate instantaneous corrosion current density (icorr). A polarization sweep from 10 to 10 mV around the corrosion potential (Ecorr) was applied to both steel electrodes at 1 mV s-1. icorr was calculated assuming values of B=26 mV for corroding steel or 52 mV for passive steel in the Stern-Geary equation:
icorr = B/Rp
The Fourier Transform Infrared Attenuated Total Reflection (FTIR-ATR) experiments were performed with a Nicolet Magna 550 Spectrometer equipped with a DTGS detector and a 45� KRS-5 ATR crystal. Spectra was collected with a resolution of 8 cm-1.
A Jeol JSM-840 Scanning Electron Microscope (SEM) was used to observe the surface morphologies.
Figure 1 shows the voltammetric response obtained with a carbon steel electrode in 0.1 M Na2CO3 solution. During the first positive scan a region between 0.2 and 0.6 V appears where the electrode shows an active state in which its surface is oxidized. From this potential of 0.6 V the passivation of the electrode surface occurs. At about 1.6 V oxygen evolution starts. The voltammetric profile changes with the number of sweeps in this range of potentials and well-defined anodic and cathodic peaks appear.
Figure 2 shows the voltammetric response of carbon steel electrode in presence of 6�10-2 M phenol in the 0.1 M Na2CO3 solution. During the first positive scan up to 1.6 V the voltammetric profile is roughly the same as the one obtained in Figure 1 (phenol free solution). From this potential up a sharp oxidation peak is obtained with a maximum approximately at 1.85V that can be associated to the oxidation of phenol. In the following sweeps, the peak disappears and the phenol oxidation is practically inhibited. Moreover, the evolution of the voltammetric profile with the number of sweeps is different to that observed in Figure 1, as can be concluded from the next observations:
The oxidation of phenol on platinum electrodes forms a polymer film on the electrode surface in the same aqueous carbonate solution . Therefore, this voltammetric behaviour obtained in Figure 2 in presence of phenol in solution could be associated to the formation of a low permeable polymeric film on the stainless steel surface.
To confirm the existence of a polymeric film on the electrode surface, the electrode was potentiostatically maintained at 1.9 V for seven minutes in the solution of phenol. The obtained electrode was then removed from the electrochemical cell and thoroughly washed with ultrapure water. After that, it was immersed in CAC filtering solutions free of phenol and cycled between 0.5 and 1.4 V (Figure 3 and Figure 4). Theses Figures show the tenth cycle for both bare and coated electrodes. It can be observed that the evolution of voltammetric profile with the number of sweeps is slower when the polymeric film exists on the electrode surface. Thus, the peak current associated to dissolution of stainless steel electrode is smaller (about 40% less) for the covered electrode than the one obtained for a clean stainless steel electrode. Then, the polymer film obtained from phenol oxidation seems to have a protective effect against the oxidation of the electrode surface. Similar results are obtained with OPC filtering solutions as medium.
The corrosion process in the presence and the absence of the protective layer on the stainless steel electrodes has been studied from the instantaneous corrosion current density (icorr).
For the corrosion tests, the polyphenol-coated samples (S=2.54 mm2) were dipped in a CAC filtering solution. After 1 hour icorr was measured by polarization resistance technique, Rp. For uncoated stainless steel electrode (used as reference), the current density obtained in solution was: icorr(SS)= 0.9 μ A cm-2, while for the polyphenol coated sample the coresponding value was: icorr(SS)= 0.2 μ A cm-2. These values confirm the partial inhibiting effect of this coating film in this particular medium.
Figure 5 shows the ATR-IR spectra of the film produced by electrooxidation of 6�10-2 M phenol in carbonate medium for stainless steel electrodes. For polymeric film formation, the electrode was submitted to one potential sweep from 0.5 to 1.9 V in this solution, and maintained at this final potential for seven minutes.
In order to assign the IR bands obtained for the polymeric film of Figure 4, the spectra of phenol monomer  and polymeric film obtained on Pt electrode in carbonate medium  are used. The characteristic bands observed in Figure 5 are:
i) At 1400-1650 cm-1 several bands appear associated to the aromatic carbon-carbon stretching vibration .
ii) At 2800-3000 cm-1 spectral region, several bands appear associated to the aromatic C-H stretching vibration 
iii) A broad band at 3300 cm-1 is also observed in both spectra attributed to the O-H stretching vibration.. A large decrease in the intensity of this band is observed with respect to the phenol monomer spectrum. This decrease can be associated to ether bond formation in the polymeric chain.
iv) At 900-1150 cm-1 spectral region, a broad and unresolved band associated to ether C-O symmetric and asymmetric stretching vibration (=C-O-C= ring) is observed in both spectra.
Therefore, from ATR-IR spectrum it can be concluded that the polymeric film formed on stainless steel electrodes maintains the aromatic character and contains ether-linked rings.
In order to obtain information about the surface morphology of the polymeric films obtained on stainless steel electrodes, the Scanning Electron Microscopy technique has been used. Figure 6 shows the SEM microphotograph of stainless steel electrode coated with the polymeric film (left region) and bare surface (right region). As it can be observed, the morphology of this film appears to be scaly. This morphology is different to that obtained on platinum electrodes [15 ]. A polymer with similar surface morphology is obtained on carbon steel electrodes.
1. The electrooxidation of phenol in carbonate medium on stainless steel electrodes causes the formation of a passivating film. This passivating film avoids further phenol oxidation and partially inhibits the electrode metal oxidation.
2. The IR spectra of the polymeric films show characteristic bands of aromatic C-H stretching vibration and aromatic C=C stretching vibration. These bands permit to propose that the polymeric films created in carbonate medium maintain the aromatic character.
3. The intensity of the band associated to O-H stretching vibration decreases in the IR spectrum. Moreover a band of ether =C-O-C= stretching vibration appears. These results indicate that the polymeric film contains ether-linked rings.
4. SEM results show the scaly looking surface morphology of such films.
5. The comparison of bare vs coated electrode voltammograms show that the latter have current densities about lower at the oxidation peaks.
6. The corrosion rates in CAC media are lower for phenol polimer coated stainless steel electrodes as compared to the bare surface electrodes.
Authors thank to the Direcci�n General de Ense�anza Superior e Investigaci�n Cient�fica (PB97-0130) and to the Generalitat Valenciana (GV-1159/93) and (AE97-2) for the financial support.
Figure 1.- Cyclic voltammograms for a stainless steel electrode immersed in: 0.1 M Na2CO3 solution. (-� -) first, (- - - ) second and (--- ) fifth cycle up to 1.7 V. (Click for a higher resolution image)
Figure 2.- Cyclic voltammograms for a stainless steel electrode immersed in 0.1 M Na2CO3 + 6�10-2 M phenol solution. . (-� -- ) first, (- - - ) second and (---) fifth cycle up to 2.05 V. (Click for a higher resolution image)
Figure 3 .- Cyclic voltammogram for a stainless steel electrode (bare surface) immersed in CAC filtering solution. Cycle 10. (Click for a higher resolution image)
Figure 4 .- Cyclic voltammogram for a stainless steel electrode (coated) immersed in CAC filtering solution. (� � ) first, (- - - ) second, ( .) fifth and (--------) tenth cycle. (Click for a higher resolution image)
Figure 5. FTIR-ATR spectrum of polymer film created on stainless steel electrode by electrooxidation of 6�10-2 M phenol in 0.1 M Na2CO3 solution during seven minutes at a fixed potential of 1.9 V. (Click for a higher resolution image)
Figure 6. Scanning electron micrographs of polymeric film created on stainless steel 0.1 M Na2CO3 + 6�10-2 M phenol solution. Left region: polymer film surface. Right region: bare steel surface.