Electro Polymerization of Polypyrrole Coatings Doped with Different Proton Acids for Corrosion Protection of 304 Stainless Steel

Polypyrrole (PPY) coatings doped with sulphuric, oxalic and phosphoric acid were prepared on 304 stainless steel (304SS) by electropolymerization. Surface morphology and anti-corrosive performance of the polymer film was investigated by scanning electron microscope and electrochemical tests. The results indicate that the phosphoric acid doped PPY coating has a compact and smooth surface which shifts the Ecorr of 304SS towards to more positive value, while the jcorr was dropped from 10.4 to 0.88 μA cm. Long-term immersion test shows that the PPY coating doped with phosphoric acid provides better and durable corrosion resistance in comparison with the PPY coatings doped with the sulphuric and oxalic acids.


Introduction
Stainless steel (SS) becomes one of the most suitable metallic materials for industrial applications in the contemporary society due to the advantages of decent conductivity, high mechanical strength, low gas permeability and reasonable cost [1,2].However, the main problem associated with SS is to resist corrosion in the aggressive medium [2,3].The occurrence of corrosion not only brings large economic loss, the dissolution of SS also leads to harmful metallic ions (Cr, Fe, Ni et al.) release out, which undoubtedly contributes to the environmental pollution.Therefore, surface modification coatings have been proposed to improve the corrosion resistance of SS [4,5].Since Deberry [6] found conducting polyaniline coating could provide corrosion protection for steel, different conducting polymers for example polyaniline (PANI) [7,8] and polypyrrole (PPY) [9,10] are intensively investigated for their corrosion protection applications [11,12].It is widely accepted that the polymer helps passivate the surface between the coating and the SS substrate [13], moreover, the conducting polymer can be regarded as barrier layer to protect the SS substrate from corrosive ions.PPY is one of the most studied conducting polymers due to its good anti-corrosive performance and easily preparation nature [14].Commonly the conductivity of PPY can be achieved by doping protonic acid during the electro polymerization process and usually the doped acid has significant effect on conductivity, capacitive characteristic and corrosion resistance of PPY [9].a Corresponding author : luhb@nju.edu.cn/mengxk@nju.edu.cn a Corresponding author : luhb@nju.edu.cn/mengxk@nju.edu.cn

ICMSNT 2017
In order to get a homogenous, compact PPY coating for the use 304 SS to decrease the economic loss caused by corrosion, in this study PPY was electro polymerized in sulphuric, oxalic and phosphoric acid solution respectively.Scanning electron microscope(SEM), electrochemical impedance spectroscopy, open-circuit potential and potentiodynamic polarization were used to explore the effect of different proton acid of the anti-corrosive properties for PPY coating in acid environment, and the protection mechanism of the PPY coating was discussed based on the results.

Experimental procedures
The 304SS plate used as work electrode was sealed with epoxy resin with an exposed surface area for 1 cm 2 and polished with emery papers, followed by rinsed with deionized water and acetone, respectively.The composition of type 304SS is shown in Table 1 [15].Three electrode system was carried out for electro polymerization process, with saturated calomel electrode (SCE) as reference electrode and a platinum plate as counter electrode.The aqueous solutions for electro synthesis consist of 0.1 M pyrrole and 0.3 M proton acid.Using cyclic voltammetry (CV) technique to achieve the electro polymerization of PPY films, the potential varied from -700mV to 1800mV versus SCE with potential scan rate of 10 mV -1 for 12 cycles.The anti-corrosion properties of the PPY coatings were investigated in 0.3M H 2 SO 4 solution through electrochemical test and SEM was used to observe the morphologies of the polymer coatings.

Electro synthesis of PPY coatings
Black PPY coatings electropolymerized with different proton acid are observed after CV process and Figure 1 shows their surface morphologies.All coatings consist of globular particles, which is in agreement to the previously reported PPY films [16].It can be seen that PPY-H 2 SO 4 film exhibits some tiny cracks while H 2 C 2 O 4 doped PPY coating shows relatively dense but not smooth surface as displayed in Figure 1a and Figure 1b.However, the film of PPY-H 3 PO 4 is observed the densest, continuous and crack free with a more homogenous surface morphology(Figure 1c), which may becuse pyrrole monomers has better solubility in phosphoric acid solution.The smooth and crack free surface of PPY-H 3 PO 4 indicating that this coating has the finest grain size, thus can offer the best physical barrier to 304SS substrate to isolate the corrosive ions.ICMSNT 2017

Potentiodynamic polarization measurements
Figure 2a shows the polarization curves for uncoted and coated 304SS after 1 h immersion in 0.3M H 2 SO 4 solution.Using Tafel extrapolation method to calculate the samples corrosion potential (E corr ) and corrosion current (j corr ).The potentiodynamic polarization parameters are partly summarized in Table 2, the lower j corr values demonstrate the decrease in corrosion rate and the more positive E corr values attributes to the substrate which is difficult to corrode in the aggressive environment [17].The E corr and j corr values for bare 304SS are -315 mV (SCE) and 10.4 μA cm -2 respectively, it is observed that all the coatings have lower j corr with more positive E corr values compare with the 304SS substrate due to their anti-corrosive nature.Especially H 3 PO 4 doped PPY coating, which has the most positive E corr and smallest j corr values of 58 mV and 0.88 μAcm -2 , respectively, suggesting that PPY-H 3 PO 4 film exhibits outstanding corrosion protection performance.Besides, the fitting polarization resistance R p for the uncoated and coated 304SS based on the polarization data in 0.3 M H 2 SO 4 solution can be acquired through the following equation : ( where β a and β c represants the Tafel slopes for the anodic and cathodic reactions, respectively.It can be see that R p of coated subtrate all considerably higher than that of bare 304SS, in particular, the R p value of the PPY-H 3 PO 4 film is the largest among all samples.The results explicitly shows that H 3 PO 4 doped PPY coating can offer better protectiveness than PPY-H 2 SO 4 and PPY-H 2 C 2 O 4 coatings, which corresponding to the densest morphology shown in Figure 1.

Open-circuit potential (E ocp ) measurements
The curves of E ocp versus time of uncoated and different PPY coated 304SS are presented in Figure 2b.
The increased E ocp values for the bare 304SS in the initial immersion stage attributes to the formation of passive oxide layer and then the value decreased to -250mV, indicating that the passive layer gradually destroyed and the substrate suffered from corrosion [11].The initial E ocp values of all PPY coatings are more than 300mV (SCE) higher than the value of uncoated 304SS substrate.Initially, the E ocp values for PPY coatings relatively stable, however the E ocp values for PPY-H 2 SO 4 and PPY-H 2 C 2 O 4 coatings significantly decreased with the prolonged immersion time, which attributes to the slow degradation of these two coatings and electrolyte gradually penetrate inward.The E ocp values for PPY-H 3 PO 4 coating fall slightly with time and always larger than other PPY coatings, suggesting that PPY-H 3 PO 4 coating can prevent corrosive ions from penetration effectively during the long-term immersion and provides better corrosion protect for 304SS as compared to PPY-H 2 SO 4 and PPY-H 2 C 2 O 4 coatings.

Electrochemical impedance measurements
Using the electrochemical impedance spectroscopy (EIS) measurements to obtain better assessment of the corrosion protection properties for the coatings.Nyquist plots for the three kinds of coatings protected 304SS after 1 day and 16 days immersion are presented in Figure 3.The plots of PPY-H 2 SO 4 and PPY-H 2 C 2 O 4 coated 304SS all consist of two depressed capacitive loops in high frequency region with a linear part in low frequency region (Figure 3a -3d), which shows the diffusion of electrolyte.Generally, the high frequency parts reflects the coatings information and the low-frequency parts tells the processes at the coating/304SS interface in EIS plots [11].Therefore, the lines from low frequency region of PPY-H 2 SO  3b, 3d) shows that the surface of the coating destroyed to a certain extent that the corrosive ions have penetrate inward, which in correspondence with OCP data(Figure 2b).However, the Nyquist curves of PPY-H 3 PO 4 coated 304SS shows capacitive characteristic instead of diffusion process in the low frequency region during immersion (Figure 3e, 3f), and the capacitance loop in high frequency region is higher than other coatings with continuously increase through the corrosion process, indicating that PPY-H 3 PO 4 exhibits preferable anti-corrosive performance among the three coatings.
In general, the conductive polymer could provides corrosion protection not only for its barrier effect, but also for anodic protection through passivate the substrate.However, when the doped anion released during the protection process, the polymer will be reduced slowly and just serve as a mechanical barrier in its reduction form.It is reproted that the electrodeposited polyaniline film in phosphoric acid solution can improve the adhesiveness of the coating on the metal surface [15], we can deduce that the electropolymerized PPY coating in phosphoric acid solution has similar properties.Therefore, the good adhesion performance and compact surface of PPY-H 3 PO 4 coating offers an excellent physical barrier to isolate corrosive ions from the electrolyte solution.Meanwhile, the dedoping process may carried out difficultly due to the strong binding force between phosphate radical thus corrosion protection of the PPY-H 3 PO 4 coating is more effective as a physical barrier and an stable oxidizer than PPY-H 2 SO 4 and PPY-H 2 C 2 O 4 coatings.

Conclusions
The corrosion protection performances of the PPY doped with H 2 SO 4 , H 2 C 2 O 4 and H 3 PO 4 were investigated in acid corrosive environment.The electrochemical tests showed that all the different coatings provide a certain corrosion protection for 304SS substrate.PPY coatings enlarged the corrosion potential of bare 304SS by more than 300 mV (SCE), particularly PPY-H 3 PO 4 shifts the corrosion potential to a more positive value and significantly decreased the corrosion current density.Moreover, PPY doped with H 3 PO 4 has a higher corrosion impedance compared to other coatings and the impedance increased continuously during the 16 days H 2 SO 4 immersion, suggesting that the PPY-H 3 PO 4 coating provides more effective corrosion protection for 304SS resulted from a physical barrier and an stable oxidizer.

Figure 2 .
Figure 2. Tafel polarization curves of bare and different PPY coated 304SS (a) and time dependence of E ocp for different samplesin 0.3 M H 2 SO 4 solution during long-term immersion (b).

Table 1 .
The composition of type 304SS substrate

Table 2 .
Polarization parameters of the bare and different PPY coated 304SS in 0.3 M H 2 SO 4 solution 4 and PPY-H 2 C 2 O 4 coatings