Corrosion and Wear Resistance of Carbon Films Obtained by Electrodeposition on Ferritic Stainless Steel

Ferritic stainless steel is one of the most important engineering materials due to its low cost and easy mechanical forming. It is an ideal material for a wide range of products, such as kitchenware, interconnections and architectural applications. However, this material shows poor performance in corrosion resistance when compared with austenitic stainless steels. A possible solution to enhance its corrosion resistance is the application of protective films1-4. Amorphous carbon coatings have been used in surface treatments due to their high corrosion resistance in several corrosive environments, as well as due to the fact that they are electric conductors5,6. Traditional methods for carbon films deposition include physical or chemical vapor phase processes, such as Physical Vapor Deposition (PVD) or Chemical Vapor Deposition (CVD) and their variants4,7-10. On the other hand, the deposition of amorphous carbon films by electrodeposition can be considered an alternative process. This technique does not use vacuum and high temperatures. Besides, more complex shapes can be produced with the film11,12. In 1992, Namba obtained carbon films by electrodeposition using ethanol as the electrolyte13. Following this discovery, Suzuki also made carbon films by electrolysis using a water-ethylene glycol solution12,14. A year later, Wang performed the experiment with methanol15. The chemical group methyl is better than ethyl because it provokes a faster growth of the film. Another important parameter is the dipole moment: the higher the dipole moment is, the easier it is to obtain C-C sp3 bonds. These bonds are favorable to make diamond like-carbon (DLC) films. Nowadays, the typical electrolytes used in the electrodeposition of carbon films are acetonitrile (CH3CN), DMF (HCON(CH3)2), nitromethane (CH3NO2), methanol (CH3OH), nitroethanol (CH3CH2OH) and ethanol (CH3CH2OH) 12,16. In the present study, the objective is to obtain a carbon layer on AISI 430 stainless steel to improve the corrosion and hardness proprieties of this material using the electrodeposition technique with N,N-dimethylformamide (DMF) and organic dopant addition as the electrolyte. In this context, the substrate was pretreated by anodization aiming to optimize the film anchoring.


Introduction
Ferritic stainless steel is one of the most important engineering materials due to its low cost and easy mechanical forming.It is an ideal material for a wide range of products, such as kitchenware, interconnections and architectural applications.However, this material shows poor performance in corrosion resistance when compared with austenitic stainless steels.A possible solution to enhance its corrosion resistance is the application of protective films [1][2][3][4] .
Amorphous carbon coatings have been used in surface treatments due to their high corrosion resistance in several corrosive environments, as well as due to the fact that they are electric conductors 5,6 .
Traditional methods for carbon films deposition include physical or chemical vapor phase processes, such as Physical Vapor Deposition (PVD) or Chemical Vapor Deposition (CVD) and their variants 4,[7][8][9][10] .On the other hand, the deposition of amorphous carbon films by electrodeposition can be considered an alternative process.This technique does not use vacuum and high temperatures.Besides, more complex shapes can be produced with the film 11,12 .
In 1992, Namba obtained carbon films by electrodeposition using ethanol as the electrolyte 13 .Following this discovery, Suzuki also made carbon films by electrolysis using a water-ethylene glycol solution 12,14 .A year later, Wang performed the experiment with methanol 15 .
The chemical group methyl is better than ethyl because it provokes a faster growth of the film.Another important parameter is the dipole moment: the higher the dipole moment is, the easier it is to obtain C-C sp³ bonds.These bonds are favorable to make diamond like-carbon (DLC) films.Nowadays, the typical electrolytes used in the electrodeposition of carbon films are acetonitrile (CH 3 CN), DMF (HCON(CH 3 ) 2 ), nitromethane (CH 3 NO 2 ), methanol (CH 3 OH), nitroethanol (CH 3 CH 2 OH) and ethanol (CH 3 CH 2 OH) 12,16 .
In the present study, the objective is to obtain a carbon layer on AISI 430 stainless steel to improve the corrosion and hardness proprieties of this material using the electrodeposition technique with N,N-dimethylformamide (DMF) and organic dopant addition as the electrolyte.In this context, the substrate was pretreated by anodization aiming to optimize the film anchoring.

Material and Methods
Ferritic stainless steel (AISI 430) disks with a diameter of 15 mm and a thickness of 0.95 mm were used as substrates.The chemical composition of the substrates is: %C: 0.104, %Cr: 16.08, %Mn: 0.455, %P: 0.0124, %S: <0.001 and %Si: 0.33.The samples were sanded on SiC abrasive paper (# 400-1200) and then mechanically polished using 1 µm alumina.In order to optimize the film anchoring on the substrate, the surface was pretreated by anodization with 150 V between the electrodes (platinum wire as cathode and AISI 430 stainless steel sample as anode) for 10 minutes, at room temperature.An acid electrolyte was used, which is currently in patent process.
The carbon films were obtained on an AISI 430 stainless steel sample (with and without anodizing treatment), with the application of a cathodic potential of 1200 V between the electrodes for 24 hours.A graphite plate was used as an anode, which was placed at 7 mm from the cathode.An N,N-dimethylformamide (DMF) with organic dopant was used as an electrolyte.The electrodeposition temperature was kept constant at 20 °C with the use of a thermostatic bath with external water circulation.
Atomic force microscopy (AFM -Shimadzu SPM-9500J3, contact mode), scanning electron microscopy (SEM -Jeol 6060, 20 kV) and optical interferometry Zygo (Zigo New View) were performed to observe the surface morphology of the samples.Raman Spectroscopy (Renishaw inVia Raman, laser 514 nm, laser in 10%, 5 accumulations) was used to obtain the microstructural characterization of the films.
The electrochemical characterization was performed using potentiodynamic polarization curves (Omnimetra Instrumentos PG-3901 potentiostat), and the open circuit potential (OCP) was monitored during one hour.For the electrochemical measurements, a saturated calomel electrode (SCE) was used as a reference, a platinum wire was used as a counter-electrode, and a 1 M ethanol: 0.5 M sulfuric acid solution was used as an electrolyte.
The wear tests were performed on a ball-on-plate tribometer at room temperature and a humidity level of 37%.The wear test was performed with a reciprocal linear motion of a 4.76 mm diameter alumina ball, using a constant force of 0.1 N, a frequency of 1 Hz and a track length of 1.5mm.The linear speed was 3 mm/s and the average Hertzian contact stress was 270.13 MPa.
The scratch test was conducted in a CSM scratch tester, using a Berkocich indenter, a linear force of 3 mN to 500 mN, a pre and post load of 3 mN.The scratch appearance and the critical loads were evaluated by optical microscopy.

Morphology study
The preparation of the substrate allowed a relatively regular surface, free of imperfections, as shown on the SEM image (Figure 1) and on the AFM image (Figure 2).The low surface roughness was measured, R a (21.8 nm) and R z (221.1 nm), as shown in Table 1.
Figure 3 and Figure 4 (image of SEM and AFM) show the anodized surface.This surface acts as an anchorage for the carbon films.By doing so, as expected, the anodized surface presented a high roughness, R a (199.0 nm) and R z (3592.3nm), as shown in Table 1, in comparison to the surface before the anodizing process, which is indicated by the consequent increase on the surface area.
The electrodeposited samples (Figure 5 and Figure 6) showed a lower roughness than the anodized surface, R a (72.3 nm) and R z (3290.34nm), as shown in Table 1.
These results indicate that the carbon film grew faster on the valleys than on the peaks, leveling the surface.two bands: the G band relates to sp 2 bonding, centered around 1500-1700 cm −1 and the D band relates to a bond-angle disorder in the sp 2 caused by the presence of sp 3 bonding, H or N, centered around 1200-1450 cm −1 .
The Raman analysis for the obtained carbon film showed two bands centered around 1382.6 ± 1.8 cm −1 and

Raman studies
The diamond spectrum has a peak at 1332 cm −1 and the graphite spectrum shows a peak at 1580 cm −1 [17,18] .In the amorphous carbon films, there are two peaks around 1345-1355 cm −1 and 1570-1590 cm −1 [18,19] , which originated 1569.0 ± 1.1 cm −1 (Figure 7).This suggests that the structure of these films was composed by mixed sp 2 , sp 3 carbon, H and N.In other words, all of the mentioned results are characteristics of amorphous carbon film materials 17,20 .
The measured intensity ratio I D /I G of the carbon film was 0.68 ± 0.002.This result is the same as the results reported on literature, as per Table 2.The Raman spectrum showed a positive slope of baseline that demonstrates the film has a certain degree of hydrogenation.

Corrosion behavior
The carbon film offers a significant improvement in corrosion resistance, in a 1 M ethanol and 0.5 M sulfuric acid solution, when compared to an anodized system and even more when compared to a substrate without surface treatment (Figure 8).There was a displacement of the open circuit potential to more noble values, namely: substrate (-512 mV), anodized (36 mV) and carbon film (163 mV).
Furthermore, the anodized system and the carbon film present a considerable reduction in the corrosion current densities, reaching five orders of magnitude higher in relation to the substrate (Figure 9 and Table 3).In addition, the resistance of polarization of film increased four orders of magnitude higher in relation to the substrate.Moreover, the anodizing surface treatment and the film deposition improved de anodic current density, presenting lower values than the uncoated substrate.
This behavior could be associated to the barrier of the oxide film, which forms a physical obstacle amidst the substrate and the corrosive environment.At the same time, the carbon film confers a nobler trait and covers the imperfections and defects of the oxide layer.

Mechanical behavior
An initial period was observed (Figure 10) with the surfaces sliding freely, and with the coefficient of friction starting at lower values.However, at a subsequent moment of wear, the upper layer was abraded and the coefficient of friction was stabilized at higher values.The oscillation around this average value can be explained by the formation    of particles resulting from the contact between the bodies and its insertion in the interface of wear 21 .
Finally, it was noted that the coefficient of friction of the polished substrate was lower than the coefficient of the anodized film and the coefficient of the carbon film.This can be explained by the insertion of hard abraded particles for the rougher anodized layer.
The start of the coefficient of friction of the carbon film was lower than of the anodized one, which may be justified by the lubrication characteristic of the carbon film.The carbon film supported the normal load up to 0.72 m,   when the value of the coefficient of friction reached the anodized value, indicating that the film is quickly worn.
Figure 11 shows a reduction of wear tracks on the substrate when compared to the carbon film.This suggests that the carbon film and the anodized film absorbed the applied load, which resulted in a less pronounced wear.
The scratch test results show that the film had a plastic deformation in the beginning of the experiment (3 mN, red line in Figure 12) and it had only lateral material displacement in 10 mN (green line in Figure 12).However, the film was not delaminated until the end of the track.This indicates that the coating has adhered well to the substrate.

Conclusions
Before the anodizing process, the substrate had an evenly wrinkled surface.Afterwards, the treatment created a surface that favors the nucleation of the film.Furthermore, the film grew preferentially on anodized valleys, leveling the surface.
The Raman spectroscopy showed the presence of a hydrogenated amorphous carbon film.
The anodized and the carbon film improved the corrosion resistance of stainless steel, diminishing the corrosion, the anodic current densities, and producing a displacement of the corrosion potential to more noble values than the values of the substrate.
In regards to the wear study, this electrodeposited carbon film did not bear the stress for a very long time.However, in the scratch test, the coating showed a good adhesion on the substrate, and the delamination did not occur until the end of the track.

Figure 1 .
Figure 1.Surface images obtained by SEM of ferritic stainless steel (AISI 430) substrate, after it was mechanically polished.

Figure 2 .
Figure 2. Morphology obtained by AFM of ferritic stainless steel (AISI 430) substrate, after it was mechanically polished.The image on the left shows the top view and the image on the right shows the 3D view of roughness.

Figure 3 .
Figure 3. Surface images obtained by SEM of the anodized surface.

Figure 4 .
Figure 4. Morphology obtained by AFM of the anodized surface.The image on the left shows the top view and the image on the right shows the 3D view of roughness.

Figure 5 .
Figure 5. Surface images obtained by SEM of the carbon film on the anodized surface.

Figure 6 .
Figure 6.Morphology obtained by AFM of the carbon film on the anodized surface.The image on the left shows the top view and the image on the right shows the 3D view of roughness.

Figure 7 .
Figure 7. Raman spectral deconvolution of the carbon film and the fitting result.

Figure 8 .
Figure 8. Open circuit potential monitoring in a 1 M ethanol and 0.5 M sulfuric acid solution.

Figure 9 .
Figure 9. Potentiodynamic polarization curves comparing substrate, anodized and electrodeposited films in a 1 M ethanol and 0.5 M sulfuric acid solution, E vs SCE.

Figure 10 .
Figure 10.Coefficient of friction in the ball-in-plate test comparing the substrate, the anodized film and the electrodeposited carbon film.

Figure 11 .
Figure 11.SEM image of the substrate track (a) and of the carbon film track (b) after wear tests.

Figure 12 .
Figure 12.Scratch test image of the carbon film.

Table 1 .
Comparative table of roughness between the studied systems by optical interferometry.

Table 2 .
The intensity radio I D /I G for the obtained film in comparison with the results presented in the literature.

Table 3 .
Electrochemical characteristics of the studied systems.