Electrochemical Behavior of Nanocrystalline Fe 88 Si 12 Alloy in 3 . 5 wt % NaCl Solution

Influence of microstructure on electrochemical behavior of nanocrystalline Fe88Si12 alloy has been investigated in 3.5 wt% NaCl solution. The results show that Fe88Si12 alloy with optimal corrosion resistance is composite of ordered Fe3Si and disordered Fe(Si) phases and grain size of 40 nm. Because the ordered Fe3Si structure is beneficial to form SiO2 film, which possesses good corrosion resistance compared with the Fe2O3 film from disordered Fe(Si). Moreover, although the decreased grain size is conducive to form preservative, as the grain size decreases to 10 nm, the grain boundary increases to above 30 vol%, which is the active sites for corrosion attack.


Introduction
Corrosion resistance is of great importance in assessing many future applications of the nanocrystalline materials.The corrosion behavior of the nanocrystalline materials has been investigated over the last two decades for a variety of materials (pure metals, alloys, and composites) [1][2][3][4][5].Mishra [6] indicated that the high micro-strain in the electrodeposition Ni with grain size of 8 nm can be related to the lower corrosion rate in the 1 mol/l H 2 SO 4 solution.Owing to the higher grain boundary density, the nanocrystalline Co coating exhibited good corrosion resistance comparing with coarse grained Co coating in the NaOH or NaCl solutions [7].Xu et al. [8] reported that the corrosion resistance of a nanocrystalline Ti 5 Si 3 C 0.8 film and a nanocrystalline Ti 5 Si 3 film with an average grain size of 15 nm was superior to that of Ti-6Al-4V alloy.However, Vinogradov investigated [9] that the corrosion behavior of the nanocrystalline Cu changed slightly compared with the coarse grained Cu.These researches indicate that the corrosion resistance of the nanocrystalline materials depends on their unique microstructure.
Fe 88 Si 12 (atom ratio) alloy has been extensively investigated due to their excellent soft magnetic properties [10,11].However, the corrosion resistance of the Fe 88 Si 12 alloy, especially about the nanocrystalline Fe 88 Si 12 alloy, has not received much attention.In this paper, the electrochemical behavior of the nanocrystalline Fe 88 Si 12 alloy has been studied by the electrochemical tests, to research influence of the grain size and phase structure on the electrochemical behavior.

Experimental
The different microstructures of Fe 88 Si 12 alloys has been fabricated by a self propagating high temperature synthesis technique (SHS) [12], and annealed treat at 900˚C and 1000˚C for 1 h with air atmosphere, respectively.The grain size and phase structured of the different Fe 88 Si 12 alloys are shown Table 1.
Potentiodynamic polarization curves of the different Fe 88 Si 12 alloys were performed with 3.5 wt% NaCl solution at 25˚C.A three-electrode cell system was employed.All the results are referred to standard hydrogen electrode (SHE).The ribbons measuring 40 mm × 8 mm were cut from the samples.They were mechanically polished with 600 emery paper and rinsed with ethanol and distilled water prior to the electrochemical test.Linear polarization curves were obtained at a scan rate of 0.01 mV/s.The sample was allowed to reach a stationary open circuit potential (90 min).Then, a potential value 200 mV lower than the corrosion potential was applied for 5 minutes and the potentiodynamic scan was initiated.The specimens were examined by X-ray diffract-meter (XRD) and transmission electron microscope (TEM).Specimens after the polarization tests were examined by the corrosive surfaces were characterized by a scanning electron microscope (SEM) and a PHI-5702 multifunctional Xray photoelectron spectroscope (XPS), respectively.

Results and Discussion
Typical anodic potentiodynamic polarization curves of the Fe 88 Si 12 alloys in 3.5 wt% NaCl solution are given in Figure 1(a), a typical active-passive-transpassive-active behavior can be clearly observed.Normally, these passive films cover the surface of the corroded samples and increase the difficulty of Fe or Si ions migrating to surface to participate in electrochemical reaction, and thus create the passivation region where the current density is almost independent of potential.
Corrosion rate of the Fe 88 Si 12 alloys are determined using the Stern-Geary equation from the polarization measurement [13].
  corr 2.303 where i corr is the corrosion current density, R p the polarization resistance, β a and β c the anodic and cathodic tafel slopes, respectively.The R p is determined from the slopes of the potential-current plots measured by the linear polarization curve in the range of ±10 mV about the open-circuit potential (E ocp ) (Figure 1(b)).The corrosion potential (E corr ) and corrosion current density (i corr ) are summarized in Table 2.
Generally, corrosion potential and corrosion current density are used to characterize the active dissolution ability of materials, while passivation current density and passivation potential are used to characterize the passivation ability of materials [14].Corrosion potential of the NC 40 significantly decreases and polarization resistance largely increases.Thus potentially much improve corrosion resistance comparing with NC 10 and CG.Moreover, NC 40 possesses lower passive current density (12.3 μA•cm −2 ) than that of NC 10 (74.1 μA•cm −2 ) and CG (51.3 μA•cm −2 ), which indicates that the NC 40 is easier to passivate than the NC 10 and CG.
Figure 2 shows the SEM morphologies of the surface of the Fe 88 Si 12 alloys after running the polarization curves.Comparing with the CG, only some slight and  It is conclude that the lower corrosion rate of nanocrystalline Fe 88 Si 12 alloys can attribute to its more protective passive film.However, some micro-crack and large corrosion hole are observed on the surface of the NC 10 (Figure 2(a)), which shows the corrosion resistance decreases as the grain size decreases to 10 nm.Palumbo [15] has shown that the passivity of the 14 at% to 20 at% Si-Fe alloy, which consist of ordered Fe 3 Si and few disordered Fe(Si) phases, is controlled by the formation of a silicon dioxide (SiO 2 ) film in 1 mol/l sulfuric acid.The research [15] indicated that the phase composition of the Fe-Si alloy influences the formation mechanisms, growth kinetics, thickness, and composition of passive layers.It also showed that high-Si content can improve the passivation behavior.Although the Si content of the Fe-Si alloy is only 12 at%, the NC 10 and NC 40 consist of ordered D0 3 Fe 3 Si and disordered Fe(Si) phases.So, the SiO 2 film was observed on the surface of the NC 10 and NC 40 (Figure 3), but only Fe 2 O 3 film was examined on the surface of the CG, which suggests that the corrosion resistance of the NC 10 and NC 40 is better than the CG.
On the other hand, with the grain size decreases to 10 nm, the grain boundaries and triple junctions increase to above 30 vol% [16], which are the active sites for corrosion attack when exposed to a corrosion environment.As a result, preferential corrosion at grain boundaries and triple junctions significantly accelerates the corrosion rate of NC 10 .It is note that the NC 10 possesses large numbers of micro-strain, which also makes for easier corrosion.However, the grain boundaries and triple junctions are decrease to below 5 vol% sharply as the grain size increases to 40 nm, and the micro-strain decreases as the annealing.Thus, although some ordered Fe 3 Si structure transformed to the disordered Fe(Si) structure, the corrosion resistance improve as the grain size increases to 40 nm.Because the CG only composed of disordered Fe(Si) solid solution.There only Fe 2 O 3 film is examined on the CG (Figure 3).The Fe 2 O 3 film is unstable compared with the SiO 2 film [17].So the corrosion resistance of the CG is worse than the NC 40 .

Conclusion
The electrochemical behavior of different microstructures of the Fe 88 Si 12 alloys in 3.5% NaCl solution has been investigated.Both the Fe 88 Si 12 alloys with grain size of 10 and 40 nm consist of ordered Fe 3 Si and disorder Fe(Si) phases.But the CG is only composed of disordered Fe(Si) phase.The order Fe 3 Si structure is favor to form SiO 2 film.It possesses excellent corrosion resistance comparing with the Fe 2 O 3 film which forms from disordered Fe(Si).On the other hand, as the grain size decreases to 10 nm, the grain boundaries and triple boundaries increase to above 30 vol%, which are the active sites for corrosion.Based on the above two sides, the corrosion resistance of the Fe 88 Si 12 alloy with grain size of 40 nm is optimal.

Figure 2 .
Figure 2. SEM morphologies of the Fe88Si15 alloys after electrochemical corrosion test.

Figure 3 .
Figure 3. XPS expectra of O2p in the Fe 88 Si 15 alloys after electrochemical corrosion test.