3.4 Influence of microstructure on electrochemical behavior

Electrochemical impedance spectroscopy (EIS) after a longer immersion time of 3300 s (Ecorr) was also carried out. EIS experiments were performed at open circuit potential over a frequency range of 0.1–100 kHz. The sinusoidal potential perturbation was 5 mV in amplitude. Figure 8 shows Nyquist plots for untreated and LSRtreated alloys in aerated 0.1 M H2SO4 at a temperature of 25°C 0.5°C, after 3300 s immersion.

Trdan and Grum [5] pointed out, which EIS technique is probably one of the most powerful nondestructive steady-state methods in electro-chemistry. EIS enables us to determine different parameters of equivalent electrochemical systems (capacitance, resistance, electrolyte interface, etc.). Moreover, Kendig et al. [22] suggested that EIS spectra obtained over a wide range of frequencies indicate that the technique is right choice, since it is applicable for evaluating complicated corrosion processes.

By analyzing the diagram of Figure 8, presence of capacitive loops is observed at high frequencies and inductive loops at low frequencies. However, it is seen that LSR-treated workpiece exhibit higher resistance compared to untreated (as received) workpiece, at all immersion times, this result is in agreement with result studied by Trdan et al. [5, 23]. Capacitive behavior of material is related to aluminum oxide layer properties, studied by Pariona et al. [7], while inductive behavior can be attributed with active state of aluminum surface present in studied electrolyte. Passive regions refer to the oxide layer on aluminum, in accordance with Zhang et al. [24] and Pariona and Micene [7] argued that LSR-treated sample resulted in reduction of current density, and this fact indicates a lower corrosion, therefore, LSR-treated workpiece showed clearly a wide passive zone.

thicker aluminum oxide when compared to untreated workpiece, argued by Pariona et al. [8]. According to result of Table 4, based on the values of Rp, LSR-treated layer presents greater resistance to charge transfer at electrode/solution interface in relation to untreated workpiece. Then, it can be emphasized that treated sample is more resistive than untreated workpiece, at 11 times higher. However, LSR-treated workpiece showed lower electric double layer capacitor values than untreated material. These results once again report that RSL treatment is an efficient technique to improve corrosion behavior of Al-2.0 wt.% Fe alloy in sulfuric acid medium, thus, Trdan and Grum [5] demonstrated improvement of corrosion resistance by means of increased pitting potential with lower intensity of pitting attack

Rp Resistance of passive layer

Treated 22 Ω 22.6 KΩ 1.94 μF Untreated 22 Ω 2.07 KΩ 3.4 μF

Effect of Microstructure on Microhardness and Electrochemical Behavior in Hypereutectic…

Electrochemical parameters for Al-2.0 wt.% Fe alloy LSR-treated and untreated.

Cdl Electric double layer capacitor

There are several opinions and controversies of authors around pseudoinductive that it presents themselves in EIS technique. According to Zhang et al. [24], the pseudo-inductive behavior was observed on microcapillaries. Silver et al. [25] argued, which in many cases, loops emerging in the low-frequency range are wrongly called inductive. In opinion of these last authors, the pseudo-inductive behaviors are caused by drift and corrosion and can be explained by so-called

equivalent circuit, designed from Matlab software, the following the model

EC proposed (Figure 9) describe this system, where is given by Rs, which represents the solution resistance, Cdl is the electric double layer capacitor, Rp is the polarization resistance, R1 is the inductive resistance and L is the inductive element. The Rs, Rp and Cdl data were obtained experimentally by potentiostat software. However, L has no way to measure, for this a computational adjustment was made by Matlab software, using the circuit of Figure 9, then, for untreated

According to impedance spectroscopy technique, it was possible to characterize electric behavior of treated layer and to design the values of equivalent circuit (EC) formed by resistors, capacitors and inductors. In Figure 9 were fitted a proposed of

on the specimen's surface due to laser shock peening (LSP).

Electric behavior of treated layer. A proposed of the equivalent circuit.

negative capacitance effect.

Material Rs

Table 4.

Figure 9.

Polarization resistance

DOI: http://dx.doi.org/10.5772/intechopen.81095

suggested by Macdonald [26].

185

Electrochemical parameters obtained for Al-2.0 wt.% Fe alloy LSR-treated and untreated are shown in Table 4. Where, Rs is the polarization resistance of M H2SO4 solution. Cdl and RP are attributed to the electric double layer capacitor and the resistance of passive layer, respectively. It can be seen that results obtained for Rp are very close to those obtained through of potentiodynamic polarization curves, investigated for same alloy by Pariona and Micene [7], thus indicating reliability of presented results. It was also noted that LSR treatment provided the formation of

Figure 8. Nyquist plots for untreated and LSR-treated alloys in aerated 0.1 M H2SO4 at a temperature of 25°C.

Effect of Microstructure on Microhardness and Electrochemical Behavior in Hypereutectic… DOI: http://dx.doi.org/10.5772/intechopen.81095


Table 4.

3.4 Influence of microstructure on electrochemical behavior

immersion.

Aerospace Engineering

rosion processes.

Figure 8.

184

Electrochemical impedance spectroscopy (EIS) after a longer immersion time of 3300 s (Ecorr) was also carried out. EIS experiments were performed at open circuit potential over a frequency range of 0.1–100 kHz. The sinusoidal potential perturbation was 5 mV in amplitude. Figure 8 shows Nyquist plots for untreated and LSRtreated alloys in aerated 0.1 M H2SO4 at a temperature of 25°C 0.5°C, after 3300 s

Trdan and Grum [5] pointed out, which EIS technique is probably one of the most powerful nondestructive steady-state methods in electro-chemistry. EIS enables us to determine different parameters of equivalent electrochemical systems (capacitance, resistance, electrolyte interface, etc.). Moreover, Kendig et al. [22] suggested that EIS spectra obtained over a wide range of frequencies indicate that the technique is right choice, since it is applicable for evaluating complicated cor-

By analyzing the diagram of Figure 8, presence of capacitive loops is observed at high frequencies and inductive loops at low frequencies. However, it is seen that LSR-treated workpiece exhibit higher resistance compared to untreated (as received) workpiece, at all immersion times, this result is in agreement with result studied by Trdan et al. [5, 23]. Capacitive behavior of material is related to aluminum oxide layer properties, studied by Pariona et al. [7], while inductive behavior can be attributed with active state of aluminum surface present in studied electrolyte. Passive regions refer to the oxide layer on aluminum, in accordance with Zhang et al. [24] and Pariona and Micene [7] argued that LSR-treated sample resulted in reduction of current density, and this fact indicates a lower corrosion,

Electrochemical parameters obtained for Al-2.0 wt.% Fe alloy LSR-treated and

untreated are shown in Table 4. Where, Rs is the polarization resistance of M H2SO4 solution. Cdl and RP are attributed to the electric double layer capacitor and the resistance of passive layer, respectively. It can be seen that results obtained for Rp are very close to those obtained through of potentiodynamic polarization curves, investigated for same alloy by Pariona and Micene [7], thus indicating reliability of presented results. It was also noted that LSR treatment provided the formation of

Nyquist plots for untreated and LSR-treated alloys in aerated 0.1 M H2SO4 at a temperature of 25°C.

therefore, LSR-treated workpiece showed clearly a wide passive zone.

Electrochemical parameters for Al-2.0 wt.% Fe alloy LSR-treated and untreated.

Figure 9.

Electric behavior of treated layer. A proposed of the equivalent circuit.

thicker aluminum oxide when compared to untreated workpiece, argued by Pariona et al. [8]. According to result of Table 4, based on the values of Rp, LSR-treated layer presents greater resistance to charge transfer at electrode/solution interface in relation to untreated workpiece. Then, it can be emphasized that treated sample is more resistive than untreated workpiece, at 11 times higher. However, LSR-treated workpiece showed lower electric double layer capacitor values than untreated material. These results once again report that RSL treatment is an efficient technique to improve corrosion behavior of Al-2.0 wt.% Fe alloy in sulfuric acid medium, thus, Trdan and Grum [5] demonstrated improvement of corrosion resistance by means of increased pitting potential with lower intensity of pitting attack on the specimen's surface due to laser shock peening (LSP).

There are several opinions and controversies of authors around pseudoinductive that it presents themselves in EIS technique. According to Zhang et al. [24], the pseudo-inductive behavior was observed on microcapillaries. Silver et al. [25] argued, which in many cases, loops emerging in the low-frequency range are wrongly called inductive. In opinion of these last authors, the pseudo-inductive behaviors are caused by drift and corrosion and can be explained by so-called negative capacitance effect.

According to impedance spectroscopy technique, it was possible to characterize electric behavior of treated layer and to design the values of equivalent circuit (EC) formed by resistors, capacitors and inductors. In Figure 9 were fitted a proposed of equivalent circuit, designed from Matlab software, the following the model suggested by Macdonald [26].

EC proposed (Figure 9) describe this system, where is given by Rs, which represents the solution resistance, Cdl is the electric double layer capacitor, Rp is the polarization resistance, R1 is the inductive resistance and L is the inductive element. The Rs, Rp and Cdl data were obtained experimentally by potentiostat software. However, L has no way to measure, for this a computational adjustment was made by Matlab software, using the circuit of Figure 9, then, for untreated

sample, the corresponds values were found, for L = 1000 H and for treated sample was L = 5000 H.

evolution, further, Cordovilla et al. [20] pointed out as essential tool to understand way in which each track affects the microstructures produced by previous one. On the other hand, Guan et al. [28] argued that overlapping is important in determining corrosion resistance due to microstructure in-homogeneities in the molten pool. However, Kalita [1] noted existence of increase in corrosion resistance comes as

Effect of Microstructure on Microhardness and Electrochemical Behavior in Hypereutectic…

a result of homogenization and microstructure refinement, which is due to the surface layer melting, as well as through decreases electrical conductivity of resultant passive layer, still these same authors argue, laser surface melting is a useful method for corrosion protection of friction stir weld surfaces as a result of improved microstructure and phase distribution. Nevertheless, Watkins et al. [19] reported, laser surface treatments offer significant potential for improvement of materials

The authors Yue et al. [14] who reported, potentiodynamic polarization tests showed that as a result of laser treatment, the corrosion current can be reduced by as much as six times, and a passive region was obtained. Besides, analysis of electrochemical impedance measurements showed that at an open-circuit potential (OCP), the polarization resistance and double-layer capacitance of the film electrolyte interface of laser-treated specimen were one order of magnitude higher and six

Since then, hypereutectic Al-2.0 wt.% Fe alloy laser-treated is very peculiar and

This research involved a study of hypoeutectic Al-2.0 wt.% Fe alloy subjected to

1. In the cast region shown a refined compact and homogeneous microstructure

2. Fine microstructure of the melt zone is attributed to high cooling rate due LSR-

3. The cast region of Al-2.0 wt.% Fe alloy showed a noticeable overlapping line of

4.The hardness of the cast region of Al-2.0 wt.% Fe alloy was about 61% higher

5. Electrochemical impedance spectroscopy parameters obtained for Al-2.0 wt.% Fe alloy LSR-treated and untreated showed presence of capacitive loops at high

6.LSR-treated workpiece exhibit higher polarization resistance than untreated, in 11 times higher and capacitive behavior of material is related to aluminum

frequencies and inductive loops at low frequencies.

a laser surface remelting (LSR) treatment. The main results are the following:

devoid of microcracks and with formation of a small protuberance,

that it has very special characteristics. Therefore, in this study the influence of microstructural characteristic on microhardness and electrochemical behavior was demonstrated clearly, thus it has an innovative character and can be applied in aerospace, aeronautical and automobile industries. Guan et al. [28] argued, which overlapping adjacent traces as a result of multiple passes using scanning laser beam is usually adequate for production of area coverage. It has long been realized that laser beam overlapping may play a significant role in influencing final surface

properties such as corrosion performance and wear resistance.

DOI: http://dx.doi.org/10.5772/intechopen.81095

times lower than that untreated specimen, respectively.

properties of laser-treated materials.

consecutive weld fillets,

than the untreated material,

4. Conclusions

treated,

187

Figures 10 and 11 show experimental results that were overlaid with simulated data for untreated and LSR-treated workpieces, respectively, thus, curves presented a good fit for the untreated case (Figure 10), where experimental and simulated values are very close.

Moreover, for LSR-treated case, curves did not present a good fit (Figure 11), experimental and simulated values are quite different. This is because, after LSRtreated, treated layer is composed of metastable phases, mainly consisting of alumina and aluminum nitride, besides, microstructure showing multiple laser tracks characteristics formed during laser treatment, however, the molten pool zone showed a fine microstructure due to high quenching rates applied, meanwhile, Guan et al. [28] argued, which laser beam tracks has significant influence on surface quality of laser-treated materials. Consequently, LSR-treated layer has a very complex feature; with certainly, EC proposed should be more complex for LSR-treated.

Different authors reported several investigations about this study, which microstructure characteristic caused by overlapping ratios and multi-track, influence on electrochemical behavior or laser multi-track overlapping and consequently in effect of corrosion process. According to previous works, He et al. [27] point out that overlapping tracks affect heat transfer and liquid flow, microstructure

Figure 10. Nyquist diagram of experimental and simulated result of a untreated sample.

Figure 11. Nyquist diagram of experimental and simulated result of a LSR-treated workpiece.
