Effects of High Pressure Torsion on the Pitting Corrosion Resistance of Aluminum Iron Alloys

Effects of High-Pressure Torsion on the Pitting Corrosion Resistance

of Aluminum

­

Iron Alloys

Hiroaki Nakano

, Hiroto Yamaguchi

, Yohei Yamada

, Satoshi Oue

,

In-Joon Son

, Zenji Horita

and Hiroki Koga

1_{Department of Materials Science & Engineering, Kyushu University, Fukuoka 812-8581, Japan} 2_{Department of Materials Process Engineering, Kyushu University, Fukuoka 812-8581, Japan}

3_{Department of Materials Science and Metallurgy, Kyungpook National University, Daegu 702-701, Korea} 4_{Mechanics and Electronics Research Institute, Fukuoka Industrial Technology Center, Kitakyushu 807-0831, Japan}

The effects of reducing the grain size by high-pressure torsion (HPT) on the pitting corrosion resistance of Al­Fe alloys with Fe contents of 0.5, 2 and 5 mass%were investigated by means of polarization curves in solutions containing 0.1 mol·dm¹3_Na

2SO4and 8.46 mmol·dm¹3NaCl

(300 ppm Cl¹) at 298 K and by surface analysis. The potentials for pitting corrosion of the Al­Fe alloys were clearly shifted to the noble direction by HPT, leading to an improvement in pitting corrosion resistance. This improvement was larger in the Al­0.5%Fe and Al­2%Fe alloys and smaller in the Al­5%Fe alloy. The Al­Fe alloys contained precipitates of Al­Fe intermetallic compounds, around which pitting corrosion occurred. The Al­5%Fe alloy, in particular, contained large precipitates tens of micrometers in size, and pitting corrosion was significant around these large precipitates. It is evident from the time-dependence of the corrosion potential and the polarization resistance of the corrosion reaction that the formation rate of Al oxidefilms increases as a result of HPT. It was therefore concluded that the improvement in pitting corrosion resistance of the Al­Fe alloys with HPT is caused by increasing the oxidation rate of Al. [doi:10.2320/matertrans.MH201301]

(Received February 8, 2013; Accepted March 25, 2013; Published May 11, 2013)

Keywords: aluminum­iron alloy, high pressure torsion, corrosion resistance, pitting corrosion, polarization curve

1. Introduction

Although aluminum is inherently an active metal, it shows excellent corrosion resistance over a neutral pH range 4­8 due to its superficial oxidefilm. In solutions containing Cl¹, however, pitting corrosion occurs locally where the oxide

film is attacked by Cl¹.1­7) Many studies of the pitting

corrosion resistance of Al alloys have been conducted, and the concentration of Cl¹and pH of the solution are known to have a large effect on this.1­7) Reducing the grain size of metallic materials to the submicron or even the nanometer size using equal-channel angular pressing (ECAP), high-pressure torsion (HPT), or continuous rotation evolutional control (CREO) is increasingly being studied with the aim of improving mechanical properties such as strength and ductility.8­13)HPT, in particular, can produce the largest strain in the Al alloys and cause the largest decrease in grain size. The pitting corrosion resistance of Al alloys is known to improve by the decrease in grain size due to severe plastic

deformation.14­18) In contrast, when the grain size of the

metal decreases, the corrosion resistance becomes worse generally because of an increase in grain boundaries,19­22) showing that there is ambiguity in the effect of grain size on the pitting corrosion resistance of Al alloys.

Al can often be contaminated by Fe impurities, e.g., in the recycling of Al. It is therefore industrially and commercially important to clarify the effects of severe plastic deformation on the pitting corrosion resistance of Al­Fe alloys. In this study, HPT was performed to decrease the grain size of the

Al­Fe alloys with various Fe contents, and the pitting

corrosion resistance was evaluated electrochemically. The

electrode potential and the electrochemical impedance were measured to understand and discuss the reasons why the pitting corrosion resistance of Al­Fe alloys is altered by HPT.

2. Experimental

Fe was added to pure Al (>99.99 mass%) to produce Al­ Fe alloys with Fe contents of 0.5, 2 and 5 mass%. According to the phase diagram of the Al­Fe binary system,23)since the

eutectic of Al and Al3Fe crystallizes at an Fe content of

1.8 mass%, each alloy is expected to be composed of primary crystals of Al and eutectic (Al­0.5%Fe), almost all eutectic (Al­2%Fe), and primary crystals of Al3Fe and eutectic (Al­

5%Fe). Each specimen was cut to 10 mm in diameter and

1 mm in thickness from the center of the extruded material, which was 20 mm in diameter. Figure 1 shows the schematic diagram of the introduction of strain by HPT. As shown in Fig. 1, 60 GPa of pressure was applied to the specimen from both the top and bottom, and the lower die was rotated 10 times at a speed of 1 rpm. As a result, the grain size of the Al was decreased by large shear strain due to applied torsion using the friction between the die and the specimen.

Specimens of 1.0 cm diameter were prepared for corrosion testing. Their corrosion resistance was investigated over an

area of 28.26 mm2; the remaining area was sealed with

waterproof tape to prevent any corrosion due to edge effects. After the Al­Fe alloys were carefully polished with No. 320, 600 and 1500 emery papers, they were immersed

in a solution containing 0.1 mol·dm¹3 Na2SO4 and 8.46

mmol·dm¹3 NaCl (300 ppm Cl¹) at 298 K for 30 min in air.

Potentiodynamic polarization curves were measured by

polarizing from ¹0.8 V versus NHE less noble than the

corrosion potential to the anodic potential direction using the potential sweep method at 0.5 mV·s¹1. In addition, the

time-+_{Present address: Mitsubishi Materials Co., LTD., Kagawa-gun, Kagawa}

761-3110, Japan

dependence of the anodic current density was measured while

being maintained at¹0.1 V versus NHE after immersion for

30 min. The electrode potentials were measured using a saturated KCl, Ag/AgCl reference electrode (0.197 V versus NHE, 298 K). Since mercury is contained in a calomel

electrode, a Ag/AgCl electrode was used as the reference

electrode in this study. Polarization curve potentials were plotted with reference to NHE. Platinum was used as the counter electrode in all electrochemical measurements.

The time-dependence of the corrosion potential of the

Al­Fe alloys was measured in a solution containing 0.1

mol·dm¹3Na2SO4and 8.46 mmol·dm¹3NaCl (300 ppm Cl¹)

at 298 K in air. Since the naturally formed Al oxide films

were of the barrier type, the time-dependence of the anodic potentials were examined during galvanostatic electrolysis

at 1 A·m¹2 in a solution containing 0.5 mol·dm¹3 H3BO3

and 0.05 mol·dm¹3 Na2B4O7·10H2O used to form a

barrier-type oxide film. The formation rate of the Al oxide film

was assumed to be indicated by the time-dependence of the

anode potentials. The thickness of the Al oxide films was

evaluated by radio frequency glow discharge optical emission spectroscopy (rf-GDOES) under the following analysis

conditions: diameter of 2 mmº, argon pressure of 600 Pa,

pulse frequency of 2000 Hz, and duty cycle of 0.125.

The AC impedance of the Al­Fe alloys was measured at

the resting potential in a solution containing 0.1 mol·dm¹3

Na2SO4 and 8.46 mmol·dm¹3 NaCl. To obtain the Nyquist

plots, the frequency dependence of the AC impedance and phase difference were measured using a frequency response analyzer at the resting potential («200 mV sine wave, 10¹1­ 104_{Hz, 10 points/decade).}

The morphology of the Al­Fe alloys with and without

HPT was observed using SEM after electrolytic polishing.

Prior to SEM observation, the Al­Fe alloys were carefully

polished with No. 1500 emery paper and immersed in a

0.75 mol·dm¹3NaOH solution at 298 K for 30 s. Next, they

were neutralized in 0.48 mol·dm¹3 HNO3 solution for 30 s

and electropolished in a 293 K solution containing methanol

and perchloric acid (MeOH : HClO4=4 : 1) at 10 V for

5 min. Pitting corrosion studies were conducted on the electropolished samples, which were held for 40 h in a

solution containing 0.6 mol·dm¹3 NaCl at 298 K. SEM

micrographs were also obtained for performing microstruc-tural observations.

3. Results and Discussion

3.1 Effect of HPT on the morphology of the Al­Fe alloys

Figure 2 shows the SEM images and EDX spectra of the Al­0.5%Fe alloy without HPT. It can be seen from the SEM image (a) that white second-phase precipitates are present in

the Al­Fe alloy. Only Al was detected in EDX spectrum (b)

obtained from the matrix without the precipitates, while Fe along with Al was detected in EDX spectrum (c) obtained

from the precipitates themselves. It is known that Fe­Al

intermetallic compounds precipitate in Al when Fe is present

as an impurity.24) _{From the phase diagram of the Al­Fe}

binary system, this precipitate is presumed to be Al3Fe and is described as such in this paper.

Figure 3 shows the SEM images of the surface

morphol-ogy of the Al­Fe alloys containing various amounts of Fe

without HPT. White punctiform Al3Fe precipitates were

observed over the entire surface of Al­0.5%Fe (a). These Al3Fe precipitates increased with the Fe content in Al­2%Fe (b) and Al­5%Fe (c). In Al­5%Fe (c), in particular, a large precipitate of primary Al3Fe crystal, shown by an arrow in Fig. 3(c), was also observed.

Figure 4 shows the SEM images of the morphology of the Al­0.5%Fe alloy with and without HPT. The grain size of the Al­0.5%Fe alloy without HPT is tens of micrometers, while that with HPT is approximately 1 µm, showing the significant decrease by application of HPT. However, even after HPT, the precipitates of Al3Fe are present and their size has barely changed.

Figure 5 shows the SEM images of the morphology of the Al­5%Fe alloy with and without HPT. The Al­5%Fe alloy

without HPT contains large precipitates of primary Al3Fe

[(a), (b)], and these large precipitates [(c), (d)] are still present without any decrease in size after HPT. ECAP and CREO, which apply a large shearing deformation to materials, have been reported to break up these second phase precipitates;18­20,25) however, in this study the precipitates are virtually unaffected by the application of HPT.

KeV

10

0 2 4 6 8

Al

Fe Fe

0 2 4 6 8 10

Al

Counts /a.u Counts /a.u

2µm

KeV (a)

(b) (c)

Fig. 2 SEM image and EDX spectra of the Al­0.5%Fe alloy without HPT.

Upper Anvil

Lower Anvil

Upper Anvil

Lower Anvil Lubricant Sample

Load

Load

Rotation

(a)

(b)

(c)

10µm

Fig. 3 SEM images of the morphology of the Al­Fe alloys without HPT: (a) Al­0.5%Fe (b) Al­2%Fe (c) Al­5%Fe.

2µm

(a)

(b)

Fig. 4 Effect of HPT on the morphology of the Al­0.5%Fe alloy: (a) Without HPT (b) With HPT.

100

μ

m

100

μ

m

10

μ

m

10

μ

m

(a)

(b)

(c)

_(d)

The surfaces of the electropolished Al­Fe alloys with HPT

after immersion in a solution containing 0.6 mol·dm¹3 NaCl

for 40 h were analyzed by SEM, and the images are shown in Fig. 6. In the Al­0.5%Fe alloy (a), many small precipitates were seen. In addition, pits several micrometers in size were observed around the precipitates, showing the initiation of pitting corrosion. In the Al­5%Fe alloy (b), there was a large precipitate tens of micrometers in size, and pitting corrosion

proceeded significantly around this large precipitate. The

immersion potentials of Al and Al3Fe in NaCl solution have been reported to be ¹0.76 and ¹0.56 V, respectively, i.e., Al3Fe is nobler.26)Since Al3Fe intermetallic compounds are nobler than the Al matrix, pitting corrosion of Al appears to occur by formation of a local-action cell in which an Al3Fe intermetallic compound acts as the cathode. Pitting corrosion is less likely to occur as the size of the precipitate decreases, i.e., with a decrease in the cathode area. It has been reported from electron microscope observations that multiple pitting corrosion occurs at the initial stage, but only the pits that grow readily at this initial stage ultimately develop into macro pits because most pits are unstable and are

immediately repassivated.2) The broken passive films are

readily repaired when the formation rate of Al oxidefilms is fast. Pits seem to readily grow around large precipitates at the initial stage and continue to grow to become macro pits. In this study, the pitting corrosion of Al occurred intensively

between Al matrix and Al3Fe intermetallic compounds.

When the electric conductivity of solution is high and the difference in immersion potential between the anode and

cathode is large, the pitting corrosion does not always occur in the vicinity of cathode. In this study, since the electric conductivity of solution is relatively low and the difference in immersion potential between Al and Al3Fe is not so large, the pitting corrosion of Al seems to occur intensively at interface

between Al matrix and Al3Fe intermetallic compounds.

3.2 Effect of HPT on the pitting corrosion resistance of the Al­Fe alloy

Figure 7 shows the effect of HPT on the polarization curve of the Al­Fe alloys. The anodic current densities of all the Al­Fe alloys rapidly increased at certain potentials when the anode potentials were shifted from their corrosion potential to the noble direction. This rapid increase in current density is caused by initiation of pitting corrosion. The pitting corrosion potentials of all the Al­Fe alloys, at which pitting corrosion is initiated, are shifted to the noble direction by HPT, indicating improvement in the pitting corrosion resistance by applica-tion of HPT. The shift in pitting corrosion potential to the noble direction by HPT was largest in the Al­0.5%Fe alloy,

and smallest in Al­5%Fe alloy. The pitting corrosion

potential of the Al­Fe alloy was shifted to the less noble

direction with increasing Fe content in Al­Fe alloy,

irrespective of having undergone the HPT process or not. HPT has hardly any effect on the current density for the initiation of pitting corrosion. The current density for the

initiation of pitting corrosion was the highest in the Al­

0.5%Fe alloy; there was scarcely any difference between the Al­2%Fe and Al­5%Fe alloys.

10µm

(a)

(b)

Fig. 6 SEM images of the Al­Fe alloys with HPT after dipping in 0.6 mol·dm¹3_{NaCl solution for 40 h: (a) Al­0.5%Fe (b) Al­5%Fe.}

-1.0 -0.6 -0.2 0.2 -1.0 -0.6 -0.2 0.2 -1.0 -0.6 -0.2 0.2 10-4

10-3 10-2 10-1 1 10 102

10-4 10-3 10-2 10-1 1 10 102

10-4 10-3 10-2 10-1 1 10 102

Potential, E/V vs. NHE Potential, E/V vs. NHE Potential, E/V vs. NHE

Curr

ent Density

,

I

/A

m

-2

(a) Al-0.5%Fe (b) Al-2%Fe (c) Al-5%Fe without HPT

with HPT

Ecorr

Epit

Epit _Epit

Fig. 7 Effect of HPT on the polarization curves of the Al­Fe alloys in a solution containing 0.1 mol·dm¹3_Na

2SO4and 8.46 mmol·dm¹3

Figure 8 shows the time-dependence of the anodic current density of the Al­Fe alloys while kept at a constant potential

of ¹0.1 V. The anodic current densities of all the Al­Fe

alloys increased sharply after certain periods of time because of the initiation of pitting corrosion. The time required before initiation occurred was longer with HPT than without; this was true for all the alloys. This increase in time caused by HPT was larger in the Al­0.5%Fe and Al­2%Fe alloys, but smaller in the Al­5%Fe alloy. This agrees with a smaller shift of the pitting corrosion potential seen in the Al­5%Fe alloy to the noble direction that is induced by HPT. The improvement in pitting corrosion resistance, caused by HPT, is therefore larger in the Al­0.5%Fe and Al­2%Fe alloys than in the Al­5%Fe alloy.

3.3 Mechanism of improvement in pitting corrosion resistance with HPT

Figure 9 shows the time-dependence of the corrosion

potential of the Al­Fe alloys containing various amounts

of Fe in a solution containing 0.1 mol·dm¹3 Na2SO4 and

8.46 mmol·dm¹3NaCl at 298 K. All the corrosion potentials

shifted in the noble direction with increased time. This can be

attributed to the Al oxide films that form naturally in the

solution. The corrosion potentials of the Al­Fe alloys shifted to the noble direction more rapidly with HPT than without, suggesting that the formation rate of Al oxidefilm increased with HPT. Irrespective of having undergone the HPT process,

the degree of shift of the corrosion potential to the noble direction was smallest in Al­0.5%Fe, and that of the Al­ 2%Fe alloy was almost identical to that of the Al­5%Fe alloy. In the previous discussion, the average magnitude of the oxidation rate of Al over the Al surface is taken into consideration, because it was estimated by the corrosion potential. Since pitting corrosion occurred around the precipitates, it is necessary to investigate the formation rate of Al oxidefilms immediately around the precipitates. In this study, however, it was assumed that the oxidation rate of Al around the precipitates was identical to the average of that of the entire Al surface.

Since the naturally formed Al oxidefilms are of the barrier type, the anode potentials were measured during galvano-static electrolysis in a neutral solution containing a boric salt to form a barrier-type oxidefilm. Figure 10 shows the time-dependence of the anode potentials of the Al­2%Fe alloy at 1 A/m2 in a solution containing 0.5 mol·dm¹3 H3BO3 and 0.05 mol·dm¹3 Na2B4O7·10H2O. The anode potentials of the Al­2%Fe alloy are shifted to the noble direction with increased duration of electrolysis, and the shift was faster

with HPT than without. The thickness of the Al oxidefilms

formed with electrolysis for 2600 s was evaluated by

rf-GDOES to confirm whether the shift of the anodic potential

corresponds to the formation rate of Al oxidefilms. Figure 11

shows the rf-GDOES depth profiles of the Al­2%Fe alloy

after electrolysis at 1 A/m2 _{for 2600 s. The oxygen profiles}

400

0 800 1200 1600 2000 0 400 800 1200 1600 2000 0 100 200 300 400 500 10-2

10-1 1

10-2 10-1 10

1

10-1 1 10

Curr

ent Density

,

I

/A

m

-2

Time, t/s Time, t/s _{Time, t/s} (a) Al-0.5%Fe (b) Al-2%Fe (c) Al-5%Fe

without HPT with HPT

T_pit

T_pit

Tpit

T_pit

T_pit

Tpit

Fig. 8 Time-dependence of the anodic current density of Al­Fe alloys at constant potential of ¹0.1V in a solution containing 0.1 mol·dm¹3_Na

2SO4and 8.46 mmol·dm¹3NaCl.

400

0 800 1200 1600 2000 Time, t/s

-0.7 -0.6 -0.5 -0.4

P

otential,

E

/V vs. NHE

400

0 800 1200 1600 2000 Time, t/s

400

0 800 1200 1600 Time, t/s

2000 -0.7

-0.6 -0.5 -0.4

-0.3 _-0.3

-0.4

-0.5

-0.6

(a) Al-0.5%Fe (b) Al-2%Fe (c) Al-5%Fe without HPT

with HPT

Fig. 9 Time-dependence of the corrosion potential of the Al­Fe alloys in a solution containing 0.1 mol·dm¹3_Na

2SO4and 8.46 mmol·dm¹3

revealed that the thickness of the Al oxide films was larger with HPT than without. The faster shift of the anode potential to the noble direction with HPT (as shown in Fig. 10) is therefore confirmed to be due to an increase in the formation rate of Al oxidefilms with HPT.

Figure 12 shows the time-dependence of the anode

potentials of the Al­Fe alloys containing various amounts

of Fe at 1 A/m2_{in a solution containing 0.5 mol·dm}¹3_H 3BO3 and 0.05 mol·dm¹3 Na2B4O7·10H2O. When the thickness of

Al oxide films was evaluated, a long-term electrolysis was

conducted as shown in Fig. 10. However, in Fig. 12, the electrolysis was conducted for a short time to investigate the formation behavior of Al oxidefilms at the initial stage. The anode potentials of all the Al­Fe alloys shifted linearly to the noble direction with increased duration of electrolysis, and the shift was faster with HPT than without. The shift of the anodic potential corresponds to the formation rate of Al oxide

films. Therefore, it is apparent that the barrier type of Al oxide

film is formed more quickly with HPT than without in a

neutral solution. These results are in complete agreement with the time-dependence of the corrosion potentials in Na2SO4 solution containing Cl¹, as shown in Fig. 9. Irrespective of having undergone the HPT process, the degree of shift in the anode potential to the noble direction was smallest in the Al­ 0.5%Fe, with the Al­2%Fe and Al­5%Fe alloys being almost the same. The formation rate of the Al oxidefilms in the Al­ 0.5%Fe alloy is expected to be slower than that of the Al­

2%Fe and Al­5%Fe alloys. Considering the relationship

between the formation rate of the Al oxide films and the

crystalline structure, it seems likely that the primary crystal of Al3Fe rarely affects the formation rate of Al oxidefilms,

0 5 10 15 20 25

0 500 1000 1500 2000 2500 3000 without HPT

with HPT

Time, t/s

P

o

tential,

E

/V vs. NHE

Fig. 10 Time-dependence of anode potentials of the Al­2%Fe alloy at 1 A/m2_{in a solution containing 0.5 mol·dm}¹3_H

3BO3and 0.05 mol·dm¹3

Na2B4O7.

0 20 40 60 80 100

0 20 40 60 80 100

0 0.1 0.2 0.3 0.4

0 0.1 0.2 0.3 0.4

with HPT without HPT

Depth, L/μm Depth, L/μm

O O

Al

Al

Content of O and

Al (mass%)

Fig. 11 Rf-GDOES depth profiles of the Al­2%Fe alloy after electrolysis at 1 A/m2_{for 2600 s in a solution containing 0.5 mol·dm}¹3

H3BO3and 0.05 mol·dm¹3Na2B4O7.

100

0 200 300 400 Time, t/s

100

0 200 300 400 Time, t/s

100

0 200 300 400 Time, t/s

-1.0 -0.5 0 0.5 1.0 1.5

P

otential,

E

/V vs. NHE

-1.0 0 1.0 2.0 3.0

-1.0 0 1.0 2.0 3.0

(a) Al-0.5%Fe (b) Al-2%Fe (c) Al-5%Fe

without HPT with HPT

Fig. 12 Time-dependence of the anodic potential of the Al­Fe alloys at 1 A·m¹2_{in a solution containing 0.5 mol·dm}¹3_H 3BO3and

and that the formation rate increases with the eutectic of Al and Al3Fe. It is well known that the oxidation reactions of metals occur more quickly at crystalline lattice defects, such as grain boundaries and dislocations, because the lattice defects have larger potential energy than normal area and include segregated impurities.1,2)The HPT process markedly increases the number of grain boundaries and the dislocation density in Al. The oxidation rate of Al appears to increase as a result of the increase in the grain boundaries and dislocation density caused by HPT. Even if the pitting corrosion occurs at the initial stage, the pits seem to be immediately repassivated and do not seem to grow when the formation rate of Al oxide

films is fast. As discussed previously, it can be assumed that the increase in the formation rate of Al oxidefilms contributes to the improvement in the pitting corrosion resistance of the Al­Fe alloys with HPT.

To evaluate the polarization resistance for the corrosion reaction of the Al­Fe alloy, the AC impedance was measured at the corrosion potential in a solution containing 0.1 mol·dm¹3 of Na2SO4 and 8.46 mmol·dm¹3 NaCl. Figure 13

shows the Nyquist plots for the Al­Fe alloys containing

various amounts of Fe measured in the solution. The

diameters of the Nyquist plots of all the Al­Fe alloys

increased with HPT, indicating that the polarization resistance to the corrosion reaction increased with HPT. The increase in the polarization resistance to the corrosion reaction caused

by HPT was most pronounced in the Al­0.5%Fe alloy and

smallest in the Al­5%Fe alloy. This trend is in agreement with the degree of shift in pitting corrosion potential to the noble direction by HPT shown in Fig. 7. The increase in polarization resistance with HPT can be attributed to the increase in the oxidation rate of Al.

As mentioned above, the effect of HPT on the pitting corrosion resistance of the Al­Fe alloys depended on the Fe content in the alloy. The improvement in the pitting corrosion resistance caused by HPT was larger in the Al­0.5%Fe and Al­2%Fe alloys, while it was smaller in the Al­5%Fe alloy. In the Al­5%Fe alloy, as the driving force for a local-action cell reaction, in which an Al3Fe intermetallic compound acts as the cathode, is larger because of the presence of the large

primary crystal of Al3Fe, the improvement in the pitting

corrosion resistance by HPT is smaller in spite of the increase

in the formation rate of the Al oxidefilms. According to the phase diagram of the Al­Fe binary system, since the eutectic of Al and Al3Fe is crystallized at an Fe content of 1.8 mass%, the improvement effect in the pitting corrosion resistance by HPT seems to decrease with increasing the primary crystal of Al3Fe above an Fe content of 1.8 mass%. On the other hand,

although the Al­2%Fe alloy contained more precipitates

of Al3Fe than the Al­0.5%Fe alloy, the pitting corrosion resistance of the Al­2%Fe alloy was almost identical to that of the Al­0.5%Fe alloy. In the Al­2%Fe alloy, even if the pitting corrosion occurs at the initial stage, the pits seem to be immediately repassivated because of the small precipitates of Al3Fe. Since the formation rate of the oxide film of the Al­2%Fe alloy is faster than that of the Al­0.5%Fe alloy, the pitting corrosion resistance of the Al­2%Fe alloy is essentially identical to that of the Al­0.5%Fe alloy in spite of thee presence of more precipitates of Al3Fe.

4. Conclusion

The effects of reducing the grain size by HPT on the pitting corrosion resistance of Al­Fe alloys were investigated using electrochemical techniques. The potentials for pitting corro-sion of Al­Fe alloys containing various amounts of Fe were clearly shifted to the noble direction by HPT, leading to an improvement in pitting corrosion resistance. This improve-ment was greater in the Al­0.5%Fe and Al­2%Fe alloys, while it was lesser in the Al­5%Fe alloy. The Al­Fe alloys

contained precipitates of Al­Fe intermetallic compounds,

around which pitting corrosion occurred. In particular, the Al­5%Fe alloy contained larger precipitates, tens of micro-meters in size, and pitting corrosion proceeded significantly around these large precipitates. It is evident from the time-dependence of the corrosion potential and the polarization resistance for the corrosion reaction that the formation rate of the Al oxidefilms increased as a result of the HPT process. Therefore, it was concluded that the improvement in the

pitting corrosion resistance of the Al­Fe alloys with HPT

was caused by the increased oxidation rate of Al. Even if pitting corrosion occurs at an initial stage, the pits seem to be immediately repassivated because of an increase in the oxidation rate of Al.

0 1 2 3 4

0 1 2 3 4 0

2 4 6

0 2 4 6

0 1 2 3 4

0 1 2 3 4

without HPT with HPT

0.1Hz 1Hz

0.1Hz 1Hz

1Hz 0.1Hz

1Hz

1Hz

0.1Hz

0.1Hz 1Hz

0.1Hz

(a) Al-0.5%Fe (b) Al-2%Fe (c) Al-5%Fe

Re, Z/ΩΩ m2

Re, Z/Ω m2

Re, Z/Ω m2

−

Im,

Z

/

Ω

m

2

Fig. 13 Nyquist plots for the Al­Fe alloys containing various amounts of Fe measured in a solution containing 0.1 mol·dm¹3_Na 2SO4and

Acknowledgments

This study was supported by the Grant-in-Aid for

Scientific Research on Innovation Areas (Research in a

Proposed Research Area No. 23102505) of the Ministry of Education, Culture, Sports, Science and Technology of Japan in 2012.

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