**3.1 Voltammetric data**

During the anodic potential scanning, the voltammogram of equiaxed zinc shows that the current is practically zero until it reaches a potential of -1 V, where the current rises sharply, starting the active dissolution of metal (Figure 2 (a)). The negative potential scan shows a hysteresis loop, suggesting that this current increase was due to the start of a process of pitting, and two cathodic current peaks at about -1.2 V and - 1.3 V (called C1 and C2). These peaks could be associated with the reduction of Zn(OH)2 and ZnO, respectively (Zhang, 1996). The composition of corrosion products formed on the zinc surface may be not uniformly distributed. The different compositions of the films formed can explain the difference in the outcomes reported in the literature.

The properties of corrosion products are a function of various material and environmental factors and thus vary essentially from situation to situation. For example, only one peak appears in the case of the columnar zinc (Figure 2 (b)). As the concentration of aluminum in the alloy increases, the definition of these reduction peaks is not clear, although the C2 peak is dominant (Figure 2 (c)).

In the case of the alloys, the values of the anodic currents are similar for the same Al concentration, independently of the structure, and the most important difference is observed in the distribution of the cathodic current peaks, which indicates the different characteristics of the films formed during the anodic scan. These results can be attributed to the aggressive/depassivating action of Cl anions (Augustynski, 1978). At present, the mechanism of film formation is still uncertain. For the case of CET structure, profiles are more complex, because the proportion of one or other structure (columnar or equiaxed) can vary from sample to sample (Figure 2 (d)). Also, as the concentration of Al increases, the voltammetric profile of the different structures tends towards the response of pure aluminum (Figure 2 (e)).

Analyzing the response of the composites (Zn-27wt%Al + 8vol%SiC, Zn-27wt%Al + 15vol%SiC (Figure 2(f)), Zn-50wt%Al + 8vol%SiC, Zn-50wt%Al + 15vol%SiC (Figure 4 (g)), Zn-27wt%Al + 8vol%Al2O3, Zn-27wt%Al + 15vol%Al2O3 (Figure 2 (h))), we observed that when the volume percent of SiC particles increase from 8% to 15% in ZA27 and ZA50 matrix, the rate of dissolution of the alloy increases. In the case of the addition of Al2O3 particles to ZA 27 matrix the rate of dissolution is approximately the same.

This different distribution of the peaks in the voltammograms gives rise to surface layers with different corrosion products, as shown in the micrographs of Figure 3, where samples with higher proportion of particles in the matrix show the formation of a thicker layer of corrosion products. Also, it is observed the formation of pitting on the electrode surface.

Three distinctive features in the potentiodynamic curves can be clearly observed (Figure 4): (i) the potential at which the anodic current during the forward anodic bias increases sharply form the passive current level (breakdown or critical pitting potential Ep); (ii) a hysteresis loop (difference between forward and reverse scans) and (iii) the potential at which the hysteresis loop is completed during reverse polarization scan after localized corrosion propagation (repassivation potential Er). Stable pits form at potentials noble to Ep and will grow at potentials noble to Er (Frankel, 1998). Also, for many years it has been recognized that Ep measurements are applicable to naturally occurring pit initiation on stainless alloys in chemical and marine environments (Wilde, 1972; Bilmes et al., 2005).

During the anodic potential scanning, the voltammogram of equiaxed zinc shows that the current is practically zero until it reaches a potential of -1 V, where the current rises sharply, starting the active dissolution of metal (Figure 2 (a)). The negative potential scan shows a hysteresis loop, suggesting that this current increase was due to the start of a process of pitting, and two cathodic current peaks at about -1.2 V and - 1.3 V (called C1 and C2). These peaks could be associated with the reduction of Zn(OH)2 and ZnO, respectively (Zhang, 1996). The composition of corrosion products formed on the zinc surface may be not uniformly distributed. The different compositions of the films formed can explain the

The properties of corrosion products are a function of various material and environmental factors and thus vary essentially from situation to situation. For example, only one peak appears in the case of the columnar zinc (Figure 2 (b)). As the concentration of aluminum in the alloy increases, the definition of these reduction peaks is not clear, although the C2 peak

In the case of the alloys, the values of the anodic currents are similar for the same Al concentration, independently of the structure, and the most important difference is observed in the distribution of the cathodic current peaks, which indicates the different characteristics of the films formed during the anodic scan. These results can be attributed to the

of film formation is still uncertain. For the case of CET structure, profiles are more complex, because the proportion of one or other structure (columnar or equiaxed) can vary from sample to sample (Figure 2 (d)). Also, as the concentration of Al increases, the voltammetric profile of

Analyzing the response of the composites (Zn-27wt%Al + 8vol%SiC, Zn-27wt%Al + 15vol%SiC (Figure 2(f)), Zn-50wt%Al + 8vol%SiC, Zn-50wt%Al + 15vol%SiC (Figure 4 (g)), Zn-27wt%Al + 8vol%Al2O3, Zn-27wt%Al + 15vol%Al2O3 (Figure 2 (h))), we observed that when the volume percent of SiC particles increase from 8% to 15% in ZA27 and ZA50 matrix, the rate of dissolution of the alloy increases. In the case of the addition of Al2O3

This different distribution of the peaks in the voltammograms gives rise to surface layers with different corrosion products, as shown in the micrographs of Figure 3, where samples with higher proportion of particles in the matrix show the formation of a thicker layer of corrosion products. Also, it is observed the formation of pitting on the electrode surface.

Three distinctive features in the potentiodynamic curves can be clearly observed (Figure 4): (i) the potential at which the anodic current during the forward anodic bias increases sharply form the passive current level (breakdown or critical pitting potential Ep); (ii) a hysteresis loop (difference between forward and reverse scans) and (iii) the potential at which the hysteresis loop is completed during reverse polarization scan after localized corrosion propagation (repassivation potential Er). Stable pits form at potentials noble to Ep and will grow at potentials noble to Er (Frankel, 1998). Also, for many years it has been recognized that Ep measurements are applicable to naturally occurring pit initiation on stainless alloys in chemical and marine environments (Wilde, 1972; Bilmes et al., 2005).

the different structures tends towards the response of pure aluminum (Figure 2 (e)).

particles to ZA 27 matrix the rate of dissolution is approximately the same.

anions (Augustynski, 1978). At present, the mechanism

**3. Results and discussion** 

difference in the outcomes reported in the literature.

**3.1 Voltammetric data** 

is dominant (Figure 2 (c)).

aggressive/depassivating action of Cl-

Corrosion Resistance of Directionally Solidified Casting Zinc-Aluminum Matrix 43

Al (Columnar) Al (Equiaxed)



(f)

(e)

ZA27-8Csi ZA27-15CSi



0.00

0.01

0.02

**CURRENT DENSITY (A/cm2**

**)**

0.03

0.04

0.05

0.06





**CURRENT DENSITY (A/cm2**

**)**

0.00

0.20

0.40

0.60

(c)

Zn-27%Al (Columnar)

Zn-27%Al (Equiaxed)



(d)

(c)

Zn-27%Al (CET)



0.00

0.01

0.02

**CURRENT DENSITY (A/cm2**

**)**

0.03

0.04

0.05

0.06

0.00

0.01

0.02

**CURRENT DENSITY (A/cm2**

**)**

0.03

0.04

0.05

0.06

(e)

(f)

Corrosion Resistance of Directionally Solidified Casting Zinc-Aluminum Matrix 45

ZA27-8%SiC ZA27-15%SiC ZA27-8%Al2O3 ZA50-8%SiC ZA50-15%SiC ZA27-15%Al2O3


(i)

(d) Zn-27wt%Al alloy with CET structure, (e) pure Aluminum with columnar and equiaxed structure, (f) Zn-27wt%Al + 8vol%SiC, Zn-27wt%Al + 15vol%SiC, (g) Zn-50wt%Al + 8vol%SiC, Zn-50wt%Al + 15vol%SiC, (h) Zn-27wt%Al + 8vol%Al2O3, Zn-27wt%Al +

(a) Zn-Columnar (b) Zn – Equiaxed

210 m 210 m

210 m 210 m

(c) Al-Columnar (d) Al – Equiaxed

Fig. 2. Voltammograms of (a) pure Zinc with equiaxed structure, (b) pure Zinc with columnar structure (c) Zn-27wt%Al alloy with columnar and equiaxed structures,


15vol%Al2O3 and (i) All types of composites.

0.00

**CURRENT DENSITY (A/cm2**

**)**

0.01

0.02

(g)

(h)

ZA50-15%SiC ZA50-8%SiC



(h)

(g)

ZA27-8%Al2O3 ZA27-15%Al2O3



0.00

0.01

0.02

**CURRENT DENSITY (A/cm2**

**)**

0.03

0.04

0.05

0.06


0.000

0.010

0.020

**CURRENT DENSITY (A/cm2**

**)**

0.030

0.040

0.050

0.060

Fig. 2. Voltammograms of (a) pure Zinc with equiaxed structure, (b) pure Zinc with columnar structure (c) Zn-27wt%Al alloy with columnar and equiaxed structures, (d) Zn-27wt%Al alloy with CET structure, (e) pure Aluminum with columnar and equiaxed structure, (f) Zn-27wt%Al + 8vol%SiC, Zn-27wt%Al + 15vol%SiC, (g) Zn-50wt%Al + 8vol%SiC, Zn-50wt%Al + 15vol%SiC, (h) Zn-27wt%Al + 8vol%Al2O3, Zn-27wt%Al + 15vol%Al2O3 and (i) All types of composites.

Corrosion Resistance of Directionally Solidified Casting Zinc-Aluminum Matrix 47


> Er (V)

ZA50-15vol%SiC -1.002 -1.093 91 -1.271 178

ZA50-8vol%SiC -0.988 -1.082 94 -1.302 220

ZA27-8vol%Al2O3 -0.974 -1.086 112 -1.297 211

ZA27-15vol%Al2O3 -0.974 -1.079 105 -1.298 219

ZA27-15vol%SiC -1.002 -1.086 84 -1.312 226

ZA27-8vol%SiC -0.981 -1.107 126 -1.245 138

Table 2. The susceptibility to corrosion, E, was measured as the difference between the

ZA27 CET -1.02 -1.06 30 -1.102 42 ZA27 Col -1.028 -1.052 24 -1.071 19 ZA27 Eq -1.038 -1.068 40 -1.068 0

∆Ep-r (mV)

Ecorr (V)

∆Er-corr (mV)

In all cases, the E / I response of the alloys shows the typical hysteresis indicates the phenomenon of pitting and found that the more susceptible are the composites than the alloys. The most susceptible are those containing neither SiC nor Al2O3 in the matrix, see

(V)

Er Ep


Alloy / Composite Ep

potential of pitting, Ep, and the repassivation potential, Er.

0.000

0.005

**CURRENT DENSITY (A/cm2)** 

Fig. 4. Representative curve.

Table 2 and Figure 5.

0.010

0.015

Fig. 3. Micrographs of different alloy samples and structures.

Fig. 4. Representative curve.

320 m 320 m 320 m

(e) Zn-27%Al-Columnar (f) Zn-27%Al – Equiaxed (g) Zn-27%Al – CET

389 m

(h) Zn-27%Al-15%SiC (i) Zn-27%Al-8%SiC

(j) Zn-50%Al-8%SiC (k) Zn-50%Al-15%SiC

389 m

460 m

460 m

389 m

389 m

(l) Zn-27%Al-15%Al2O3 (m) Zn-27%Al-8%Al2O3

Fig. 3. Micrographs of different alloy samples and structures.

In all cases, the E / I response of the alloys shows the typical hysteresis indicates the phenomenon of pitting and found that the more susceptible are the composites than the alloys. The most susceptible are those containing neither SiC nor Al2O3 in the matrix, see Table 2 and Figure 5.


Table 2. The susceptibility to corrosion, E, was measured as the difference between the potential of pitting, Ep, and the repassivation potential, Er.

Corrosion Resistance of Directionally Solidified Casting Zinc-Aluminum Matrix 49

ZA27 (CET) ZA27 (Col) ZA27 (Eq) ZA27-8%CSi ZA27-15%CSi ZA27-8%Al2O3

0.00E+00 5.00E+01 1.00E+02 1.50E+02 **IMAGINARY PART (kcm2**

The whole set of experimental impedance spectra can be discussed according to the

*Z R Z <sup>W</sup> ct W* . <sup>1</sup> <sup>1</sup>

where R is the ohmic solution resistance, = 2f; Cdl the capacitance of the electric double layer, Rct the charge transfer resistance and ZW the diffusion contributions in impedance

ZW = RDO (jS) -05 for semi-infinite diffusion contribution and ZW = RDO (jS) -05 coth (jS) -05 is related to diffusion through a film of thickness d, formed on the electrode, where RDO is the diffusion resistance and the parameter S= d2/D, where d and D are the diffusion thickness

The good agreement between experimental and simulated data according to the transfer function given in the analysis of Eqs. 1 and 2 using non-linear least square fit routines is shown in Figure 8 (a) at high frequencies. At low frequencies (less than 10-1 Hz) is not achieved a good fit with this model, since the impedance measurement does not give us enough information to define a new input capacitance, this occurs for samples ZA27-8% SiC, ZA50-8% SiC and ZA27-Al2O3. This process can represent by an equivalent circuit in Figure

and diffusion coefficient related to the transport process (Fedrizzi et al., 1992).

**)**

Zt (jw) R Z (1)

(2)

*j C*

0.00E+00

following total transfer function.

with:

spectra.

8 (a).

Fig. 6. (a) Nyquist Diagram for different samples.

5.00E+01

**REAL PART (k**

**cm2**

**)**

1.00E+02

1.50E+02

Fig. 5. Ep-r as a function of concentration and alloy structure.

The susceptibility to corrosion was measured as the difference between the potential of pitting, Ep, and the repassivation potential, Er, as Ep-r and the difference between the repassivation potential and the corrosion potential of each sample through Table 2 it is possible to observe that the values of repassivation potential for materials without particles in the matrix are near the corrosion potential, but not in the case of the the other samples.

## **3.2 Electrochemical impedance spectroscopy data**

Impedance spectra are strongly dependent on the composition and structures of the alloys and composites. Figure 6 shows the experimental Nyquist diagrams for all the alloys and composites used. All the diagrams show one capacitive time constant at high frequencies and a non-well defined time constant at low frequencies, probably associated with diffusion processes also reported in the literature (Deslouis et al., 1984; Trabanelli at al., 1975). It can be seen that as the concentration of aluminum in the alloy increases, the second time constant approximates the response associated with a diffusion process in finite thickness, due to the formation of a more compact oxide.

In some cases, the shape of the Nyquist diagrams for CET structure in alloys resembles that of those with equiaxed structure and in others those with columnar structure, depending on the relative amount of each phase in the CET structure, which in turn depends on the region where the specimen was obtained.

Fig. 6. (a) Nyquist Diagram for different samples.

The whole set of experimental impedance spectra can be discussed according to the following total transfer function.

$$\mathbf{Z}\_{\text{t}}(\text{jw}) = \mathbf{R}\_{\text{\alpha}} + \mathbf{Z} \tag{1}$$

with:

48 Corrosion Resistance

**ZA27+8%CSi**

**ZA27+8%Al2O3**

**ZA27+15%Al2O3**

**ZA27+15%CSi**

Fig. 5. Ep-r as a function of concentration and alloy structure.

**3.2 Electrochemical impedance spectroscopy data** 

due to the formation of a more compact oxide.

where the specimen was obtained.

**ZA27(Equiaxed)**

**ZA27(CET) ZA27(Columnar)**

**ZA50+8%CSi**

**ZA50+15%CSi**

0 10 20 30 40 50 60 70 80 90 100 **ALLOY / COMPOSITE**

The susceptibility to corrosion was measured as the difference between the potential of pitting, Ep, and the repassivation potential, Er, as Ep-r and the difference between the repassivation potential and the corrosion potential of each sample through Table 2 it is possible to observe that the values of repassivation potential for materials without particles in the matrix are near the corrosion potential, but not in the case of the the other samples.

Impedance spectra are strongly dependent on the composition and structures of the alloys and composites. Figure 6 shows the experimental Nyquist diagrams for all the alloys and composites used. All the diagrams show one capacitive time constant at high frequencies and a non-well defined time constant at low frequencies, probably associated with diffusion processes also reported in the literature (Deslouis et al., 1984; Trabanelli at al., 1975). It can be seen that as the concentration of aluminum in the alloy increases, the second time constant approximates the response associated with a diffusion process in finite thickness,

In some cases, the shape of the Nyquist diagrams for CET structure in alloys resembles that of those with equiaxed structure and in others those with columnar structure, depending on the relative amount of each phase in the CET structure, which in turn depends on the region

0

20

40

60

**Ep-r**

80

100

120

140

$$\frac{1}{Z} = \frac{1}{R\_{cl} + Z\_W} + j\_W \, ^\circ C \tag{2}$$

where R is the ohmic solution resistance, = 2f; Cdl the capacitance of the electric double layer, Rct the charge transfer resistance and ZW the diffusion contributions in impedance spectra.

ZW = RDO (jS) -05 for semi-infinite diffusion contribution and ZW = RDO (jS) -05 coth (jS) -05 is related to diffusion through a film of thickness d, formed on the electrode, where RDO is the diffusion resistance and the parameter S= d2/D, where d and D are the diffusion thickness and diffusion coefficient related to the transport process (Fedrizzi et al., 1992).

The good agreement between experimental and simulated data according to the transfer function given in the analysis of Eqs. 1 and 2 using non-linear least square fit routines is shown in Figure 8 (a) at high frequencies. At low frequencies (less than 10-1 Hz) is not achieved a good fit with this model, since the impedance measurement does not give us enough information to define a new input capacitance, this occurs for samples ZA27-8% SiC, ZA50-8% SiC and ZA27-Al2O3. This process can represent by an equivalent circuit in Figure 8 (a).

Corrosion Resistance of Directionally Solidified Casting Zinc-Aluminum Matrix 51

R<sup>Ω</sup>

Rct

C1

Cdl

R1

The corrosion current can be related to the Rct in the case of mixed control (Epelboin et al., 1972), where the polarization resistance technique fails, according to the following

 Rct = ba bc/2.303(ba+bc)Icorr (3) However, it is important to note that the Rct values are not directly related to the susceptibility to corrosion of the different alloys and composites. They are related to the rate of charge transfer reactions that give rise to the formation of a passive layer on the surface of the samples (the impedance measurements are at open circuit potential only). The protective characteristics of these passive films depend on the preparation conditions of the alloys, the distribution of elements in the alloy and the presence on the surface of active sites for

Zn Zn+2 + 2 e-

H2O + ½ O2 + 2 e- 2 OH-

The ZA27 alloy with different structures is less resistant to corrosion and its susceptibility to corrosion is dependent of the structure. The ZA27 – 15%CSi composite has the highest corrosion resistance. Also, discriminating by type of composite materials, the MMCs with

320 m

SiC are more corrosion resistant than those MMCs prepared with alumina particles.

(a) (b)

Fig. 8. Equivalent circuit of EIS for different samples.

Rct W

Cdl

Ions are formed during the anodic dissolution of alloy

Fig. 9. Micrograph of ZA27 – 8%SiC sample after EIS test.

expression:

R<sup>Ω</sup>

adsorption of chloride ion.

which can react with hydroxyl ions

and can be generated Zn(OH)2

For ZA27samples and those containing 15%SiC at high frequencies seems to be defined one second capacitive loop corresponding to corrosion processes controlled by precipitation and dissolution of ions Zn, see Figure 7 (b, c and d). The equivalent circuit corresponds to that showed in Figure 8 (b).

The values of Cdl, C1 and Rct determined from the optimum fit procedure are presented in Table 3.

The analysis of the impedance parameters associated with the time constant at low frequencies is difficult because in some cases the loop it is not complete. However, it was possible to calculate from by fitting an approximate value of diffusion coefficient D 10-10 – 10-12 cm2/s.

High values of capacity confirm the formation of porous corrosion products, as can be seen in Figure 9. These high values of capacity may also be correlated with an increase in the area.

Fig. 7. Bode y Nyquist plot for – (a) ZA50-8%SiC (b) ZA27 L(CET) (c) ZA27 L(Columnar) (d) ZA27-15%SiC

Fig. 8. Equivalent circuit of EIS for different samples.

For ZA27samples and those containing 15%SiC at high frequencies seems to be defined one second capacitive loop corresponding to corrosion processes controlled by precipitation and dissolution of ions Zn, see Figure 7 (b, c and d). The equivalent circuit corresponds to that

The values of Cdl, C1 and Rct determined from the optimum fit procedure are presented in

The analysis of the impedance parameters associated with the time constant at low frequencies is difficult because in some cases the loop it is not complete. However, it was possible to calculate from by fitting an approximate value of diffusion coefficient D 10-10 –

High values of capacity confirm the formation of porous corrosion products, as can be seen in Figure 9. These high values of capacity may also be correlated with an increase in the

10

1

(c) (d)

10

100

**IMPEDANCE |Z| (.cm2**

**-PHASE ANGLE/DEGREE**

Fig. 7. Bode y Nyquist plot for – (a) ZA50-8%SiC (b) ZA27 L(CET) (c) ZA27 L(Columnar) (d)

**)**

1000

100

experimental fitting experimental

(a) (b)

0 50 100 150 200 **REAL PART (.cm2 )**

0 100 200 **REAL PART (.cm2 )**

**IMPEDANCE |Z| (.cm2**

**-PHASE ANGLE/DEGREE**

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 -3 -2 -1 0 1 2 3 4 5 6 **FREQUENCY (Hz)**

> -300 -250 -200 -150 -100 -50 0

**IMAGINARY PART (.cm2**

**)**



**IMAGINARY PART (.cm2**

**)**

experimental fitting experimental

1

1000

**IMPEDANCE |Z| (.cm2**

**)**

10

experimental fitting experimental

0 200 400 600 **REAL PART (.cm<sup>2</sup> )**

10

100

0 100 200 300 400 500 600 700 **REAL PART ( .cm<sup>2</sup> )**

**IMPEDANCE |Z| (.cm2**

**)**

1000

10000

**)**

1000

showed in Figure 8 (b).

Table 3.

10-12 cm2/s.

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00

ZA27-15%SiC

**-PHASE ANGLE/DEGREE**


> -100 -80 -60 -40 -20 0

**IMAGINARY PART (.cm2**

**)**


experimental fitting experimental -200 -150 -100 -50 0

**IMAGINARY PART (.cm2**

**)**

area.

**-PHASE ANGLE/DEGREE**

The corrosion current can be related to the Rct in the case of mixed control (Epelboin et al., 1972), where the polarization resistance technique fails, according to the following expression:

$$\mathbf{R\_{ct}} = \mathbf{b\_a} \,\mathrm{b\_c} / 2.503 \,\mathrm{(b\_a + b\_c)}\mathrm{I\_{corr}} \tag{3}$$

However, it is important to note that the Rct values are not directly related to the susceptibility to corrosion of the different alloys and composites. They are related to the rate of charge transfer reactions that give rise to the formation of a passive layer on the surface of the samples (the impedance measurements are at open circuit potential only). The protective characteristics of these passive films depend on the preparation conditions of the alloys, the distribution of elements in the alloy and the presence on the surface of active sites for adsorption of chloride ion.

Ions are formed during the anodic dissolution of alloy

$$\mathbf{Zn} \rightarrow \mathbf{Zn^{\*2} + 2e^{-}}$$

which can react with hydroxyl ions

$$\text{H}\_2\text{O} + \text{V}\_2\text{O}\_2 + 2\text{ e} \rightarrow 2\text{ OH}\cdot\text{h}$$

and can be generated Zn(OH)2

The ZA27 alloy with different structures is less resistant to corrosion and its susceptibility to corrosion is dependent of the structure. The ZA27 – 15%CSi composite has the highest corrosion resistance. Also, discriminating by type of composite materials, the MMCs with SiC are more corrosion resistant than those MMCs prepared with alumina particles.

Fig. 9. Micrograph of ZA27 – 8%SiC sample after EIS test.

Corrosion Resistance of Directionally Solidified Casting Zinc-Aluminum Matrix 53

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Table 3. Principal parameters obtained from the EIS analysis.
