**2.3 Corrosion tests**

For the electrochemical tests, samples of 20 mm in length of each zone and for each concentration were prepared as test electrodes (see Figure 1), polished with sandpaper (from SiC #80 until #1200) and washed with distilled water and dried by natural ow of air.

All the electrochemical tests were conducted in 3wt% NaCl solution at room temperature using an IM6d Zahner®-Elektrik potentiostat coupled to a frequency analyzer system.

Chemical composition of Zn Element Weight percent, wt% Zn 99.98 0.2 Fe 0.010 0.01 Si 0.006 0.0001 Pb 0.004 0.001 Others 0.001 0.0001 Chemical composition of Al Element Weight percent, wt% Al 99.94 0.2 Fe 0.028 0.0001 Si 0.033 0.001 Pb 0.001 0.0001 Others 0.001 0.0001

Table 1. Chemical composition of the Zn and Al used to prepare the alloys.

columnar-to-equiaxed transition (CET), see Figure 1 (a).

The alloy samples were melted and solidied directionally upwards in an experimental setup described elsewhere (Ares et. al., 2007). It was designed in such a way that the heat was extracted only through the bottom promoting upward directional solidication to obtain the

In order to reveal the macrostructure, after solidication the samples were cut in the axial direction, polished, and etched using concentrated hydrochloric acid for 3 seconds at room temperature for the zinc-aluminum alloys, followed by rinsing and wiping off the resulting black deposit. The microstructures were etched with a mixture containing chromic acid (50 g Cr2O3; 4 g Na2SO4 in 100 ml of water) for 10 seconds at room temperature (Vander Voort, 2007). Typical longitudinal macrostructure of different areas of the sample are shown in

The position of the transition was located by visual observation and optical microscopy. The distance from the chill zone of the sample was measured with a ruler. It is noted in Figure 1 that the CET is not sharp, showing an area where some equiaxed grains co-exist with columnar grains. As was reported before, the size of the transition area is in the order of up to 10 mm (Ares et al., 2007, 2010). The grain structure was inspected by visual observation

For the electrochemical tests, samples of 20 mm in length of each zone and for each concentration were prepared as test electrodes (see Figure 1), polished with sandpaper (from SiC #80 until #1200) and washed with distilled water and dried by natural ow of air. All the electrochemical tests were conducted in 3wt% NaCl solution at room temperature using an IM6d Zahner®-Elektrik potentiostat coupled to a frequency analyzer system.

**2.2 Directional solidification** 

Figure 1 (b) to (d).

**2.3 Corrosion tests** 

under Arcano® optical microscopy.

A conventional three-compartment glass electrochemical cell with its compartments separated by ceramic diaphragms was used. The test electrodes consisted of sections of the ZA ingots (see Figure 1) were positioned at the glass corrosion cell kit (leaving a rectangular area in contact with the electrolyte). The potential of the test electrode was measured against a saturated calomel reference electrode (0.242 V vs NHE), provided with a Luggin capillary tip. The Pt sheet was used as a counter electrode.

Voltammograms were run between preset cathodic (open circuit potential -1.500 V) and anodic (Es,a = -0.700 V) switching potentials at potential sweep rates (v), at 0.002 V.s-1. Impedance spectra were obtained in the frequency range of 10-3 Hz and 105 Hz at open circuit potential.

For comparison purposes, experiments using pure metals and aluminum-based alloys with different structures were conducted under the same experimental conditions. All the corrosion tests experiments were triplicate and the average values and graphical outputs are reported.

Fig. 1. Experimental device for electrochemical tests. (f) A glass corrosion cell kit with a platinum counter electrode and a sutured calomel reference electrode (SCE).

Corrosion Resistance of Directionally Solidified Casting Zinc-Aluminum Matrix 41

Zn (L- Equiaxed)



(b)

**C1 C2**

(a)

Zn (Columnar)



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
