**1. Introduction**

34 Corrosion Resistance

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Research Confidential Technical Reports, August 26, 2009, September 8, 2009, November 11, 2009, February 5, 2010, September 23, 2010, November 12, 2010, Generally solidification leads to two types of grain morphologies: columnar and equiaxed. The origin of each one has been the subject of numerous theoretical and experimental researches in the field of metallurgy for many years. Columnar grains often grow from near the mold surface, where the thermal gradients are high, and the growth is preferentially oriented in a direction close to the heat flux. When the gradients are reduced near the center of the casting, equiaxed grains grow in all space directions leading to a material with more isotropic macroscopic mechanical properties and a more homogeneous composition field than with columnar structure. Depending on the application, one type of grain is preferred and thus favoured, e.g. equiaxed grains in car engines and columnar grains in turbine blades as reported by Reinhart et al, 2005 and McFadden et al., 2009.

Since the grain structure inuences the properties of a casting, a great deal of effort has been devoted in the last decades to understand the mechanism behind the development of the macrostructure during solidication. Thus, equiaxed grains can nucleate and grow ahead of the columnar front causing an abrupt columnar to equiaxed transition (CET) whose prediction is of great interest for the evaluation and design of the mechanical properties of solidified products. As a consequence, it is critical for industrial applications to understand the physical mechanisms which control this transition during solidification (Spittle, 2006).

In order to realize the control of the columnar and equiaxed growth, it is necessary to understand the columnar to equiaxed transition (CET) mechanism during solidification, and make clear the CET transition condition. Fundamentally, it is necessary to have knowledge of the competition between nucleation and growth during solidification. Qualitatively, the CET occurs more easily when an alloy has a high solute concentration, low pouring temperature (for casting), low temperature gradient, high nucleation density in the melt and vigorous melt convection.

Corrosion Resistance of Directionally Solidified Casting Zinc-Aluminum Matrix 37

The biggest susceptibility to corrosion of the alloys with columnar structure can be observed by analyzing the values of Rct (charge transfer resistance) obtained using the electrochemical impedance spectroscopy (EIS) technique. In Zn-4wt%Al and Zn-27wt%Al, the corrosion susceptibility depends on the structure of the alloy. The alloy with 16wt%Al is less resistant to corrosion and their susceptibility to corrosion is independent of the structure. The alloy with 27wt%Al and the CET structure is the alloy which has the most corrosion resistance. When the critical temperature gradient becomes more negative, the Rct values increase. In the case of the correlation of Rct values and the structural parameters such as the grain sizes and secondary dendritic spacing, Rct values increase when the grain size and secondary dendritic spacing increase. But this does not happen for ZA16 alloy.

Composite materials obtained by solidification of alloys have made remarkable progress in their development and applications in automotive and aerospace industries in recent decades. Among them the most current applications are the zinc and aluminum base composite materials (Long et al., 1991; Rohatgi, 1991). It is well-known that the corrosion behavior of MMCs is based on many factors such as the composition of the alloy used, the type of reinforcement particles used, the reinforcement particle sizes and their distribution in the matrix, the technique used for the manufacture, and the nature of the interface between the matrix and reinforcement. A very slight change in any of these factors can

In short, there is little research related to the study of mechanical and electrochemical properties of Zn-Al alloys as well as Zn-Al alloys MMCs containing SiC and Al2O3 particulations with different grain structures in the matrix. Also there is lack of fundamental study on the performance of Zn-Al alloys and their MMCs in corrosive environments when both solidification microstructure and type of particle distribution are in consideration. In the present research, Zn-Al-SiC and Zn-Al-Al2O3 composites are prepared and solidified by vertical directional solidification method. By means of voltammograms and electrochemical impedance spectroscopy, the corrosion resistances of Zn-Al matrix composite materials with

different types of particles are obtained and analyzed and the results are compared.

Zinc-Aluminum (ZA) alloys of different compositions were prepared from zinc (99.98 wt pct), aluminum (99.94 wt pct), and composites were prepared by adding SiC and Al2O3 particles to the alloys. The compositions of the alloys and composites prepared and directionally solidified are: Zn-27wt%Al, Zn-50wt%Al, Zn-27wt%Al + 8vol%SiC, Zn-27wt%Al + 15vol% SiC, Zn-50wt%Al + 8vol%SiC, Zn-50wt%Al + 15vol%SiC, Zn-27wt%Al +

The chemical compositions of the commercially pure metals used to prepare the alloys are presented in Table 1. The molds were made from a 23 mm i.d. and 25 mm e.d. PYREX (Corning Glass Works, Corning, NY) tube, with a flat bottom, a cylindrical uniform section and a height of 200 mm. The sample was a cylinder 22 mm in diameter and 100 mm in

seriously affect the corrosion behavior of the material.

**2.1 Alloys and metal matrix composites preparation** 

**2. Materials and methods** 

height.

8vol%Al2O3, Zn-27wt%Al + 15vol%Al2O3.

However, a quantitative understanding of the CET requires a thorough comprehension of all physical mechanisms involved.

In 1984, Hunt first developed an analytical model to describe steady-state columnar and equiaxed growth, and to qualitatively reveal the effects of alloy composition, nucleation density and cooling rate on the CET. On the other hand, he used a very simple empirical relationship to describe the variation of the undercooling with alloy composite and solidification rate. Cockcroft et al. (1994) used a more recent growth theory for the columnar and equiaxed growth but without considering high velocity non-equilibrium effects under rapid solidification. Recently, based on Hunt's CET model, Gäumann et al. (1997, 2001) developed a more comprehensive model by combining KGT model (Kurtz et. al., 1986) for directional solidification with LKT model (Lipton et. al., 1987) for the undercooling melt growth, with high velocity non-equilibrium effects to be taken into account. Gäumann et al. (2001) succeeded in applying their model to epitaxial laser metal forming of single crystal.

In previous research, the authors of this work carried out experiments in which the conditions of columnar to equiaxed transition (CET) in directional solidication of dendritic alloys were determined The alloy systems in this work include Pb–Sn (Ares & Schvezov, 2000), Al–Cu (Ares et. al., 2011), Al–Mg (Ares et. al., 2003), Al–Zn and Zn-Al alloys (Ares & Schvezov, 2007). These experiments permit to determine that the transition occurs gradually in a zone when the gradient in the liquid ahead of the columnar dendrites reaches critical and minimum values, being negative in most of the cases. The temperature gradients in the melt ahead of the columnar dendrites at the transition are in the range of -0.80 to 1.0 ºC/cm for Pb–Sn, -11.41 to 2.80 ºC/cm for Al–Cu, -4.20 to 0.67 ºC/cm for Al–Si, -1.67 to 0.91 ºC/cm for Al–Mg, -11.38 to 0.91 ºC/cm for Al–Zn. Two interphases are dened; assumed to be macroscopically at, which are the liquidus and solidus interphases. After the transition, the speed of the liquidus front accelerates much faster than the speed of the solidus front; with values of 0.004 to 0.01, 0.02 to 0.48, 0.12 to 0.89, 0.10 to 0.18 and 0.09 to 0.18 cm/s, respectively. Also, the average supercooling of 0.63 to 2.75 1C for Pb–Sn, 0.59 to 1.15 1C for Al–Cu, 0.67 to 1.25 1C for Al–Si, 0.69 to 1.15 1C for Al–Mg, 0.85 to 1.40 1C for the Al–Zn and was measured, which provides the driving force to surmount the energy barrier required to create a viable solid–liquid interface (Ares et al., 2005). A semi-empirical model to predict the columnar to equiaxed transition is developed based on experimental results obtained from measurements during solidication of lead–tin alloys directly upwards (Ares et al., 2002). The measurements include the solidication velocities of the liquidus and solidus fronts, and the temperature gradients along the sample in the three regions of liquid, mushy and solid. The experimental data was coupled with a numerical model for heat transfer. With the model, the predicted positions of the transition are in agreement with the experimental observations which show that the transition occurs when the temperature gradient reaches values below 1ºC/cm and the velocity of the liquidus front increases to values around 0.01cm/s.

In addition, the thermal parameters, type of structure, grain size and dendritic spacing with the corrosion resistance of Zn-4wt%Al, Zn-16wt%Al and Zn-27wt%Al alloys were correlated (Ares et al., 2008). The polarization curves showed that the columnar structure is the most susceptible structure to corrosion, in the case of the alloy with only 4wt%of Al. The rest of the structures presented currents of peaks in the same order which were independent to the concentration of Al composition presenting in the alloy.

The biggest susceptibility to corrosion of the alloys with columnar structure can be observed by analyzing the values of Rct (charge transfer resistance) obtained using the electrochemical impedance spectroscopy (EIS) technique. In Zn-4wt%Al and Zn-27wt%Al, the corrosion susceptibility depends on the structure of the alloy. The alloy with 16wt%Al is less resistant to corrosion and their susceptibility to corrosion is independent of the structure. The alloy with 27wt%Al and the CET structure is the alloy which has the most corrosion resistance. When the critical temperature gradient becomes more negative, the Rct values increase. In the case of the correlation of Rct values and the structural parameters such as the grain sizes and secondary dendritic spacing, Rct values increase when the grain size and secondary dendritic spacing increase. But this does not happen for ZA16 alloy.

Composite materials obtained by solidification of alloys have made remarkable progress in their development and applications in automotive and aerospace industries in recent decades. Among them the most current applications are the zinc and aluminum base composite materials (Long et al., 1991; Rohatgi, 1991). It is well-known that the corrosion behavior of MMCs is based on many factors such as the composition of the alloy used, the type of reinforcement particles used, the reinforcement particle sizes and their distribution in the matrix, the technique used for the manufacture, and the nature of the interface between the matrix and reinforcement. A very slight change in any of these factors can seriously affect the corrosion behavior of the material.

In short, there is little research related to the study of mechanical and electrochemical properties of Zn-Al alloys as well as Zn-Al alloys MMCs containing SiC and Al2O3 particulations with different grain structures in the matrix. Also there is lack of fundamental study on the performance of Zn-Al alloys and their MMCs in corrosive environments when both solidification microstructure and type of particle distribution are in consideration. In the present research, Zn-Al-SiC and Zn-Al-Al2O3 composites are prepared and solidified by vertical directional solidification method. By means of voltammograms and electrochemical impedance spectroscopy, the corrosion resistances of Zn-Al matrix composite materials with different types of particles are obtained and analyzed and the results are compared.
