3.1. Effect of zirconia content of A 356 Al alloy/ZrO2 composites on their corrosion in 3.5 % NaCl solution
3.1.1. Potentiodynamic polarization measurements
The polarization curves of A 356 Al alloy and its vortex cast composites containing different vol. % of ZrO2 in 3.5 % sodium chloride solution are shown in Figure 1. The cathodic and anodic curves of the studied materials are similar i.e., they exhibit the same anodic and cathodic processes. The corrosion parameters which are the corrosion potential (Ecorr), corrosion current density (Icorr), corrosion rate in mpy, slope of the cathodic branch (βc) and slope of the anodic branch (βa) were estimated and listed in Table 1.
The value of the corrosion potential tends to increase as the zirconia content of the composites increases up to a maximum value at 20 vol. % and then decreases at 30 vol. % as shown in Table 1. This shift in the value of Ecorr to the noble value conforms a decrease in the corrosion rate of the composites as the vol. % of ZrO2 increases. It has been found in the literature  that the corrosion potential of the Al-6061 composites in 3.5 % NaCl solution increases with increasing the weight percentage of zirconia in the composites which attributed to the decrease in the corrosion rate.
Also, it can be seen from Table 1 that the value of Icorr decreases and consequently the corrosion rate decreases as zirconia content in the composites increases in a manner like that of the corrosion potential behavior. This may be due to the formation of a protective layer of corrosion products, consist mainly of less soluble aluminium compounds in sodium chloride solution . Figure 2a and b shows the SEM micrographs for the vortex cast composite containing 5 % ZrO2 after the mechanically polishing of its surface and after immersion it in 3.5 % NaCl for 10 days, respectively. It is clear that the defects and notches which appear in Figure 2a were decreased due to growth of a protective passive film on the composite surface in the chloride solution as shown in Figure 2b.
Also, the results of the EDS analysis shown in Figure 3a and b for the surface of mechanically polished vortex cast composite (3a) and for the same surface after immersion it in sodium chloride solution for 10 days (3b) reveal that the passive film formed on these surfaces consists mainly of aluminium oxide layer.
3.1.2. Electrochemical impedance measurements
The EIS spectra have been recorded for the unreinforced A 356 Al alloy and its vortex cast composites containing different percentages namely 5, 20, and 30 vol. % of ZrO2 in the chloride solution at open-circuit potential. The results are shown in Figure 4 as Nyquist plots which are a part of the imperfect capacitive semicircles due to the frequency dispersion . The capacitive semicircle increases in diameter with increasing the percentage of ZrO2 added to A 356 Al alloy up to 20% then decreases again slightly at 30%. This means that the corrosion resistance of vortex cast composites surfaces in the chloride solution increases as ZrO2 concentration increases to 20 % then decrease.
The impedance data were analyzed using the equivalent electrical circuit (EEC) in Figure 5. This circuit is similar to that used before in our work on titanium in NaF solutions . The components of this EEC are as follows: Rs solution resistance; Cdl double layer capacitance; Rct charge transfer resistance that related to the corrosion process; Cf capacitance due to the dielectric nature of the surface film and Rf resistance due to the surface film.
Since there is a variance between real capacitance and pure capacitance therefore, computer simulation of the EIS spectra can be carried out by replacing the capacitance C, with a constant phase element (CPE). The impedance of CPE is described by the following expression [13,14]:
ZCPE = [(jω)α Y]-1
where Y is the frequency independent real constant of the CPE, ω the angular frequency, j = Ö-1, and α is an adjustable empirical exponent which varies between 1.0 for a perfect smooth surface with pure capacitive behavior and 0.5 for a porous surface.
The fitted EIS parameters for the tested materials in the sodium chloride solution were listed in Table 2. It is obvious from this Table that the Rf increases and simultaneously Cf decreases as the percentage of zirconia in the composites increases up to 20% then Rf decrease and Cf increases at 30%. These variations can be attributed to the increase of the thickness of primary passive film on these materials as the zirconia content increases to 20% and it decreases when this content equals 30%.
3.2. Effect of casting pressure of A 356 Al alloy/ 5 vol. % ZrO2 composites on their corrosion in 3.5 % NaCl solution
3.2.1. Potentiodynamic polarization measurements
The polarization curves for the vortex and squeeze cast composites of A 356 Al alloy reinforced with 5 vol. % of ZrO2 in 3.5 % sodium chloride solution are given in Figure 6. The squeeze composites cast under different pressures namely 20, 50 and 88 MPa. Figure 6 shows similar polarization curves and passivity characteristics for the vortex and squeeze cast composites. This means the same corrosion processes occur for the two types of composites tested here in the chloride solution.
The polarization parameters were listed in Table 3 in which Ecorr increases and corrosion rate decreases with increasing of the pressure used for the squeeze cast composites due to decreasing of their porosity. Also, the values of the corrosion rate for the squeeze composites are lower than that for vortex cast composite of the same percentage of zirconia. Previously, it was found that the values of corrosion current density for A 356-10 vol. % SiC composite cast by gravity are greater than those for composite cast by squeeze in 0.05 and 0.1 M H2SO4 solutions . Therefore in sodium chloride solution the squeeze cast composite is less exposure for the corrosion process than the vortex one because of the lower porosity content of squeeze one.
Figure 7a and b shows the SEM micrographs for the mechanically polished surfaces of the vortex cast composite (7a) and squeeze cast composite at 50 MPa (7b) before immersion in the chloride solution. It is obvious from this Figure that the pores of different sizes spread in large amount on the surface of vortex cast composite than on the surface squeeze one. Many researchers had been found that the percentage of porosity for the squeeze composite decreases with the increase in the pressure during the preparation process [7, 15-17].
3.2.2. Electrochemical impedance measurements
The EIS results for the vertex and squeeze cast composites containing 5 vol. % of ZrO2 in 3.5 % NaCl solution are given in Figure 8 as Nyquist plots. The diameter of the capacitive semicircle increases as the squeeze pressure increases due to increase of corrosion resistance. Fitting of EIS results were carried out using the EEC in Figure 5 and the results are given in Table 4. Both the charge transfer resistance, Rct, and passive film resistance, Rf, increase as the pressure of squeeze cast composites increases and they are being greater than those of vortex composite. This means that the increases of casting pressure of the composite from 20 to 88 MPa leads to a decrease of the porosity within the composite and consequently results in formation of more passive film on its surface.
Figure 9a and b shows the SEM photographs for the surfaces of the vortex and squeeze cast composites after immersion in sodium chloride solution for 10 days. From this Figure it can be seen that the repairing of the surface due to growth the pre-immersion passive film in case of squeeze composite is good than in the case of vortex one which in consistent with the polarization and EIS results.