Investigation of Effect of Mold Mechanical Vibration During ‎Solidification on the Structure ‎and Corrosion Resistance of ‎ Zn-4Si Alloy

In the current study the effect of mold mechanical vibration during the solidification was studied on microstructure and corrosion behavior of Zn-4Si composite. According to the microstructural observation results, mechanical vibration substantially improved the SiP particle distribution and refined them. The image analysis results showed that mechanical vibration at 20, 40, and 60 Hz reduced the average size of SiP particles by 34, 55, and 75%, and increased their number density by 6, 16, and 36 times, respectively. Mechanical vibration at the 20, 40, and 60 Hz also decreased the average grain size by 50, 68, and 76%, respectively and increased the equiaxed zone in castings. The results of Tafel and impedance corrosion tests at 3.5 wt. % NaCl solution implied on increasing the corrosion current and shifting the corrosion potential to the more negative values in mechanically vibrated samples. The corrosion current of as-cast and 60 Hz samples were determined as -1.3610-5 and -2.3310-5 A, respectively. Mechanical vibration also reduces the corrosion resistance of samples where the resistance of 60 Hz sample (about 76 ohm) is lower than that of as-cast sample by about 45%. The increased density of grain boundaries and fine distribution of primary Si particles (as cathodic points) in the composite matrix are characterized as the most important factors decreasing the corrosion resistance of the composite. This is because they increased the number and interspacing of the galvanic cells within the matrix and exhibited appropriate locations for pitting.