Effetto del trattamento ad ultrasuoni di leghe allo stato liquido sulla resistenza a corrosione

The introduction of ultrasonic waves in the liquid bath, that induce cavitation effects, allows to refine and degas the alloy in a single step without the addition of nucleants agents, with a subsequent reduction of the production costs and the obtainment of an easier process. Moreover, this technology can be applied for the obtainment of semi-solid alloys, characterized by globules of primary phase surrounded by the eutectic, as confirmed by recent studies. The aim of this work is to evaluate the effect of ultrasound treatment (UST) on the corrosion resistance of Aluminium and Zinc foundry alloys. The different alloys have been treated at the liquid state and then cast in a small laboratory die. The corrosion resistance of the treated samples was evaluate by means of potentiodynamic tests in NaCl 0,5M solution and acetic salt spray tests. The results were compared with those obtained from not ultrasound treated samples. Further samples analyses were carried out by means of optical microscope (OM) and scanning electron microscope (SEM), in order to better understand the corrosion phenomena occurred on the samples surfaces during salt spray test. For the Aluminium alloy AISi5 it was observed that: - UST induces a refinement and a homogenization of the microstructure, with the formation of globular grains; - porosity strongly decreases; - intermetallics particles have same chemical composition but smaller size in the treated samples respect to the not treated ones; - higher micro-hardness values are observed in the treated samples. Concerning the corrosion tests: - US treated alloys show a better corrosion behaviour and a higher corrosion potential, probably related to a more uniform distribution of the solute; - in both samples, corrosion begins and propagates through the eutectic, favoured from silicon and intermetallics particles; - the higher porosity of not treated samples makes worse the corrosion resistance. For the Zinc alloy ZA27 it was observed that: - UST does not produce a substantial microstructure variation, even if dendrites appear slightly rounded; - microhardness tests show a light improvement of mechanical properties of the US treated alloy, characterized by a more homogeneous structure; - as for AISi5, the corrosion rate of the treated samples appears lower than that of not treated ones, even if not treated samples have a more noble potential; - for both samples, the corrosion propagation is through the transformed eutectic, rich in copper, extending inside the grains, where the Al composition is higher; - the depth of corrosion attack is higher in the case of not treated samples.