Corrosion behaviour of a high pressure die cast Al-Si-Cu secondary alloy.

In recent years, in order to reduce harmful emissions the transport sector has largely focused on a wider use of light alloys, possibly obtained from scrap recycling. The alloys obtained from recycling of aluminium scraps are termed secondary. The production of secondary aluminium alloys shows significant advantages such as an important saving of natural resources with a consequent material cost reduction and a considerable energy-saving, associated to reduction in pollution and CO2 emissions. In this context, the aluminium alloys used to replace the traditional cast iron represent a very important class of materials, thanks to their low density, excellent combination of mechanical properties, castability, workability and recyclability and interesting properties of corrosion resistance. In particular, Al-Si-Cu alloys are now the most widely employed secondary alloys in the automotive industry. The presence of Si provides a high fluidity to these alloys and therefore an easy filling of the mould, while the presence of Cu increases their mechanical strength, but reduces their ductility and corrosion resistance. Up to now, the corrosion behaviour of Al-Si-Cu alloy components has not yet been widely studied. Therefore, this research aims at evaluating the susceptibility to stress corrosion cracking (SCC) in a neutral chloride solution (3.5% NaCl) of tensile specimens obtained by high pressure die-casting (HPDC), starting from a secondary alloy of composition AlSi9Cu3(Fe). Moreover, the local corrosion behaviour of the tensile specimens was investigated in correspondence of the ingate, gauge and overflow regions, in order to determine the influence of segregation phenomena in the longitudinal direction. To this aim, small cylinders, coming from the three different regions of the tensile specimen (at different distances from the die-casting injection area), were prepared in order to perform electrochemical tests. The results showed that the studied Al-Si-Cu alloy was immune to SCC in 3.5 % NaCl solution, at a strain rate of both 10-6 and 10-7 s-1. However, localized corrosion phenomena were observed, strongly dependent on the position in the mould. In particular, the intensity of the corrosion phenomena varied in the longitudinal direction and was higher in the overflow and gauge portions of the specimen. Moreover, in each of the three studied regions, the corrosion resulted particularly localized in surface bands corresponding to the joining lines of the two die halves. The causes of this peculiar behaviour were correlated to segregation phenomena of elements such as Cu, Si and Fe.