The aluminium-silicon alloys are widely used
in the automotive industry, due to their excellent castability, good corrosion
resistance and mechanical properties, which can be further improved by eutectic
Si modification with Sr, Na or Sb as modifying elements and the precipitation
of nanometric Mg2Si precipitates during T6 heat treatment.
Recently, the use of secondary alloys has
attracted considerable interest, as these alloys offer several advantages over
conventional primary alloys, such as cheaper raw material, less energy consumption during
their manufacturing and protection of resources.
However, these types of alloys usually contain
high levels of
alloying elements and/or impurities such as Fe, Cu and Zn which may affect
adversely the microstructure, mechanical properties and corrosion resistance.
The effect of these compounds on the corrosion
behaviour was studied.
Different alloys have been fabricated containing variables
amounts of the target chemical elements (see table 1).
Table 1. Chemical
composition of the alloys (wt.%)
Salt spray test have been conducted as per ASTM B177
with 336 hours of exposure. Results from macroscopic observations agree quite
well with the weight loss measurements.
It is observed that corrosion occurs
preferentially in the eutectic regions at both the Si particles and iron
intermetallics interfaces (Fig. 2), while no corrosion attack was observed in
the a-aluminium. In the initial stage localized corrosion
occurs at the iron intermetallic interface and subsequently corrosion
progresses in the eutectic Al matrix at the Si particle interface.
Figure 3: a) Polarization curves of the ref.
alloy A, alloy D and alloy E in 0,03 M NaCl. b) Polarization curves of the ref. alloy A, alloy F, alloy I, alloy J and L in 0,03M NaCl. |
The corrosion current density is very similar in all the cases. These
materials present a high susceptibility to localized attack and the pitting
potential corresponds to the corrosion potential.
The addition of Fe and/or MnCrV in the concentration
used in the present work appears to have no detrimental effect on the corrosion
behavior with respect to the reference alloy.
The results suggest that the addition of Cu in the
concentrations evaluated do not modify the corrosion behaviour of the reference
alloy.
while the highest amount of Zn (3.13 wt.%) added shows a slight increase
in the corrosion kinetics. This alloy revealed also in the salt spray test a
significant higher weight loss than the other Cu and Zn modified alloys.
For
automotive applications that require high strength, ductility and good
corrosion resistance, the improvement of strength that could be obtained by
addition of Cu, would be possible in concentrations of about Cu 0.5 wt.% where
corrosion resistance remains still acceptable.
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