Effect of brazing on the microstructure and corrosion behaviour of a twin roll cast Al-Mn-Fe-Si alloy system

The Al-Mn-Fe-Si alloy systems, produced via Twin Roll Casting (TRC), are mostly used in applications where a good corrosion resistance is required like brazing applications, for example heat exchanger systems in vehicles. While the TRC manufacturing method allows for a high production rate, it can cause some complications for the microstructure, such as centre line segregation (CLS). This CLS is the chemical segregation of the alloying elements occurring at the centre plane of the thickness and readily has an influence on corrosion characteristics of the materials.
In this work the effect of a brazing step on the microstructure and corrosion behaviour of a TRC Al-Mn-Fe-Si alloy system was studied. Moreover, the effect of brazing on the corrosion properties of the centre line segregation of alloying elements (CLS) and the interaction of the intermetallic particles with the surrounding matrix was studied. In this study optical microscopy, SEM/EDS (scanning electron microscopy/ energy dispersive spectroscopy), HAADF-STEM/EDS (high-angle annular dark field-scanning transmission electron microscopy/ energy dispersive spectroscopy), AFM/SKPFM (atomic force microscopy/ scanning Kelvin probe force microscopy), potentiodynamic polarization and potentiostatic polarization were used to characterize the microstructure and corrosion behaviour. It was shown that the cross sections are significantly more active than the surface of the fin material, showing the importance of the CLS on the corrosion behaviour. Furthermore, it was shown that the pitting corrosion activity and penetration depth of the formed pits on the exposed cross sections decreased considerably after brazing, especially near the CLS. SKPFM measurements showed that after brazing the contact potential difference between the intermetallic particles and the matrix reduced significantly.