In-situ Synchrotron X-ray Analysis of Corrosion Reactions on Surfaces of High Pressure Die Cast Aluminium-Silicon Alloy EN-AC 46000

Sammanfattning

Corrosion is a type of naturally occurring material degradation which can become detrimental if uncontrolled. Understanding how corrosion occurs can help in calculating a product’s lifespan. This master’s thesis takes a closer look at the corrosion mechanism of the recycled and high pressure die cast aluminium alloy EN-AC 46000, with specific consideration for differences in machined surfaces and as-cast surfaces with a ”cast skin”. The work was done with the aim to understand the differences between the surfaces in terms of corrosion behaviour, corrosion mechanism, corrosion products formed, surface layer structure and chemical composition of said layer structure. It was also possible to utilize the HIPPIE beamline at MAXIV to perform Ambient-Pressure XPS, which coupled with data analysis could give insight in what phases were present on the surface at both vacuum conditions and in-situ corrosion conditions giving insight on corrosion mechanisms. SEM and EDS-imaging showed differences in surface microstructure, with as-cast samples being finer and more varied in phases. Notable differences being possibly improperly formed Al-Si eutectic, and a more branched Cu-rich intermetallic phase. Intermetallic phases (IMPs) were also more concentrated along casting crevices. Potentiodynamic polarization done in aerated NaCl solution revealed more negative open circuit potential (OCP) and corrosion potential (ECorr) for the as-cast as compared to the polished samples. This also rung true when measuring in de-aerated NaCl solution, with an overall less negative result in OCP for both but less so for the as-cast sample. Preliminary qualitative data analysis of AP-XPS and UHV-XPS revealed the surface oxides for both polished and as-cast surfaces consist of Al- and Si-oxides and hydroxides/hydroxylated oxides with additional Mg-oxides for as-cast sample. Neither Fe nor Cu could be detected efficiently, which were of interests for the recycled alloy. Finally a post-corrosion imaging showed two types of corrosion attacks, one with dissolution of Al-part of Al-Si eutectic and the other of dendritic Al dissolving surrounding Al-Si eutectic often in proximity to a Cu-intermetallic.