Characterization of the β-phase (Al3Mg2) in 5XXX Aluminum Alloys

The 5XXX series aluminum alloys used in lightweight Navy vessels are prone to sensitization at standard service temperatures due to the preferential precipitation of the β-phase (nominally Al₃Mg₂) at grain boundaries.  Sensitization results in susceptibility to intergranular corrosion and intergranular stress corrosion cracking.  Characterization of the electrochemical properties of the β-phase can lead to further understanding and prediction of the nature of the precipitates in the bulk aluminum alloy and use in future sensitization models.  However, the β-phase present at grain boundaries is on the size scale of 100nm.  In order to characterize its behavior without interference from the surrounding matrix, bulk β-phase has been synthesized by induction melting.  This work reports the results of the characterization of pure β-phase samples via qualitative and quantitative analytical methods.  The purity of the sample was determined using x-ray diffraction, proton induced x-ray emission spectroscopy, and optical microscopy.  Preliminary electrochemical tests were conducted to evaluate the corrosion behavior in concentrated nitric acid, phosphoric acid, ammonium persulfate, and sodium phosphate solutions.  Scanning electron microscopy and energy dispersive X-ray spectroscopy were used to assess the effects of trace impurities from precursor materials post electrochemical testing.