Characterization of Intergranular Corrosion in AA 5xxx Al-Mg Alloys

Intergranular corrosion refers to a selective corrosion attack at the grain boundaries of polycrystalline materials. In engineering Al-Mg alloys of the AA 5xxx series it is caused by the formation of chemicallyless noble β-Al Mg phases along the grain boundaries. The sensitivity of a material to intergranular corrosion can be assessed based on the mass losses in the so-called (NAMLT) according to ASTM G67. However, a detailed investigation of the underlying corrosion mechanisms requires that the β phases are made directly visible in the microstructure by metallographic methods. In the present work, the NAMLT mass losses of three AA 5xxx alloys with different Mg contents are compared with the results from two different etching methods. On the one hand, the alloys are etched in diluted phosphoric acid, a substance routinely used to examine the grain boundary occupancy in AA 5xxx materials. On the other hand, a newer etching method using a dilute ammonium persulfate solution is tested which etches the β-Al Mg phases in the microstructure in a way that they can be examined at higher magnifications; even examinations in the scanning electron microscope are possible.