Mechanism of Etching of Al-4.5Mg-1.0Mn Alloy

Abstract 5xxx series aluminum alloys, as Al-4.5Mg-1.0Mn (AA5083), are strengthened by Mg solid solution and work hardening. A drawback of this alloy is the fact thatβphase, Al₃Mg₂, can precipitate on grain boundaries causing sensitization and intergranular corrosion, which is detrimental to the integrity of the structure. Metallography is an important technique to study the grain structure and highly sought for intergranular corrosion evaluation; however, revealing the grains of completely un-sensitized AA5083 is challenging. This paper introduces a new procedure to etch AA5083 samples that were solutionized at 450°C for 1.5 h. The new procedure is a two-step etching method, including a phosphoric acid pre-etching step and a Weck’s reagent coloring step. Solutionized, lightly sensitized, and as-received AA5083 were evaluated, and the grains were observed using optical microscopy. The microetching mechanism was further studied by optical profilometry, atomic force microscopy, scanning electron microscopy, and energy dispersive spectrometry. The phosphoric acid created a surface profile determined by the grain orientations and its reactivity, and the Weck’s reagent was then able to color grains by preferential MnO₂formation over some pre-etched grains. Moreover, the final polishing with colloidal silica was essential to reach a high contrast image.