The role of magnesium in the electrochemical behaviour of 5XXX aluminium-magnesium alloys

Abstract

An investigation concerning the effects of magnesium on the intergranular corrosion susceptibility of AA5XXX aluminium alloys was carried out. In the present work, magnesium is found to be highly mobile in the bulk metal as well as in the aluminium oxide. This mobility is also found to be dependent on the temperature and bulk magnesium concentration. Interestingly, the corrosion susceptibility of the AA5XXX series appears to be directly related to the magnesium mobility as well. It was found that magnesium is the most favoured alloying element in the aluminium metal that actively contributes to the formation and composition of the oxide layer. Nevertheless, the extent of this contribution strongly depends on the magnesium content of the alloy and the applied temperature. Auger composition-depth profiles performed on alloys with different Mg contents indicate that when the bulk magnesium concentration is high, the Mg enrichment on the oxide layer becomes stronger. This situation brings as a consequence the formation of thicker oxide layers. At grain boundaries, magnesium enrichment was measured and quantify. A mechanism for intergranular corrosion is proposed. This mechanism is based on the combination of three main factors: magnesium-rich precipitates at grain boundaries, magnesium enrichments at grain boundaries as free-atoms, and hydrogen diffusion into the grain boundaries.