Combined Effect of the Sc/Zr Ratio and Mg Concentration on the Intergranular Corrosion Resistance of Al–Mg–Sc–Zr Alloys: A Case of Cast Alloys and Ultrafine-Grained Alloys

The aim of this study was to investigate the effect of the Sc/Zr ratio (Sc/Zr = 0.45–2.2) on the intergranular corrosion (IGC) resistance of Al–Mg alloys with different Mg content (2.5, 4, and 6%) and with a Sc + Zr = 0.32%. A change in the Mg concentration led to a change in the number of β-phase particles. A change in the Sc/Zr ratio led to a change in the composition of Al3(Sc,Zr) particles. The IGC resistance of Al–Mg–Sc–Zr alloys was investigated by Tafel electrochemical tests and stationary tests. It has been demonstrated for the first time that two types of IGC defects appear during electrochemical tests. Large Type I defects were associated with the destruction of primary β-phase particles located along the dendrite boundaries. Fine Type II defects were associated with the grain boundaries (GBs). It has been demonstrated that during the stationary tests, Type I defects are formed. ECAP and subsequent annealing affect the ratio of the number of Type I and II defects. Increasing the Sc/Zr ratio reduced the depth of Type I defects, increased the fraction of Type II defects, and reduced the corrosion current density icorr. It has been shown for the first time that the dependence of icorr(T) had a three-stage character with a maximum at 450 °C in alloys with 2.5% and 4% Mg. A two-stage dependence of icorr(T) is observed in alloys with 6% Mg. Increasing icorr at T < 450 °C is due to the precipitation of the secondary β-phase particles on Al3(Sc,Zr) particles and due to the effect of solid-phase wetting of the GBs by β-phase, which leads to an increase in the proportion of GBs containing thin layers of β-phase. Decreasing icorr at T > 450 °C is associated with the dissolution of β-phase particles.