Effects of Zn addition on the microstructure and corrosion behavior of as-extruded Mg–Al–Ca–Mn-based alloys

The influence of Zn addition (0–0.6 wt.%) on the microstructure and the corrosion behavior of as-extruded Mg–1Al–0.3Ca–0.3Mn (wt.%) alloys was investigated by X-ray diffraction, optical microscopy, scanning electron microscopy combined with energy-dispersive X-ray spectroscopy, immersion tests and electrochemical measurements in 3.5 wt.% NaCl solution. All the as-extruded alloys consist of an α-Mg matrix and a small fraction of Al4Ca particles distributed along the grain boundaries; no Zn-bearing phase is detected over the composition range studied, so that Zn is retained in solid solution. Immersion tests show that a small Zn addition markedly improves the corrosion resistance: after 168 h the corrosion rate of the alloy containing 0.2 wt.% Zn is 4.77 × 10⁻⁴ g cm⁻² h⁻¹, which is only 14.3% of that of the Zn-free alloy. Potentiodynamic polarization confirms the same ranking, the corrosion potential shifting from −1.490 V to −1.440 V and the corrosion current density decreasing from 71.1 μA cm⁻² to 15.4 μA cm⁻². Electrochemical impedance spectroscopy, however, ranks the four alloys differently. Because the grain-size sequence does not track the corrosion-rate sequence—the alloy with the finest grains does not exhibit the lowest corrosion rate—grain refinement alone cannot account for the observed improvement. The beneficial effect is instead attributed to Zn dissolved in the α-Mg matrix, which modifies the growth of the corrosion-product film and limits the area over which micro-galvanic coupling at the Al4Ca particles can operate; this effect is optimal at 0.2 wt.% Zn and is weakened when a higher Zn content coarsens the grains.