Effect of Al Content on Microstructure and Corrosion Performance of Mg-2Sr Alloys in Different Saltwater Environments

Abstract

The corrosion behavior of Mg-2.0Sr-xAl alloys with Al contents between 6 and 14 wt.% and minor alloying additions was systematically investigated to address the limited understanding of high-Al containing Mg-Sr systems. Three alloys Mg-2.0Sr-6.0Al-0.2Si-2.0Sn, Mg-2.0Sr-10.0Al-0.2Si-0.5Zn, and Mg-2.0Sr-14.0Al-0.2Si-0.2Zn were fabricated by stir-casting and evaluated in chloride and mixed chloride-bicarbonate electrolytes using immersion testing, hydrogen evolution, potentiodynamic polarization, and, for the first time in this system, electrochemical noise analysis. Increasing Al content promoted coarser grains, enhanced connectivity and volume fraction, as well as increased the aspect ratios of Mg17Al12 and Al4Sr intermetallic phases. The Mg-2.0Sr-6.0Al-0.2Si-2.0Sn alloy consistently exhibited the lowest corrosion current density, reduced noise fluctuations, and diminished pit density, reflecting superior resistance due to refined grain size and disrupted intermetallic connectivity. Surface analyses identified Mg(OH)2, Al(OH)3, MgCO3, and hydromagnesite as common corrosion products, with reduced intensity in the Mg-2.0Sr-6.0Al-0.2Si-2.0Sn alloy. These findings demonstrate that corrosion resistance in Mg-Al-Sr alloys is governed primarily by intermetallic morphology and spatial connectivity rather than isolated elemental effects, while bicarbonate-containing environments further stabilize protective films. The superior corrosion resistance of the Mg-2.0Sr-6.0Al-0.2Si-2.0Sn alloy is primarily due to finer intermetallic phase fractions, morphology, and a finer aspect ratio that reduced the cathode-to-anode ratio, resulting in reduced micro-galvanic coupling and localized attack.