Corrosion Behavior Of Mg-Gd-Zn Alloys With Long Period Stacking Ordered Structures

The long period stacking ordered structure (LPSO) containing alloy have attracted much attention due to their excellent room and elevated temperatures properties. However, it is well known that poor corrosion resistance of magnesium alloy has often baffled it from further applications, and this high strength magnesium alloy with long period stacking ordered structure is no exception. Therefore, in order to solve this problem, the study of the corrosion behavior of magnesium alloy with long period stacking ordered structure has become one of the urgent requirements.Mg-Gd-Zn and Mg-Gd-Zn-Zr, alloys with LPSO structures were prepared by conventional permanent mold casting method combined with heat treatment. The microstructures and corrosion surfaces of Mg-Gd-Zn and Mg-Gd-Zn-Zr alloys were analyzed by OM, XRD、SEM、EDS and TEM. The corrosion performances of Mg-Gd-Zn and Mg-Gd-Zn-Zr alloys were studied by combining collecting gas test, immersion test and electrochemical measurements. Results are shown below:(1) The LPSO structures phases in the Mg-Gd-Zn alloy transformed from eutectic structures after heat treatment, and its volume fraction increase and the distribution change from continuous to discontinuous with the increase of content of Gd.(2) The aged Mg95.5Gd3.5Zn1alloy with eutectic structures has a higher corrosion resistance compared with as-cast Mg95.5Gd3.5Zn1alloy with LPSO structures. This may be because of the difference of the constituent and structure of eutectic structures and LPSO structures, making the potential difference between second phase (eutectic structures or LPSO structures) and a-Mg is different. Thus, the accelerating corrosion effect of aged alloy and as-cast alloy is not the same. Then, with theoretical analyses, the cathodic reaction is easy occur on the eutectic structures, and this can accelerate the dissolution of Mg, thus, the corrosion resistance of as-cast alloy decrease.(3) The LPSO phase, like all second phases in common magnesium alloys, acting as cathodic phase forming micro-galvanic couple with α-Mg phase, and accelerating the corrosion rate around the a-Mg.(4) The as-cast (F) Mg95.8Gd3Zn1r0.2alloy has a highest corrosion resistance compared with T4and T6alloys. This because of the continuous distribution of the eutectic structures in the F alloy, and it can hinder the developing of the corrosion. Due to the addition of Zr, the compactness and uniformity of the corrosion film increase, thus, the corrosion resistance of the as-cast alloy improved. The corrosion resistance of T4Mg95.8Gd3Zn1Zr0.2alloy is between as-cast alloy and T6alloy. This is due to the change of the morphology, quantity and distribution of the second phases in T4alloy compared with the as-cast alloy. The second phases in T4alloy mainly act as cathodic phases accelerating the corrosion rate of α-Mg, and because of the inhomogeneity of the microstructure and electrochemistry, reducing the uniformity of the corrosion film, thus, making the ability of protecting magnesium matrix from erosion decrease. The T6Mg95.8gGd3Zn1r0.2alloy has a lowest corrosion resistance. This is that the cathodic-to-anodic area ratio in T6alloy is much higher than in T4alloy. Consequently, the galvanic corrosion of T6alloy is severe. And that the corrosion film on the T6matrix is loosened, so, it would not hinder the Cl-through the film, thus, the corrosion resistance of T6alloy decrease.