Influence Of Microstructure On The Corrosion Behaviour Of AZ91D Magnesium Alloys In Particular Environments

Magnesium alloys have potential as structural materials for aerospace, 3C and other transport applications due to their high specific properties, low density and high damping capacity. However, the use of Mg alloys in these fields is still limited to some extent because of their susceptibility to corrosion. Despite such concern, very few studies are currently dedicated to the atmospheric corrosion of Mg alloys, especially in a polluted environment. Mg alloy parts are mainly formed by cast, high pressure die-casting and semi-solid processing. Previous studies have been extensively carried out on the corrosion behaviour of Mg alloys in NaCl solution. However, the corrosion processes of Mg alloys in solutions containing SO₄^2- ions still remain ambiguous. Although there are several works studied the corrosion behaviour and microstructure evolution of Mg alloy produced by semi-solid processes during heat treatment, there are rare reports concerning on the thixomolding Mg alloy. We aim to study the influence of microstructure on the corrosion behaviour of AZ91D magnesium alloys in particular environments in great detail. The results of our investigation may find wider applications for advancing materials.In this paper, the atmospheric corrosion behaviour of cast and high pressure die-cast AZ91D alloy in a polluted environment, the corrosion behaviour of die-cast AZ91D magnesium alloys in sulphate solutions with various pH value, the corrosion behaviors of the thixomolding and high die-casting AZ91D alloy in NaCl solution and the microstructure evolution of thixomolding AZ91D magnesium alloy during heat treatment were investigated by OM, SEM, TEM, XRD, FTIR, GDS and polarization measurements in order to provide further details regarding the corrosion processes and mechanisms of a die-cast Mg alloy in solutions containing the SO₄^2-. The results are discussed in terms of the overall Mg corrosion theories.The kinetics of atmospheric corrosion of the ingot and the die-casting AZ91D alloys could be expressed as C=1.533t0.602 and C=1.661t0.507, respectively. The corrosion rates of the ingot and the die-casting AZ91D alloys exposed to the polluted environment were about 6.24 g·m^-2·y^-1 and 5.97 g·m^-2·y^-1, respectively. SO₂ gas played an important role in atmospheric corrosion. The main corrosion products of AZ91D alloys were Mg(OH)₂, Mg₂Al₂(SO₄)₅·39H₂O, and MgCO₃. The microstructure played an important role in the corrosion process. The initial corrosion rate was mainly influenced by surface defects. The thickness of the product layer and the diameter of the crack were greatly influenced by the grain size. The distribution of theβphase acts on the later corrosion rate. The pitting takes place on the die-casting specimen surface immersed in a NaCl solution when the immersion time is less than 48 h. In contrast, in Na₂SO₄ and MgSO₄ solutions, pitting corrosion does not occur, and the corrosion appears generalized because the aggressiveness of SO₄^2- is lower than that of Cl^-. The corrosion rate on the whole surface is approximately uniform in the sulphate solution. The corrosion rate order of the die-cast AZ91D Mg alloy in the three aqueous solutions is: vNaCl > vMgSO₄ > vNa2SO₄. The control factor of the corrosion process in the sulphate solution is the cathodic hydrogen-evolution corrosion. The main corrosion products are Mg(OH)₂ and MgAl₂(SO₄)₄·22H₂O in the sulphate solution. The precipitation of the MgAl₂(SO₄)₄·22H₂O needs a threshold immersion time. When the immersion time is extended to 6 h, the concentration of MgAl₂(SO₄)₄·22H₂O in the solution gradually became supersaturated and then began to precipitate from solution. The order of corrosion rate of Mg alloy in Na₂SO₄ solutions with various pH value was acidic solutions > neutral solutions > alkaline solutions. The corrosion products of the die-cast Mg alloy were mainly Mg(OH)₂ and MgAl₂(SO₄)4·22H₂O. The pH value could influence the corrosion rate and morphology of the corrosion products. Pitting occurred on the specimen surface which immersed in the acidic solution after immersed for about 330 h.There was unmeltedα-phase in the thixomolding specimen, and the Al content of which was low, so it made the Al content of the eutecticα-phase become higher than the die-casting specimen. The unmeltedαphase was a major corrosion phase at the early stage, with the corrosion time prolonged, the corrosion process was prevented due to the corrosion products covering on the unmeltedα-phase. And then, the eutecticα-phase corroded and dissolved. Electrochemical testing results showed that the corrosion rate of the thixomolding specimen was 28% lower than that of the die-casting specimen. The AZ91D Mg alloy specimen processed by thixomolding exhibits a fasterβphase dissolution rate than the die-casting alloy at 415℃solution treatment. The Al-rich halos have been seen at the early stage of the solution treatment because of Al diffusing from theβphase to the unmeltedα-globules. The thixomolding AZ91D alloy has fasterβphase dissolution kinetics in comparison with the die-casting alloy, showed as d3-d03=588t and d3-d03=324t, respectively. The reason is that initial secondaryα-Mg grain in the thixomolding specimen is equiaxed and fine, which makes the dissolution rate of theβ-Mg17Al12 faster.