| Recently, magnesium alloys have received extensive recognition because of their corrosion resistance problem. Nowadays, the researchers in various countries did a lot of studies on the corrosion behavior and protective method of magnesium alloys, which broadened the application of magnesium alloys to some extent. With the development of research, people would like to understand the corrosion behavior of magnesium alloys in particular environment and identify the main factors of controlling corrosion. The environmental factors which influence the corrosion behavior of magnesium alloys include solution composition, gas composition, temperature, humidity, pressure and so on. A consensus appears to have been reached regarding the corrosion mechanism underlying the aggressive behavior of sodium chloride against magnesium. However, the corrosion process of magnesium alloys in solutions containing SO42- ions remains ambiguous. With more and more magnesium alloys exposed to the atmospheric environment, the effects of acid rain on magnesium alloys have become the focus of various studies. Further more, H+ions could influence both the anode reaction and the cathode reaction, the corrosion behavior of magnesium alloys vary with the pH value changed. In this paper, a study of the corrosion behavior of die-cast AZ91D in Na2SO4 solution has been done and the influences of pH value and SO42- ions on the corrosion behaviour of die-cast AZ91D are systematically studied. The results of this study may help pave the way for wider applications of magnesium alloys. As for the temperature, humidity, pressure and other factors, there are several works studied the corrosion behaviours of magnesium alloys in salt spray at normal temperature and pressure, but there isn't a clear understanding of the corrosion behaviours of magnesium alloys in harsh salt spray conditions. In this paper, We chose cast and extrude magnesium alloy AZ61 and studied the influences of temperature, humidity, pressure and NaCl atmosphere on the corrosion products and corrosion mechanism by means of SEM, EDS and XRD. The main results can be summarized as follows:1. The die-cast AZ9lD magnesium alloys were immersed in 0.1 M Na2SO4 solutions with pH values of 2,4,7,9, and 12, the corrosion rate of the specimens increased with the decreasing pH values. The solution pH value can influence the morphologies of corrosion products, and the corrosion products of the die-cast Mg alloy were mainly Mg(OH)2 and MgAl2(SO4)4-22H2O.2. The morphologies of the corrosion product films changed as the immersion time increased. Hemisphere-like films formed on the surface of specimens in acidic solutions. In alkaline solutions, the morphology of the products changed from white reticulated lines to an acicular film and finally to a flocculation film.3. In acidic solutions, pitting occurred on the surface of the specimens, but chloride ions and sulphate ions had different pitting initiation times. The volume of a sulphate ion is much larger than that of a chloride ion, and MgAl2(SO4)4-22H2O, as the corrosion product, is semi-permeable. Both the volume and the semi-permeable property decreased the penetration of sulphate ion. So the sulphate ions had longer pitting initiation time.4. After the cast and extrude AZ61 magnesium alloys immersed in sodium chloride atmosphere at high temperature and high pressure, we could indicate the corrosion rate of extruded AZ61 magnesium is faster than that of cast AZ61 magnesium.5. Both the rearrangement of second phase and smaller grain size, which owing to the extrusion process, change the morphologies of corrosion products. After immersed in sodium chloride atmosphere, extrude AZ61 magnesium alloys form circular corrosion pits. The corrosion products are not changed by the extrusion process.6. The environmental factors play an important role in the corrosion behavior of magnesium alloys. Higher temperature, humidity, and sodium chloride atmosphere work together to accelerate the corrosion rate of Mg alloys.
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