Corrosion morphology of AZ91D exposed in an atmospheric environment
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EXISTING studies have demonstrated that the a matrix phase of a magnesium alloy is always preferentially corroded[1–5] in an aqueous solution due to the galvanic effect. In practice, most magnesium components are used in atmospheric environments and are likely to suffer from atmospheric corrosion. In these cases, it is unclear if the galvanic corrosion mechanism still operates and the a phase is preferentially attacked. Although there have already been some investigations on the atmospheric corrosion of magnesium alloys,[6–10] the detailed atmospheric corrosion morphology of magnesium alloys is rarely reported. Illustration of the detailed morphologic characteristics in corroded areas is an important step to comprehensive understanding of the atmospheric corrosion mechanism and behavior. In this article, a dual beam focused ion beam (FIB) system with an energy-dispersive X-ray (EDX) probe was used to reveal the detailed corrosion morphology that could not be illustrated by conventional methods. The AZ91D specimens used in this study were polished, cleaned, and then dried before tests. To simulate the corrosion of the alloy in an industrial and marine atmospheric environment, a NH4Cl-ethanol solution (1.6 g NH4Cl dissolved in 1 L ethanol-water solvent with water/ethanol 5 1/9) was sprayed, resulting in about 2 mg " cm$2 of the solution deposited on the surfaces of the specimens. After being dried, the specimens were exposed to a clean atmosphere in a humidity chamber whose relative humidity was controlled at 85 pct 6 2 pct and temperature at 25 °C for 2 days. The specimen surfaces were then observed under scanning electron microscopy (SEM). To look inside a corrosion cavity or pit, a NOVA 200 FIB system was also used. YE WAN, Postdoctoral Student, JUN TAN, Research Fellow, and CHUANWEI YAN, Professor, are with the Institute of Metals Research, Chinese Academy of Sciences, Shenyang, 10016, People’s Republic of China. GUANGLING SONG, Senior Research Fellow, is with the School of Engineering, University of Queensland, Brisbane, Qld 4072, Australia. Contact e-mail: [email protected] Manuscript submitted May 31, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
The specimens were trenched in some areas using a gallium beam at a current of 1 nA and a voltage of 30.0 kV. The most outstanding characteristic of the morphology of AZ91D after being exposed in the simulated atmospheric environment for 2 days was that all the corroded areas were adjacent to the intermetallic precipitates, indicating that the atmospheric corrosion was caused by the galvanic effect of the intermetallics. Figure 1 displays a typical corroded area with such a characteristic. Apart from the basic feature, the atmospherically corroded AZ91D also displayed the following morphologic characteristics: (1) Some areas surrounding the intermetallic precipitates were slightly corroded or not corroded. These areas are representatively indicated by (S) on the SEM photo; (2) Some areas surrounding the intermetallic precipitates were severely corroded, but th
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