Characterization and Improvement in the Corrosion Performance of a Hot-Chamber Diecast Mg Alloy Thin Plate by the Remova
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MAGNESIUM alloys are increasingly used for automotive applications because of their capacity to meet vehicle performance requirements.[1] Diecasting is one of the most extensively used approaches for producing magnesium alloy parts. Two basic diecasting processes—hot chamber and cold chamber—are used.[2] The former process has some advantages over the latter, including rapid cycling, improved fluidity, the use of a lower injection pressure, etc.[2] The former process, therefore, offers a favorable method for the mass production of electronic/appliance housings with thin sections. However, a comprehensive microstructural study and the dependence of the microstructure on the improved corrosion resistance of a hot-chamber diecast
JUN-YEN UAN, Associate Professor and CHING-FEI LI, Graduate Student are with the Department of Materials Science and Engineering, National Chung Hsing University, Taichung, Taiwan, Republic of China. Contact e-mail: [email protected] BING-LUNG YU, formerly Graduate Student with Department of Materials Science and Engineering, National Chung Hsing University, is now an Engineer with ProMOS Technologies Inc., Taichung, Taiwan, Republic of China. Manuscript submitted October 29, 2006. Article published online January 23, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
thin plate (1.4-mm thick, in this work) have been addressed briefly.[3,4] The Mg alloy exhibits high electrochemical activity,[5] leading to poor corrosion resistance, such as in a chloride environment. Chemical conversion is an important surface pretreatment for improving the corrosion resistance of the Mg alloy.[2] Small cracks are distributed on the conversion coating layer during dehydration, improving the adhesion of subsequent paint layers or organic coatings to the surface of the Mg alloy substrate. The conversion coating on magnesium has been traditionally based on hexavalent chromium compounds.[2] However, the hexavalent chromium (Cr6+) is a toxic substance that pollutes the environment and detrimentally affects health.[6] Recently, many investigations have studied an alternative, chrome-free conversion coating process to protect magnesium against corrosion.[7–11] In the new conversion treatments, the Mg sample was immersed in a bath of permanganatestannate phosphate (KMnO4 + MnHPO4),[9,10] (NaOH + K2SnO3 Æ 3H2O + NaC2H3O2 Æ 3H2O + Na4P2O7),[12,13] cerium nitrate (Ce(NO3)3),[8,11] CeCl3/ H2O2,[14] La(NO3)3,[8] Pr(NO3)3,[8] or cobalt-III hex coordinated complex (Co(NO3)2 + NH4NO3 + NH4OH + NH3).[15] The Mg sample that was treated in the various aforementioned baths is coated with various compounds. For example, the conversion coating VOLUME 39A, MARCH 2008—703
films may be composed of Mg3(PO4)2/MgMn2O8,[9] MgSnO3,[13] cerium oxide/hydroxide,[8,11,14] lanthanum oxide/hydroxide,[8] or praseodymium oxide/hydroxide.[8] The cited investigations have contributed substantially to the success of the Cr6+-free conversion coating process. However, with respect to recycling magnesium scraps from automotive components or any post
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