Morphological and Microstructural Characterization of the Aragonitic CaCO 3 /Mg,Al-Hydrotalcite Coating on Mg-9 Wt Pct A
- PDF / 1,549,097 Bytes
- 13 Pages / 593.972 x 792 pts Page_size
- 31 Downloads / 171 Views
I.
INTRODUCTION
MAGNESIUM alloy has high electrochemical activity,[1] leading to poor corrosion resistance in a chloride environment. Many studies have elucidated the electrochemical and corrosion properties of Mg[2,3] and Mg alloys.[4–7] A method of improving corrosion resistance is to coat an Mg-based substrate to prevent contact with a chloride environment. Chemical conversion is an effecting surface pretreatment for improving the corrosion resistance of Mg alloys.[8] Conversion coating on Mg has traditionally used hexavalent chromium compounds.[8] However, hexavalent chromium (Cr6+) is a toxic substance that pollutes the environment and has adverse effects on human health.[9] Many recent investigations have explored several chrome-free conversion coating processes.[10–14] For instance, Mg alloy samples were immersed in a bath of permanganate-phosphate (KMnO4 + MnHPO4),[12,13] stannate (NaOH + K2SnO3Æ cerium 3H2O + NaC2H3O2Æ3H2O + Na4P2O7),[15,16] nitrate (Ce(NO3)3),[11,14] CeCl3/H2O2,[17] La(NO3)3,[11] JUN-YEN UAN, Associate Professor, and XIN-LIANG PAN, Graduate Student, are with the Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan, Republic of China. BING-LUNG YU, formerly Graduate Student, Department of Materials Science and Engineering, National Chung Hsing University, is Engineer, ProMOS Technologies Inc., Taichung 402, Taiwan, Republic of China. Contact e-mail: jyuan@ dragon.nchu.edu.tw Manuscript submitted March 12, 2008. Article published online October 25, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
Pr(NO3)3,[11] or cobalt-III hex coordinated complex (Co(NO3)2 + NH4NO3 + NH4OH + NH3).[18] These investigations[10–14] contributed substantially to the success of the Cr6+-free conversion coating process. Importantly, using an environmentally friendly electrolyte solution containing no chromate, heavy-metal ions, phosphates, or fluoride, Zhang et al.[19] and Lin et al.[20] generated new surface coating processes to protect Mg alloys from corrosion. However, the studies of Mg conversion coating from environmentally clean baths are still limited.[19,20] Generating a hard conversion coating on Mg alloys using a clean solution remains challenging. The idea of the present work is based on the formation of aragonitic CaCO3 in vivo that incorporates mineral components Ca2+ (or Mg2+) and CO3-2 .[21] This study examines the effectiveness of using an environmentally friendly aqueous bath with Ca2+, Mg2+, and CO3-2 for a CaCO3 conversion coating on Mg alloys that protects against corrosion. Magnesium alloys react with water, following Eq. [1], in aqueous solutions with pH 1 hour, the peak of aragonite structure at a 2h angle of 29.1 deg had a strong preferred orientation. The intensities of peaks at a 2h angle of 29.1 deg increased as immersion duration METALLURGICAL AND MATERIALS TRANSACTIONS A
increased (spectra (c) through (i) in Figure 5). Figure 6 shows the surface microstructures on the AZ91D samples immersed in the Ca2+/HCO3- solution for different duration
Data Loading...
