Please cite this article in press as: J.-Y. Wang, et al., J. Alloys Compd. (2008), doi:10.1016/j.jallcom.2007.12.049 ARTICLE IN PRESS +Model JALCOM-17313; No. of Pages 4 Journal of Alloys and Compounds xxx (2008) xxx–xxx Investigation of the photo-catalytic coating on AZ91 alloy Jian-Yih Wang a,∗ , Hao-Jan Tsai a , Jun-Yen Uan b , Ming-Show Wong a , Shyi-Kaan Wu c a Department of Material Science and Engineering, National Dong Hwa University, No. 1, Sec. 2, Da-Hsueh Road, Shoufeng, Hualien County 974, Taiwan b Department of Material Science and Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung 402, Taiwan c Department of Materials Science and Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei, 106 Taiwan Received 14 June 2007; received in revised form 12 December 2007; accepted 12 December 2007 Abstract A TiO 2 thin film was deposited on AZ91 alloy to enhance the corrosion–resistance and photo-catalytic properties. Experimental results showed that the TiO 2 -coated AZ91 exhibited hydrophilicity. The corrosion potential of TiO 2 -deposited AZ91 was higher than that of the uncoated AZ91, suggesting an increase in the intrinsic corrosion resistance. Compared with the uncoated samples, the corrosion resistance of TiO 2 -deposited samples, on the other hand, deteriorated. FE-SEM observation showed polycrystalline characteristics as well as the presence of pinholes on the film. EDS analysis revealed the pinholes to be locales of incomplete deposition, where accelerated corrosion occurred. © 2007 Elsevier B.V. All rights reserved. Keywords: AZ91 alloy; Magnetron sputtering; Contact angle; Corrosion resistance; Photo-catalytic 1. Introduction Magnesium alloys are well known for such outstanding prop- erties as low density, high specific strength, vibration absorption ability, and electromagnetic shielding [1]. Besides their use as 3C products, magnesium alloys have found many applica- tions in transportation industries. AZ91 (9% Al–1% Zn), among all magnesium alloys, is the most widely used because of its good castability and mechanical properties, with especially large demand for the shells of Notepad PC in the future [2]. However, because of the poor corrosion resistance of magne- sium alloys, their applications can be severely limited without proper corrosion-prevention treatment [3]. Currently, chemical conversion process followed by painting [4] or anodizing treat- ment [5] is the most widely used surface treatment, yet the results have not been sufficient. Several researches – including such surface treatment as micro-arc anode oxidation [6,7], new con- version coating [8], and cladding of pure metal elements on the ∗ Corresponding author. Tel.: +886 922 233553; fax: +886 3 8634200. E-mail address: jy-wang@yahoo.com.tw (J.-Y. Wang). surface of magnesium alloys [9–11] – were conducted in indus- try to address this issue. Among other active research goals are the deposition of layers of AlN + AlN + AlN, or AlN + TiN on magnesium alloys using PVD [12], purification of magnesium alloys, and the effect of second phase [13]. The present study investigates the deposition of a TiO 2 photo- catalytic film on AZ91 alloy to improve the corrosion resistance and impart photo-catalytic effect for use in dirt-prevention (self cleaning function), which is a property that can be conveniently assessed by measuring the contact angles as an indication of the thin film’s hydrophilicity. The effectiveness of dirt-prevention, and therefore the photocatalytic characteristic, increases with decreasing contact angles. Moreover, the use of TiO 2 as a depositing material offers the benefit of shunning environmen- tally harmful materials that are usually used in conventional surface treatments. 2. Experimental procedures AZ91 billets, 7 ′′ in diameter, were first heated to 350 ◦ C and then extruded into a sheet metal 100 mm in thickness. The sheet metal was further hot rolled at 300 ◦ C to a thickness of 2 mm. Specimens of AZ91, 30 mm × 3 mm, were cut for photo-catalytic deposition. 0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.12.049