Effect of Thermal Oxidation on Corrosion Resistance of Commercially Pure Titanium in Acid Medium M. Jamesh, Satendra Kumar, and T.S.N. Sankara Narayanan (Submitted August 24, 2010; in revised form April 11, 2011) This article addresses the characteristics of commercially pure titanium (CP-Ti) subjected to thermal oxidation in air at 650 °C for 48 h and its corrosion behavior in 0.1 and 4 M HCl and HNO 3 mediums. Thermal oxidation of CP-Ti leads to the formation of thick oxide scales (20 lm) throughout its surface without any spallation. The oxide layer consists of rutile- and oxygen-diffused titanium as predominant phases with a hardness of 679 ± 43 HV 1.96 . Electrochemical studies reveal that the thermally oxidized CP-Ti offers a better corrosion resistance than its untreated counterpart in both HCl and HNO 3 mediums. The uniform surface coverage and compactness of the oxide layer provide an effective barrier toward corrosion of CP-Ti. The study concludes that thermal oxidation is an effective approach to engineer the surface of CP-Ti so as to increase its corrosion resistance in HCl and HNO 3 mediums. Keywords corrosion resistance, surface modification, thermal oxidation, titanium 1. Introduction Titanium and its alloys have become the materials of choice in many industries due to their excellent corrosion resistance in a wide variety of environments (Ref 1). The formation of a thin self-adherent passive oxide film on their surface is considered responsible for this attribute. The corrosion rate of titanium and its alloys, however, is significant in hydrofluoric acid, caustic solutions, and uninhibited concentrated hydrochloric or sulfuric acid solutions that dissolve the protective oxide film or limit its formation. The increased use of titanium and its alloys in extractive metallurgy has prompted research into their corro- sion behavior in various acids (Ref 2-9). Since the passive oxide film is responsible for the excellent corrosion resistance of titanium and its alloys, it is obvious that any treatment or modification that facilitates its formation and thickening would improve their corrosion performance. Addition of strongly oxidizing inorganic compounds such as K 2 Cr 2 O 7 , KMnO 4 , KIO 3 , Na 2 MoO 4 , NaClO 3 , Cl 2, and H 2 O 2 tend to promote passivation of titanium. Addition of anions, such as iodate (IO 3  ), metavanadate (VO 3  ), and molybdate (MoO 4 2 ) promoted passivation of Ti-6Al-4V alloy in 2.5 M H 2 SO 4 and 5.0 M HCl (Ref 5). Tomashov et al. (Ref 10, 11) have shown that the greatest increase in corrosion resistance occurs when titanium is alloyed with palladium and other noble metals, which are capable of shifting the corrosion potential in the noble direction. Alloying of titanium with molybdenum, chromium, aluminum, zirconium, and tantalum, which increases its tendency to passivate, also enables a beneficial influence (Ref 10-14). Surface modification is a promising way to increase the surface hardness, corrosion resistance, and wear resistance of titanium and its alloys. Numerous surface modification meth- ods, such as chemical treatment (acid and alkali treatment) (Ref 15, 16), electrochemical treatment (anodic oxidation) (Ref 17), chemical vapor deposition (Ref 18), physical vapor deposition (Ref 19), sol-gel coatings (Ref 20), plasma spray deposition (Ref 21), ion implantation (Ref 22), thermal oxidation (Ref 23), etc., have been explored. Among these methods, thermal oxidation is considered as a cost-effective method to deliber- ately generate a barrier oxide layer of relatively higher thickness (20-30 lm) on titanium compared to the naturally formed oxide layer (typically of 4-6 nm). Thermal oxidation of titanium is aimed to produce in situ ceramic coatings, mainly based on rutile, in the form of a thick, highly crystalline oxide film, which is accompanied by the dissolution of oxygen beneath them. The thermally formed oxide layer enables an increase in hardness, wear resistance, and corrosion resistance of titanium and its alloys (Ref 24-31). The ability of thermally oxidized (TO) commercially pure titanium (CP-Ti) and Ti-6Al- 4V alloy in improving the corrosion and fretting corrosion resistance in RingerÕs solution has been reported in our earlier articles (Ref 29-33). This article aims to evaluate the corrosion behavior of the TO CP-Ti in HCl and HNO 3 (0.1 and 4 M) by open circuit potential (OCP)-time measurement and potentio- dynamic polarization studies for industrial application. 2. Experimental Details The CP-Ti (Grade 2, chemical composition in wt.%: N, 0.01; C, 0.03; H, 0.01; Fe, 0.20; O, 0.18; and Ti: Balance) having a dimension of 3 9 4 9 0.2 cm 3 was used as the substrate material. Before thermal oxidation, the CP-Ti samples were abraded using successive grit size of SiC-coated abrasive papers (60, 100, 220, 320, 400, 600, 800, and 1000 lm, M. Jamesh, Satendra Kumar, and T.S.N. Sankara Narayanan, National Metallurgical Laboratory, Madras Centre, CSIR Madras Complex, Taramani, Chennai 600 113, India. Contact e-mail: tsnsn@rediffmail.com. JMEPEG (2012) 21:900–906 ÓASM International DOI: 10.1007/s11665-011-9970-8 1059-9495/$19.00 900—Volume 21(6) June 2012 Journal of Materials Engineering and Performance