Investigations of corrosion on the surface of titanium substrate caused by combined alkaline and heat treatment Vukoman Jokanovic ´ a,⇑ , Miroljub Vilotijevic ´ a , Bojan Jokanovic ´ b , Monika Jenko c , Ivan Anz ˇel d , Dragoslav Stamenkovic ´ e , Vojkan Lazic e , Rebeka Rudolf d,f a Vinc ˇa Institute of Nuclear Sciences, Mike Petrovic ´a Alasa 12-14, Belgrade, Serbia b SGL Carbon, Meitingen, Germany c Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia d University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, 2000 Maribor, Slovenia e University of Belgrade, High School of Dental Medicine, Dr. Subotic ´a 8, 11000 Belgrade, Serbia f Zlatarna Celje d.d., Kersnikova 19, 3000 Celje, Slovenia article info Article history: Received 4 June 2013 Accepted 21 January 2014 Available online 30 January 2014 Keywords: A. Titanium C. Alkaline corrosion C. High temperature corrosion B. XRD B. IR spectroscopy B. AES abstract In this research, the structure changes along the depth of gradient layers of titanium substrate, after etch- ing with NaOH and subsequent thermal treatment at various temperatures between 300 and 800 °C, were investigated by XRD, FTIR and AES. Particularly, the changes of Ti substrate after etching with NaOH, sub- sequent ionic exchange of Na + with Ca 2+ ions and thermal treatment at 700 °C were analysed. Due to this approach, it was possible to get insight into the chemical changes and changes of Ti oxidation states and consequent phase analysis, along the depth of the titanium oxide coatings. In addition, Secondary Electron Imaging (SEI) showed very interesting nanotopology of all samples. Particularly interesting topology, consisting of very thin nano-designed walls between mutually inter- connected pores, was observed for the sample in which Na + were replaced with Ca 2+ ions. This structure might be suitable for deposition of hydroxyapatite by biomimetic or plasma methods and as an appropri- ate scaffold for cell adhesion and proliferation. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Titanium and titanium alloys have been used widely as implant materials for total hip replacements, bone fixation and replace- ments of various bone defects, due to their good properties like low module of elasticity, good fatigue strength, formability, mechanical workability and corrosion resistance [1,2]. Despite all this, titanium and its alloys cannot fulfil all necessary clinical requirements, particularly those related with biological properties of the material [3,4]. Therefore, surface modification of titanium and its alloys is necessary, by formation of thin oxide layers, using various methods of chemical and electrochemical preparation [5–7]. Kim et al. first introduced alkali and heat treatments, which have been further widely studied in many papers [8]. The treat- ments of titanium and titanium alloys using NaOH should be adjusted to obtain a titanium oxide layer of optimal thickness which enables gradual transition from the mechanical and corro- sion properties of titanium implant to the properties of top oxide layer. Chemical composition of the layer changes with the distance from the surface (with the maximal quantity of bound oxygen on the top layer and minimal on the contact with pure metal) causing the changes of physical, particularly mechanical properties. Besides the improvement of mechanical stability of gradient oxide layer, this approach enables the optimization of implant biological properties, and formation of a reliable and reproducible titanium oxide/hydroxide surface. To the present, the knowledge about the quantitative composi- tion of the oxide/hydroxide overlayer and the mechanism of the overlayer formation is not satisfactory. The distribution of various titanium oxidation state oxides was subjected to thermodynamic analysis of the oxygen, which had been used to oxidize the tita- nium to a range of oxidation states ((IV), (III) and (II), respectively) under chosen temperature and pressure conditions [9]. Generally, Ti oxidation is characterized by fast oxygen adsorption, followed by slower oxygen uptake until saturation is reached. Obviously, a systematic study of the oxidation and hydroxylation of the tita- nium surface has a particular importance as a first step in the full understanding of the process of oxide/hydroxide formation as overlayers on titanium surfaces [9,10]. It is expected that the formation of sodium titanates on tita- nium and titanium alloys‘ surfaces, by their etching with NaOH solution and a subsequent heat treatment, can be very helpful for 0010-938X/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2014.01.014 ⇑ Corresponding author. Tel.: +381 113408724; fax: +381 113408607. E-mail address: vukoman@vin.bg.ac.rs (V. Jokanovic ´). Corrosion Science 82 (2014) 180–190 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci