* A. Robin, J. P. Meirelis Departamento de Engenharia de Materiais, Escola de Engenharia de Lorena – Universidade de Sa ˜o Paulo – EEL/USP Estrada Municipal do Campinho, CEP – 12.601-810 Lorena – SP (Brazil), E-mail: alain@demar.faenquil.br Influence of fluoride concentration and pH on corrosion behavior of Ti-6Al-4V and Ti-23Ta alloys in artificial saliva A. Robin* and J. P. Meirelis Titanium alloys exhibit an excellent corrosion resistance in most aqueous media due to the formation of a stable oxide film and some of these alloys (particularly Ti-6Al-4V) were chosen for surgical and odontological implants for this resistance and their biocompat- ibility. Treatments with fluorides (F  ) are known as the main meth- od to prevent plaque formation and dental caries. Toothpastes, mouthwashes and prophylactic gels can contain from 200 to 20 000 ppm F  and can present neutral to acidic character, which can affect the corrosion behavior of titanium alloys devices present in the oral cavity. In this work, the behavior of Ti-6Al-4V and the new experimental Ti-23Ta has been evaluated in artificial saliva of pH 2, 5 and 7 and different F  concentrations (0, 1000, 5000 and 10 000 ppm), through open-circuit potential measurements, poten- tiodynamic polarization and electrochemical impedance spectro- scopy. A defined correlation between pH and F  concentration settled the active or passive character of the materials. For both al- loys, an active behavior was observed for pH 2 and 1000 to 10 000 ppm F  and for pH 5 and 5000 and 10 000 ppm F  . The pas- sive behavior was observed for the other investigated conditions. The F  concentration increase and pH decrease reduced the corro- sion resistance of the alloys and decreased the stability of their pas- sive film. The corrosion behavior of both alloys was very similar, but the Ti-23Ta alloy generally presented slightly higher corrosion resistance. 1 Introduction Titanium and its alloys (specially Ti-6Al-4V) are widely used in restorative surgery such as dental and orthopedic im- plants, pacemakers and heart valves, because they present high corrosion resistance in physiological media and biocom- patibility [1]. Their corrosion resistance is due to the sponta- neous formation of a superficial oxide film (mainly formed of titanium dioxide TiO 2 ) [2], which exhibits high stability in biological fluids. Another interesting property of Ti and its alloys is their high strength to density ratio. In orthodontics, Ti and Ti-6Al-4V are usually employed as dental implants [1, 3–4] and more recently, as brackets [5]. There is an increasing use of prophilactic products in dental treatments to prevent plaque and caries formation. Most of these products contain fluoride ions (F  ) at different concen- tration levels (e.g. 200 ppm F  in bucal rinses and 1000–1500 ppm F  in toothpastes and 10 000–20 000 ppm F  in gels), and their pH can range from neutral to acidic va- lues. Several works about the influence of both F  ions and pH on the corrosion behavior of Ti and Ti-6Al-4V alloy in arti- ficial saliva are avaliable [6–12]. Although these studies were generally conducted under narrow ranges of F  concen- tration and pH and the data are dispersed and difficult to be compared, all agree that the corrosion resistance of these ma- terials can be affected by the presence of F  ions, probably due to the formation of stable and soluble titanium fluoride or oxi-fluoride complexes [13, 14]. In some works, low pH was shown to increase the corrosion rate of Ti in presence of F  ions, due to hydrofluoric acid formation [7]. Some studies showed that aluminum and specially vanadi- um release from the implants may have toxic effects [15, 16]. Consequently, vanadium-free alloys containing non-toxic ele- ments such as niobium and iron were developed, e.g. Ti-6Al- 7Nb or Ti-5Al-2.5Fe and more recently, alloys without vana- dium and aluminum, such as Ti-13Nb-13Zr [16–17] and Ti-Ta [18–19] alloys were produced. The aim of this study was to evaluate the simultaneous ef- fect of F  concentration and pH on the electrochemical behav- ior of Ti-6Al-4V and the experimental Ti-23Ta alloy in arti- ficial saliva. The study was carried out in saliva of pH from 2 to 7 containing up to 10 000 ppm F  , using open-circuit po- tential measurements, potentiodynamic polarization and elec- trochemical impedance spectroscopy. 2 Experimental procedure The materials used in this work were the commercially available Ti-6Al-4V alloy and a new Ti-23Ta alloy produced in our laboratory. The Ti-23Ta alloy was obtained from commercially pure Ti and Ta sheets by triple arc-melting, followed by heat treatment under vacuum at 1200  C (temperature above the b transus temperature [20]) for 48 h and natural cooling to room tem- perature in the furnace. The microstructures of both alloys are shown in Fig. 1. Ti-6Al-4V presents a coarse a plate-like mi- crostructure with intergranular b-phase (Fig. 1a). Ti-23Ta shows a two-phase (a þ b) Widmansta ¨tten-type microstruc- ture (Fig. 1b), due to the partial transformation of b-phase to a-phase during cooling from 1200  C to room temperature. Cylindrical test specimens (8 mm diameter  15 mm length) were machined from Ti-6Al-4V and Ti-23Ta ingots and mounted in PTFE holders. Materials and Corrosion 2007, 58, No. 3 DOI: 10.1002/maco.200604004 173 www.wiley-vch.de/home/wuk F 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim