Titanium and its alloy have been widely utilized for excellent corrosion-resistance, high melting point, high strength and biocompatibility. However, Ti and Ti alloys are non-bioactive after being implanted in bone. Thus, for further improvement in biocompatibility the various implant surface modifications have been investigated. These surface modifications have included deposition of Ti coating using plasma spraying, and deposition of calcium phosphate or hydroxy-apatite (HA) coating, sandblasting, acid ething, oxidation, ion implantation and alkaline treatment.
One of these surface modifications that alkaline solution is formed on Ti in 5 M NaOH solution at 60℃, and it can be converted into an amorphous sodium titanate layer by heat treatment that induces a bone-like apatite formation on its surface in simulated body fluid. Although electrochemical and chemical treatments of Ti have been carried out in alkaline solutions to form surface oxide film for medical application, the detailed electrochemical behavior of titanium in various concentrations of sodium hydroxide has not been reported.
In this work, electrochemical behavior of commercially pure titanium in alkaline solution was investigated as a function of NaOH concentration by open-circuit potential transients, cyclic polarization curves, galvanostatic (constant current) method and surface morphological study using SEM and CLSM.
Commercially pure titanium specimen (99.6% ASTM grade1) of 1.77 cm2 surface area was used as the working electrode. The specimen was ground successively with silicon carbide papers from 400 to 2000 grit for 30 seconds and then rinsed with ethanol for 30 minutes by using ultra sonic and distilled water. A platinum mesh and saturated calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively.
The open-circuit potential transients of Ti in different concentrations of NaOH were measured. The open-circuit potential showed an increase with time in the solutions lower than 0.1 M NaOH, while it showed a decrease with time in the solutions higher than 0.1 M, which are attributed to the growth and dissolution of surface oxide film, respectively. It is noted that the open-circuit potential value obtained at 2000 seconds of immersion time decreases with increasing concentration of NaOH. This indicates that the dissolution of surface oxide film is easier in more concentrated NaOH solutions.
Cyclic polarization curves of Ti were obtained at 5 ㎷/s in different concentrations of NaOH. Current peaks were clearly observed during the positive going scan of potential in concentrated NaOH solutions. The magnitude of the peak current was higher but peak potential became lower with increasing concentration of NaOH.
Galvanostatic potential transients obtained form Ti at 0.5 ㎃/㎠ in different concentrations of NaOH. The potential under the anodic current increased with time in the initial stage and then reached a steady-state value in all the concentrations of NaOH. The rate of initial increase and steady-stated value of potential was lowered with increasing concentration of NaOH, which suggest that the dissolution of Ti metal through the anodic oxide film is easier in more concentrated NaOH solutions. Surface morphological observation revealed that the dissolution of Ti is enhanced by the increase of NaOH concentration.