Pitting Corrosion Behavior of Zirconium in a Simulated Body Fluid Containing Chloride Ions
Authors: Tsukada Junichi , Tsutsumi Yusuke , Ashida Maki , Chen Peng , Doi Hisashi , Sugawara Yu , Muto Izumi , Hara Nobuyoshi , Hanawa Takao 
Published: February 2020

[Introduction]
Zr has similar properties to those of Ti that is widely used as metallic biomaterials. On the other hand, Zr shows much lower magnetic susceptibility than Ti. In addition, Zr does not form calcium phosphate on its surface in body fluid, so that new bone formation is not accelerated unlike Ti. These unique properties of Zr are considered as advantages in magnetic resonance imaging during implantation and removal surgery after healing. Thus, Zr is expected as a new implant material whichcan substitute some medical devices consisting of Ti. For metallic biomaterials, corrosion resistance is always necessary because metal ions may cause toxicity on living body. Nevertheless Zr is known to show excellent corrosion resistance, it also shows localized corrosion sensitivity by chloride ion while Ti does not corrode in the same environment. However, there is extremely limited information about corrosion reaction of Zr in chloride-present conditions. The corrosion behavior of Zr in the chloride solution must be elucidated for the application of Zr to implant devices because chloride ion is major component of body fluid.
Thus, in this study, the pitting corrosion reaction has been investigated in a simulated boy fluid. To clarify the initiation point of the pitting corrosion reaction on Zr, the combination of the microscopic observation and micro-area electrochemical measurement were performed. The effect of heat treatments on the pitting potential of Zr was also investigated.
[Materials and Methods]
Pure Zr (99.2%, Nilaco Corp., Japan) was employed in this study. In addition to the as-received specimen, the heat-treated specimens with the solution treatment and quenching (STQ) and the solution treatment and annealing (STA) were also prepared. The specimen surfaces were mechanically ground with #800 grit SiC abrasive paper. Some of the specimens were additionally polished to expose mirror surface with 0.04 mm SiO2 paste. Surface observations were performed using a laser microscope (OLS4000, Olympus Corp., Japan) and a scanning electron microscope with energy-dispersive X-ray spectrometer (SEM/EDS, S-3400NX, Hitachi High-Technologies Corp., Japan) prior to the electrochemical measurement. Potentiodynamic polarization was performed in physiological saline (0.9mass% NaCl aq.). Ag/AgCl and Pt were used as reference and counter electrodes, respectively. After exposure to the solution, linearly-increasing anodic potential with the scanning rate of 1 mV min-1 was applied. The pitting potential (E
pit) was determined from the polarization curve.
[Results and Discussion]
The E
pit and corrosion potential (E
corr) of each specimen are shown in Fig. 1. The E
pit of the as-received Zr in the saline with standard measurement area (0.35 cm2) condition ranged to be 0.9-1.4 VSCE. The E
pit of the Zr was much higher than that of stainless steel used for implant devices (type 316L), however, it was widely scattered. It was almost no difference in the E
pit among the as-received and the heat-treated specimens at this experimental condition. When the measurement area was limited to be less than 0.001 cm2, the E
pit of the as-received Zr raised and the E
pit invariably occurred at very narrow potential range between 2.0 and 2.1 VSCE. From the results of the microscopic analyses, there were a number of small inclusions containing 5 to 20% Fe exposed on the specimen surface. We found that one of these inclusions acted as an initiation site of the pitting at the potential range between 2.0 and 2.1 VSCE. These Zr/Fe inclusions were disappeared after STQ, and the E
pit of this Zr specimen was higher than 2.1 VSCE. From these results, we confirmed that the corrosion resistance of Zr is hindered by Fe as an impurity element. On the other hand, when the electrochemical measurement was performed with the macroscopic area (15.2 cm2), the pitting corrosion often occurred at the lowest potential around 0.4 VSCE. This means that the most critical defect must be still hidden and it rarely expose to the Zr surface unless the testing is performed on much larger area than that of general condition. To reveal this rare defect which totally determines the corrosion resistance of Zr, further experiments and more-detailed microscopic analyses are necessary.

Figure 1