Electrochemical Impedance and Polarization Corrosion Studies of Tantalum Surface Modified by DC Plasma Electrolytic Oxidation

Abstract

Tantalum has recently become an actively researched biomaterial for the bone reconstruction applications because of its excellent corrosion resistance and successful clinical records. However, a bare Ta surface is not capable of directly bonding to the bone upon implantation and requires some method of bioactivation. In this study, this was realized by direct current (DC) plasma electrolytic oxidation (PEO). Susceptibility to corrosion is a major factor determining the service-life of an implant. Therefore, herein, the corrosion resistance of the PEO coatings on Ta was investigated in Ringer’s solution. The coatings were formed by galvanostatic anodization up to 200, 300 and 400 V, after which the treatment was conducted potentiostatically until the total process time amounted to 5 min. Three solutions containing Ca(H2PO2)2, Ca(HCOO)2 and Mg(CH3COO)2 were used in the treatment. For the corrosion characterization, electrochemical impedance spectroscopy and potentiodynamic polarization techniques were chosen. The coatings showed the best corrosion resistance at voltages low enough so that the intensive sparking was absent, which resulted in the formation of thin films. The impedance data were fitted to the equivalent electrical circuits with two time constants, namely R(Q[R(QR)]) and R(Q[R(Q[RW])]). The inclusion of W in the circuit helped to fit the low-frequency part of the samples PEO-ed at 400 V, hinting at the important role of diffusion in the corrosion resistance of the PEO coatings described in the research.

Effect of DC Plasma Electrolytic Oxidation on Surface Characteristics and Corrosion Resistance of Zirconium

“…Generally, it was higher in the electrolytes having higher ionic strength. Similar observations were noted for the PEO of Ta in the comparable electrolytes [ 57 ]. However, it was found that the addition of magnesium acetate to the first solution led to the higher rates of voltage increase than those that were observed after the addition of calcium formate (14.1 and 16.0 V/s, respectively).…”

“…The authors of the research have already conducted some studies on DC PEO of zirconium in silicate solutions [ 54 , 55 ]. This paper is a continuation of the two previous reports concerning PEO of niobium [ 56 ] and tantalum [ 57 ] that were carried out in the electrolytes that contained calcium hypophosphite, calcium formate, and magnesium acetate. Ha et al [ 46 ] also treated Zr under DC conditions in the presence of Mg(CH 3 COO) 2 , however, their investigations were limited to the surface characterization of the coatings, and their subsequent chemical modification in H 2 SO 4 and NaOH.…”

“…While in the case of the samples after PEO at 400 V, it was found that the presence of the Ca and Mg salts in the electrolytes was more effective at increasing the surface roughness of zirconium than their concentration. It was found that at this U L , in addition to the surface pores, oblong strut-like bulges contributed to the roughness of the PEO coatings that were formed from the Ca2P1 and (CaMg)2P1 solutions, which were also observed in the case of Ta and Nb specimens [ 56 , 57 ].…”

“…PEO of the Zr specimens was realized in a two-step DC method, as described earlier for Nb and Ta [ 56 , 57 ]. The method consisted of an initial galvanostatic stage (constant current density; i = 75 mA/cm 2 ), which was performed until one of the three limiting anodization voltages was reached ( U L = 200, 300 or 400 V).…”

“…At this point, the samples were washed in tap water and then in deionized water, which was followed by drying in the stream of warm air. The second step of the method was applied to improve the tightness of the oxide coatings and to minimize the imperfections that were obtained at the peak voltage of the previous step [ 56 , 57 , 58 ]. The PEO process was performed in one of three studied aqueous electrolytes: Ca1P2: 0.5 M Ca(H 2 PO 2 ) 2 solution; Ca/P molar ratio = 0.5; Ca2P1: 0.1 M Ca(H 2 PO 2 ) 2 + 0.3 M Ca(HCOO) 2 solution; Ca/P molar ratio = 2.0; and, (CaMg)2P1: 0.5 M Ca(H 2 PO 2 ) 2 + 1.5 M Mg(CH 3 COO) 2 solution; (Ca+Mg)/P molar ratio = 2.0.…”