Eiji Kawada

Corrosion mechanism of Ti–Cr alloys in solution containing fluoride

OBJECTIVE The objective of this study was to clarify the influence of chromium content on surface reaction of Ti-Cr alloys in an acidic fluoride-containing saline solution. METHODS Four Ti-Cr alloys containing 5, 10, 15 or 20 mass% chromium were characterized in terms of dissolution of metals in an acidic fluoride-containing saline solution and surface structure by X-ray photoelectron spectroscopy and Auger electron spectroscopy. RESULTS Total amount of metals dissolved from each alloy decreased with increase in chromium content. The surface oxide films of Ti-Cr alloys before and after immersion in an acidic fluoride-containing saline solution consisted of titanic and chromic species, such as oxide, hydroxide, and hydrate. The [Cr]/([Ti]+[Cr]) ratio in the surface oxide film on as-polished Ti-Cr alloys was closely correlated with chromium content. However, the ratio in any alloy approximately doubled after immersion. Although thick oxide films were observed after immersion, all alloys showed a thinner oxide film than commercially pure titanium. SIGNIFICANT In all alloys, concentration of chromic species such as oxide and hydroxide in the surface oxide film was associated with chromium content, and chromic species improved corrosion resistance to fluoride.

The influence of colored zirconia on the optical properties of all-ceramic restorations.

This study investigated the effects of different colored tetragonal zirconia polycrystal (TZP) core on the optical properties of TZP framework restorations. Three various colors of TZP discs (Katana Zirconia) 14 mm in diameter and 0.5 mm thickness were layered with 2 shades of veneering ceramics (shade A1 and A4: Cerabian ZR). These specimens were polished to approximately 1.5 mm. CIE L*a*b* coordinates, translucency (TP), and opalescence (OP) on the TZP restorations were evaluated. Consequently, TZP core color affected CIE L*a*b* values of TZP restorations however TP and OP did not significantly differ among the 3 core colors. Translucency and opalescence for colored TZP framework restorations were not influenced by the underlying TZP core color when veneering ceramics were layered to thicknesses of 1.0±0.1 mm.

Influence of sulfide concentration on the corrosion behavior of titanium in a simulated oral environment

This study assessed the corrosion behavior of titanium in response to sulfide by determining the effects of sulfide concentration and pH over immersion period. Corrosion was evaluated through changes in color, glossiness, surface characterization, and titanium release. Sulfide solutions were prepared in 3 different concentrations with Na2S, each in pH unadjusted (sulfide-alkaline) and pH adjusted to 7.5 (sulfide-neutral). Titanium discoloration increased and glossiness decreased as sulfide concentration and immersion period increased in sulfide-alkaline solutions. Coral-like complexes were observed on the surface of these specimens, which became more pronounced as concentration increased. Small amounts of titanium release were detected in sulfide-alkaline solutions; however, this was not affected by immersion periods. Corrosion was indicated through considerable surface oxidation suggesting the formation of a thick oxide layer. No significant changes in color and glossiness, or titanium release were indicated for titanium specimens immersed in sulfide-neutral solutions indicating that pH had a significant effect on corrosion. Our findings suggest that a thick oxide layer on the titanium surface was formed in sulfide-alkaline solutions due to excessive oxidation.

Suppression of fluoride‐induced corrosion of titanium by albumin in oral modified environment

Although fluoride induces corrosion in titanium in an oral environment, the influence of protein on this corrosion remains to be clarified. The objective of this study was to investigate suppression of fluoride-induced corrosion of titanium where albumin was either present in a solution or where albumin was preadsorbed on titanium. Titanium dissolution in titanium specimens and surface characterization of each specimen were determined. Dissolution in a saline solution containing both albumin and fluoride was less than that in only fluoride-containing saline solution. The titanium specimen was covered in an albumin film. The morphology of the titanium in the fluoride-containing saline solution revealed jagged edges, whereas titanium immersed in the saline solution containing albumin and fluoride showed a round and plate-like morphology. When albumin-adsorbed titanium was immersed in a fluoride-containing solution, dissolution within 6 h was less than that in non albumin-adsorbed titanium. Dissolution increased with desorption of adsorbed-albumin from the titanium surface. The results suggest that albumin in a solution suppresses dissolution of titanium compounds, thus influencing their morphology. Albumin adsorbed on titanium reduces fluoride attack and suppresses dissolution.

Effects of calcium hydroxide reagent on the bond strength of resin cements to root dentin and the retention force of FRC posts

The aim of this study was to determine the effects of calcium hydroxide (Ca(OH)2) treatment on bond strength of resin cements to root dentin and retention force of fiber-reinforced composite (FRC) posts. Bovine root dentin was endodontically prepared and treated with Ca(OH)2 for 7 days. Root dentin for bond strength test was adhered to resin-composite with resin cements. For pull-out test, posts consisting of FRC posts and resin-composites were fabricated and cemented to root. Shear bond and pull-out tests were performed using a universal testing machine. No significant differences in bond strength and post retention force were found between Ca(OH)2 treated and untreated groups. Significant differences were found among the cements. A positive correlation was indicated between bond strength of cements and retention force of FRC posts. In conclusion, Ca(OH)2 treatment on root dentin did not affect bond strength of resin cements and retention force of FRC posts.

Resin Bonding of Self-Etch Adhesives to Bovine Dentin Bleached from Pulp Chamber.

This study evaluated the microtensile bond strength (μTBS) of 1-step self-etch adhesives (1-SEAs) and 2-step self-etch adhesives (2-SEAs) to pulp chamber dentin immediately after bleaching with 2 types of common bleaching techniques. Pulp chamber dentin of bovine teeth was bleached using 30% hydrogen peroxide (H2O2) solution with quartz-tungsten-halogen light-curing unit (Group 1) and 3.5% H2O2-containing titanium dioxide (TiO2) (Pyrenees®) activated with 405-nm violet diode laser for 15 min (Group 2). Unbleached specimens were placed in distilled water for 15 min and used as controls. After treatment, dentin was bonded with resin composite using 1-SEA or 2-SEA and stored in water at 37°C for 24 h. Each specimen was sectioned and trimmed to an hourglass-shape and μTBS was measured. Fractured specimens were examined under a scanning electron microscope to determine fracture modes. All specimens in Group 1 failed before proper bonding tests. In Group 2, the μTBS of 2-SEA was significantly greater (with no failed specimens) than 1-SEA (where 21 out of 36 failed). These results indicate that 2-SEA is a better adhesive system than 1-SEA on bleached dentin. Our results also demonstrated that application of H2O2 significantly decreases bond strength of resin to dentin; however, in the case of nonvital tooth bleaching, Pyrenees® is a better alternative to the conventional 30% H2O2 bleaching.

Corrosion Resistance of Ti-29Nb-13Ta-4.6Zr Alloy in a Fluoride-Containing Solution

The objective of this study was to evaluate the corrosion behavior of Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) with immersion in an acidic saline solution containing fluoride by investigating change in color and the surface structure of the oxide film. With immersion in fluoride-containing solution, TNTZ showed a less marked change in color than commercially pure titanium (TI), and a smaller decrease in glossiness. The outermost surface was covered with oxides from its constituent elements at before and after immersion in solution with or without fluoride. When immersed in fluoride-containing solution, the film consisted of larger niobium and tantalum oxides than that before or after immersion in solution without fluoride. In summary, TNTZ showed superior resistance to discoloration to TI after immersion in fluoride-containing solution. The results suggest that the subsequent increase in niobium and tantalum fractions in the oxide film in TNTZ improves resistance to corrosion.

Surface Reaction of Titanium Oxide Film to Fluoride and Hydrogen Peroxide in Simulated Oral Environment

When titanium and titanium alloys are used in prosthodontic devices, exposure to the oral environment may result in discoloration or corrosion. This phenomenon may be due to fluoride, found in prophylactic agents, or peroxide, which is either produced by inflammatory cells or present in denture cleaning agents. The aims of this study were to clarify the process of discoloration or corrosion of titanium in an oral environment, and investigate the reaction of surface titanium oxide to fluoride and hydrogen peroxide by analysis of electrochemical behavior and X-ray photoelectron spectroscopy. Surface oxide film on commercially pure titanium (CP-Ti) in fluoride-containing solution was dissolved and corroded. In the peroxide-containing solution, a gradual development and thickening of the surface oxide film with subsequent discoloration of the titanium was observed