The Study of Aluminum and Mo/Al Etching in Phosphoric Acid-Nitric Acid Solution and Anodization Treatment of NiTi Alloy
Date Issued
2012
Date
2012
Author(s)
Wang, Yi-Chen
Abstract
This study examined the etching behaviors and electrochemical characteristics of aluminum in phosphoric acid-based etchants. The etchant used in the study was primarily composed of phosphoric acid and nitric acid. The surface analysis of the etched aluminum sample revealed the existence of Al2O3, AlO(OH), and PO43-. Other than the contribution of phosphate ions, the experimental results showed that hydrogen ions in the etchant influenced the etching rate greatly. By including nitric acid in the etchant, a synergetic effect on the etching rate was observed. Hence, this study proposes an etching mechanism in which the passivated film is dissolved by hydronium ions and phosphate ions, respectively. In the etchant, aluminum becomes passivated AlO(OH) and a dissolvable complex simultaneously. The adsorbed H(ads) produced on the Al surface is removed by nitric acid, which leads to acceleration in the etching rate. The kinetics model which was established according to the mechanism proposed in this study illustrated the changes in aluminum etching rates. To simplify the kinetics model, an equation of etching rate was used to explain the contributions of phosphoric acid and nitric acid in aluminum etching. In the study of electrochemical impedance measurements, an equivalent-circuit model with two capacitor elements has been established. The etching behavior of Mo/Al was also investigated in this study. The dramatic change in open circuit potential explained the different aspects for discrete etching durations.
In the research of modification of NiTi alloy, NiTi alloy was anodized in two different electrolytes NH4F(aq) and CH3COOH(aq). The formation of oxidant TiO2 was observed after anodization treatment. For the NiTi samples anodized in NH4F(aq), formation and dissolution of TiO2 took place at the same time. This phenomenon resulted in high roughness and the porous structure on the surface oxide layer. The XRD analysis showed the amorphous TiO2 on the surface. Under the anodizing voltage ranged from 2V to 20 V, the NiTi samples treated at 5 V had the lowest surface nickel content, the best corrosion resist property, the highest hardness, and the best level of wear resist. However, after 14-day immersion in NaCl(aq), the SEM images revealed serious pitting corrosion, the rough surface of the anodized NiTi at 5V caused extremely high nickel release. For the NiTi samples anodized in CH3COOH(aq), the surfaces became even and fine. The obvious increases in corrosion potential and decreases in anodic current all illustrated the excellent corrosion resist. The NiTi samples anodized at 20 V had the lowest roughness, the highest corrosion potential, and the lowest nickel release. Moreover, the modified NiTi samples at 20 V also had the highest hardness and the best ability of wear resist. Despite the high nickel content on the surface, the fine surface effectively prevented pitting corrosion to improve both corrosion resist ability and mechanical properties.
Subjects
Aluminum etching
Etching mechanism
Mo/Al etching
NiTi alloy anodization
Type
thesis
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