Titanium is a material with both high strength and low density, which has been used in aerospace, petrochemical and medical fields. Titanium has excellent corrosion resistance in most aqueous solutions, because a few nanometers thick passivation film will naturally form on the surface when oxygen is present. Like all other valve metals, this natural oxide film can be grown thick by anodization to form a thin (interference color) film or a thick porous film layer. Depending on the applied oxidation conditions (such as electrolyte type, concentration and electrolysis voltage, etc.), a dense or porous, amorphous or crystalline state, or thin film arranged in nanotubes can be generated. In the process of titanium anodization, due to the effects of thermal energy and electric field, ions migrate and diffuse in the film to grow oxides. The film layer grows between the metal/oxide and oxide/electrolyte interfaces. The cations mainly migrate in TiO2. As the current density increases, the migration amount (the growth ratio at the oxide/electrolyte interface and the entire oxide layer) reaches close to 0.5. Defects that occur in the film layer due to crystallization and formation result in high internal stresses, because high electric fields cause electrostriction. Research suggests that the stability of the oxide amorphous layer formed on the surface of TiSi and TiAl in borate or phosphoric acid solution is attributed to the electrolyte content and alloy elements in the matrix. At present, there is still lack of information on the influence of electrolyte content on the crystallization of the anodic oxide film on the titanium surface.
Research shows that two kinds of solutions of 95%~97% sulfuric acid and 85% phosphoric acid are used as electrolytes to perform anodization of pure titanium tape (1 mm thick). Raman spectroscopy and transmission electron microscopy were used to analyze the microstructure of the film; glow discharge light emission spectroscopy was used to analyze the distribution of chemical elements along the depth of the film. In the previous work, we have studied the anodized film formed in these two electrolytes, and conducted a comparative study on their growth and chemical composition. This work proposes a growth model based on the microstructure of the anodized film grown early, with the purpose of clarifying the relationship between the content from the electrolyte and the stable crystallization of the titanium oxide amorphous. The results show that during the growth of the oxide layer in both the inward and outward directions, the electrolyte content enters the oxide layer. Most of these inclusions are sulfates and phosphates, which are related to the initial crystallization inside the oxide layer and have an influence on the amorphous stability of the titanium oxide. At an anode oxidation voltage of 20 V, the sulfur-containing contents of the sulfuric acid electrolyte migrate to the amorphous portion of the stable oxide near the metal/oxide interface, while the middle and outer layers of the film show an anatase phase; Phosphorus-containing inclusions in the electrolyte almost play a role in the stability of the entire layer of oxide amorphous, only a very small amount of nanocrystals are generated in areas with extremely low P content, such as the oxide/metal interface; In this electrolyte, nano-sized oxidized bubbles were observed in the middle of the formed oxide. In the previous literature, there have been no reports of the formation of oxygen bubbles in the titanium anodic oxide even in areas where no crystal exists.
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