What is Titanium Anodizing and How Does it Work

The most prominent characteristics of titanium and titanium alloys are low density and high specific strength. However, titanium and its alloys are relatively susceptible to hydrogen and hot salt stress corrosion, and are prone to galvanic corrosion when paired with other materials. In the past 30 years, many countries have intensified research on surface treatment technologies for titanium alloys. Among various surface treatment techniques, anodizing is widely used in industrial production due to its simple process, the excellent wear resistance of the resulting oxide film, and its strong resistance to contact corrosion and hydrogen embrittlement. Furthermore, after anodic oxidation, a transparent oxide film forms on the titanium alloy surface. By controlling the electrolytic conditions, the film thickness can be controlled, and due to the interference of light, this can present a variety of brilliant colors.

Titanium Anodizing Process

The process flow for the anodic oxidation coloring of titanium alloy plates is as follows:

Step 1: Ultrasonic cleaning in petroleum ether → Step 2: Pickling in 5% HF aqueous solution (I) → Step 3: Water rinsing → Step 4: Pickling in 1% HF + 10% H₂O₂ + 3% HNO₃ aqueous solution (II) → Step 5: Water rinsing → Step 6: Anodic oxidation → Step 7: Water rinsing → Step 8: Post-treatment → Step 9: Drying.

This process was designed based on a large number of preliminary experimental results. Specifically, ultrasonic cleaning in petroleum ether is for removing grease from the material surface; pickling (I) is for reducing surface roughness, and controlling the pickling time can achieve a good fine-finished surface; pickling (II) is for increasing the surface activity of the titanium material to promote the anodic oxidation reaction. During pickling, the pickling solution must be stirred to ensure the reaction proceeds uniformly. Post-treatment can significantly enhance the oxide film’s anti-fouling capability, color stability, and wear resistance.

How Does Titanium Anodizing Work?

The process used an aluminum plate as the cathode and the titanium alloy plate as the anode, with the cathode area being the same as the anode sample area. Current density was controlled through voltage, with the voltage ranging from 10～45V. As the electrification time was prolonged, the surface color changed. After exceeding a certain period, the oxide film thickness essentially remained unchanged, and the color stabilized due to passivation. The anodic oxidation treatment time was set to 10 min. The electrolyte was a 1% phosphoric acid and 2.5% sulfuric acid aqueous solution. To improve the uniformity of the sample surface color, gas must be introduced into the electrolyte and thoroughly stirred to ensure a homogeneous mixture.

Titanium alloy plates treated by anodic oxidation for 10 min at different voltages showed different surface colors. The specific correspondence is as follows:

Voltage, V	10	15	25	45
Color	Yellow	Purple	Blue	Golden yellow

A transparent, insulating oxide film forms on the surface of titanium and titanium alloys after oxidation. According to the principle of light interference, the primary reflected light wave from the film’s upper surface interferes with the secondary reflected light wave from the film’s lower surface, thereby forming interference colors. Different oxide film thicknesses result in different interference colors. During the anodic oxidation process, when the composition and concentration of the electrolyte change within a certain range, it only alters the oxide film’s pore size, with minimal effect on the stable film thickness, and the color remains essentially unchanged. The most significant factor affecting film thickness is current density, which is usually controlled by adjusting the voltage in practical applications. As the voltage increases, the stabilized film thickness increases linearly. Due to the difference in light interference effects, the surface presents different colors.

A surface treated at low voltage is unstable when subsequently treated at high voltage, and the color will continue to change. Conversely, a surface treated at high voltage remains stable when subsequently treated at low voltage, and the color is essentially unchanged. This result provides an approach for fabricating complex, multi-color patterns on the surface of a single sample.

Properties of Titanmium Anodizing

Titanium alloys are suitable for anodizing, which carries out many advantages such as superior film properties and rich functional expandability. These features make titanium alloy components broadly applicable in many industries.

Exceptional Chemical Stability

The oxide film created on titanium alloy surfaces through anodic oxidation (anodizing) exhibits exceptionally high chemical stability. Its corrosion resistance significantly surpasses that of the bulk titanium alloy, providing an effective barrier against degradation in various aggressive chemical environments. This robust protection substantially extends the material’s service life. In demanding conditions involving strong acids, strong bases, or corrosive ions, the oxide film acts as an impenetrable “shell,” ensuring the reliable integrity of the titanium alloy component.

Versatile Decorative Coloring

By meticulously controlling the process parameters, titanium alloy anodizing enables the achievement of a wide range of decorative coloring effects. The resulting oxide film can display vibrant colors, including gold, blue, and purple. This not only meets the aesthetic requirements across different industries but also enhances the visual appeal of the product. Such versatile decorative finishing expands the material’s potential applications in consumer goods, architectural elements, and jewelry.

Enhancement in Hardness and Wear Resistance

Anodic oxidation substantially increases the surface hardness of titanium alloys, typically achieving values between 300 ～ 800 HV. Concurrently, the oxide film demonstrates excellent wear resistance, effectively mitigating damage from external friction and abrasion. In practical engineering uses, anodized titanium alloy parts—such as critical mechanical components or tools—maintain a longer operational lifespan despite frequent sliding and impact, consequently reducing replacement costs and downtime.

Flexibility in Electrolyte Selection

The titanium anodic oxidation process offers flexibility in selecting electrolytes to meet specific application needs. Acidic electrolytes, such as sulfuric acid or oxalic acid, are utilized to form oxide films with tailored properties. Likewise, alkaline electrolytes, such as sodium hydroxide, can be employed to facilitate oxide film formation under different conditions. The choice of electrolyte profoundly influences the oxide film’s composition, structure, and performance characteristics, making the process highly adaptable to diverse industrial requirements.

Precision through Adjustable Parameters

Beyond the choice of electrolyte, the key anodic oxidation parameters—voltage, temperature, and time—are highly and precisely adjustable based on performance targets. Varying combinations of these parameters directly control the oxide film’s growth rate, final thickness, and resulting color. For instance, raising the voltage generally increases the film growth rate, though exceeding a critical threshold can compromise film quality. Precise control over temperature and time is crucial for accurately tailoring the oxide film’s thickness and physical properties to suit particular end-use specifications.