I. Pre-treatment: The core objective of the key pre-treatment process for ensuring the adhesion of the coating layer is to thoroughly remove oil stains, oxide scales, and impurities from the surface of the titanium alloy, ensuring a clean and uniform base surface for subsequent film formation. This stage includes three key steps: degreasing, acid washing, and water washing. The quality of pre-treatment directly affects the density and adhesion of the oxide film, and is an indispensable part of the process. 1. Degreasing - Alkaline degreasing: Use a mixed solution of sodium hydroxide and sodium carbonate (concentration 5% - 10%), soaking at 50 - 80℃ for 10 - 20 minutes. This method is suitable for workpieces with light oil stains and has low cost and simple operation. Organic solvent ultrasonic degreasing: Using acetone or ethanol as the medium, ultrasonic cleaning for 5 - 10 minutes is suitable for workpieces with heavy oil stains or complex structures. The cavitation effect of ultrasonic can penetrate micro-pores and crevices, improving cleaning efficiency. Titanium House reported that an aerospace component enterprise optimized the ultrasonic degreasing parameters, increasing the compliance rate of cleaning for complex structure workpieces to 98%. 2. Acid washing - Use a mixed acid solution of hydrofluoric acid and nitric acid (volume ratio approximately 1:3 - 1:5) at room temperature for 1 - 5 minutes. Hydrofluoric acid can quickly dissolve the oxide scales on the surface, while nitric acid inhibits excessive corrosion of the base by the acid solution until the surface of the workpiece presents a uniform silver-gray metallic luster. The acid washing time must be strictly controlled to avoid increased surface roughness due to over-corrosion. The stability of the acid washing process is crucial for the uniformity of the film layer. A chemical equipment manufacturer reduced the acid washing defect rate to below 0.5% by introducing an online concentration monitoring system. 3. Water washing - After degreasing and acid washing, rinse with tap water and deionized water successively to avoid contamination of the subsequent electrolyte by residual chemicals. Inadequate water washing may result in uneven film formation or film layer defects. We have improved the water washing efficiency by adopting multi-stage counter-flow rinsing technology, increasing the efficiency by 30% while reducing water consumption.
II. Anodic Oxidation: The core coating process of anodic oxidation achieves precise control over the thickness, color, and performance of the coating layer by regulating the composition of the electrolyte and process parameters. This stage consists of three parts: clamping and circuit connection, electrolyte selection, and process parameter control. The refined control of the anodic oxidation process is the key to enhancing the performance of the coating layer. 1. Clamping and Circuit Connection: The cleaned titanium alloy workpiece is used as the anode, and stainless steel plates or graphite plates (with a cathode area typically 1.5 to 2 times that of the anode) are selected as the cathode. The two electrodes are placed parallelly with a spacing of 10 to 30 centimeters. A reasonable clamping method ensures uniform current distribution and avoids local overheating or uneven coating. An automotive component manufacturer optimized the design of the clamping fixture, increasing the uniformity of current density for large workpieces to over 95%. 2. Electrolyte Selection: Based on application requirements, select the type of electrolyte: Sulfuric acid type: 10% to 20% sulfuric acid water solution, which forms a fast-coating layer without color or transparency, suitable for corrosion-resistant and insulating components. Oxalic acid type: 2% to 5% oxalic acid water solution, which can change the color of the coating layer (golden yellow → blue → green → purple) through voltage regulation, suitable for decorative or aerospace components. Phosphoric-chromium acid type: The coating layer has excellent corrosion resistance and is suitable for harsh environments such as marine and chemical industries. Minor adjustments to the electrolyte composition can significantly affect the performance of the coating layer. For example, an ocean equipment manufacturer increased the salt spray corrosion resistance time of the coating layer to over 2000 hours by adding trace amounts of rare earth elements to the phosphoric-chromium acid electrolyte. 3. Process Parameter Control: Voltage: adjustable from 5 to 100V. Low voltage (such as 10 to 20V) is suitable for thin-layer coating, while high voltage (such as 60 to 100V) can increase the coating layer thickness but requires prevention of ablation. Temperature: 10 to 35℃. Excessive temperature will accelerate the dissolution of the coating layer, while too low a temperature will result in slow coating speed and uneven coating. Time: 10 to 60 minutes. Excessive time is likely to cause the coating layer to crack, and the thickness growth will tend to saturate. Current density: 0.5 to 2A/dm². Excessive current density is likely to cause local overheating and ablation, and it is necessary to adjust the temperature control simultaneously to avoid ablation. In the oxalic acid electrolyte, increasing the current density from 1A/dm² to 1.5A/dm² can increase the growth rate of the coating layer by 40%, but it is necessary to optimize the temperature control simultaneously to avoid ablation.
III. Post-treatment: Enhancement of Film Layer Performance The post-treatment process for improving the performance of the film layer involves filling the pores of the oxide film to enhance corrosion resistance and wear resistance, and prevent discoloration of the film layer. This stage consists of three steps: water washing, sealing treatment, and drying. Post-treatment is the final hurdle for enhancing the practical performance of the film layer. 1. After the oxidation process is completed, immediately rinse the surface of the workpiece with deionized water to remove any residual electrolyte and prevent it from causing corrosion to the film layer. 2. Sealing treatment: Immersing the workpiece in 90-100°C deionized water for 10-20 minutes to utilize the hydration reaction to fill the pores. This method is simple to operate and has a low cost. Salt solution sealing: Using a sealing solution containing nickel salt or cobalt salt can further enhance the sealing effect and is suitable for components with high corrosion resistance requirements. An electronic component manufacturer discovered through comparative experiments that the salt solution sealing treatment can reduce the contact angle of the film layer to below 15°, significantly improving hydrophilicity and meeting specific application requirements. 3. Drying: Place the sealed workpiece in a 60-80°C oven for drying for 10-15 minutes, or allow it to dry naturally at room temperature. It is necessary to avoid residual moisture causing the film layer to turn yellow and prevent high temperatures from causing stress cracking of the film layer. A precision instrument manufacturer reduced the film layer cracking rate to below 0.1% by adopting a low-temperature gradient drying process. Conclusion: The titanium alloy anodizing process can significantly improve the surface performance of the material through systematic pre-treatment, precise anodizing control, and scientific post-treatment, meeting the application requirements of different fields. In actual production, process parameters need to be optimized based on the material, shape, and usage environment of the workpiece to ensure stable and reliable film layer quality. With the advancement of materials science and electrochemical technology, the titanium alloy anodizing process will develop towards higher precision, lower cost, and more environmental friendliness, providing stronger technical support for the high-end manufacturing field.