In the production of titanium and titanium alloy materials, removing the oxide scale is a crucial step, and pickling is the main method to achieve this. Before pickling, titanium materials usually need pre-treatment, such as molten alkali cleaning, mechanical descaling, or sandblasting, to remove most of the surface dirt and thicker oxide layers, creating favorable conditions for subsequent pickling.
For the slight titanium oxide film formed below 600°C, pickling is a very effective way to remove it. During pickling, titanium reacts chemically with the acid, forming metal ions and thereby removing the oxide layer. However, titanium pickling is not just a simple chemical reaction-it also involves electrochemical phenomena.
When pickling in mixed acids containing hydrofluoric acid or fluorides, special attention needs to be paid to the reduction reaction of hydrogen ions. These hydrogen ions can potentially be reduced to atomic hydrogen or molecular hydrogen. Molecular hydrogen tends to diffuse into the metal, and once it enters, it can cause hydrogen embrittlement, severely affecting the performance and service life of titanium.
Therefore, when pickling titanium, it is essential to strictly control the composition and ratio of the acid solution to prevent hydrogen absorption. Specifically, the ratio of nitric acid to hydrofluoric acid should always be maintained above 5. Controlling this ratio is crucial to preventing hydrogen embrittlement. At the same time, the nitric acid content should not be too high, since excessive nitric acid slows the reaction, decreases production efficiency, and generates a lot of fumes, which makes the process more difficult.
Additionally, other process parameters such as temperature and duration must be monitored during pickling. Too high a temperature or too long a time can excessively corrode the titanium, affecting surface quality. Therefore, in practice, pickling parameters should be reasonably adjusted based on the titanium material, thickness, and the severity of the oxide layer to ensure optimal pickling results.
