Surface treatment is mostly required during and after heat treatment to remove metal surface oxide scale and various pollutants, reduce the activity of bare metal surfaces, and also before and during the application of protective layers and various functional coatings on titanium and its alloys. The purpose of applying this coating is to improve the performance of the metal surface, such as preventing corrosion, oxidation, and wear.
The pickling conditions of titanium and its alloys depend on the types (characteristics) of the oxide layer and existing reaction layer, which are influenced by the high-temperature heating process and the increase in processing temperature (such as forging, casting, welding, etc.). At low processing temperatures or high heating temperatures below approximately 600X, only a thin oxide layer is generated. At high temperatures, an oxygen rich diffusion zone is formed near a certain oxide layer, which must also be removed by acid washing. Various methods for removing oxide scale can be used: mechanical methods for removing thick oxide layers and hard surface layers, methods for removing oxide scale in a molten salt bath, and methods for removing oxide scale through acid washing in an acid solution.
In many cases, a combination of several methods can be used, such as mechanical removal of oxide scale and subsequent pickling, Alternatively, a combination of salt bath and subsequent acid washing can be used to remove oxide scale. In the case of oxide and diffusion layers formed at higher temperatures, special methods should be used. However, when heated to 600X at high temperatures, most of the oxide layers formed can be dissolved by ordinary acid washing.
