GR5 titanium alloy has a series of advantages such as excellent corrosion resistance, small density, high specific strength, good toughness and weldability, and has been successfully applied in aerospace, petrochemical, shipbuilding, automotive and pharmaceutical sectors. During the thermoforming process of GR5 titanium alloy, a layer of scale is formed on the surface. The temperature of the thermoforming is different, and the color of the scale obtained is different. Among them, the scale is blue when the thermoforming temperature is about 600℃; the scale is reddish gray when the thermoforming temperature is about 850℃; and the scale is gray when the thermoforming temperature is 900℃. As the temperature increases, the difficulty of removing oxide scales gradually increases. At home and abroad, the removal of titanium alloy oxide scales mostly adopts a two-step method, that is, molten alkaline cleaning and then acid cleaning; however, the temperature of the molten alkaline cleaning medium is high, which can reach about 450 ℃, and it is easy to cause semi-finished products of α+β and β titanium alloys. The premature aging strengthening of the steel will bring difficulties to further processing. In addition, hot alkali cleaning and then cleaning with nitric acid and hydrofluoric acid may also cause hydrogen embrittlement corrosion on the surface of the titanium alloy.
The researchers conducted a study on the process of oxide scales obtained at three temperatures of 590, 850, and 920°C, first washed with a low-temperature alkaline solution, and then pickled to remove the oxide layer on the surface. The morphology of the oxide scale was characterized by an optical microscope, the influence of time on the scale removal was studied, and the hydrogen mass fraction of the titanium alloy after the scale removal was carried out by the internationally advanced LECO RH-404 hydrogen analyzer Determination.
The lye used in the experiment is a mixed supersaturated solution of industrial pure NaOH and industrial pure NaNO2. The acid washing uses a mixed solution of hydrofluoric acid and nitric acid (volume ratio HF:HNO3:H2O=1:1:3). The GR5 titanium alloy after hot alkali washing and washing with water was completely immersed in the acid solution for 20s, then taken out, and then washed with tap water. Finally, the above sample was ultrasonically cleaned with acetone or alcohol, then dried and weighed, and the mass of the sample before and after hot alkali washing and acid washing was measured with an electronic balance, with an accuracy of 0.001 g. The analysis shows that the scale on the surface of the GR5 titanium alloy can be completely removed by the process of hot alkaline cleaning followed by pickling. conclusion as below:
(1) The thickness of the scale on the hot-formed surface of GR5 titanium alloy is less than 2μm.
(2) The scale on the surface of the GR5 titanium alloy can be completely removed through the process of hot alkaline washing and pickling. The ratio of alkaline solution is 700g/L NaOH + 250g/L NaNO2. The temperature when boiling is about 160℃. Supersaturated hot alkaline solution Soak for 6h to achieve the removal effect.
(3) The GR5 titanium alloy with oxide scale does not change much in the mass fraction of hydrogen through hot alkali washing and pickling, which meets the production needs.