Long service life of engine tanks is necessary for development of aerospace technology. Commercially pure titanium TA2 is<br />an important structural material for rocket engines. However, there has been insufficient research on the mechanism of long-term<br />compatibility of TA2 with liquid propellants, making it impossible to assess the service life of propellant tanks or similar structure<br />materials. The electrochemical corrosion behavior of commercially pure titanium TA2 with typical liquid propellant DT-3 was studied<br />through potentiodynamic polarization tests and the electrochemical impedance spectrum(EIS). The structure of the passive film on the<br />surface of TA2 was characterized by scanning electron microscopy(SEM) and x-ray photoelectron spectroscopy(XPS). The results of<br />potentiodynamic polarization tests showed that the corrosion current density of TA2 in DT-3 was approximately 58 nA/cm2, which<br />was converted to a corrosion rate of approximately 0.5 mu m / a. There were two-stage potentials for passivation in the anodic area<br />before the pitting potential of 2.15 V. The surface morphology of the sample after polarization curve testing was observed using SEM.<br />When the principal components of hydrazine and hydrazine nitrate existed alone, the passive film did not rupture even if the electrode<br />potential reached 4 V. However, considerable pitting corrosion occurred when they coexisted above 2.15 V. The SEM graphs showed<br />that small pits rapidly developed and connected to form large circular pits. The EIS results showed that the passive film of TA2 had<br />high electrochemical reaction impedance. With an increase in anode potential from -0.35 V to 2 V, the Nyquist diagram had only one<br />capacitance arc, indicating that the corrosion process had only one dominant electrochemical reaction. The components and structure<br />of TA2 oxide film in different oxidation conditions were characterized using the XPS Ar+ sputtering depth analysis method. The<br />results showed that the thickness and structure of the oxide film were different from those of the lower potential in the two-stage<br />potentials. The following conclusions can be drawn from this experimental study: (1) TA2 has a low self-corrosion current density high electrochemical reaction impedance, and low annual corrosion rate in DT-3. It has good resistance to uniform corrosion in DT-3.<br />(2) TA2 has a high pitting potential and a large difference between pitting potential and self-corrosion potential in DT-3, resulting in<br />strong pitting resistance. When the electrode potential reaches the pitting potential, hydrazine and hydrazine nitrate have a synergistic<br />effect on the dissolution process, causing the passive film to rupture, leading to local corrosion. Thus, with use of TA2 in liquid<br />propellant DT-3, an excessive local potential difference must be prevented. (3) As the electrode potential increases, the passivation<br />film thickens; however, the passivation mechanism of TA2 in DT-3 does not noticeably change. In the corrosion process, the valence<br />state of titanium is oxidized from zero to two, three, and ultimately four. The electrode potential affects the structure of the passive<br />film. The XPS analysis of the oxide film showed that a passive film formed under higher passivation potential became more perfect;<br />the passive film eventually became a double-layer structure characterized by an outer layer of TiO2 and an inner layer of Ti2O3 and TiO. The evolution of the passivation film structure of TA2 in liquid propellant DT-3 plays a crucial role in the surface treatment of<br />titanium alloys
