Electrochemical corrosion and impedance studies of Ti-30Zr- x Nb ( x = 7, 10, 13 at.%) alloy in simulated downhole environment

The microstructure and corrosion behavior of Ti-30Zr- x Nb ( x = 7, 10, 13 at.%) alloy for oil and gas downhole tools are studied. The microstructure of alloy is characterized by X-ray diffractometer (XRD), optical microscope (OM), and scanning electron microscope (SEM). The results show that with the increase of Nb content, the alloy changed from α″ phase and β phase to a single β phase, and the addition of Nb reduced the crystalline grain size. In addition, sub-grains are found in the original grains of TZN alloys, which may be due to the phase decomposition caused by the Nb content exceeding the solid solubility limit. The electrochemical behavior of the alloy is studied by measuring the open circuit potential (OCP), cyclic voltammetry curve (CV), Tafel curve (TF), and electrochemical impedance spectroscopy (EIS). Cyclic voltammetry curve and Tafel curve showed that the addition of Nb could stabilize the passivation film of Ti-Zr alloy and reduce the pitting sensitivity, among which Ti-30Zr-10Nb alloy has the best corrosion resistance, and its corrosion potential and current density are 0.36 (V) and 2.813 (× 10 −8 A cm 2 ) respectively. EIS results show that Nb can significantly improve the corrosion resistance of the alloy by optimizing the microstructure of the passivation film.