Study On The Corrosion Resistance Of Titanium Alloy Oil Well Pipe
The essence of titanium alloy corrosion resistance is that titanium is a thermodynamically unstable element with a standard electrode potential of only -1.63 V (standard hydrogen electrode HSE). Therefore, titanium and titanium alloys are very easy to form a continuous, dense and very thin surface oxide film in air or even water, which is composed of an inner layer of Ti2O3 and an outer layer of TiO2, and it continues to thicken as the redox reaction proceeds. The oxide film covering the surface of the titanium alloy hinders the reaction charge transfer and reduces or inhibits the dissolution of the titanium alloy in the corrosive medium, resulting in passivation.
However, titanium alloy has a higher positive potential than other alloys. When coupled with different alloys, titanium alloy is protected as a cathode, which accelerates the corrosion of the coupled metal and can lead to structural damage. Therefore, domestic and foreign scholars have also conducted certain research on the corrosion resistance of titanium alloy in drill pipes and oil casing.
1. Titanium alloy drill pipe
Peng et al. evaluated the fatigue performance of titanium alloy drill pipes. The results showed that in air, with the increase of steel grade, the fatigue life of the drill pipe will be extended, while in drilling mud, the fatigue performance of titanium alloy drill pipe is the best. Figure 3a shows the fatigue curves of different drill pipe samples under room temperature H2S mud. The presence of H2S mud will greatly reduce the fatigue life of each drill pipe sample, indicating that the drill pipe has a high sensitivity to H2S mud. In the H2S mud environment, the fatigue life of titanium alloy drill pipe is significantly higher than that of steel drill pipes such as G105, S135 and V150. Figure 3b plots the S-N curves of different drill pipes in H2S mud at 100 °C. Compared with room temperature air, the fatigue life of G105, S135, V150 and Ti samples is significantly reduced. The coupling factor of H2S mud and temperature has a greater impact on the fatigue life of drill pipe than a single factor. Under this coupling condition, the fatigue life of titanium drill pipe still has a greater advantage than other drill pipes.
Fig.3 Fatigue curves of G105, S135, V150 and Ti drill pipe specimens under different working conditions
Chen et al. used a new surface treatment micro-arc oxidation technology to add different concentrations of sodium tungstate to the oxidation solution to perform micro-arc oxidation on the surface of TC4 titanium alloy drill pipe. Studies have shown that tungsten doping can effectively improve the hardness and corrosion resistance of TC4 titanium alloy drill pipe. And when the concentration of sodium tungstate is 3 g/L, the comprehensive performance of the micro-arc oxidation layer on the titanium alloy drill pipe is the best.
In summary, the corrosion fatigue life of titanium alloy drill pipe in high temperature and high sulfur environment is better than that of steel drill pipe, and the surface treatment of TC4 titanium alloy can effectively improve the hardness and corrosion resistance of the drill pipe. However, there are still few studies on improving the corrosion resistance of titanium alloy drill pipe by surface treatment, which also provides a direction for future research.
2. Titanium alloy oil casing
Wang et al. studied the titanium alloy material TC4 that can be used as oil casing. They found that in an acidic corrosion environment, there is local electrochemical corrosion on the surface of TC4 alloy, mainly pitting corrosion. In the completion fluid containing CO2, the corrosion degree of TC4 alloy is more serious, but the corrosion resistance is better in the formation water containing CO2. In the above two CO2-containing corrosive media, TC4 alloy has excellent resistance to stress corrosion cracking. Compared with the terrestrial environment, TC4 alloy is more sensitive to stress corrosion cracking in the deep sea environment.
At the same time, Wang et al. also studied the corrosion resistance mechanism of TC4 titanium alloy under different stress loading conditions and found that pits appeared on the surface of the sample loaded with elastic stress, but the degree of pitting was relatively light, and the surface film layer showed n-type semiconductor properties and had cation selective permeability. When the surface pits of the sample subjected to plastic stress were deeper and wider, and the semiconductor type of the surface film layer was transformed into p-type, anions such as Cl- and CO32- were more easily adsorbed and destroyed the protective film, and contacted with the substrate through the protective film, resulting in a decrease in the corrosion resistance of TC4 titanium alloy.
At present, the working conditions of non-conventional oil and gas fields are harsh. High temperature will reduce the yield strength and elastic modulus of tubing and casing, and high pressure will increase the pressure of tubing and casing. Under the action of H2S, CO2 and Cl- alone or together, the corrosion of tubing and casing is becoming more and more serious. Titanium alloy tubing and casing can effectively solve the problem of downhole corrosion failure, but the current research on the corrosion resistance of titanium alloy tubing and casing is still incomplete and needs further research.
3. Titanium alloy oil well pipes
Schutz et al. compared the corrosion resistance of UNS R55400 alloy pipe strings with other oilfield titanium alloy pipe strings. The laboratory corrosion test data of UNS R55400 pipeline development showed that the titanium alloy had improved resistance to SSC and local pitting and crevice corrosion in highly acidic and non-acidic chloride-rich water environments related to the oilfield industry.
Table 2 shows the approximate environmental service limits of different types of titanium alloys in different oilfield environments. It can be seen that UNS R55400 and UNS R56404 titanium alloys have the best performance in acidic and non-acidic chloride-rich water environments, and the highest strength is UNS R58640 beta titanium alloy.
Wei et al. studied the effect of annealing temperature on the microstructure evolution and corrosion behavior of Ti-Mo titanium alloy in hydrochloric acid. They found that when the annealing temperature exceeded 850 °C, the MoO3 and TiO2 passivation films formed on the surface of the titanium alloy accelerated dissolution, the corrosion rate increased, and α-phase and β-phase microgalvanic cells were formed. In addition, the passivation film shows n-type semiconductor properties that are independent of annealing temperature.
Through the above research results, it is found that annealing temperature, high acid, and non-acidic chloride-rich water environment will affect the corrosion resistance of titanium alloys. This conclusion has guiding significance for the optimization of titanium alloy materials in the future.
