Composition Design And Corrosion Resistance Performance Study Of A New Corrosion-resistant Titanium Alloy For Oil Well Pipes

In order to increase the production of sedimentary oil fields in lakes and rivers in China,it is necessary to dissolve rocks through acid solution to expand the permeability channels of oil reservoir rocks,which puts forward stricter requirements for oil well pipe materials.Titanium and titanium alloys have low density,high specific strength and good corrosion resistance,which can meet the needs of oil and gas exploitation under extreme conditions.However,the high price of titanium alloy for oil well pipe in China and the performance can’t fully meet the needs of oil and gas conditions,which seriously restricts the large-scale application of titanium alloy oil well pipe in China.Therefore,the design and development of low-cost titanium alloys with corrosion resistance,high pressure,strong acid and other extreme environments is one of the key issues for the large-scale application of titanium alloy oil well pipes in China.In this paper,the effects of common elements（Mo,Al,Zr,Cr,V,Nb,Ni,Cu）in eight kinds of titanium alloys on the corrosion resistance of titanium alloys were studied.A new type of titanium alloy Ti-5542（Ti-4.6Al-5.4Nb-3.9Zr-1.5Cr）was designed by combining strength theory and crystal structure design method.The effects of different heat treatment processes on the corrosion resistance of Ti-5542 alloy were studied.The main contents and results are as follows:Through the analysis of OCP diagram,electrochemical impedance spectroscopy and polarization curve,it can be seen that the addition of Mo,Al,Zr,Cr,V,Nb,Ni and Cu elements can reduce the corrosion rate of TA1 alloy and improve the corrosion resistance of TA1 alloy.Based on the electrochemical corrosion experimental data of Ti-X（Mo,Al,Zr,Cr,V,Nb,Ni,Cu）binary titanium alloy,combined with the existing strength theory and crystal structure design method,the specific addition amount of the new titanium alloy Ti-5542 was determined to be W_Al=4.6%,W_Nb=5.4%,W_Zr=3.9%,W_Cr=1.5%,and the remainder Ti.Electrochemical experiments and corrosion weight loss experiments were carried out on Ti-5542 and TC4 alloys.It was found that compared with TC4,Ti-5542 alloy had higher OCP value,self-corrosion potential,polarization resistance and lower corrosion current density,and the corrosion rate was reduced by about 14%,which indicated that Ti-5542 had better corrosion resistance than TC4 alloy.In addition,the tensile strength was increased by 62 MPa.The Ti-5542 alloy annealed at 850°C for 2 h has the best corrosion resistance.Through the analysis of corrosion products,it can be seen that the reason for the rupture of Ti-5542 natural oxide film in HCl solution is the dissolution of Al₂O₃ and Ti O₂.The microstructure of Ti-5542alloy before and after corrosion was observed.With the increase of annealing temperature,the microstructure of Ti-5542 alloy changes from equiaxedαstructure to lamellarαstructure,which indicates that the corrosion resistance of equiaxedαstructure is better than that of lamellarαstructure.Through the analysis of the corrosion mechanism of Ti-5542 alloy,it is found that Al₂O₃ is concentrated on the surface ofαphase,which leads to the preferential corrosion ofαphase,and then the formation of micro-galvanic effect betweenαandβphases further accelerates the corrosion ofαphase.