Study On Typical Service Behaviors Of Surface Thermal Oxidation Modifed Ti6Al4V Alloy Used For Oil Casing Tube

Oil casing tube is an important foundation member of oil/gas well which occupies three quarters of the petroleum tube consumption.The oil casing tubes are prone to deterioration and failure that induced by corrosion and/or wear when they are operated under the complex and harsh work conditions in oil/gas wells,which have greatly threaten safe operation of the oil/gas wells.Using oil casing tube made of anticorrosion alloy has been considered as the most effective way to guarantee high efficient and safe production.However,anticorrosion alloy tubes usually contain expensive alloy elements,and this can inevitably increase the cost on operation and production.Due to their promising merits of high strength to weight ratio,excellent thermal stability,good corrosion resistance and exceptional biocompatibility,Ti6Al4 V alloy has been considered as the most frequently and successfully used titanium alloy.Ti6Al4V alloy has been used as the interior wall of Ti6Al4 V alloy-carbon steel composite oil casing tube in exploitation.In this structure,the carbon steel was used as the basic material of the composite tube to bear the pressure from the tube system because of its excellent mechanical performance and low price.Ti6Al4 V alloy has been used as the interior wall of Ti6Al4 V alloy-carbon steel composite oil casing tube owing to its outstanding advantages of high strength to weight ratio and excellent corrosion resistance.The Ti6Al4 V alloy-carbon steel composite oil casing tube which can significantly reduce the long-term operating costs of raw materials and tube has a wide application prospect.However,the directive usage of Ti6Al4 V as Ti6Al4 V alloy-carbon steel composite oil casing tube would have a strong impact on the safety and reliability of the structure by its downsides of low heat conductivity,high friction coefficient,low surface hardness/low surface bearing capacity,high sensitivity to adhesive wear and fretting wear.The mentioned issues are inevitable problems and must be taken into account.Taking the advantage of high chemical affinity between oxygen(O)and titanium(Ti),thermal oxidation(TO)is an ideal surface technique that can improve the wear and corrosion resistance of Ti6Al4 V.Its popularity is mainly assign to its simplicity and rapidity,cost effectiveness.This work focus on the engineering application background of Ti6Al4 V used as the interior wall of Ti6Al4V-carbon steel composite oil casing tube,TO technology was applied to conduct surface modification on Ti6Al4 V.The effects of process parameters of TO temperature and soaking time were systematically studied,aimed to get an optimal TO process.Typical service behaviors of erosive wear and corrosive wear behaviors of the obtained TO layer were estimated under the simulated oilfield conditions.This work is hoped to create database which can provide reference information for practical application of the TO process on Ti6Al4V-carbon steel composite oil casing tube.The main contributions are as follows:(1)According to the results of thickness values,wear and corrosion resistance of the obtained TO layers,the optimal TO process parameters for Ti6Al4 V were achieved by comprehensively evaluation.The optimal TO process parameters were obtained as follows: a TO temperature of 973 K and a holding time of 30 h.Under this TO condition,a received TO layer which reached a more than 12 ?m thickness was mainly built up by rutile TiO2.The TO layer revealed a surface hardness of 868 HV.The whole TO layer presented a multilayer structure characteristic and could be orderly divedided into four parts from the surface to the inside: a mixed crystal layer of ?-Al2O3 and rutile TiO2,a nearly pure rutile TiO2 layer,a hybrid layer of TixOy and a O-diffusion layer.(2)The XPS patterns of the 973 K-30 h TO layer were analyzed.The hardness distribution profile along the cross-sectional TO layer and dynamic mechanical property of the TO layer were measured.The Mott-Schottky curves of Ti6Al4 V alloy and TO layer in CO2-saturated simulated oilfield brine were obtained in order to investigate the semi-conductive property and reveal the corrosion resistant essence.The results showed that both the component and hardness presented gradient distribution characteristic,which were favorable to improve the surface bearing capacity and wear resistance of Ti6Al4 V alloy.Bonding strength test results showed that critical loads of dense aluminum-depleted oxide layer and the oxygen diffusion layer were about 6 N and 10 N obtained from scratch test.Mott-Schottky curve of Ti6Al4 V alloy could be fitted to two linear features,and the n-type semiconductor characteristic of Ti6Al4 V alloy was identified by the two positive slopes of the two fitted lines in its Mott-Schottky curve.However the Mott-Schottky curve of TO layer consisted of a fitted linear feature and a near linear relationship.Branch(I)revealed obvious linear feature with a positive slope,indicating its n-type semiconductor characteristic.The p-type semiconductor characteristic was presented in branch(II)according to its near linear relationship with a negative slope.Taking the advantage of bipolar semiconductor feature,p-n type semiconductive TO layer which could prevent migration of cations from the matrix and meanwhile could inhibit damage of matrix from anions immigrating,showed promising corrosion resistance.(3)The erosive wear behaviors of Ti6Al4 V alloy and TO layer were investigated under the solid-liquid two phase flow conditions.The cooperation of the erosive action of solid particle and the corrosive effect of the liquid medium contributed to the damages of the Ti6Al4 V alloy and TO layer.The Ti6Al4 V alloy with low surface hardness showed lower erosive wear resistance in comparison with TO layer.The TO layer prevented Ti6Al4 V alloy from erosive corrosion.The protective effect of the TO layer was attributed to its higher surface hardness and chemical stability.(4)There were obvious variations in corrosive wear behaviors between the tested samples under different experimental conditions,which could be revealed in the results of friction coefficient-sliding time curves,mass losses and wear trace characterizations in sliding.Taken as a whole,TO layer on the surface has remarkably enhanced the anti-corrosive wear property of Ti6Al4 V alloy.