| The Ti-6Al-3Nb-2Zr-1Mo（Ti80）alloy,newly developed by China for the marine engineering,serves as one of the ideal materials for the fabrication of seamless tubes for the offshore oil mining.In offshore oil exploration service environments,including not only the complex and harsh marine environment,but also strong corrosive medias（for instance,the residual acid HCl environment of the oil layer acidification treatment）,there are high requirements on the corrosion resistance of Ti80 alloy.However,there are relatively few studies regarding the corrosion behavior and mechanism of Ti80 alloy in offshore oil exploration service environments.Therefore,it’s necessary to systematically investigate the corrosion behavior of Ti80 alloy,reveal its corrosion mechanism,and then provide insights into exploring the practical and effective ways to improve its corrosion resistance in offshore oil exploration service environments.To this end,this thesis mainly focuses on the elaboration of the corrosion behavior and mechanism of Ti80 alloy,and the optimization in the corrosion performance of Ti80 alloy in offshore oil exploration service environments.We thoroughly investigated the corrosion behavior and mechanism of Ti80alloy in the artificial seawater containing various concentrations of F-,and HCl solutions with different concentrations.In the artificial seawater,the increase of F-concentration seriously deteriorates the corrosion resistance of Ti80 alloy,whereas the increase of pH obviously improves the corrosion resistance of Ti80 alloy.The cathodic branches are mainly associated with the hydrogen evolution reaction（HER）,and the discharging of H+on the electrode surface to form Hads,acts as the rate determining step of HER.The F-concentration has few effects on the cathodic reaction but noticeable promoting effects on the anodic process,and the increase of pH plays a significant role in inhibiting both cathodic and anodic processes.The exchange current density（i0）of HER is closely related to the pH,and elgi0/epH is obtained to be around-0.90.The anodic process is closely related to both F-concentration and pH.Specifically,the maximum anodic current density（im）is strongly dependent of F-concentration and pH,i.e.,elgim/epH and elgim/ecF-are determined to be-0.23 and 68,respectively.The critical potential（Em）is independent of the F-concentration,but has a good linear relationship with the pH:eEm/epH=-62.Besides,the ip lineally increases with increasing the F-concentration,following the relationship:eip/ecF-=0.014.Increasing either HCl concentration or temperature can accelerate the corrosion process of Ti80 alloy.There is a relatively good linear relationship between icorr（im）and HCl concentration（C）:logi=a+nlog C.In addition,icorr（im）and temperature follow the Arrhenius formula:logi=（log A-Ea）/2.303RT.The corrosion mechanism of Ti80 alloy is mainly related to three aspects:（Ⅰ）Firstly,the dense protective oxide film formed on the surface is completely destroyed due to the chemical dissolution reactions;（Ⅱ）then,without the protection of the oxide film,the Ti80 alloy substrate will be corroded through the chemically active dissolution reactions;（Ⅲ）the formation of galvanic corrosion between different phases accelerates the corrosion of Ti80 alloy substrate.We investigated the effects of annealing and solution-aging treatments on the corrosion behavior of Ti80 alloy.The corrosion resistance increases with increasing the annealing temperature due to the fact that increasing the annealing temperature enhances the volume fraction of relatively more corrosion resistantβ-phase,and alleviates the galvanic corrosion betweenα-andβ-phases.The solid solution-treated alloy exhibits a significantly improved corrosion resistance since there is little elemental segregation betweenámartensite andβ-phase owing to the rapid cooling,which effectively restricts the occurrence of galvanic corrosion.Although the corrosion resistance decreases after the subsequent aging treatments,it is still superior to that of the traditional wrought alloy.In addition,there is a relationship between the corrosion current density icorr（corrosion rate Ⅰ）andα（á）-phase thickness（d）:icorr（Ⅰ）=A+Bd-0.5.We carried out the electron beam surface remelting（EBSM）technology to process the wrought Ti80 alloy.The EBSM alloy displays a significantly improved corrosion resistance,compared to the wrought alloy,due to the formation of a more stable protective oxide film on the surface of EBSM alloy.Besides,the extremely high cooling rate during the EBSM process limits the diffusion of alloy elements,and thus reduces the segregation of elements between different phases,which further inhibits the formation of galvanic corrosion.In addition,we employed the electron beam freeform fabrication（EBF3）to manufacture the Ti80 alloy.The EBF3-manufactured alloy also displays a better corrosion resistance than the wrought alloy,attributed to the formation of a more stable oxide film on the surface of EBF3-manufactured alloy.Besides,first-principles calculations reveal that the difference in Fermi energy level betweenα-andβ-phases of EBF3-manufactured alloy is smaller than that of wrought alloy,which thermodynamically clarifies that the galvanic corrosion betweenα-andβ-phases can be alleviated by the EBF3 technology.We analyzed the effects of Zr and C elements on the corrosion resistance of Ti80 alloy.The corrosion resistance monotonously improves along with increasing the Zr content,which is a result of the modified composition of oxide film,increased volume fraction of corrosion resistantβ-phase and alleviated galvanic corrosion betweenα-andβ-phases.The corrosion mode changes from the uniform corrosion to the mixed uniform and pitting corrosion with the Zr addition up to 40 wt.%.The addition of interstitial C greatly improves the corrosion resistance of Ti80 alloy,which is attributed to the effective inhibition of active dissolution and significant improvement in the corrosion resistance of prior-βgrain boundaries. |
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