Research On Deep Sea Corrosion Resistance And Crack Tip Dissolution And Self-healing Behavior Of Passive Film Of Titanium Alloy

Titanium alloy has become an important structural material in the deep-sea environment due to its high strength to density ratio,excellent corrosion resistance and non-magnetic properties.Its excellent corrosion resistance is attained by the rapid formation of a stable passive film on the material surface.However,there are few studies on the corrosion resistance of passive film of titanium alloy in the deep-sea environment.Furthermore,the dissolution and self-healing behaviors of the passive film formed on flat surfaces and crack tip surfaces have not been revealed.In this paper,the corrosion resistance of titanium alloy and its passive film in simulated seawater environments at various depth was systematically studied in an environmental simulation test apparatus of deep sea.The conventional electrochemical measurements involving dynamic polarization,electrochemical impedance spectroscopy(EIS),and Mott-Schottky were carried out.The compositions of the passive film were also investigated by using X-ray photoelectron spectroscopy(XPS).The electrochemical performances of passive films of titanium of three typical microstructures,i.e.equiaxed,widmanst?tten,and duplex were compared.The evolution of significant factors of local electrolyte at the crack tip was monitored by micro-electrode technology.Based on that,the effects of p H value,Cl^-concentration and dissolved oxygen(DO)on the dissolution and self-healing behavior of passive films in simulated crack tip solutions were also investigated using microelectrode method,conventional electrochemical test,and friction-electrochemical simultaneous detection.The dynamic polarization test,EIS and Mott-Schottky test were performed on titanium alloys in simulated shallow sea,1000 m and 3000 m deep sea environments.The results showed that,the passive current density(i_p)of titanium alloy in the simulated deep-sea environment was one order of magnitude higher than that in the simulated shallow sea environment.The passivation ability and corrosion resistance of the alloy decreased with the increase of seawater depth,which was reflected in the decrease of passive film resistance(R_f),as well as the increase of film capacitance(CPE_f),and the donor density(N_D).The results of XPS analysis showed that the passive film was mainly composed of Ti O₂with intermediate oxides of non-stoichiometric ratio.Moreover,in the inner layer of the passive film and the passive film formed in the deep-sea environments,the content of Ti O₂ decreased,while the content of titanium oxide in lower valence state increased,leading to the point defects increased.The dynamic polarization test,EIS,Mott-Schottky test and crystal phase analysis were performed on widmanstatten,equiaxed and duplex structure titanium alloys in simulated shallow sea,1000 m and 2000 m deep sea environments.The results showed that,titanium alloys of the three microstructures all showed excellent passivation performance and corrosion resistance in the simulated shallow sea environment.In the simulated deep-sea environment,the passive film formed on duplex alloy had n-type semiconductor property,but the film was observed to be composed of inner p-type film and outer n-type film in terms of widmanst?tten and equiaxed alloys.Therefore,the corrosion resistance of the duplex alloy is better than those of the other two microstructures,which was thought to be related to the higher proportion of?phase in the duplex microstructure.The physical and chemical properties of the stress-corrosion crack tip environment of the titanium alloy in a 1000 m deep sea environment were tested.The dynamic polarization test,EIS and Mott-Schottky test were performed on the titanium alloy in the simulated crack tip micro-region solution.The results demonstrated that,the local electrolyte in the crack tip area can be rapidly acidified and enriched with Cl^-.With the ND values and i_p of the film increased,the impedance decreased correspondingly.The dissolution of the passive film was enhanced,while the ability of self-healing was weakened,resulting in the decrease of corrosion resistance.Friction and wear experiments were performed on titanium alloys in simulated crack tip solutions with different DO,Cl^-concentrations,and p H values.At the same time,open circuit potential and EIS tests were performed.The self-healing tests showed that the passive film of titanium alloy could be repaired instantly in the simulated seawater solution.But the self-healing process was mitigated in the solution with low dissolved oxygen,because of the limited reduction reaction of dissolved oxygen.The dissolution and self-healing of the film will be accelerated by increasing Cl^-concentration in the alkaline environment of simulated seawater,while slowed down the self-healing of film in the acidified environment of crack tips.It can be concluded that low DO,high Cl^-concentration,and low p H all can lead to the deterioration of corrosion resistance of the self-healed film.