Research On Behavior Of Cavitation Corrosion For TA2 In LiBr Solution

In this paper,the ultrasonic apparatus was applied to simulate cavitation environments and TA2 was used as the test material.The cumulative mass loss rate,microstructure,surface roughness and three-dimensional profile in Li Br solution were obtained by using mass loss method,SEM,roughness profiler,three-dimensional video microscope.The surface mechanical properties of TA2 were analyzed by X-ray stress analyzer and hardness tester,and the electrochemical behavior during cavitation corrosion process was explored through the electrochemical and micro electrochemical tests.According to the above results,the synergistic effect of mechanical effect and corrosion,and the evolution of cavitation corrosion were discussed.There are three stages of cavitation corrosion process of TA2.At initial stage,local plastic deformation occurs,grain boundaries are destroyed after cavitation for 30 min,a small amount of cracks appear,then the surface in local areas falls off by expanding and spreading of cracks to the gains.In the initial 70 min,the mass loss is large,the roughness increases rapidly,from 0.360 ?m to 0.790 ?m.At the transition stage,the depth of cavitation increases,the surface of TA2 falls off seriously,and the surface has a honeycomb appearance at 180 min.The growth rate of roughness decreases at this stage.At 210 min,the surface roughness reaches the highest,0.998?m.At the stable stage,the roughness and the mass loss rate tend to level off.After 540 min,the value of mass loss is 14.17 mg in Li Br solution,but the value of mass loss is 12.33 mg in pure water,indicating TA2 is easier to be destructed in LiBr solution compared with pure water.It can be observed that the OCP value shifted towards negative direction by almost 150 mV,and could not return back.Under static conditions,compared with pure water,the corrosion potential of TA2 in LiBr solution moves towards negative direction by about 359.5 mV,passivation current increases and pitting potential decreases,the rate of pitting increases,suggesting the destroy of TA2 in LiBr solution is easier than in pure water.In pure water,compared to static condition,the corrosion potential of TA2 under cavitation condition shifted towards negative direction by about 111.3 mV,the passive rage shortens,passivation current density increases by about 3.503 ?A·cm-2.In Li Br solution,the corrosion potential of TA2 under cavitation condition decreases by about 138.5 mV,the passive rage shortens,and the curves shows oscillation phenomena,passivation current density increases by about 23.246 ?A·cm-2.The pitting potential decreases,leading to the increase of pitting corrosion possibility.After 30 min,the charge transfer resistance of the passive film increases firstly and then decreases,while the film resistance decreases with the cavitation time,from 1×1020 ?·cm2 to 1.844×105 ?·cm2.This implies that the anti-corrosion of passive film decreases gradually,the donor carrier density become bigger,the corrosion is accelerated.These are in agreement with the results of polarization curves and EIS.The value of surface residual stress increases sharply.It reaches the maximum value n LiBr solution at 5 min,339.6 MPa,but in pure water the highest value occurs at 20 min,the value is 213.6 MPa.Then it tends to be stable and at later period it is reduced.The vickers hardness increases in pure water and LiBr solution at early stage,reaches the maximum at 60 min,the value is 242.17 MPa in pure water,and 245.73 MPa in LiBr solution.After that,work hardening phenomenon takes place,and the value decreases to 235.5 MPa in pure water,213.17 MPa in LiBr solution.The potential difference of TA2 in LiBr solutios increases with cavitation time,at 120 min it is 240 mV.As the progression of cavitation erosion,the anode regions and cathode regions on the surface become more obvious,and the corrosion effect is increased.The potential difference in LiBr solution has little difference with the potential difference in pure water,showing the influence of corrosion effect of LiBr solution is not obvious.TA2 surface is covered with a dense n-type semiconductor oxide film with a good self-healing capability.The mechanical effect of cavitation destroyed the outer layer of passive film.After 60 min,the inner film is destroyed,and the passive film is changed from n-type semiconductor to p-type semiconductor,and the passive film is locally detached from the metal surface.When the dissolution rate of passive film is faster than the rate of repassivation,the film is thinning until it is completely damaged,and the fresh titanium substrate is exposed to Li Br solution.There is stirring effect caused by mechanical impact,the species transfer rate in the solution increases,the corrosion effect is accelerated.The existence of plastic deformation results in the uneven surface electrochemical properties.As the increasing of the cavitation time,the galvanic-couple cells occur on the material surface,the degree of corrosion is further promoted.At the same time,the local damage on the surface of TA2 favors the gathering of the shock wave.This makes the stress of the defect more concentrated,and conversely promoting the mechanical effect.Therefore,the synergistic effect exists in the process of the cavitation erosion process of TA2,which accelerates the destruction of TA2.