Research On Behavior At Initial Stage And Influencing Factors Of Cavitation Corrosion For Ti-6Al-4V Alloy In LiBr Solution

In this paper, the environment of cavitation corrosion for Ti-6Al-4V alloy in LiBr solution with high concentration was simulated by ultrasonic apparatus. The initial behavior of cavitation corrosion of Ti-6Al-4V alloy in Li Br solution and the effect of solution temperature were discussed by means of SEM, roughness profiler, threedimensional video microscope. Meanwhile, cavitation mechanism and temperature influence were analyzed from the perspective of synergistic effect of corrosion and cavitation using electrochemical measurements. In addition, with the aid of residual stress analyzer and micro vickers hardness tester observed the variation of residual stress and hardness for Ti-6Al-4V alloy surface after cavitation at different time, and discussed the electrochemical corrosion behavior of it in LiBr solution, in order to study the effect of cavitation on the factor of electrochemistry corrosion.The results of the variation of surface roughness value(Rq), mean cavitation corrosion depth and morphological features indicate that there are three stages of cavitation corrosion process of Ti-6Al-4V alloy. At initial stage, the value of Rq increases linearly with time and plastic deformation increases. When reach the end of the initial stage, the falling of material in local surface area causes cavitation corrosion pits, and finally make the degree of surface concave and convex gradually increase. The period of transition stage is from 160 min to 320 min, cavitation corrosion pit keeps increasing and the material of surface falls off obviously. In addition, the falling off of the former convex bodies at surface leads to the decrease of roughness when the mean depth of cavitation corrosion increases slowly. The value of Rq tends to stabilize and the mean depth of cavitation corrosion began to greatly increase after cavitation corrosion 320 min, which is the steady-state stage. At initial stage of cavitation corrosion for Ti-6Al-4V alloy, the damage of the low intensive α phase occurs preferentially. Electrochemical corrosion micro-cell is formed by α and β phase on the surface of titanium alloy, and the synergistic effect of mechanical and electrochemical corrosion promotes surface damage.Temperature is an important factor of cavitation corrosion behavior for Ti-6Al-4V alloy. The temperature effect increases gradually as cavitation corrosion time increases, it is most obvious at 240 min. From 25 ℃ onwards, each increase of 1 ℃ increases the surface roughness value by 1% and the mean cavitation corrosion depth by 3% until the maximum is reached at about 55 ℃. When the temperature is higher than 65 ℃, increase of 1 ℃ decreases the surface roughness value and the mean cavitation corrosion depth by 10%. Continue to raise the temperature after the damage of cavitation corrosion reaches a maximum at 55 ℃, electrochemical corrosion factor will still be promoted, but lower cavitation corrosion as a result of mechanical factor is abated. The reasons are not only cushioning effect for the increase of steam content, but also the destruction of the surface passivation film is weakened results in the impact strength of micro-jet attenuates. Since the mechanical factor plays a leading role in cavitation corrosion for Ti-6Al-4V alloy, and therefore cavitation corrosion rate is mitigated.At initial stage of cavitation, the value of surface residual stress increases sharply. After the maximum value of surface residual stress of Ti-6Al-4V alloy is reached, i.e. cavitation 20 min, the surface residual stress tend to be stable and later period to be reduced and the increasing rate of vickers hardness is reduced. With the increasing of cavitation time, uneven deformation of Ti-6Al-4V alloy surface in the process of cavitation and the damage area of the passive film on the surface are both increasing gradually, causing the inhomogeneity of the electrochemical properties of surface aggravated. Eventually, Ti-6Al-4V alloy after cavitation occurs active corrosion dissolution in lithium bromide solution, and longer of cavitation will causes a serious corrosion damage of material surface.In summary, plastic deformation of the low intensive α phase of Ti-6Al-4V alloy occurs preferentially due to absorption of impact energy generated by bubble collapse, causing uneven deformation of material surface. Passive film on the surface of α phase in local area is easily attacked and fresh titanium alloy substrate is exposed. Small anode and large cathode form due to relatively low electric potential of α phase in comparison with β phase, so cavitation improves the tendentiousness of surface electrochemistry corrosion. However, in the corrosive medium, corrosives and corrosion products diffuse fast with the aid of agitation of cavitation, leading to the acceleration of the corrosion dissolution. The growth of pits causes the concentration of local internal stress, which strengthens the mechanical factor, accordingly increases the degree of surface concave and convex. The synergetic effect of mechanics and electrochemical corrosion results in the development of the existing pits and emergence of new pits. Eventually β phase distributed at the boundary of pits falls off and the degree of surface concave and convex becomes small. The influence of temperature on cavitation is the main reason for the influence of temperature on cavitation corrosion behavior.