| Aqueous lithium bromide is widely used as an absorption working fluid in recent years. Because it has lots of excellent characteristics, such as high thermal efficiency, low energy consumption and zero discharge, lithium bromide has gradually taken place of Freon to be the choice for refrigerator and heat exchanger. However, cavitation phenomenon would occur due to the change of pressure of lithium bromide solution while Li Br-Refrigeratory is running. Since high concentration LiBr solution is an aggressive medium, the metal materials which contact with the medium will suffer from serious cavitation corrosion, such as stainless steel, carbon steel, copper and copper alloy, etc. It will directly affect the equipment life and reliability. Therefore, it is important to choose a metal material to be applied in Li Br-Refrigeratory which has good cavitation resistance. In this paper, cavitation corrosion process was simulated by ultrasonic cavitation test machine. The cavitation corrosion behavior and mechanism of TC4 alloy and inhibiting effect of NaNO2 in 55% LiBr solution were disscussed through cumulative loss test, microhardness test, residual stress test, electrochemical measurement. In addition, the microstructure and morphology were observed by means of SEM, AFM, three-dimensional video microscope.The results revealed that TC4 alloy has good ability to resist cavitation corrosion,. The cumulative mass loss after 8h cavitation test in 55% Li Br solution was only 6.86 mg, the process of cavitation can be divided into two periods: incubation period and rise period. During the first period(3h), there is little mass loss, and mechanical impact of cavitation caused the surface layer of metal to plastically deform, micro-cracks and pits began to generate on the surface of titanium alloy, microhardness increased rapidly and reached a maximum at the end of the incubation period and then dropped gradually, The mass loss increased rapidly when the cavitaion process turned into rise period, the cracks on the layer of metal began to extend laterally and then gradually fell off. The morphology of metal surface present like a honeycomb due to the mechanical attack and hardening layer moved to the depth of matrix in.Pure mechanical effect is the dominant factor for the damage of material in the process of cavitation corrosion whose proportion is more than 77%, while the contribution of synergistic effect between the mechanical attack and corrosion is 15.09%-22.51%. Corrosive medium would accelerate the growth of crack and reduce the mechanical properties to some extent. The electrochemical behavior of titanium alloys has been significantly affected by the micro-jet and shock wave generated from bubbles' breaking down, the result shows that the free corrosion potential of TC4 alloy shifted negatively rapidly due to the damage of passivation film, and the electrochemical impedance decreased, resulting in acceleration of the electrochemical corrosion loss.NaNO2 is an inhibitor of anodic type help to reduce the mass loss of TC4 alloy after prolonged cavitation corrosion in 55% LiBr solution to some degree and has significant effect on electrochemical corrosion behavior of titanium alloy in 55% LiBr solution. It can also increase the thermodynamic stability of titanium alloy under static and cavitation condition and reduce the free energy of system, in addition, it can promote the formation of passive film when NO2 – was absorbed on the active corrosion part of titanium alloy, the electrode corrosion potential shifted positively and significantly increase its linear polarization resistance. Consequently the inhibitor was able to reduce the cavitation corrosion current efficiently and avoid massive pitting of the material. |
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