Response Of Surface Layer Properties Of Nickel-aluminum Bronze Alloy To Cavitation Effect

Nickel-aluminum bronze(NAB)alloy has become the most used material for modern ship propellers due to its great corrosion resistance and mechanical properties.During the working process,the NAB propeller is subjected to cavitation corrosion due to the synergistic effect of cavitation mechanics and electrochemical corrosion,resulting in performance degradation.In this paper,to study the cavitation corrosion behavior and surface property changes of as-cast NAB,the cavitation process is simulated by an ultrasonic cavitation tester,combined with material surface analysis and testing technology,electrochemical testing technology and surface nano-mechanics testing technology.The main conclusions are as follows:(1)Under the effect of cavitation,the as-cast NAB alloy has basically the same rate of mass loss with time in the initial stage;after that,the rate of mass loss increases sharply with time;as the cavitation time continues to extend,the rate of mass loss increases slowly;the mass loss rate shows a gradually decreasing trend after cavitation for 10 hours.Therefore,the cavitation corrosion mass loss process of the NAB alloy is divided into three stages:the incubation period,the rising period,and the stable period.However,the cavitation mass loss decreases,after the electrochemical effect of the cavitation corrosion process is inhibited;when the electrochemical effect is increased,the cavitation mass loss increases.The cavitation erosion process accounts for the largest proportion of erosion mass loss,which belongs to the erosion control caused by corrosion.(2)The as-cast NAB alloy is composed of ?,? and ? phases.the ? phase and its eutectoid transformation ? phase are corroded first,which leads to selective phase attack occurs and the content of ? phase increased slightly during cavitation process.The grain size decreases,the dislocation density and micro-stress increase with the cavitation time increases.The change trend of grain size,dislocation density and micro-stress under polarization cavitation is the same as the trend of cavitation for different time,but the degree of change caused by cathodic polarization is more than anodic.(3)When the time of cavitation is 1h,the nano-hardness(Hnano)and the nano-elastic modulus(Enano)increase,and the phenomenon of work hardening appears.Plastic deformation occurs after experiment for 3 hours,the Hnano and the Enano decreases.Both of Hnano and Enano increase,and their distribution range is narrow after inhibiting the electrochemical effect of the cavitation erosion process;Both of nano-hardness and Hnano sdecrease and have a larger distribution range,resulting in a shallower hardened layer and deeper softened layer,after increasing the electrochemical effect of the cavitation erosion process.The curve of(H/E)nano changes with depth under different cavitation conditions is more orderly than the curve of nano-hardness and nano-elastic modulus.Using(H/E)nano can more easily analyze the changes in the comprehensive mechanical properties of materials.The(H/E)nano increases first and decreases overall with the extension of cavitation time.The comprehensive mechanical properties of the surface(H/E)nano under different cavitation conditions are positively correlated with the reciprocal of the corresponding cumulative mass loss,in other word,the larger the(H/E)nano,the smaller the mass loss.(4)The polarization resistance under static immersion increases greatly with the extension of the immersion time;the polarization resistance under cavitation erosion is smaller,and it increases slightly with the extension of the cavitation time.Both static immersion and static immersion under polarization electrochemical process have two time-constants;the electrochemical process under cathodic polarization cavitation have two time-constants,and the electrochemical process under anodic polarization cavitation has two time-constant.Cathodic polarization has an inhibitory effect on the electrochemical process of nickel-aluminum bronze,while anodic polarization can enhance the electrochemical process.