The Passivity And Piting Of Cu-25Zn-15Ni Alloy Based On The PDM

With the introduction of China’s major national strategies such as ocean power,the development and utilization of marine resources has been accelerated.New marine industries such as offshore construction,marine resource extraction and underwater operations are developing rapidly,and various types of new deep-sea drilling,submarines,offshore equipment and other facilities have become the fundamental guarantee for the development of marine resources and the maintenance of national security.Copper alloys have excellent mechanical properties and corrosion resistance,so they are widely used in marine ships,power transmission,large equipment manufacturing,aerospace and other fields.As the service life of marine vessels and other fields increases,higher demands are placed on the reliability and safety of copper alloys.Copper alloys are also constantly being improved and upgraded,from early purple copper to bronze and brass with better corrosion resistance,as well as the white copper we now use widely.However,due to the harsh marine environment with high chloride ion content,the passivation film generated on the surface of copper alloys in the natural environment may be locally eroded by chloride ions,resulting in the generation of deeper localized corrosion,such as pitting corrosion,decomposition corrosion,fatigue corrosion,galvanic corrosion,etc.Therefore,it is of great engineering significance to carry out the corrosion behavior of the new Cu-25Zn-15Ni alloy in a chlorine-containing environment for the development of copper alloys in the field of marine transportation.In this paper,a series of electrochemical tests were performed on Cu-25Zn-15Ni alloy to address the above issues.The passivity and semiconductor properties of the surface passivation film of Cu-25Zn-15Ni alloy in borate solutions of different pH and the effect of different chloride ion concentrations on passivation film rupture were investigated.The point defect model was used to explain the Cu-25Zn-15Ni alloy passivity and calculate the distribution law of pitting potential as well as the important parameters required to simulate the localized corrosion damage on the surface of Cu-25Zn-15Ni alloy.The main results of the study are as follows:（1）A series of electrochemical tests on Cu-25Zn-15Ni alloy in boric acid solutions at different pH showed that the Cu-25Zn-15Ni alloy exhibited a wide passivation interval and a small passivation current at different pH,indicating that the Cu-25Zn-15Ni alloy passivation film was well protected.The Mott-Schottky test results showed that the quadratic performance of the applied potential and the capacitance of the passivation film are inversely proportional and linear,which qualitatively indicates that the Cu-25Zn-15Ni alloy passivation film is a p-type semiconductor.It is found that the Cu-25Zn-15Ni alloy passivation film has a principal density of 0.966～4.87×10²⁰（88）^-3,and the diffusion coefficient in boric acid solution is about 10^-16（88）²/.（2）Cu-25Zn-15Ni alloy is susceptible to pitting in chloride containing solutions,and the pitting potential of Cu-25Zn-15Ni alloy decreases with increasing chloride ion concentration or increases with increasing solution pH.The average pitting potentials of Cu-25Zn-15Ni alloy were 0.798 V_SCE,0.774 V_SCE,0.077 V_SCE,0.03 V_SCE,when the solution pH=10 and the chloride ion concentrations were 0.01 mol/L,0.1 mol/L,0.5 mol/L,and 1mol/L,respectively.The critical pitting potentials of Cu-25Zn-15Ni alloy in solutions of pH 8,9,and 10 were-0.107 V_SCE,-0.079 V_SCE,and 0.03V_SCE,respectively,when the chloride ion concentration in the solution was1 mol/L.The critical metal cation vacancy concentration required to cause the passivation film rupture of the Cu-25Zn-15Ni alloy was estimated to be about 1.9452×10¹⁴No/cm² based on the relevant equations of the PDM.This value is closer to the results based on the dotted surface densities of copper atoms in metallic copper and cuprous oxide(1.1529×10¹⁵No/cm²,7.7159×10¹⁴No/cm²,respectively)results are closer.（3）Based on the experimentally obtained test results at different chloride ion concentrations and solution pH values,the cumulative probability distribution curves of the passivation film rupture potential of Cu-25Zn-15Ni alloy were made by using origin software.By comparing the experimental curves with the PDM theoretical calculation results,it is clear that the two curves have good overlap.The results strongly demonstrate the reliability of the pitting corrosion of Cu-25Zn-15Ni alloy in a chlorine-containing environment obtained using the PDM calculations.