Modification Design Of Nickel Based Alloying Surface On Nickel-Aluminum Bronze And Its Corrosion Resistance

Nickel-aluminum bronze?NAB?alloys are widely used in the marine condition due to its good mechanical properties and corrosion resistance.However,the complex multi-phase microstructure could easily result in the selective phase corrosion,which would accelerate the material damage.Besides,cavitation erosion-corrosion behavior under dynamic loading is one of the most prominent engineering problems.Therefore,it is of great significance to seek a surface modification method to improve the static corrosion resistance and the cavitation erosion corrosion resistance on the NAB alloys.It could also improve the comprehensive service performance of the properller components.The cast NAB alloys are usually heat treated at 675?to eliminate the?',thus improving the corrosion resistance.According to that,a new mothed of nickel based surface modification layer on the NAB alloy was designed in this paper.The research project was the Ni/NAB diffusion system.The completely Ni-Al-Cu layer and the gradient Ni-Cu layer were obtained through controlling and adjusting the thickess of the Ni coating and the thermal diffusion parameters.In addition,the growth mechanism of Ni-Cu layer and Ni-Al-Cu layer was revealed by the in-depth analysis of the diffusion process.Besides,the static corrosion and cavitation erosion corrosion performance of the modified layers were also systematically studied.The main results are as follows:?1?The completely Ni-Cu layer/Ni-Al-Cu layer and the single Ni-Al-Cu layer were obtained by controlling the thickness of Ni coating and the heat treated parameters under 675?.The microstructure of the modified layers was evenly distributed and refined.According to the thermodynamic and kinetic analysis of the surface diffusion process,the diffusion of Ni,Cu and Al was persistent during300?⁶75?and the Al preferentially reacted with Ni.Besides,the relationship of the diffusion coefficient of Ni,Cu and Al wasD_{A l}<D_{C u}<D_{N i}.The growth rate constant of the Ni-Cu layer was smaller than that of the Ni-Al-Cu layer,which was controlled by the diffusion rate.The diffused Cu and Ni formed solid solution area.Thus,the activation energy of the Ni-Cu layer was lower.Besides,the physical model was also established to analyze the diffusion process.It showed that the diffusion process was dominated,which meant that the Ni and Cu firstly diffused to form the solid solution area and then Al diffused into Ni-Cu layer.?2?The salt spray tests showed that the Ni-Cu layer exhibited uniform corrosion and the Ni-Al-Cu layer exhibited pitting corrosion,eliminatting the elective phase corrosion of the NAB alloy.The corrosion resistance of Ni-Cu layer was much better,due to the formation of compact passive film mainly consisting of Ni?OH?₂ and Cu₂O.Thus,for the static corrosion environment,the Ni-Cu layer has geater advantages.The corrosion mechanism of the gradient Ni-Cu layer was also investigated.The passive film formed on the gradient Ni-Cu layer was stable.And the gradient Ni-Cu layer exhibited gradient potential distribution.When pitting corrosion occurred,the interface potential drop between the outer passive film and the bare Ni-Cu within pits was also gradiently distributed.Besides,the interface potential drop decreased with the pits growth.Therefore,the driving force for galvanic corrosion of the gradient Ni-Cu layer decreased,thus decreasing the anodic dissolution within the pits and the growth of the pits,which in turn increased the static corrosion resistance.?3?Cavitation erosion-corrosion process is the result of the synergistic effect of electrochemical corrosion and mechanical impact.The nano-hardness of Ni-Al-Cu layer was about 5.5 GPa,which was about 2.5 times of the gradient Ni-Cu layer and the NAB substrate,as well as the wear resistance.The mass loss of the Ni-Cu layer was significantly higher than that of the NAB substrate and the Ni-Al-Cu layer after cavitation erosion for 2h.While increasing the cavitation erosion time to 12h,the mass loss of the Ni-Al-Cu layer in pure water and 3.5 wt.%NaCl solution was15.66%and 18.14%of the NAB substrate,respectively.Thus,for the cavitation erosion-corrosion process,the Ni-Al-Cu layer has geater advantages.The improvement of cavitation erosion-corrosion resistance of the Ni-Al-Cu layer came from two aspects.The Ni-Al-Cu layer was able to resist mechanical shock due to its high hardness in the pure water;the increased weight loss of Ni-Al-Cu layer was not obvious due to its uniform and detailed microstructure in 3.5 wt.%NaCl solution.The percentage weight loss caused by mechanical factor(W_E+E_CIE)was more than80%,which indicated that the mechanical effect was the main factor affecting the cavitation erosion-corrosion behavior.