Study On The Construction And Corrosion Resistance Of Multi Diffusion Layers On Brass Surface

Recently,the country has greatly increased the investment in ocean development for its extremely rich natural resources.As an important marine engineering material,brass alloys are widely used in valves,pipes,and marine heat exchangers.In harsh marine environments,corrosion is the most important mode of metal failure.Although the complex brass based on element addition has been developed,it is prone to casting defects and cannot completely avoid the dezincification corrosion.Therefore,it is of practical significance to study the corrosion resistance design on the brass surface and the related corrosion mechanism.In this paper,based on the element diffusion technology,the Ni-P alloy was introduced on the brass surface by electroless plating.The structure of multi diffusion layers was formed with inter-diffusion of the four elements Ni,P,Cu and Zn.The construction of multi diffusion layers was explored from process parameters as time,temperature and thickness of source coating.Corrosion resistance of different layers and corrosion protection to substrate were emphatically studied.The main research points are as follows:1.Under thermal diffusion,the structure with multi diffusion layers of the Ni-P layer,Ni-P-Zn layer,quaternary layer and Ni-Cu-Zn layer on brass surface was constructed.The speed of construction increased with the increase of time and temperature and the decrease of thickness of source coating.The source coating prepared in this paper was amorphous and the coating would crystallize during thermal diffusion,mainly forming the Ni₃P crystal phase.The composition phases of the Ni-P-Zn layer presented a dispersion distribution.The Ni-Cu-Zn layer was a Zn-rich layer from uphill diffusion.2.Micro-zone electrochemical results showed that the corrosion current density of diffusion layers was lower than the substrate;The structure of multi diffusion layers in this paper presented advantageous distribution of Volta potential.As the two outer layers,the Ni-P and Ni-P-Zn layers showed good electrical stability and were difficult to lose electrons.The Ni-Cu-Zn layer could provide sacrificial anode protection due to Zn enrichment;Impedance spectroscopy showed all the diffusion layers had better corrosion resistance than brass.The two outer layers had highest charge transfer resistance and corrosion film resistance,which indicated best corrosion resistance,followed by the Ni-Cu-Zn layer;In salt spray test,compact corrosion films could be formed on both the surfaces of the Ni-P-Zn layer and the Ni-P layer.The protection of the film on the Ni-P-Zn layer was more effective.Pitting was observed on the Ni-Cu-Zn layer.A large amount of corrosion products were found on brass during salt spray test in a short time;After a long-term immersion test,the film on the Ni-P layer was damaged to be connected.The film on the Ni-P-Zn layer was also broken to pieces.Brass showed a cluster of corrosion products in immersion test.The Ni-Cu-Zn layer showed much less products than brass,but grain boundary corrosion cracking was observed on the surface of Ni-Cu-Zn layer.3.The Ni-P-Zn layer and the Ni-P crystallization layer showed excellent cavitation performance due to dispersion strengthening and high corrosion resistance.At early stage,there only showed pit damage morphology.The local corrosion characteristic of the Ni-P-Zn layer could be maintained longer,and the mass loss was slightly lower than that of the Ni-P crystallization layer;The entire surface of brass was destroyed in a short time with high porosity.The mass loss was much bigger than other samples;The amorphous layer suffered less damage than the substrate.The surface roughness increased with the increase of cavitation erosion time;The Ni-Cu-Zn layer exhibited reticulate ridge morphology originating from phase selection corrosion at early stage.The mass loss was close to the amorphous layer.Finally,partial surface of the Ni-Cu-Zn layer was penetrated.The substrate was exposed and exhibited a sharp rise in mass loss.