Corrosion Behavior Of Cu-Ti Alloy Under Different Heat Treatment Conditions In S^2- Contaminated Seawater

Copper alloys are widely used in marine environments due to their excellent electrical and thermal conductivity and good corrosion resistance to sea water.However,the marine corrosive environment is complex and changeable.Sulphide,as the most common pollutant in natural seawater,causes great uncertainty to the service performance of copper alloy in the Marine environment.Cu-Ti alloy is expected to be one of the alternative materials of Cu-Be alloy,showing a broad application prospect.When the content of Ti is in the range of 1 wt.%?6 wt.%,the equilibrium microstructure at room temperature is Cu+Cu₄Ti.The corrosion resistance of the alloy is usually affected when there is a second phase in the alloy.In order to investigate the effect of sulfide concentration change and precipitated phase on the corrosion behavior of Cu-Ti alloy in seawater,Cu-4wt.%Ti alloy with different microstructure was obtained by solution and aging treatment.3.5 wt.%Na Cl solution containing S^2-with a concentration of 2.5,20,60 and 100 ppm was prepared to simulate seawater under normal state and mildly,moderately and seriously polluted by S^2-respectively,and 3.5 wt.%Na Cl solution without S^2-was set as blank control.The corrosion behavior of Cu-4wt.%Ti alloys with different heat treatment conditions in simulated polluted seawater containing different concentrations of sulfide was studied by immersion test and electrochemical test.By means of scanning electron microscopy(SEM),X-ray diffraction(XRD)and X-ray photoelectron spectroscopy(XPS),the corrosion products on the surface of Cu-Ti alloy were tested and analyzed to explore the synergistic mechanism of S^2-and Cl^-corrosion and the effect of?-Cu₄Ti precipitated phase on the corrosion of Cu-4wt.%Ti alloy.The main conclusions are as follows:(1)The corrosion types of Cu-4wt.%Ti alloy in 3.5 wt.%Na Cl solution with different S^2-concentrations are different in the two heat treatment states.In solid solution state copper alloy contains no pitting corrosion occurred in the solution of S^2-,with the increase of concentration of S^2-,corrosion gradually to uniform corrosion,it is because the S^2-in copper solid solution state happens on titanium alloy surface pitting than Cl^-is more serious,when the concentration of S^2-reaches 60 ppm,pitting corrosion pit distribution in the entire surface present a uniform corrosion morphology.The corrosion type of overaged copper-titanium alloy in 3.5 wt.%Na Cl solution with different S^2-concentration is mainly micro-galvanic corrosion,and the corrosion is not uniform.Due to the aging treatment of Cu-4wt.%Ti alloy,lamellar microstructure is formed by the alternating distribution of Cu matrix phase and precipitated?-Cu₄Ti phase.When immersed in solution,the Cu matrix phase contacted with?-Cu₄Ti precipitates to form a micro-galvanic corrosion cell.As the anode of the corrosion cell,the Cu matrix phase is more easily corroded,and the surface shows a groove shape after removing the corrosion products.Because the corrosion type is mainly micro-galvanic corrosion,the corrosion resistance of copper titanium alloy decreases and the corrosion rate increases after aging treatment.(2)No matter what kind of heat treatment state Cu-4wt.%Ti alloy is in,S^2-will promote its corrosion in 3.5 wt.%Na Cl solution.With the increase of S^2-concentration,the corrosion degree becomes more and more severe.There is a competitive adsorption relationship between S^2-and Cl^-on the corrosion of copper-titanium alloy,but S^2-has a stronger adsorption capacity and a faster reaction rate,which can cause rapid corrosion on copper-titanium alloy.The adsorption sites of S^2-and Cl^-of Cu-Ti alloy under two heat treatment conditions are changed.Due to the uniform surface composition of the solid solution Cu-Ti alloy,the adsorption sites of S^2-and Cl^-also show uniformity.However,due to the uneven surface composition and structure of the over-aged Cu-Ti alloy,micro-cells are generated.The Cu matrix phase loses electrons during the micro-galvanic corrosion,which makes it easier for S^2-and Cl^-to adsorb.(3)The corrosion products of Cu-4wt.%Ti alloy in 3.5 wt.%Na Cl solution containing S^2-are the same,mainly Cu S,Cu₂S,Cu₂O and Cu₂(OH)₃Cl.Because of the different corrosion types under the two heat treatment conditions,the corrosion occurs in different locations,so the distribution of corrosion products is different.(4)The higher the concentration of S^2-,the more OH^-generated by hydrolysis,which increases the thickness and density of corrosion product film of the solid solution copper-titanium alloy.When the concentration of S^2-reaches 100 ppm,the copper-titanium alloy will be passivated and the corrosion rate will be reduced.Therefore,when the solid solution Cu-Ti alloy is applied to the seawater polluted by S^2-,the concentration of OH^-in its environment can be improved by appropriate methods to promote the formation of Cu₂O protective film,thus reducing its corrosion rate.However,due to the uneven chemical composition on the surface the over-aged Cu-Ti alloy can not be passivated when the concentration of S^2-reaches 100 ppm,and the corrosion product film has no obvious inhibition effect on corrosion.