Composite Protection Technology For Titanium Alloy Surface And Study On Galvanic Corrosion Behavior

Titanium alloy has the characteristics of low density,high specific strength and good corrosion resistance,which is known as“marine metal”and has been widely used in marine engineering equipment in recent years.However,when titanium alloy is connected with dissimilar metals with low potential,galvanic corrosion is prone to occur,resulting in accelerated corrosion of dissimilar metals and great potential safety hazards.Therefore,it is urgent to protect the titanium alloy and systematically study its galvanic corrosion behavior,which lays a theoretical and practical foundation for the application of titanium alloy in marine field.In this paper,the TC4 titanium alloy was treated by micro-arc oxidation(MAO)combined with multi-arc ion plating,and the galvanic corrosion behavior of its connection with AH36,7075 and AZ31 alloys was studied.The morphology and phase composition of the film and corrosion products were detected and analyzed by SEM and XRD.The adhesion and roughness of MAO coating were measured by automatic scratch tester and roughness instrument.The galvanic corrosion tendency of TC4connected with the above three materials was studied by polarization curve method.The immersion test of bolted connection was carried out,and the corrosion current was monitored by the galvanic corrosion meter.The galvanic corrosion rate was tested by the weight loss test.Finally,the effect of galvanic corrosion on the tensile strength of TC4 titanium alloy and dissimilar metals was studied.The main research results are as follows:(1)TC4 titanium alloy was treated by micro-arc oxidation at different voltages.The results show that with the increase of voltage,the pore diameter of MAO coating gradually increases,the pore size first decreases and then increases,and the surface roughness gradually increases.When the voltage is 450 V,the porosity is 4.5%,and the surface roughness is 1.52?m.The MAO coating consists of anatase Ti O₂(101)and rutile Ti O₂(110).After micro-arc oxidation of TC4 titanium alloy,the average galvanic corrosion rate of AH36,7075 and AZ31 alloys can be reduced by 150%,409%and327%,respectively.The immersion test shows that the micro-arc oxidation treatment of TC4 titanium alloy can greatly reduce the galvanic corrosion sensitivity between AH36 and 7075 alloy,and change it from uniform corrosion to local corrosion.However,the reduction of galvanic corrosion on AZ31 is relatively low;the galvanic corrosion grade of TC4 titanium alloy with AH36 steel and 7075 aluminum alloy can be improved from grade D to grade A.However,the effect of micro-arc oxidation treatment on the galvanic corrosion of TC4 titanium alloy and AZ31 alloy is limited,and the grade is still grade E.After 15 days of galvanic corrosion,the tensile strength of the three alloys increased by 1.2%,4.5%and 4.2%.(2)After micro-arc oxidation,the TC4 titanium alloy was treated by multi-arc ion plating under different chromium target currents.The results show that the porosity of the composite coating decreases with the increase of chromium target current.When the chromium target current is 75 A,the porosity of the coating can be reduced by85.3%,the adhesion of the composite coating is 67N,and the minimum surface roughness is 1.45?m.The phase composition of Cr N film is Cr N,Cr₂N,Cr and a small amount of Cr oxides.Using multi-arc ion plating,the galvanic corrosion rates of AH36,7075 and AZ31 alloys connected with TC4 titanium alloy after micro-arc oxidation can be increased by 34.5%,79.0%and 25.0%,respectively.After composite treatment,the galvanic corrosion grade of AH36 and 7075 alloys reduced from grade A to grade B,and the galvanic corrosion grade of AZ31 alloy remained at grade E.After 15 days of galvanic corrosion,the tensile strength of the three alloys increased by 3.4%,3.0%and 11%,respectively.(3)After MAO,the TC4 titanium alloy was treated by multi-arc ion plating under different N₂:Ar ratios.The results show that with the increase of nitrogen ratio,the surface porosity and surface roughness of the coating gradually decrease.When N₂:Ar is 10:1,the surface porosity and roughness of the coating are 0.12%and 1.40?m,respectively.The phase composition of Cr N film is Cr N and Cr₂N.After composite treatment,connecting the TC4 alloy with AH36 steel,7075 aluminum alloy and AZ31magnesium alloy,the galvanic corrosion rates of the three alloys are reduced by 18.2%,55.8%and 20.1%,respectively.The TC4 alloy connected with AH36 steel and 7075aluminum alloy after composite treatment,the galvanic corrosion grade is increased to A level.When connected with AZ31,the galvanic corrosion grade remains at E level.After 15 days of galvanic corrosion,the tensile strength of the three alloys was increased by 0.4%,11.0%,8.0%,respectively.(4)The TC4 alloy was treated by micro-arc oxidation and multi-arc ion plating.When the micro-arc oxidation voltage is 450 V,the chromium target current of multi-arc ion plating is 75 A,and the N₂:Ar ratio is 10:1,the galvanic corrosion sensitivity of titanium alloy to dissimilar metals is significantly reduced.