Research On Corrosion Behavior Of Ti35 Titanium Alloy In Nitric Acid Containing Fluoride Ions

As a safe,efficient,green and economic new energy,nuclear energy has been developed and utilized on a large scale in China.In order to achieve the efficient recycling of nuclear fuel,China has clearly proposed the implementation of closed cycle technology of nuclear fuel,nuclear spent fuel reprocessing is the most important part of this route.In the process of spent fuel reprocessing,acid treatment vessel has been in the environment of high concentration boiling nitric acid,high oxidizing ions and high radioactivity for a long time,so the corrosion resistance of acid treatment vessel is strictly required.At present,titanium alloy,zirconium alloy and stainless steel are the main materials used in acid treatment container for spent fuel reprocessing in the world,and titanium alloy is used as acid treatment container in China.Because uranium oxides in nuclear fuels contain fluorine,there will inevitably be fluoride ions in the process of spent fuel reprocessing.Moreover,fluorine has strong electronegativity and is easy to combine with hydrogen ions in nitric acid to form hydrofluoric acid.A very small amount of hydrofluoric acid will cause serious corrosion to the titanium alloy,and increase the risk of equipment use.Therefore,it is significant to study the corrosion behavior,corrosion regularity and corrosion mechanism of titanium alloys in boiling nitric acid solution containing fluoride ions.In this paper,commercially pure titanium?CP-Ti?and Ti35 alloys are selected as experimental materials.The electrochemical corrosion test and uniform corrosion test are used to study the electrochemical properties and uniform corrosion behavior of the two metals in 6 mol/L boling nitric acid solution containing different fluoride ion concentrations?0 ppm,10 ppm,50 ppm,100ppm,200 ppm?,respectively.The scanning electron microscopy?SEM?,X-ray diffraction?XRD?and X-ray photoelectron spectroscopy?XPS?are used to analyse the surface morphology and the composition of the passive film of two metals.And the corrosion mechanisms of two metals in boling nitric acid solution containing fluoride ions are revealed.This accumulates experimental data for the use of reprocessing equipment,and it is significant for improving the safety and stability of nuclear spent fuel reprocessing engineering.The main results are as follows:?1?The corrosion behaviors of CP-Ti and Ti35 alloys in nitric acid solution containing fluoride ions are studied by means of uniform corrosion test.It is found that Ti35 alloy has better corrosion resistance than CP-Ti.With the increase of fluoride concentration,the corrosion rates of both metals increase,and the corrosion rates of Ti35 alloy increased less than that of CP-Ti;The distribution of Ta in passive film on Ti35 alloy surface is uniform whether fluoride ion exists or not.However,when the concentration of fluoride ion is greater than or equal to50 ppm,the distribution of Ti and O in passive film on CP-Ti and Ti35 alloy surface are not uniform.?2?It is found that when the concentration of fluoride ion is more than 50ppm,the surface of CP-Ti hanging piece is gradually coated with a layer of white adhesives,and white powders appear at the bottom of flask as the corrosion proceeded,while Ti35 alloy doesn't appear the above phenomena at all fluoride ion concentrations.The results show that the white attachment is TiO₂.And the formation mechanism is that when the concentration of fluoride ion is greater than or equal to 50 ppm,porous film with the minimum pore diameter of 20 nm is formed on the surface of CP-Ti;the TiO₂ nucleates on porous membranes at high energy regions and falls off under the scouring effect of boiling nitric acid.?3?The electrochemical corrosion behavior of two metals in nitric acid containing fluoride ion was studied by electrochemical method.It was found that the critical concentration of fluoride ion was 50 ppm,which caused a significant change in the corrosion resistance of the two metals.With the increase of fluoride ion concentration,the open circuit potential shifts toward negative,the corrosion current density increases and the capacitance arc radius decreases of the two metals,and the corrosion resistance of two metals decrease.Compared with CP-Ti alloy,Ti35 alloy has a positive open-circuit potential and a higher polarization resistance,so the corrosion resistance of Ti35 alloy in nitric acid containing fluoride ion is better.?4?The compositions of passive film on the surface of CP-Ti and Ti35 alloys after corrosion in nitric acid solution containing fluoride ions are analyzed by XPS.It is found that the passive film CP-Ti surface is mainly composed of TiO₂,and the passive film between the surface layer and the matrix are composed of TiO₂,Ti₂O₃,TiO and Ti matrix.While the passive film on Ti35 alloy surface are mainly composed of TiO₂ and Ta₂O₅,and the passive film between the surface layer and the matrix are composed of TiO₂,Ti₂O₃,TiO,Ta₂O₅ and TaO_x.Ti oxides in passive film on metal surface are easy to react with fluoride ion,which destroys the integrity of passive film,but not with Ta oxides.It shows that the addition of Ta element makes the passive film on Ti35 alloy surface uniform and compact,and the corrosion resistance of the alloy is excellent.