Anodic Behavior Of Ti And Ti Alloys In AlCl₃-EMIC Ionic Liquid And Its Application

Ti alloys have been widely used in many high-tech industries like aerospace,medical implants and devices,high-end automotive and so on due to their attractive properties like low mass,high strength,and excellent resistance to corrosion etc.Especially notorious are Ti6Al4V and NiTi alloys.Ti6A14V accounts for more than 50%of total titanium usage.It offers a combination of high strength,light weight,formability and corrosion resistance which have made it a world standard in aerospace applications.NiTi is the most widely known and used shape memory alloy.It has been used for its shape memory properties for couplings and actuators,although its contemporary use has been mostly applied for medical devices.However,during the engineering applications Ti alloys show some problems such as the protective coatings needed for Ti6A14V alloy for solving its contact corrosion and wear poor resistance or the development of new surface treatments for NiTi which do not infract any damage affecting fatigue resistance for NiTi alloy.Electrochemical processes are a relatively economical,reliable,efficient,and applicable to a wide variety of shapes and sizes,comparing with other methods.The high electrochemical activity of Ti makes it difficult to perform surface treatments of Ti alloys.It has become an important topic in the research area of surface science and technology of reactive metals to look for new medium suitable for the surface treatments of reactive metals and to study the related mechanisms.In the past,electrochemical processes had some limitations associated with the electrolytes used during the process.There are thousands of different electrolytes,though the ones most suitable for anodic-related applications include aqueous solutions,organic solvents and recently-discovered ionic liquids.Anodic etching of Ti or electrodeposition of Al does not occur in an aqueous solution.In the past,this was partially solved using organic solvents instead,but during electrochemical processes in alcohol solutions dangerous,explosive components such as perchloric acid need to be handled.Process efficiency is not high enough sometimes as well.The term ionic liquid has come to mean an ionic material that is liquid below 100 ?.Ionic liquids have many advantages such as wide potential windows,the high solubility of metal salts,avoidance of metal/water chemistry,high conductivity,and a large number of possible formulations and properties(so-called designer solvents),so they are being broadly investigated for the electrochemistry of metals.Alloy deposits that are difficult or impossible to produce from aqueous electrolytes are available from ionic liquids.Besides ionic solvents are considered a green chemistry product and handling them is not dangerous.Electrodeposition on Titanium alloys,electrochemical polishing and electrochemical forming of Titanium materials using ionic liquids are all related to anodic behavior of Titanium in ionic liquids.Anodic behavior is a science that involves interactions between electrolyte and materials imposed by an external electric field.The anodic behavior of Ti,Ti6A14V and Niti alloys in 2:1 acidic AlCl3-1-ethyl-3-methyl-imidazolium chloride(AlCl3-EMIC)ionic liquid was firstly investigated in this paper.Electrochemical testing like cyclic voltammetry,chronoamperometry,and chronopotentiometry...was measured using three-electrode and two-electrode system.The surface morphology of the three materials was characterized using a scanning electron microscopy(SEM).The results showed that the oxide film on the surface of Ti alloys exerted great effect on the anodic behavior of Ti alloys in AlCl3-EMIC ionic liquid,restraining the dissolution of Ti alloys.After the oxide film was removed,all the three materials started to dissolve.A new phenomenon never previously acknowledged was reported for the first time in this kind of ionic liquids.During the anodic electrochemical experiments two kind of passivation layers are observed on the electrode.The firstly formed yellow layer is associated with the existence of an oxide layer on the surface after mechanical polishing outside the glove box.The red TiCl3 passivation layer formed after the yellow layer is formed for longer etching times was as well reported on other literature.What is most interesting is that when the red TiCl3 layer deposits onto the previously formed yellow layer,the electrode does not show a passivation behavior as in other papers,but instead the anodic reaction can still continue.In case we remove the yellow layer from the electrode,this is not able to form anymore and we have the same passivation behavior of the TiCl3 passivation layer reported in other papers.In addition,the red layer deposited on the electrode was characterized and confirmed to be TiCl3 through Raman spectrum measurement.So we can confirm that the product of anodic dissolution of Ti existed in the form of Ti(?).The passivation was caused by the coverage of TiCl3 on the Ti electrode without traces of oxide layer.Ti dissolved into Ti(?)and consume the Al2Cl7-on the surface,which made the interface became neutral.Then TiCl3 particles formed on the surface and caused the passivation.The electrochemical behavior of the three materials was similar in this Ionic Liquid.After the yellow layer was removed,all the three materials showed a similar electrochemical behavior but slightly different current and potential values.Surface morphology evolution for the three materials was as well studied,showing different morphology for the three of them.This is associated to the different alloy phases presented in all of them.Secondly,electrodeposition of Al protective coatings on Ti6Al4V substrate surface was performed.After electrolytic etching pretreatments at low current densities(0.2A/dm2)the electrodeposited Al coating on the surface of Ti6Al4V showed a high adhesive strength of up to 11.5 MPa.When the samples were mechanical polished with sand paper inside the glove box the coatings were teared from the surface during the ultrasonic cleaning indicating a poor adherence to the substrate.The high adhesion of electrodeposited Al coating can be attributed to the facts that the oxide film on the surface of Ti6A14V was completely removed without any Ti-Al compounds adhering on the surface.Besides the complete removal of oxide film,the high adhesion of Al coating was also attributed to the coarse surface of the substrates due to the different corrosion rates of the different phases.The anodic electrolytic etching pretreatment of Ti6Al4V had great effects on the electrodeposition of Al on its surfaces.After the complete removal of oxide films on the surface of Ti6Al4V by anodic electrolytic etching pretreatment,the cross section SEM images of the Al coating-substrate interfaces shows a good adherence without crevices on it.