Electrochemical Reduction Mechanism,structure-control And Oxidation Resistance Of Ir Coating Prepared By Electrodeposition From Molten Salts

Noble metal iridium?Ir?is considerized as one of the most excellent oxidation resistance coating materials,which is widely used for fabricating rocket chambers and thermal protective systems in the space craft.In this paper,under the background of development of high temperature and long lifetime oxidation protective Ir coatings,focusing on optimization of the preparation process of Ir coatings,regulation of the microstructure of the Ir coatings and improvement in oxidation resistance of Ir coatings,relationship between the deposition parameters and the microstructure of the diepsoited Ir coatings and effects of the microstructure on the oxidation resistance of Ir coatings have been discussed.Ionic structure of Ir³⁺ions and effects of temperatures and composition of the salt on ionic structure of Ir³⁺ions has been studied by the cryoscopic method and spectral analysis.The results showed that two kinds of Ir-Cl complexes,[IrCl₄]^-and[IrCl₆]^3-,were found in the melting CsCl-IrCl₃ salts,accompaning with a transition from the[IrCl₄]^-to[IrCl₆]^3-with increaseing of temperatures and concentration of Ir³⁺ions.In the melting CsCl-IrCl₃-CsF salts,because of replacement of some Cl^-in the Ir-Cl complexes by F^-ions,[IrCl₄]^-and[IrCl₆]^3-converted to be the corresponding[IrCl_xF_y]^-and[IrCl_xF_y]^3-mixed complexes.Ir³⁺ions in the melting CsCl-Cs₃IrCl₆ salts existed in the form of multinuclear complexes{[IrCl₆]^3-}ⁿ.The addition of K⁺and Na⁺ions into the melts had no obvious effects on the ionic structure of Ir³⁺ions,however,because of the stronger porlarization of Na⁺and K⁺,structure stability of the Ir-Cl complexes in the melts decreased and their“lifetime”were shorter.The electrochemistry of Ir eletrodeposition were studied by electrochemical techniques and organizational observation.The electrochemical reduction of Ir³⁺ions in the melting chlorides was a diffusion-control irreverisible reaction at 823K⁸83K,while converted to be a quasi-reversible one when the temperature increased to about913K.It found by electrode dynamic analysis that increasing of temperatures and concentration of Ir³⁺ions promoted the mass transfer and electron transfer step,which resulted in decreasing of the needed cathodic over-potentials for the electrochemical reduction of Ir³⁺ions.Owing to difference in ionic structure of the Ir-Cl complexes and absorption of ions in the melts?such as F^-etc.?,The rate of mass transfer and electron transfer steps of Ir³⁺electrochemical reduction in various melts decreased in the turn of NaCl-KCl-CsCl-IrCl₃?IrCl₃-chlorides?,NaCl-KCl-CsCl-Cs₃IrCl₆?Cs₃IrCl₆-chlorides?and NaCl-KCl-CsCl-CsF-IrCl₃?IrCl₃-flourides/chlorides?,which illustrated the electrochemical activities of Ir³⁺in these melts decreased and the needed over-potentials for Ir³⁺electrochemical reduction increased accordingly.Moreover,it found by studies of electrocrystallization of Ir³⁺in the melts that nucleation of Ir grains positively correlated to the applied cathodic over-potentials.The higher the applied cathodic over-potential was,the higher the nucleation ability?including nucleuses density and nucleation rate?of Ir atmos was.Decreasing of temperature and Ir³⁺concentration,increasing of the applied cathodic current density and addition of F^-ions into the melts would remarkably increase the nucleation ability of Ir³⁺ions.Specially,when Ir³⁺ions was electrochemically reduced from the melting IrCl₃-flourides/chlorides,Ir nucleuses could formed continuously in the whole electrodeposition.Based on the results of the ionic structure and electrochemistry of Ir³⁺ions in the melts,according to difference in raction mechanism,factors,such as temperatures,Ir3+concentration,current parameters and melts system,could been divided into two groups.The first group contained those factors who affected the microstructure of the Ir coatings by modification of the dynamic activity of electrochemical reaction of Ir³⁺in the melts.Therefore,they were defined as the“dynamic mechanisms”,such as temperatures,concentration of Ir³⁺ions in the melts and current parameters.The second groups included those factors who affected the microstructure of the Ir coatings by modification of structure of the electrode/electrolyte interface.Therefore,they were defined as the“structure inhibition mechanism”,such as composition of the meltss.Electrodeposition of Ir coatings from the melts consisted of two different step,namely nucleation and Ir grain growth,which co-determined the microstructure of the Ir coatings.If the nucleation step only occurred at the initial stage of Ir electrodeposition,most of the Ir grains grown along the vertical direction of substrates and the Ir coatings were mainly characterized as the field-oriented texture structure with coarse grains?FT type?.If the nucleation step occurred from beginning to end of Ir electrodeposition,the Ir coatings were characterized as the unoriented dispersion structure with fine grains?UD type?.Taking these factors into considerization,unoriented dispersion structure and dense and continuous Ir coatings with smooth surface could be prepared by pulse current electrodpeosition from the melting IrCl₃-fluoride/chlorides.Failure of Ir coatings on the Re substrates was resulted from the formation of penetrating holes on the boundaries of Ir coatings when oxidized at high temperature in stagnant air.High temperature oxidation failure of Ir coatings on the Re substrates included two mechanisms,uniform oxidation of Ir grains to reduce the thickness of Ir coatings and preferential oxidation of grain boundaries caused by the Re diffusion along the Ir grain boundaries.Oxidation temperature,atmosphere,thickness and microstructure of Ir coatings had great effects on its oxidation resistance.The atomspher factor mainly affected the oxidation rate of Ir grains.The lower the partial pressure of Oxygen was,the lower oxidation rate of Ir grains was.Microstructure of Ir coatings thrown effects on the oxidation resistance of Ir coatings through changing the Re diffusion rate along Ir grain boundaries.The longer the boundary diffusion distance from the substrate to surface of Ir coating was,the better the oxidation resistance of the Ir coatins was.Temperature and thickness of Ir coatings affected the oxidation resistacne through the two mechanisms.Comparably,preparation of equiaxed grain Ir coatings was the most efficient method to improve its oxidation resistance.It demonstrated that the lifetime of 100?m-thick Ir coating with equiaxed grains was about 420min oxidized at 2273K in the stagnant air,which was far longer than that of columnar grain Ir coatings with the same thickness at the same conditions.