Electrochemical Synthesis Mechanism Of Zirconium Diboride Coating In Molten Salt And Its Corrosion Resistance To Molten Metals

Transition metal borides?TMBs?are a group of materials with excellent combination properties like high hardness,oxidation and corrosion resistance.Various covalent bonds between boron atoms and high valence-electron density of transition metals induce multiple electron transfers and speciality of chemical bond formations and thus lead to the rich structures and potential multiple properties of TMBs.Of TMBs,ZrB₂ has promising applications in machinery,chemistry,electronics,aerospace industries due to its low density,high melt temperature,high thermal conductivity,good thermal shock resistance and low coefficient of thermal expansion.In this dissertation,the electrochemical reduction mechanism of Zr???and B???in NaCl-KCl molten salt systems and the electroreduction process of ZrB₂ have been studied by electrochemical measurement techniques for the aim of providing theoretical foundation for the preparation of ZrB₂ coatings by molten salt electrolysis.ZrB₂ coatings have been synthesized on different materials by electrodeposition in a NaCl-KCl-K₂ZrF₆-KBF₄ molten salt.Scanning electron microscope?SEM?,energy dispersive spectrometer?EDS?and X-ray diffraction?XRD?have been applied to investigate the influence of technological parameters including current density,temperature and deposition time on the surface morphology,microstructure characteristic and phase composition of the electrodeposited coatings.Meanwhile,the effects of ZrB₂ coatings on the corrosion of stainless steel in molten aluminum and molten zinc have been studied.The main results are as follows:The electrochemical reduction mechanism of B???in molten NaCl-KCl-KBF₄ has been studied using cyclic voltammetry,chronopotentiometry and square wave voltammetry methods.The reduction potential of B???appears at-0.9 V?vs.Pt reference electrode?.The reduction process of B???is a diffusion-controlled single-step three-electron quasi-reversible process,but with a better reversibility observed at lower scan rates.The diffusion coefficient of B???at 750? is calculated to be the order of magnitude of 10-5 cm²·s-1.The diffusion activation energy can be calculated to be 65.2 kJ·mol-1.The electrochemical reduction mechanism of Zr?IV?in molten NaCl-KC1-K₂ZrF₆ has been studied using cyclic voltammetry,linear sweep voltammetry and chronopotentiometry methods.The reduction potentials of Zr?IV?appear at-0.8 V,-1.2 V and-1.6 V.Potentiostatic electrolysis at the three potentials suggests that no solid products are obtained at-0.8 V and-1.2 V,while gray deposits are obtained under the potential of-1.6 V and proved to be Zr by SEM and XRD.The reduction of Zr???is a three-step diffusion-controlled complex process.Zirconium ions can be reduced to Zr at the last step and intermediate products exist as complex zirconium ions with different valences in molten salts.The reduction process of ZrB₂ in molten NaCl-KCl-K₂ZrF₆-KBF₄ has been studied using cyclic voltammetry method.The molar ratio of electroactive species K₂ZrF₆ and KBF₄ can significantly influence the reduction process.When the molar ratio of Zr and B is high,the reduction peaks for Zr???,B???and the formation of ZrB₂ can be identified in the cyclic voltammograms.When the molar ratio is comparatively low,however,these reactions merge into one peak.ZrB₂ coatings have been deposited on stainless steel substrates by galvanostatic electrolysis and the influence of some parameters like current density,temperature and deposition time on the quality of the coatings has been investigated.The results reveal that too large or too small current density is unfavorable for the preparation of ZrB₂ coatings,with an optimized value of 100 mA·cm^-2.This is also applicable for the operating temperature and the proper value is 750 ?.ZrB₂ coatings can also be prepared on graphite and C/C composite substrate,respectively,but with obvious cracks observed on the surface of ZrB₂ coatings deposited on the C/C composite.The corrosion behaviors of 201 stainless steel and ZrB₂ coating in molten aluminum at 800 ? have been studied using static corrosion tests.201 stainless steel shows poor corrosion resistance and partly dissolves into aluminum after 4 h immersion,forming double-layered corrosion products composed of inner?Fe,Cr?₂Al₅ intermetallic compounds and outer?Fe,Cr?Al₃ intermetallic compounds.ZrB₂ coatings exhibit good corrosion resistance and can protect the substrate from aluminum attack.After 96 h immersion,however,localized corrosion is also observed due to partial cracking and spallation of the coating,especially along edges.The corrosion behaviors of 201 stainless steel and ZrB₂ coating in molten zinc at 550 ? have been studied using static corrosion tests.201 stainless steel suffers severe corrosion in molten zinc,forming double corrosion layers composed of inner ?-phase??Fe,Cr?Zn₇?and outer ?-phase(?Fe,Cr?Zn₁₃).ZrB₂ coatings act as an effective barrier against molten zinc and greatly improve the corrosion resistance of the substrate.Although cracks and spallation emerge after long-term immersion,the remained ZrB₂ coating can still play its barrier role.