| Molybdenum is a type of rare refractory metals.Molybdenum and its coatings are widely used.Compared with other methods for preparing molybdenum coatings,molten salt electrolysis has the advantages of high current efficiency,high deposition rate,strong adhesion and good corrosion resistance.In this paper,molybdenum coating was prepared in fluoride molten salts by electrolysis.The electrochemical behavior of molybdenum ions has been studied in molten potassium fluoride and cryolite.The influence of different electrolysis conditions on the quality of molybdenum coating has also been investigated.A simple KF-Mo O3 molten salt system was selected for electroplating of molybdenum.The electrochemical behavior of Mo6+in this system has been tested using cyclic voltammetry at 900°C.The results show that the reduction process of molybdenum ions on a platinum electrode is a one-step process with an exchange of six electrons.The cathode process is a quasi-reversible process controlled by diffusion,and the diffusion coefficient was calculated as 0.91×10-5cm2·s-1.Using KF-Mo O3 as the electrolyte and molybdenum as the soluble anode,molybdenum coatings were prepared on copper and nickel substrates.Molybdenum coatings with high purity and good quality have been successfully obtained without additives.In this system,the coatings obtained at lower current densities are smoother,denser,and better in quality.The most suitable current density range is 11?33 m A·cm-2.The electrolysis time has an effect on the grain size and compactness,Coatings with electrolysis time of 30?60 min are better.The increase in temperature makes the grain size uniform and reduces the consistency of the coating.The best electrolysis temperature is 900?.The surface of the coating obtained by the periodic reverse current method is flat and the grains are finer.The preferred orientation of grains on the copper substrate is more obvious and the copper is dissolved in the system.Nickel sheet is more suitable as a base material.In the Na3Al F6-Na F-Al2O3-Mo O3 system with cheap raw materials and a wide range of sources,the cyclic voltammetry has been used to study the electrochemical behavior of Mo6+on a platinum electrode at 930°C.The cathode process of molybdenum in the Na3Al F6-Na F-Mo O3 system without addition of alumina is a quasi-reversible process controlled by diffusion,and the diffusion coefficient was calculated as 1.25×10-5cm2·s-1.After adding alumina,the reversibility of the cathode process is greatly reduced.In view of the irreversibility of the cathode reaction,molybdenum coatings were prepared in Na3Al F6-Na F-Al2O3-Mo O3 molten salts,using molybdenum metal as a soluble anode,on different substrate materials.The effects of current density,electrolysis time,electrolysis temperature and substrate materials on the quality of the coating have been studied.Under the conditions used in the experiment,the best current density for obtaining the optimal coating is 30 m A·cm-2,the electrolysis time should be60min,and the optimal electrolysis temperature is 950°C.Both nickel substrates and316 stainless steel substrates can obtain good coatings.In addition,it has been found that in this system,the irreversibility of the cathode reaction not only improves the quality of the coating under various conditions,but also provides the possibility of preparing thick coatings with acceptable current efficiency. |
PDF Full Text Request