Study On Zirconium Coatings Electrodeposited From Molten Salt And The I-SCC Properties

Nuclear energy is one of the important energy sources because it has many advantages such as cleanliness,high efficiency,and abundant reserves.With the increasing application of nuclear power in the world,people are paying more and more attention to improving the safety of nuclear power during operation.Zirconium is used as the nuclear fuel cladding of nuclear reactors due to its low cross-section for neutron absorption and good mechanical properties.This nuclear fuel cladding is the first barrier to prevent the fission products from leaking.However,with the increase of reactor power,the thermal expansion of fuel pellets and the corrosion of fission product iodine will make the zirconium alloy cladding prone to fail because of iodine-induced stress corrosion cracking(I-SCC),which will increase the risk of fission product leakage.Therefore,it is of great significance to improve the I-SCC performance of the zirconium alloy cladding.In this paper,Zr coatings were prepared in the two molten salt systems,pure fluoride molten salt and chloride-fluoride mixed molten salt,on alloy ZIRLO with different power supply methods,deposition temperatures,current densities,duty cycles and frequencies by molten salt electrodeposition method.The influence of different process parameters on the phase composition,microstructure and bonding strength of the Zr coatings was systematically studied.Moreover,the I-SCC behavior of the Zr-coated samples was tested by slow strain rate tensile(SSRT).The main research results are as follows:In the FLiNaK-K2ZrF6 pure fluoride molten salt,Cu-Zr-Al and Cu-Zr diffusion coatings can be obtained on the copper substrate without applying power.However,an external power source is necessary to obtain the Zr coating on the surface of alloy ZIRLO.The size of Zr grain obtained by direct current(DC)electrodeposition was large,and the bonding strength with the substrate was poor.The size of Zr grain obtained by pulse current(PC)electrodeposition was small,and the bonding strength with the substrate was better than the Zr grain obtained by DC electrodeposition.The Zr coating obtained by the bidirectional pulse current electrodeposition had the best performance,hardness value,and bonding strength.The Zr coating with the same surface quality as the sheet sample can be obtained on the inner wall of the zirconium alloy tubular by PC electrodeposition.In FLiNaK-K2ZrF6 pure fluoride molten salt,the effect of current density on the structure and properties of Zr coating was studied.As current density increased,grain size decreased progressively,and the crystal orientation changed from(00L)-orientated to(110)-direction tendency.Zr coating can improve the ultimate tensile strength of alloy ZIRLO in iodized butanol solution.Especially,the sample with Zr coating obtained under a current density of 90 mA/cm2 had the highest ultimate tensile strength and ductility during the SSRT test in iodized butanol solution.I-SCC susceptibility indices of ultimate tensile strength and failure time were reduced from 0.23 to 0.12 and 0.55 to 0.40,respectively.Zr coatings could reduce the diffusion rate of iodine perpendicular to the surface from 3.258×10-12 m2/s to 0.175×10-12 m2/s.The effect of the duty cycle on the structure and properties of Zr coating was studied in FLiNaK-K2ZrF6 pure fluoride molten salt.As the duty cycle increased from 20%to 80%,the average grain size of zirconium deposits increased,but current efficiency decreased progressively.The Zr coating of 50%duty cycle had the highest bonding strength with the substrate.Furthermore,it had the highest ultimate tensile strength and ductility during the SSRT test in iodized butanol solution.I-SCC susceptibility indices of ultimate tensile strength and failure time were reduced from 0.23 to 0.09 and 0.55 to 0.40,respectively.In the KF-NaCl-LiF-K2ZrF6 chloride-fluoride mixed molten salt,the effects of different electrodeposition parameters on the structure and properties of Zr coating were studied.The Zr coating could be obtained on the surface of alloy ZIRLO substrate when the temperature was between 943 K and 1023 K.Temperature has a great influence on the Zr.coatings.As the temperature increased,both the bonding strength and the I-SCC performance gradually decreased.Current density and frequency have little effect on the Zr coatings,relatively.The I-SCC performance of the Zr coated sample with a temperature of 943 K,a current density of 60 mA/cm2 and a frequency of 100 Hz was the best.The ultimate tensile strength was 383.38 MPa,the elongation reached 34.97%,and the failure time could be increased to 24.22 hours.Iodine ions will diffuse non-directionally on the surface of the zirconium coating because there are no other alloying elements in the coating.In addition,Zr coating can increase the diffusion path of iodine,reduce the diffusion rate of iodine,and effectively inhibit the occurrence of pitting corrosion,because the columnar crystal structure of the zirconium grains in the coatings can provide wide grain boundaries.