Zirconium alloys have been extensively used as a cladding material for fuel rods in nuclear reactors, due to their low thermal neutron absorption cross-section, excellent corrosion resistance and good mechanical properties at high temperatures. Zircaloy-4 is a specific zirconium based alloy containing, on a weight percent basis, 1.2 % to 1.7 % Sn, 0.18 % to 0.24 % Fe and 0.07 % to 0.13 % Cr, and was developed for use in pressurized water reactors (PWR). It is well known that the cladding tube exposed to PWR reactor environment is subject to an oxidation with power rate law at initial stage and with a linear kinetics at later stage. Also, the Zircaloy tubes often failed by fretting corrosion occurring at the tube-grid contact due to flow-induced assembly vibration and also by erosion (debris-induced fretting) caused by debris such as $Fe_3O_4$ and $Fe_2O_3$ flowing and moving in coolant. Since these corrosion failures result in the reduction of life time of the fuel cladding tube and cause a contamination of radio active materials, it is necessary to improve the surface properties of fuel cladding tube such as wear and corrosion resistance and hence to increase the life time of the tubes during reactor operation. Alloy developments and surface treatments have been attempted to improve the corrosion and mechanical properties of Zircaloy tube.
In this work, the effects of nitrogen ion implantation and laser treatments on the microstructure and corrosion behavior of Zircaloy-4 were studied. These surface treatments are effective technologies capable of improving the surface properties of fuel cladding material without noticeably affecting the bulk properties of the alloy. Also, the semiconducting properties of passive films formed on Zircaloy-4 were examined based on in-situ photo-electrochemical analysis since the corrosion resistance of metals and alloys. has a close relationship with the structure and composition of passive films formed on these alloys.
A. Effects of Nitrogen Ion Implantation on the Corrosion Resistance of Zircaloy-4.
The influence of nitrogen implantation on the corrosion behavior of Zircaloy-4 was examined by potentiodynamic polarization tests in a chloride and an acid solution, and the results were discussed with structural and compositional variations of implanted layer that was determined by X-ray diffraction (XRD) and Auger electron spectroscopy (AES). The resistance to localized corrosion of Zircaloy-4 was very sensitive to the ion dose and the substrate temperature during the implantation. At substrate temperatures above 200℃, the pitting potential of the nitrogen implanted alloy in deaerated 4 M NaCl at 80℃ increased from $350mV_{SCE}$ for the unimplanted sample to values of 900 to about $1400 mV_{SCE}$ for the implanted alloy with an increase in a ion dose. In contrast to this, the alloy implanted at 100℃ exhibited an inferior corrosion resistance to the unimplanted sample irrespective of the ion dose. A drastic increase in the resistance to pitting corrosion in chlorides were found to be associated with the formation of a continuos ZrN layer and a significant reduction in the passive current density in acid solution of the implanted alloy be associated with $ZrO_2$ layer. The low resistance to localized and general corrosion of the alloy implanted at 100℃ was attributed to the increase in structural defects produced by ion bombardment and to low atomic mobility in the implanted layer.
The influences of nitrogen implantation on the corrosion resistance of Zircaloy-4 were examined by immersion tests in high temperature water at 350℃, and the results were discussed with structural and compositional variations of implanted layer that were determined by X-ray diffraction (XRD) and Auger electron spectroscopy (AES). The nitrogen-implanted layer is composed of ZrN and $ZrO_2$ layers. The corrosion resistance of nitrogen implanted Zircaloy-4 depends on the thickness of $ZrO_2$ layer formed during the implantation. Since ZrN in the implanted layer is decomposed with releasing of nitrogen into water, the formation of ZrN during the nitrogen implantation was found to be detrimental to the corrosion resistance of Zircaloy-4 in high temperature water.
B. Influence of Laser Surface Alloying with Niobium on the Corrosion Resistance of Zircaloy-4.
The influence of laser surface alloying (LSA) with niobium (Nb) on the corrosion and mechanical properties of Zircaloy-4 was examined by potentiodynamic polarization tests in a chloride solution at 80℃ and microhardness test. The results are discussed with structural and compositional variations of LSA layer determined by X-ray diffraction (XRD), scanning electron microscope (SEM) and wavelength dispersive spectrometer (WDS). LSA on the Zircaloy-4 precoated with Nb produced a Nb-alloyed layer 200∼300 μm thick with 1.3∼2.5 wt % Nb, depending on laser beam power. The alloyed layer was composed of a mixed structure of α-Zr and β-Zr phases with the β-Zr phase being increased with the Nb content in the alloyed layer. LSA with Nb increased the microhardness of Zircaloy-4, which was attributed primarily to a grain-size refinement by rapid cooling and also to a solid solution hardening with Nb. The resistance to localized corrosion of Zircaloy-4 in a chloride solution was significantly improved by LSA with Nb, which was attributed to the combined effects of fine rapidly cooled microstructure and Nb alloying. However, LSA with Nb reduced the corrosion resistance of Zircaloy-4 in steam, which appeared to be associated with the β-Zr phase formed by LSA with Nb and hydrides formed in oxide of laser surface alloyed samples.
C. Photo-electrochemical Study on the Passive Film of Zircaloy-4.
Semiconducting properties of the passive film formed on Zircaloy-4 in pH 8.5 buffer solution were examined by the photocurrent measurement. Passive film formed on Zircaloy-4 at low potentials below 0 V was found to be a hydrous oxide with band gap energies of 3.2±0.14 eV and 2.98±0.29 eV. In contrast, the passive film formed at potentials above 0.25 V was composed of duplex layer with external the hydrous oxide layer and internal an anhydrous oxide($ZrO_2$) layer with band gap energy of 4.30±0.15 eV close to the value (4.77 eV) for the crystalline $ZrO_2$ formed in air at 400℃. Photo current spectra($i_{ph}$ vs. hν) of the passive film as a function of passivation time was also measured. In the initial times(within 1 h), the hydrous oxide has primarily grown and the anhydrous oxide did at later times(after 2 h). It was observed that the flat band potential of the hydrous oxide(-0.6 V) is different from that of the anhydrous oxide(-1.2 V) from $i_{ph}$ vs. hν plots measured for the passive film formed on Zircaloy-4 at 1 V at various applied potentials. Donor density of the passive film formed at low potential (-0.5 V) evaluated from the Mott-Schottky plot was higher than that of the film formed at high potential (1 V).
