Electrochemical and surface studies on zircaloy-4 exposed to concentrated nitric acid medium

Abstract

Zircaloy-4 has been considered as a candidate material for dissolver and evaporators in nuclear fuel reprocessing plants involving nitric acid of high concentrations at high temperatures. The present work deals with the electrochemical studies and surface analysis of passive films on zircaloy-4 and its welds in 11.5 M nitric acid medium. Potentiodynamic polarization technique was employed to study the corrosion behaviour of the alloy in both base and welded (Manual TIG and electron beam (EB) welding) conditions. The polarization curves for zircaloy-4 and its welds exhibited a distinct breakdown of passivity at 1.5 V (SCE) and repassivation, after showing a region of stable passivation. The characterization of passive films by SEM, XRD and LRS, grown at different potentials in 11.5 M nitric acid are presented. The surface morphology and phase analysis of passive films grown at higher potentials showed the presence of monoclinic ZrO 2 . The phases present in the passive film grown electrochemically in 11.5 M nitric acid at 2.0 V and by autoclaving are found to be similar. The presence of characteristic 1040 cm -1 Raman peak from the sample passivated at 1.2 V indicate that the oxide present consist of nanometer size grains of monoclinic zirconia.

Corrosion issues in nuclear fuel reprocessing plants

“…19.22 [57]). This confirms the proposed mechanism for zirconium passivation involving the development of a quasi-isolating film in which a high electric field prevails (in the order of 10 6 V/cm), according to the Cabrera, Mott and Verwey model [60][61][62][63][64][65][66][67][68]. However, if the passive layer is of excellent quality at the potential at equilibrium, the film formed at a higher potential and under concentrated and hot nitric solutions tends to be less stable and porous [57,60,64,65].…”

“…19.22), the oxidation current is always high and thick layers of zirconia are formed and tend to scale off [59]. In this range of potential, monoclinic zirconia is the major phase and tetragonal/cubic zirconia are the minor phases detected [66]. Contrary to the passive film where it is absent, the presence of the nitrate ion was detected in the outside part of these layers [65,67,68].…”

“…photoelectrochemistry, impedance, ellipsometry, X-ray electron spectroscopy (XPS), atomic force microscope, Raman spectroscopy) provide some information on crystallographic structure, chemical composition and electronic structure [66][67][68][69][70][71][72][73][74][75][76], summarized in [77].…”

“…With regard to the crystallographic structure of passive films on zirconium, photoelectrochemical [75] and capacity [60] measurements suggest that the film is highly de-oriented or amorphous. However, Raman spectra indicate that the oxide could consist of nanometer grains of monoclinic zirconia [66]. Finer microellipsometric measurements show that it depends heavily on the crystallographic structure of the substrate [76].…”

Role of beam oscillation on electron beam welded zircaloy-4 butt joints

“…Potentiodynamic polarisation curves (Figure 4) showed the presence of breakdown potential and a rapid re-passivation in all test samples, which indicated the prevalence of pitting corrosion [13]. The passive film recovered quickly after being damaged to prevent Zr grade alloy being corroded.…”