Electrochemical Impedance Spectroscopic Study of Passive Zirconium

Abstract

The development of deterministic models for predicting the accumulation of corrosion damage to zirconium and Zircaloys in boiling water reactor coolant environments requires the acquisition of values for various model parameters. In the present work, the point defect model ͑PDM͒ was further developed to account for the properties of passive films comprising oxide barrier layers and porous oxide outer layers that form on zirconium and Zircaloys in high-temperature, deaerated aqueous solutions. The model parameter values were extracted from electrochemical impedance spectroscopic data for zirconium in deaerated, borate buffer solution ͓0.1 M B͑OH͒ 3 + 0.001 M LiOH, pH 6.94͔ at 250 °C by optimization. The results indicate that the corrosion resistance of zirconium in high-temperature, deaerated aqueous solutions is dominated by the porosity and thickness of the outer layer. The impedance model based on the PDM provides a good account of the growth of the bi-layer passive films described above, and the extracted model parameter values might be used, for example, for predicting the accumulation of general corrosion damage to Zircaloy fuel sheath in BWR operating environments.

Mechanistic Analysis of Anodic Dissolution of Zr in Acidic Fluoride Media

“…There is a significant amount Zr 3+ ads (2) in the active region, and its fractional surface coverage continuously decreases with overpotential. Figure 9 also shows that the surface coverage of Zr 4+ ads (1) and Zr 4+ ads (2) increases slowly in the initial phase, but at higher overpotentials, it increases rapidly. These findings are comparable to the surface coverage plots obtained for 10 mM HF solution using the mechanism shown in Eq.…”

“…However, in the active region, the reverse reactions cannot be neglected. A comparison of the chemical dissolution steps by the two pathways show that k 6 is significantly larger than k 5 and that the chemical dissolution rate via Zr 4+ ads (2) intermediate species will be more than the corresponding dissolution rate via Zr 4+ ads (1) species. On the other hand, k 8-dc is almost two orders of magnitude less than k 7-dc and hence electrochemical dissolution Zr crease in the current in the active region and the continuous decrease in current in the passive region.…”

“…The fractional surface coverage of the bare metal is very small as compared to the coverage of other entities and it decreases further with an increase in the overpotential. The fractional surface coverage of Zr 3+ ads (1) initially increases up to a potential of about 0.15 V vs. OCP and then it decreases with an increase in overpotential. There is a significant amount Zr 3+ ads (2) in the active region, and its fractional surface coverage continuously decreases with overpotential.…”

“…[1][2][3][4] The wide acceptance of this metal and its alloys is due to their superior mechanical strength, and high resistance toward corrosion, and H 2 embrittlement. [5][6][7][8] These properties are due to the formation of a stable, protective passive film of ZrO 2 .…”

“…Since these rate constants are independent of potential, the chemical dissolution rate follows the increasing trend of the surface coverage of Zr and Zr 4+ ads (2) . The electrochemical dissolution is a function of the rate constants k 7 , and k 8 as well as the surface coverage of Zr 3+ ads (1) and Zr 3+ ads (2) . The rate constant k 8 is three orders of magnitude smaller than k 7 and the total electrochemical dissolution rate is determined by the dissolution step involving k 7 and Zr 3+ ads (1) .…”