Methodology for a mechano-electrochemical evaluation of the coupling at the crack tip. Application of halide-induced Stress Corrosion Cracking of Zircaloy-4

Emilien Durif,Marion Fregonese,Julien Réthoré,Alain Combescure,Catherine Alemany-Dumont

doi:10.1016/j.corsci.2015.01.010

Abstract

In order to evaluate the relative contribution of both mechanical and electrochemical processes at the crack tip during Stress Corrosion Cracking propagation, an original combination of controlled pre-cracking followed by thermal pre-oxidation of SCC specimens was developed. It allowed localizing dissolution at the crack tip for Zircaloy-4 tested in halide solutions under potentiostatic polarization. Digital Image Correlation and electrochemical methods were then carried out to study correlations between the stress intensity factor, the crack length and the measurements of dissolution current.

Highlights

Stress Corrosion Cracking (SCC) in metallic alloys is still a serious problem in many industrial domains such as the oil and gas, chemistry and nuclear industries
It was chosen to oxidize the pre-cracked specimens at 470 °C for 40 h in air and to further polarize the pre-oxidized specimen above the pitting potential of the non-oxidized metal, under tensile stress. This temperature being lower than the recrystallization temperature of Zircaloy-4, it was considered that the mechanical properties were not affected by the thermal treatment
From the results presented in the previous paragraph, it can be observed that electrochemical activity is concentrated on new non-oxidized surfaces generated during crack propagation

Summary

Introduction

Stress Corrosion Cracking (SCC) in metallic alloys is still a serious problem in many industrial domains such as the oil and gas, chemistry and nuclear industries. Most studies related to SCC consider the crack rate propagation ðddatÞ the main interest. Due to the synergetic nature of SCC, the crack propagation rate appears to depend on both mechanical (r⁄) and electrochemical (v⁄) parameters [2,3,4,5]: da dt 1⁄4 f ðrÃ ; vÃ Þ ð1Þ. V⁄ parameter estimation mainly consists of evaluating the dissolution current density j at the crack tip while da dt can be related to j by local Faraday’s law [2]: z MÁj ÁFÁq ð2Þ. (M: molar mass of the dissolved species, z: number of electrons exchanged during dissolution, F: Faraday’s constant, q: density of the corroding metal.) From an electrochemical point of view, three possibilities of evaluating j are offered:

Objectives

Methods

Results

Conclusion