Kinetics of hydrogen permeation through a Ni-base alloy membrane exposed to primary medium of pressurized water reactors
Résumé
The stress corrosion cracking (SCC) of Ni-base alloys in high temperature water is an intricate phe-nomenon in which numerous metallurgical, mechanical and environmental variables play a role. The role of hydrogen in the SCC of Ni-base alloys exposed to high temperature water has been discussed in numerous studies. Among the H-alloy interactions leading to a loss of mechanical strength, an em-brittling role of hydrogen absorbed during primary water exposure has been suggested in different works. However, such effect would involve large values of the hydrogen activity at the crack tip. Such large values could only be provided by the cathodic reactions concomitant to the oxidation process.In this context, the present study aims at establishing the kinetics of H permeation through a Ni-base alloy membrane from the first hours of its exposure to simulated PWR primary medium in order to evaluate if the evolution of the H flux measured in these conditions is consistent with high transient hydrogen activities at the alloy/oxide interface. The permeation experiment was carried out in a recir-culation autoclave equipped with ion exchange resins. The H2 overpressure was monitored to ensure the desired dissolved hydrogen concentration at 325DC. On one side the specimen (permeation mem-brane) was exposed to the primary water, and the other side pumped under vacuum and connected to a mass spectrometer. The final oxide scale formed on the primary water side was TEM-characterized in order to determine the continuous protective oxide layer thickness and composition.The H permeation flux JH passing through the Alloy 690 membrane during its exposure to simulated PWR primary medium in isothermal conditions at 325 DC shows a decrease of tending towards quasi stationary regime. It is suggested that the H permeation kinetics are, at least partially, governed by a diffusion step through the oxide scale growing on the Alloy 690 membrane during its exposure to simulated primary medium. Permeation kinetic laws are proposed in this work, taking into account the transport across the oxide layer then transfer into the alloy. The proposed model allows for the study of the evolution of P(H2) at the oxide/alloy interface during the permeation test based on the experi-mental data. The results and kinetic modelling of H permeation showed for the first time that at the very beginning of oxidation, high H partial pressure could be reached at the oxide/alloy interface (15 bar), compatible with hydrogen involvement in the SCC mechanisms of Ni-base alloys at such temperatures. This cou-pling between modelling and experimental approaches seem to be a promising way to access to the evolution of H activity at the alloy surface during the oxide scale growth, therefore in further works to assess the local H activity at the SCC crack tip during its propagation.This study was performed with the financial support of Framatome and EDF.