About the effect of high hydrazine chemistry on FAC occurrence in PWR's Steam Generators

摘要

Flow Accelerated Corrosion (FAC) of carbon steels has been studied for twenty years in the EDF laboratories and is now fairly well understood. It's well known that reducing environment is effective to increase the resistance of steam generator tubing to intergranular attack / stress corrosion cracking and pitting. In that way, secondary water chemistry specifications have recently been modified changing from low hydrazine to high hydrazine chemistry in the steam-water circuit. Nevertheless, increasing hydrazine levels could have a detrimental effect enhancing the both FAC process and tube fouling in steam generators. Therefore, several Research and Development investigations have been carried out by EDF and have resulted as follows. First, a non linear effect of hydrazine on FAC rate of carbon steels has been exhibited on the CIROCO loop in strictly controlled water chemistry and mass transfer conditions with electrochemical potential measurements showing the reducing character of the studied environment. Next, characterisation of produced oxide layers due to FAC have shown that changes in oxide surface morphology (i.e. both oxide surface area and porosity) have been related to the influence of hydrazine on the oxide solubility. In that way, several improvements of the FAC modelling have been proposed to take into account the effect of reducing environment. These results could explain FAC degradations of French PWR steam generator internals and could allow secondary water chemistry specifications to be optimised.