Experimental assessment of flow accelerated corrosion in nuclear components

International audienceFlow accelerated corrosion (FAC) has been extensively described in the specific case of carbon steel exposed to flowing water or wet stream as FAC has caused a large number of failures in piping and equipment in all types of fossil, industrial steam, and nuclear power [1]. FAC degradation, which results in wall thinning, is mainly attributed to dissolution of normally protective magnetite film that normally forms on the internal pipe wall surface. It depends on factors such as flow conditions (flow velocity), flow geometry, chemistry condition and material. Engineers use empirical correlations incorporated in numerical codes such as CHECWORKS developed by EPRI, BRT-CICERO® developed by EDF or more advanced codes based on Computational fluid dynamics to predict the actual corrosion rates due to FAC in carbon steel tubes. However efficient for carbon steels, these programs are only valid for a limited operational window and cannot be transposed to other types of materials. Experimental data are clearly needed for any material exposed to flowing water to observe if FAC may occur and to identify the main parameters influencing the dissolution rate. Although trials have been made to collect data using lab experiments such as electrochemical devices using a submerged impingement jet system, very few representative set-ups have been developed. The FACETT device (for Flow Accelerated Corrosion Experiments for Tube Testing) at CEA Paris-Saclay allows to investigate FAC in PWR primary water (300 - 350 °C, appropriate chemistry). Outer surface of tubes as well as flat samples can be tested. The circulation pump enables water flow at the specimens up to about 7 m.s-1. Sampling of the media can be performed to follow the chemistry change.The present work reports data obtained in the FACETT corrosion loop for SiCf/SiC cladding tubes and titanium alloys. FAC rates and mechanisms were established based on weight loss measurements and material surface examination. A comparison with the results obtained in static media is done and clearly shows the significant influence of flowing water. [1] V. Kain, Flow Accelerated Corrosion: Forms, Mechanisms and Case Studies, Procedia Engineering 86 (2014) 576 - 58