FLOW ACCELERATED CORROSION OF CARBON STEEL IN THE FEEDWATER SYSTEM OF PWR PLANTS - BEHAVIOUR OF WELDS AND WELD ASSEMBLIES - 2

摘要

Flow Accelerated Corrosion (FAC) of carbon steel is a phenomenon that has been studied for many years. However, to date, the specific behavior of welds and weld assemblies of carbon steel towards this phenomenon has been scarcely examined. An experimental program of FAC of welds and weld assemblies is being conducted by EDF and CRIEPI. A previous paper had described the influence of welding method and chromium content on the behavior of welds towards FAC as well as the sensitivity to FAC of various weld assembly configurations. The study of different configurations showed that the chromium content is the main parameter affecting the behavior of the weld specimens. This paper describes the impact of temperature on FAC of welds and weld assemblies and the behavior of weld assemblies in the presence, or not, of an artificial step, in order to study the influence of penetration welds on FAC. The experimental device used for this work permits to follow online the FAC rate by gammametry measurements. Tests were performed under ammonia medium at pH_(25°C) 9.0, and oxygen concentration lower than 1 ppb. Chemical parameters, as the pH, the conductivity and oxygen concentration, are measured in situ during the test and surface characterizations are performed after each test. To study the impact of temperature on FAC of welds, tests were conducted on weld metal and base metal between 100°C and 220°C. For both metals, results showed that the highest FAC rate is measured at 150°C. For the impact of an artificial step, CFD calculations were carried out in order to determine the optimal step height and suspected high metal loss rate areas to be followed by gammametry during tests. Several specimen configurations were studied where the varying parameter was the chromium contents in the materials. Results of FAC rate measurements showed that the difference in chromium content with the adjacent metal does not affect a weld or base metal FAC sensibility and that only its own content affects FAC rate. This paper also presents the geometrical impact on FAC upstream and downstream of the step. Experimental results are compared with calculation predictions. They both showed that FAC is higher downstream the step.