Overview on the CANDU steam generator materials corrosion

Full text: The Steam Generators (SG) equipment that ensures the connection between the primary and the secondary circuits, create several safety problems during operation, mainly due to corrosion and mechanical damages. Maintenance is also difficult in the SG because of the limited access to various components and because of the presence of the high radiation field existing on the primary circuit side. The SG tubing degradation caused by corrosion and other age-related mechanisms continues to be a significant safety and cost concern for many SGs. Understanding ageing mechanisms is the key to effective management of SG ageing and consists in the knowledge of SG materials and materials properties, stressors and operating conditions, likely degradation sites and wear mechanisms. Corrosion and the mechanical damage in the SG are the result of complex interaction between various factors: strict control of water chemistry, adequate thermo-hydraulic design, and selection of generator construction materials, utilisation of compatible materials for the entire secondary circuit, conditions of achieving equipment and facilities, the operation technique. This is why a careful analysis of corrosion problems is required, necessary both from an economic point of view and for the safe operation. The investigation of the structural materials corrosion in correlation with the water chemistry, as well as the impurities and corrosion products concentration and deposition and their removing from the CANDU SG is a very active field and both the experimental works and the understanding of the mechanisms involved are submitted to some rapid changes and permanently open to researches. To provide information about the corrosion behaviour of the structural materials from CANDU SG under normal and abnormal conditions of operation and to identify the failure types produced by the corrosion, a lot of corrosion experiments were performed. These experiments consisted in chemical accelerated tests, static autoclaving and electrochemical methods. The gravimetric method, optical metallographic microscopy, XRD analysis, as well as electrochemical measurements have been used to evaluate the corrosion behavior of the tube material (Incoloy-800) and the tubesheet material (SA 508 class 2 carbon steel). The experimental data allowed the correlation of different corrosion processes with the testing parameters. Based on the results obtained by the materials testing the scientist may appreciate the corrosion intensity of the steam generators materials or may draw up an experimental program, which could lead at immediate results. (authors)