Investigation On Corrosion Resistance Of Alloy 690 Steam Generator Heat Exchange Tube

Steam Generator (SG) is one of the most important equipments of Pressurized Water Reactor (PWR) nuclear power plant. SG heat exchange tubes are the heat exchange interface between primary and secondary loop as well as pressure boundary of primary coolant, whose integrality has great effects on operation safety of nuclear plant. Therefore, the materials for SG heat exchange tubes must meet stringent technical requirements, among which resistance to the corrosion in primary and secondary coolant corrosion is of the greatest concern.In this paper, we study on intergranular corrosion, pitting,stress corrosion cracking and general corrosion resistance of Alloy 690 made in Baosteel comparing with French and Japanese ones by changing the variants like rolling deformation, annealing solution temperature and time, thermal treatment time, carbon content. We do the comprehensive study using low-temperature chemical etching methods, electrochemical methods, high-pressure autoclave experiments and scanning electron microscopy (SEM) techniques.The results show that, deformation and solution treatment processes have great effects on grain boundary structure and corrosion resistance of Alloy 690. Grain boundary carbide morphology, electrochemical characteristics and corrosion resistance are showing remarkable improvement after solution treatment and thermal treatment. We can get the lowest corrosion rate with 10 hours'thermal treatment.Carbon content has great concern with gain size control and grain boundary carbide precipitation. The grain growth is slow and grain size distribution is uneven when carbon content is high. The inner wall of tubular specimen with 0.023% carbon distributes a fine-grained layer of smaller grain size when the solution temperature is low after rolling. The width of"fine-grained layer"gradually decreases as solution temperature and time increase. When carbon content is lower, the grain growth is faster to be easy to get isometric crystal structure and the grain boundary carbide precipitation amount is less, which can bring out grain boundary carbide morphology optimization to avoid the appearance of fine-grained layer.The research shows that, medium conditions like temperature, sulfuric acid concentration, potassium thiocyanate concentration influence intergranular corrosion sensitivity and passivation properties of Alloy 690 evidently. We can find from electrochemical potentiodynamic reactivation tests that, passive current increases as temperature increases, reactivation rate gets lowest at 40℃. As sulfuric acid concentration increases, passive current and reactivation rate become higher and higher. Potassium thiocyanate concentration has complex effects.Localized tubes and imported ones of French and Japanese are investigated in simulated PWR primary coolant. Their general corrosion rate is equal to each other, getting relatively stable after 1600 hours. Both materials exhibit good resistance to stress corrosion cracking.