Failure analysis and prevention of corrosion occurring during storage on steam generator tube sheet for advanced PWR, Part I: Root causes analysis

Abstract The manufacturing cycle of a large nuclear equipment like steam generator always lasts for at least forty months, so its components including the heat transfer tubes and the tube sheet have to be stored in the factory for several months before assembly, requiring a series of methods for corrosion prevention of them during this period. However corrosions, especially uniform corrosion are still inevitable. On account that the corrosion extent is not so severe, this issue is commonly not paid enough attention to by the manufacturer, only manual cleaning is complementally performed on the components for removing the corrosion products before assembly. However in this case, since the construction project of the whole nuclear power plant was delayed due to some technical factors, the steam generator tube sheet had to be stored in the manufacturer’s factory for at least one year. Thus, the corrosion extent of the drilled holes for fixing the heat transfer tubes was aggravated and likely to be even involved with pitting corrosion. Also, considering the large quantity of the drilled holes in the tube sheet (over 20,000), great effort must be taken for the cleaning work, which would further influence the schedule and the cost of the project. Hence, failure analysis and prevention was called for by the manufacturer to identify the root causes and mechanisms of the corrosion as well as to avoid the costly cleaning work in the future. This paper is the first one of two companion papers, focusing on the causes analysis and mechanisms discussion of corrosion inside the drilled holes of the steam generator tube sheet, while the sequel will be dedicated to the design and experiment of the diverse corrosion prevention methods. In this paper, based on the investigation results including examination of the matrix materials, observation of the macro- and microscopic morphologies, detection of the corrosion products, and inspection of the process media, it was finally determined that the uniform corrosion induced from inappropriate corrosion prevention methods was the predominant failure mechanism, and the possibility of the localized corrosions like pitting corrosion was ruled out.