Influence of Operating Time on the Corrosion Rate in Single-Phase and Two-Phase Media
Modern foreign computer codes forecast a linear increase in pipe wall thinning over time due to the process of flow-accelerated corrosion (FAC), i.e., erosion-corrosion wear (ECW). However, the linear time-thinning dependence and the corrosion rate constancy are not typical of the operating conditions of NPP pipelines. And the associated excessive conservatism of the residual lifetime estimates leads to increased economic costs of repeated monitoring. In the domestic software tools, EKI-02 and EKI-03, the effect of the operating time is taken into account by introducing an appropriate coefficient into the Chexal-Horowitz model based on the yield of corrosion products into the coolant. This reflects the fact that the ECW rate increases in the early years of operation, when the yield of iron compounds in the feed water is very high, and that this process slows down significantly over time. The analysis showed that this dependence is valid for nuclear power plants with various types of reactor facilities. However, improving operating conditions, carrying out preventive measures, refining the water chemistry, etc. can reduce the FAC intensity in the components of equipment and pipelines, and the use of the dependences once obtained may turn out to be too conservative. Based on a large number of repeated measurements as well as data from corrosion indicators, the authors show that the effect of time can be described by a certain function, the coefficients of which differ for different units, types of components and subsystems. This makes it possible to determine the ‘aging function’ according to the control data, and then use it in a targeted way for specific elements. It is shown that the conservatism of such estimates is significantly lower.
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