Influence of operating time on the corrosion in single-phase and two-phase media

Modern foreign computer codes predict a linear growth in the pipeline wall thinning with time due to the process of flow-accelerated corrosion (FAC), i.e. erosion-corrosion wear (ECW). Linear time-thinning dependence and corrosion rate constancy are not however typical of the NPP piping operating conditions. And the associated excessive conservatism of the residual life estimates leads to increased economic costs of repeated inspections. In domestic software tools, EKI-02 and EKI-03, the influence of operating time are taken into account by introducing the respective coefficient into the Chexal-Horowitz model based on the yield of corrosion products into the coolant. The ECW intensity can be however reduced through improvements in operating conditions, preventive measures, improvements in water chemistry, etc., 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 on data from corrosion testers, it has been shown that the influence of time can be described by the function of a particular form, the coefficients of which differ for different units and component and subsystem types. This makes it possible to determine the ‘aging function’ based on inspection data, and then use it in a targeted way for particular components. It has been shown that such estimates are much less conservative.