Evaluation of the effects of pH and oxygen on mitigation of wall thinning of carbon steel due to flow-accelerated corrosion

摘要

A one dimensional (1D) flow-accelareted corrosion (FAC) code developed byapplying 1D mass transfer coefficient (MTC) and geometrical factors can determineregional maximum wall thinning rate, which could evaluate the high FAC risk zone.As a result of velification and validation evaluation, it was confirmed that estimationaccuracy of the wall thinning rate obtained with the 1D FAC code was within a factorof 2. At the indicated high FAC risk zone, precise flow patterns around the structuresurface are calculated with a 3D computational fluid dynamics (CFD) code,distributions of MTCs at the surface are obtained, and then, 3D wall thinning ratesare calculated with the coupled models of static electrochemical model and dynamicoxide layer growth model.One of the promising countermeasures for FAC risk mitigation is to increase pH inthe cooling water. Increasing pH from 9.2 through 10 might extend the time marginfor pipe rupture. At present time not only ammonia but also morpholine, ethnolamineand so on are injected to control pH in the PWR secondary coolant. Hightemperature magnetite solubility applied for the 1D and 3D FAC codes wasexpressed as a function of pHRT determined by ammonia. Temperature dependentpHs for many chemicals were calculated to evaluate high temperature pH which wasapplied to determine suitable pH level to mitigate wall thinning.