Influence of corrosion processes on the head loss across ECCS sump strainers

Corrosion of hot-dip galvanized grating treads, installed in the containment, may exacerbate ECCS strainer clogging problems during LOCA incidents in LWR. In order to identify the corrosion mechanisms as well as the nature of particulate corrosion products formed, corrosion investigations were carried out in boric acid solutions as well as in demineralized water using hot-dip galvanized steel samples. For the related experiments, tailored bench-scale facilities have been constructed. The results of batch experiments as well as of long-time corrosion investigations in the KorrVA test facility at FZD suggest that there is a multi-stage corrosion process. The first stage comprises the dissolution of the Zn layer in the acidic coolant forming zinc ions, which can generate crystalline zinc borate in boric acid solutions, if the ion concentration exceeds the saturation concentration. The base material (steel) is cathodically protected as long as the zinc layer is present. During the second stage, a slight head loss rise may occur due to the release of hard zinc (Zn/Fe) particles. After the complete removal of the hard zinc layer, the base material dissolves forming insoluble corrosion particles similarly to a flow accelerated corrosion process, which can subsequently lead to acceleration of the strainer clogging. The main influences on corrosion were identified as jet impact, water chemistry and Zn surface / coolant volume ratio.