ECP and Localized Corrosion of Steam Generator Tube Materials Under Simulated Crevice Conditions

Abstract

The electrochemical corrosion potential (ECP) of tube materials under faulted SG water and MULTEQ-calculated crevice chemistry conditions, which include crevice conditions resulting from seawater ingress, and simulated Bruce nuclear generator station A (BNGS-A) crevice conditions, was measured in order to provide input into attempts to determine the stress corrosion craclung (SCC), pitting and under-deposit corrosion susceptibility of steam generator (SG) tube materials. The effect of magnetite sludge, and of minor impurities in the sludge, on the ECP of the SG tube materials was studied by comparing the ECP of sludge-coated and uncoated samples. Experimental results revealed that various sludge deposits, including magnetitecontaining nickel ferrite, lead oxide and zinc silicate can increase the ECP by more than one hundred mV. Significant positive shifts in the ECP of the SG tube materials were also observed in simulated BNGS-A crevice chemistries. In these simulated chemistry environments, 100 ppm of PbO and 30 g/l of SiO, were present. In most cases, the measured ECP values for tube materials under crevice chemistry conditions were lower than the passive film breakdown potential of SG tube materials. This is because localized corrosion had already taken place in the aggressive crevice electrolyte. The kinetics of localized corrosion depend on many factors. This makes it difficult to reproduce the observed ECP values of SG tube materials. For instance, the measured value in simulated seawater crevice solution varied between -590 and -250 mV (SHE). The SG tube in the crevice is coupled with the tube free-span. The ECP of tube alloy in the crevice will also be influenced by the ECP at the tube fiee-span area. The existence of an IR drop will shift the ECP inside the crevice to a more negative value than at the tube free span. Tests performed in a simulated heated crevice showed this potential shift to be in the range 100 to 160 mV. The existence of steam bubbles will also add extra resistance to the ionic path in the corrosion system and influence the crevice ECP. The rate of crevice corrosion propagation is controlled by both the anodic and cathodic kinetics of corrosion-related reactions. The existence of oxidizing sludge inside the crevice enhances cathodic reactions which would normally take place at the free surface outside the crevice. Therefore the presence of oxidizing sludge inside the crevice will provide a short circuit to the corrosion system and increase the localized corrosion rate significantly