Accelerated corrosion of carbon steel and zinc in oxygen-free groundwater: Due to the microbiological activity?

In Finland the low and intermediate level decommissioning metal waste from Olkiluoto Nuclear Power Plant located on an island, Olkiluoto is planned to be disposed in concrete boxes into bedrock silos. Most of this metal waste is carbon steel. The corrosion behavior of carbon steel in groundwater simulating repository conditions has been studied continuously since 1998. The aim of these studies is to get a reliable data for the dissolution of carbon steel in repository conditions. On the grounds of the results the long term corrosion behavior of activated carbon steel and zinc containing waste material from the decommissioning of plant can be evaluated. Carbon steel specimens are situated in oxygen free ground water in bedrocks of Olkiluoto Island. The test site is in the underground tunnel of operational waste repository of the power plant that is in operation. These specimens immersed in 1998 are placed together in the same drill hole as concrete specimens (drill hole KR21). In 2007 new specimens were immersed in another drill hole in the bedrock at the same site (drill hole KR19). This drill hole does not include concrete specimens. In the same drill hole also a previous test was performed during the years 2001-2007. Besides the behavior of carbon steel specimens also that of zinc and zinc coated steel specimens was studied in conditions that simulate the repository. The water in this drill hole is quite similar to the water of the drill hole KR19, Table 1. Zinc is known to be toxic to variety of micro-organisms. However, there are cases where sulfate reducing bacteria have increased the corrosion rate of zinc.1 In these cases the corrosion of zinc has been localized and also the under laying steel has had pitting corrosion in the areas where zinc had been dissolved away.