Carbon steel is a corrosion-allowance metal that is expected to have a rela­tively low corrosion rate in a mild, near-neutral pH, and reducing environ­ment such as granite and clay (Jung et al., 2011). One localized corrosion process in carbon steel is pitting corrosion. This pitting process is empiri­cally represented by a ‘pitting factor’, which is defined as a ratio of pit penetration depth to the uniform corrosion depth. Therefore, degradation by pitting corrosion of a carbon steel container can be represented by adjusting the magnitude of the general corrosion rates.

Even in an underground repository, which is planned in the long term to have a reducing environment, it will initially have an oxidizing condition due to the excavation conducted before closure (which provides oxygen). The general corrosion rate of carbon steel in an oxidizing environment is very high: in the range of 10-100pm/year (3.94 x 10-4 to 3.94 x 10-3 inch/ year) at room temperature in simulated mild initial groundwater (Jung et al., 2011). During 30-year oxidizing conditions applicable for carbon steel corrosion, a general corrosion rate of 50 pm/year (1.97 x 10-3 inch/year) will
result in a small penetration depth of 0.15 cm (0.02 inch). Estimating the carbon-steel general corrosion rate using laboratory data and analogue data indicates corrosion rates in the range of 0.1-10 pm/year (3.94 x 10-6 to 3.94 x 10-4 inch/year) in the reducing environment (Yoshikawa et al., 2008; David et al., 2002). Figure 7.2 shows the data collected from Yoshikawa et al. (2008) from Japan and David et al. (2002) from France.

Carbon steel is susceptible to pitting corrosion in an oxidizing environ­ment. In addition, carbon steel is expected to corrode at a higher general

Period (year)

1000 [22]

7.2 Measured corrosion rates of carbon steel in simulated solutions and correlation with archaeological analogue data up to 1,000 years: the first analysis was conducted in Japan (a) (used with permission from Elsevier), and the second in France (b) (used with permission from Maney Publishing).

rate in an oxidizing compared to a reducing environment. However, the effect of pitting corrosion can be accounted for by using an enhanced general corrosion rate. The pitting factor, which is a ratio of pit propagation depth to general corrosion penetration depth, will approach unity during the oxidizing period as the general corrosion proceeds deeper. The pitting does not enhance the corrosion penetration at this point. An example case is shown in Fig. 7.3 (Johnson and King, 2000).