It is expected that the inflow of ground water to the deposition holes and tunnels in the Forsmark repository will be very slow. Thus, it might take some few hundred years up to thousand years before the deposition holes are filled with ground water and it might take 6000 years or more before the bentonite buffer is fully water saturated and pressurized. The copper canisters will therefore meet to two completely different environments;
- an initial period of several hundreds of years when copper is exposed to gaseous corrosion
- and then to aqueous corrosion
From a corrosion point of view the first 1000 years are the most critical for the copper canister since pure, or phosphorus alloyed copper, is not designed to cope with corrosion at elevated temperatures. The outer copper surface temperature is expected to reach 100°C within some decades after closure of the repository and then slowly cool down to around 50°C after 1000 years.
The gaseous corrosion is treated in SKB´s safety assessment as being only dependent on oxygen gas and thus easily estimated by an oxygen mass-balance calculation. This simple model has no scientific support since several corrosive trace gases, such as sulphurous and nitrous compounds, operates together with water molecules (moisture) and the corrosion product consists mostly of oxides and hydroxides derived from water molecules. These trace gases are known to have an accelerating effect on copper corrosion. Any corrosion model describing the gaseous copper corrosion period must therefore be based on experimental data.