The possibility for formation of macro-cell corrosion in a liquid with low electrical conductivity

The possibility of electrochemical corrosion of carbon steel at the rate of 3.25 x 10(-5) A/cm(2) in water was examined under the conditions present during an accidental pipe rupture at the Mihama nuclear power plant: liquid conductivity, 7.5 mu S/cm; dissolved oxygen concentration, 5 ppb or less; pH 8.6 similar to 9.3; ferrous ion concentration, 20 ppb or less; temperature, 142 degrees C. The corrosion rate of iron in a micro-cell with a dissolved oxygen reduction cathode was estimated to be only 1/400 of the preceding rate. On the other hand, that in a micro-cell with a hydrogen ion reduction cathode was estimated to be as high as 1/10 of the preceding rate, that is, 3.25 x 10(-6) A/cm(2). Two important factors may have influenced the corrosion rate: the remarkable wall thinning, which must have been the direct cause of the rupture of the pipe, was located close to, and downstream from, an orifice; and, the water temperature was in the range at which carbon steel makes a transition from the active to the passive state. Taking these facts into consideration, it appears possible that micro-cells with different corrosion rates might be generated and integrated into a macro-cell, where the iron dissolution rate might be accelerated to as much as 10 times that of the micro-cell.