A closer look at the corrosion of steel liner embedded in concrete

Containment wall of nuclear power plants is an example of a concrete structure enveloping a steel liner plate. In this research, the situations observed in connection with liner failures were investigated, namely the presence of a piece of wood (foreign matter) and a delamination gap between steel liner and concrete, with the purpose to study and define the corrosion mechanism of the steel liner plate and identify which factors promote or impede the occurrence of corrosion. Specimens were concrete slabs containing a steel plate, an inlay to modify the conditions at the steel surface, and electrodes for electrochemical measurements. The inlays represented "normal" concrete, low-pH concrete, a piece of wood, and a delamination gap between concrete and steel, and the same systems supplemented with chlorides. The results from two-year tests with wetting-drying cycles revealed localized corrosion in steel underneath the piece of wood, close to the inlay perimeter. The attack could be explained mainly by the mechanism of crevice corrosion. In contrast, no corrosion attack of steel liner could be detected in specimens with the delamination gap. The results from the experiments and steel liner characterization are provided in this paper, together with the reasoning behind the proposed degradation mechanism.