The beneficial secondary effects of conventional and pulse cathodic protection for reinforced concrete, evidenced by X-ray and microscopic analysis of the steel surface and the steel/cement paste interface

The present study reports on the investigation of conventional and pulse cathodic protection (CP), in terms of steel surface analysis and investigation of the product layers at the steel/cement paste interface, after a long term (460 days) of conditioning and monitoring. The techniques used were: X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM) and Energy-dispersive X-ray (EDX). Wet chemical analysis was used as supportive evidence for ion concentrations in the vicinity of the steel re-bars. Generally, CP promotes beneficial secondary effects i.e. enhanced OH(-) concentration and reduced Cl(-) concentration near the steel surface. Cathodic polarization also results in additional deposition of portlandite, which stabilizes the protective properties of the product layer on the steel surface. Consequently, the fundamental mechanisms, underlying the efficiency of CP techniques in. reinforced concrete, are strongly related to beneficial secondary effects of CP, affecting the morphology and transformations of these product layers. Since the experimental evidences to support the aforementioned beneficial effects are rather limited, this study investigates the morphology and composition of the "naturally" formed steel surface layers, along with the properties of the steel/cement paste interface, on corroding and protected steel reinforcement (in comparison to reference, non-corroding, non-protected conditions) after 460 days of conditioning. For the corroding specimens, the formation and substantial deposition of voluminous corrosion products, with low adherence to the steel surface is relevant (low protective ability), whereas in the protected specimens, a compact and adherent product layer of more stable high valent iron oxides, or calcium substituted such, was observed. To this end, the present work provides the experimental evidence for the fundamental mechanisms, related to the otherwise recognized positive secondary effects of CP in reinforced concrete.