Investigation of Corrosion and Cathodic Protection in Reinforced Concrete: II. Properties of Steel Surface Layers

Abstract

The present study explores the formation of corrosion products on the steel surface (using as-received low carbon construction steel) in reinforced concrete in conditions of corrosion and subsequent transformation of these layers in conditions of cathodic protection (CP). Of particular interest was to investigate whether the introduced pulse CP (a cost-effective alternative to CP) will lead to similar or even more favorable conversion of the product layers on the steel surface, compared to conventional techniques. Qualification and quantification of the studied layers was performed using X-ray diffraction, X-ray photoelectron spectroscopy, and energy dispersive analysis, visualization of morphology and products distribution was achieved using environmental scanning electron microscopy. The steel surface was found to be covered by a layered, nonhomogeneous formation of products, differing in crystallinity and composition, comprising an inner layer, similar to , and an outer layer, composed of iron (oxy)hydroxides and iron (oxy)hydroxy-chlorides [i.e., a combination of α-, β, , Fe(O,OH,Cl), and ]. The product layer in corroding specimens is a combination of low valent oxides and iron-oxy(hydroxy)chlorides, exhibiting a relatively rough morphology. The product layers in the protected specimens were far more compact. Cathodic protection reduces salinity around the steel bars, hence the inner product layer (mostly ) remains more uniform, whereas the outer layer exhibits reduced crystallinity. The favorable transformation phenomena were found to be more apparent under pulse CP conditions, attributed to the obviously beneficial effects of pulse CP in terms of enhanced chloride withdrawal from the steel surface and minor influence (less side effects) on the bulk concrete microstructure.