Investigation of Corrosion and Cathodic Protection in Reinforced Concrete

Abstract

The present study explores the formation of corrosion products on the steel surface ͑using as-received low carbon construction steel͒ in reinforced concrete in conditions of corrosion and subsequent transformation of these layers in conditions of cathodic protection ͑CP͒. Of particular interest was to investigate whether the introduced pulse CP ͑a cost-effective alternative to CP͒ 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 Fe 3 O 4 , and an outer layer, composed of iron ͑oxy͒hydroxides and iron ͑oxy͒hydroxy-chlorides ͓i.e., a combination of ␣-, ␤, ␥-FeOOH, Fe͑O,OH,Cl͒, and Fe 2 O 3 ͔. The product layer in corroding specimens is a combination of low valent oxides and iron-oxy͑hydroxy͒chlorides, 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 Fe 3 O 4 ͒ 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 effects͒ on the bulk concrete microstructure. The most common and important causes for reinforcement corrosion are either localized depassivation of the steel surface due to chloride ingress or more uniform corrosion due to acidification of the pore solution as result of carbonation of the cement paste. Cathodic protection ͑CP͒ has been found to be one of the most useful techniques for inhibiting chloride-induced corrosion in reinforced concrete.1 The fundamental mechanisms underlying the efficiency of CP techniques are strongly correlated to the morphology and transformations of product layers on the steel surface. The steel reinforcement used in the present study was as-received construction steel FeB500HKN ͑rebars, d = 12 mm, C Ͻ 0.22 wt %. Electrochemical impedance spectroscopy ͑EIS͒, polarizations resistance ͑PR͒ method, and potentio-dynamic polarization ͑PDP͒ were used for a comparative analysis of electrochemical parameters and corrosion behavior of the embedded steel in the reinforced concrete specimens presented here, in the relevant conditions of corrosion and CP, and the outcomes were reported previously.2 This paper pursues exploration of the formation, distribution, and morphological alterations of corrosion products in the reinforced c...

Corrosion performance of reinforced mortar in the presence of polymeric nano-aggregates: electrochemical behavior, surface analysis, and properties of the steel/cement paste interface

“…The physical size for the reference region of each image is 226 mm in length and 154 mm in width, with resolution of 0.317 mm/pixel, corresponding to a magnification of 5009; the sample preparation and relevant procedures are as reported elsewhere [28,29,33,34]. ESEM imaging combined with quantitative image analysis provides microstructural information of pore space, e.g., porosity and pore size distribution (more details are reported in [28,33,35,36]). The image analysis in this study complies with generally used methodology for pore structure analysis of cement-based materials [28,29,33,37].…”

“…The elements of the equivalent circuit present the following physical meanings: R el?b is the mortar resistance, including the contribution of the electrolyte resistance; the first time constant (R pn and Q pn ) is attributed to the properties of the bulk matrix in terms of connected pore network; the second time constant (Q ct and R ct ) is normally used to evaluate charge transfer process in combination with mass transport process. When there are no mass transport limitations, R ct equals the global polarization resistance R p [35,45] and is related to the electrochemical reaction on the steel surface (widely accepted for reinforced cement-based materials is to use constant phase element (Q) instead of pure capacitance to account for steel surface, product layers, and bulk matrix heterogeneity [49,50]). …”

“…The common products on a steel surface, previously conditioned in cement-based materials, include except a mixture of iron oxides/hydroxides [wustite (FeO, as part of FeOÁFe 2 O 3 ), hematite (Fe 2 O 3 ), magnetite (Fe 3 O 4 ), goethite (a-FeOOH)], also portlandite (Ca(OH) 2 ) and calcite/aragonite (CaCO 3 ), corresponding to Ca-substituted iron oxides/hydroxides on the steel surface (it is well known that the Ca-substituted layers provide better corrosion resistance [35,45]). For the control specimens, similar features and peak intensity of the XRD patterns were observed, mainly featuring wustite (W, as part of MG), corresponding to peaks 2h 49.2° [54] and magnetite (MG), corresponding to peaks 2h 35.5°and 41.6°, respectively [54,55].…”

Macro and micro aspects of the transport of chlorides in cementitious membranes

“…6, because the pore blocking can compensate the increase in conductivity due to the ingress of chlorides. More subtle is the difference between AC and DC regimes, also evident in Fig 4. The dissimilar effect in concrete's microstructure during AC and DC chloride extraction and cathodic protection experiments has already been reported in literature [25,26]. However, no interpretation to this fact has been given so far.…”

The Synergy of Electrochemistry and Concrete Material Science in Evaluating Corrosion Resistance of Wastes-Containing Reinforced Cement-Based Systems

“…For more details on the fundamental aspects and image analysis of cement-based materials, including mortar, plain concrete and reinforced concrete, please see (Serra J. 1982, Ye G. 2003, Hu J. et al 2003, Koleva D. A. 2007c).…”