M.C. Pérez

Electrochemical behaviour of steel rebars in concrete: influence of environmental factors and cement chemistry

Four series of reinforced concrete specimens have been studied over 3 years exposure in a 100% relative humidity atmosphere. Addition of CaCl2, NaNO2, and a mixture of CaCl2 and NaNO2 changed the cement chemistry with respect to an ordinary portland cement series of samples used as reference series. The study, based on low-scan rate cyclic voltammetry and electrochemical impedance spectroscopy, confirms previous results obtained in alkaline medium, i.e. the redox activity in the rebar’s oxides layer greatly influences the electrochemical behaviour of rebars in the passivity potential domain. Different redox processes also influence the active and cathodic protection domains that make corrosion rate estimations very difficult. The possibilities of estimating corrosion rate are discussed in terms of cement chemistry and corrosion potential of the system.

Use of EIS, ring-disk electrode, EQCM and Raman spectroscopy to study the film of oxides formed on iron in 1 M NaOH

Abstract Different electrochemical techniques (electrochemical impedance spectroscopy (EIS), ring-disk electrode, electrochemical quartz crystal microbalance (EQCM), and in situ Raman spectroscopy) have been employed to study the behaviour of the passive film formed on iron in alkaline medium simulating pore solution in fresh concrete. The study, based on low scan rate cyclic voltammetry performed over the entire electrolyte stability domain, allows for establishing the influence of the redox activity developing in the oxides layer on the electrochemical behaviour of the system and, thus, to get valuable information on the applicability of classical electrochemical techniques employed to assess corrosion of steel in concrete. The passive film is based on a magnetite-type structure which, in partially reversible processes, can be oxidised and reduced depending on the electrode potential. Those redox processes mask the corrosion process itself.

Galvanic coupling between carbon steel and austenitic stainless steel in alkaline media

The galvanic currents between carbon steel and two types of stainless steels have been studied in solutions of pH close to that of concrete. The situations analysed were passivity and active corrosion induced by chlorides. In all cases the galvanic effects recorded were of minor importance indicating that no significant risk of galvanic corrosion exists when carbon steel and stainless steel are electrically coupled in reinforced concrete structures. Oxygen reduction current is lower for stainless steel than for passive carbon steel. The results have been interpreted by means of EIS in terms of the resistivity of the passive layer formed on steels.

Effect of protective oxide scales in the macrogalvanic behaviour of concrete reinforcements

Abstract The influence of macrocell activity on the corrosion of steel in concrete is studied using concrete beams exposed to the atmosphere for six years. In these beams, the corroding areas (embedded in chloride containing concrete) behave as anodes with respect to the segments embedded in the chloride-free concrete. However, after six years of ageing in the laboratory (dry atmosphere), the segments in contact with chlorides reach a kind of passivation state that allows them to act as cathodes when the macrocell is connected. This behaviour is interpreted in terms of the activity of a Fe 2+ \Fe 3+ redox process, the amount of rust accumulated in those sites and the electrical conductivity of this rust, as well as the oxygen availability.

Characterisation of the electrochemical behaviour of cerium implanted stainless steels

Passive layers formed on a Ce implanted stainless steel in alkaline media are studied to clarify its beneficial effect on stainless steel corrosion resistance. An important decrease in iron and chromium activity peaks is detected by electrochemical techniques. X-ray photoelectron spectroscopy (XPS) analysis shows passive film thickness decreasing together with changes in chemical composition. Cerium implantation modifies the conductivity properties of the passive film, as it is inferred from EIS measurements.

Electrochemical Impedance Spectroscopy as a Tool for Studying Steel Corrosion Inhibition in Simulated Concrete Environments—Red Mud Used as Rebar Corrosion Inhibitor

Red mud is the main by-product of the Bayer process for alumina extraction from bauxite. Rich in iron, aluminum and silicon oxides, it is believed to have corrosion protection properties. Some are studied in the present paper. The corrosion behavior of steel rebars in chlorine containing solutions and mortars is studied and compared with the same systems containing red mud in suspension (case of solutions) or added as powder to the mix (case of mortars). The results in solution show that red mud is able to maintain steel passivity for more than 90 days in highly chlorinated alkaline solutions while in NaOH and Ca(OH)2 solutions having the same pH pit formation is observable at 25 and 55 days, respectively. Additions of red mud to mortar as powder representing 2 % by weight of cement are able to block chloride depassivation. EIS data allow understanding the protecting mechanism and comparing the behavior in mortar and in solution.

Modifications of the stainless steels passive film induced by cerium implantation

In this work X-ray photoelectron spectroscopy studies and cyclic voltammetry are combined to elucidate the effect of Ce implantation on the formation and evolution of passive layers generated on AISI 430 and AISI 304L steels in alkaline medium. The passive films develop in implanted steels result thinner and less defective than in unimplanted. This result is interpreted in terms of a cerium-induced inhibition of the passive film re-oxidation (formation of iron and chromium oxides). The main difference between the behaviour of the two types of steel (ferritic and austenitic) seems to be related to the enrichment in nickel promoted by cerium in the outermost layer of AISI 304L.

Microstructure of the Passive Layer Formed on Different Austenitic Stainless Steels Implanted with Molybdenum

Two austenitic stainless steels have been implanted at 150 keV with Mo at a fluence of 3.5x1015 ions/cm2. The effects of ion implantation in the chemical composition of the passive films was evaluated by x-ray photoelectron spectroscopy (XPS) and glancing angle x-ray diffraction (GAXRD) was used to determine the induced structural modifications. The results of the pitting corrosion studies carried out in neutral chloride medium as well as the morphology of the localized attack are discussed.

An Insight on the Influence of Ion Implantation on the Pitting Corrosion Resistance of AISI 430 Stainless Steel

Research on the effect of ion implantation on the corrosion behaviour of metals has been carried out for years, but some difficulties arise in the comparison of the obtained results due to variations in experimental conditions (alloys, surface preparation, doses, experimental techniques...). This work tries to overcome those differences, presenting the effect of several elements (Ce+, N+, Cr+ and Cr+ N+) implanted in similar conditions on the pitting corrosion resistance of AISI 430 stainless steel. Potentiodynamic measurements in 1M NaCl demonstrate the beneficial effect of all the implanted elements, showing that Ce+ is the less efficient ion, while Cr+ N+ co-implantation gives the best results in terms of localized attack resistance. Pitting morphology is explained in terms of the XPS and GIXRD data that allow chemical and structural characterization of the implanted layer. Those results help to enlighten the protection mechanisms involved in the considered implantations.

The effect of the Cerium ion implantation in the passive films properties of a duplex stainless steel

Abstract The effect of Ce+ implantation on the electrochemical behaviour of SAF 2205 DSS is studied in alkaline medium. XPS studies have detected the implanted cerium as Ce3+ throughout the oxide films. The peak current of the magnetite formation peak is directly related to the passive film thickness. Nevertheless, the Cr3+ oxidation process is not affected by cerium implantation, which suggests cerium incorporation in the iron spinel.