M.F. Montemor

Chloride-induced corrosion on reinforcing steel: from the fundamentals to the monitoring techniques

One of the most important causes for reinforcing steel corrosion is the presence of chloride ions. They cause localised breakdown of the passive film that initially forms on steel as a result of the alkaline nature of the pore solution in concrete. The harmful chloride ions can be originated from the use of contaminated mix constituents or from the surrounding environment. The determination of a critical level, above which serious problems can occur, has been one of the main goals of investigation. Unfortunately, it is difficult to establish such a value since the chloride level is influenced by several factors. Thus, after concrete contamination, it is of fundamental importance to follow the activity of chlorides and the state of the reinforcing rebars. In this respect, the use of electrochemical techniques such as polarisation resistance, electrochemical impedance, galvanostatic pulse and potential measurements have shown to be powerful tools. Nevertheless, the interpretation of the results becomes sometimes a difficult task. A large number of authors have dedicated several studies to the interpretation of such measurements and a highly dispersed number of interpretations can be found in literature. The aim of this paper is to present an overview on the state-of-the-art of the most important aspects of the corrosion process initiated by chlorides, its development and monitoring techniques.

Chemical composition and electronic structure of the oxide films formed on 316L stainless steel and nickel based alloys in high temperature aqueous environments

Abstract Auger analysis, capacitance measurements and photoelectrochemical measurements have been used in order to study the oxide films formed on 316L stainless steel, Alloy 690 and Alloy 600 in high temperature aqueous environments at pH 8 and 10. The analytical study reveals the duplex character of the films formed on 316L stainless steel and Alloy 690. For these materials it is assumed that the capacitance and the photoelectrochemical results can be related to the semiconducting properties of mixed iron-nickel oxide and chromium oxide which represent the outer and the inner parts of the film, respectively. In the case of the Alloy 600 the presence of nickel oxide controls the semiconducting properties of the film formed at pH 10, but for the film formed at pH 8 the presence of mixed iron-nickel oxide in the outer layer still controls those properties. Finally, it is demonstrated that the capacitance behaviour of the thin passive films formed at room temperatures could serve as a useful basis for interpreting the higher temperature results.

Semiconducting properties of thermally grown oxide films on AISI 304 stainless steel

Abstract The oxide films formed on AISI type 304 stainless steel in the temperature range between 50 and 450°C, have been studied by Auger electron spectroscopy, capacitance measurements and photoelectrochemistry. The analytical results show that the oxide films are in all cases composed of an inner region of a mixed chromium-iron oxide and an outer region of iron oxide. An increase in temperature of film formation changes most particularly the thickness of the outer iron oxide region. Capacitance results show that when the temperature of film formation is decreased the doping density increases and reaches, for films formed at 50°C, the characteristic values of very thin passive films. It also appears that, although very different in thickness and doping density, thermally grown and passive films show identical capacitance and photoelectrochemical behaviour. The electronic structure model previously proposed to explain the semiconducting properties of very thin passive films can be applied to explain the semiconducting properties of the relatively thick well crystallised and nondegenerate oxides.

Composition and behaviour of cerium films on galvanised steel

The composition and corrosion performance of galvanised steel treated by immersion in cerium nitrate solution was investigated by electrochemical techniques and surface analysis. The surface film consists of a mixture of Ce(III) and Ce(IV) compounds, being very rich in Ce(III) in the first instants of the deposition process and becoming gradually enriched in the more oxidised form, Ce(IV). The presence of this film on the surface hinders the corrosion reaction by reducing the rate of both the cathodic and the anodic reactions. The film becomes thicker but more uneven when the time of film growth increases, with the development of defects in the film, which in contact with electrolyte behaves anodic with respect to the covered areas of the surface. These thicker films have revealed lower resistance to corrosion initiation.

Hydroxyapatite microparticles as feedback-active reservoirs of corrosion inhibitors.

This work contributes to the development of new feedback-active anticorrosion systems. Inhibitor-doped hydroxyapatite microparticles (HAP) are used as reservoirs, storing corrosion inhibitor to be released on demand. Release of the entrapped inhibitor is triggered by redox reactions associated with the corrosion process. HAP were used as reservoirs for several inhibiting species: cerium(III), lanthanum(III), salicylaldoxime, and 8-hydroxyquinoline. These species are effective corrosion inhibitors for a 2024 aluminum alloy (AA2024), used here as a model metallic substrate. Dissolution of the microparticles and release of the inhibitor are triggered by local acidification resulting from the anodic half-reaction during corrosion of AA2024. Calculated values and experimentally measured local acidification over the aluminum anode (down to pH = 3.65) are presented. The anticorrosion properties of inhibitor-doped HAP were assessed using electrochemical impedance spectroscopy. The microparticles impregnated with the corrosion inhibitors were introduced into a hybrid silica-zirconia sol-gel film, acting as a thin protective coating for AA2024, an alloy used for aeronautical applications. The protective properties of the sol-gel films were improved by the addition of HAP, proving their applicability as submicrometer-sized reservoirs of corrosion inhibitors for active anticorrosion coatings.

Composition and corrosion behaviour of galvanised steel treated with rare-earth salts: the effect of the cation

The corrosion performance and the composition of galvanised steel treated by immersion in nitrate solutions of rare-earth metals (cerium, yttrium and lanthanum) were evaluated by electrochemical techniques and surface analysis. The surface film consists of a mixture of rare-earth oxides/hydroxides, which hinders the corrosion reactions of the substrate by reducing the rate of both the cathodic and the anodic reactions. Although all the films provide corrosion protection, the effect seems to be more pronounced for lanthanum. The behaviour of the films formed on the surface is dependent on the rare-earth cation and on the treatment time.

EFFECT OF FLY ASH ON CONCRETE REINFORCEMENT CORROSION STUDIED BY EIS

Abstract The corrosion process of steel embedded in concrete with various amounts of fly ash (up to 50% of the total binder) was tested under complete and partial immersion, in sodium chloride solution. The corrosion process was followed by monitoring of open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS). Fly ash addition has led to a raise of concrete resistivity and of the time for corrosion initiation and to a decrease of corrosion rate.

The role of Mo in the chemical composition and semiconductive behaviour of oxide films formed on stainless steels

Abstract The oxide films grown in air, inside a furnace, during two hours at temperatures between 250°C and 450°C, on the surface of commercial alloys (AISI 304 and AISI 316) and on the surface of high purity alloys with and without Mo addition (Fe-17Cr, Fe-17Cr-5Mo, Fe-17Cr-25Ni and Fe-17Cr-25Ni-10Mo) were characterised by analytical (AES and XPS) and capacitance measurements (Mott-Schottky plots). The analytical results have shown that the oxide films formed on the stainless steels are composed by an external iron oxide region and an inner chromium oxide region. Mo was not detected in the former region, whereas the later region reveals the presence of Mo. Whenever Mo is present in the alloy there is an enrichment of chromium oxide in the film. The film thickness is practically independent of the presence and content of Mo. The raise in the temperature of film growth from 250°C to 450°C led essentially to the thickening of the external iron oxide region, whereas the thickness of the inner chromium oxide region remains unchanged. The capacitance measurements indicate that the influence of Mo on the film capacitance can be related to a decrease of the number of donors in the iron oxide layers of the film. The presence of Mo also affects the defect structure of the inner chromium oxide region.

Corrosion resistance of a composite polymeric coating applied on biodegradable AZ31 magnesium alloy

The high corrosion rate of magnesium alloys is the main drawback to their widespread use, especially in biomedical applications. There is a need for developing new coatings that provide simultaneously corrosion resistance and enhanced biocompatibility. In this work, a composite coating containing polyether imide, with several diethylene triamine and hydroxyapatite contents, was applied on AZ31 magnesium alloys pre-treated with hydrofluoric acid by dip coating. The coated samples were immersed in Hanks solution and the coating performance was studied by electrochemical impedance spectroscopy and scanning electron microscopy. In addition, the behavior of MG63 osteoblastic cells on coated samples was investigated. The results confirmed that the new coatings not only slow down the corrosion rate of AZ31 magnesium alloys in Hanks solution, but also enhance the adhesion and proliferation of MG63 osteoblastic cells, especially when hydroxyapatite nanoparticles were introduced in the coating formulation.

Corrosion behaviour of rebars in fly ash mortar exposed to carbon dioxide and chlorides

Addition of fly ash has beneficial effects on some mechanical properties of concrete, as well as on the corrosion process induced by the chloride ion. The aim of this study was to investigate the effect of fly ash addition on the corrosion process occurring in reinforced concrete exposed simultaneously to carbon dioxide and chloride. The corrosion process of steel rebars embedded in mortar with 15% and 30% of fly ash was tested under carbon dioxide and sodium chloride contamination. Monitoring of open circuit potential and electrochemical impedance spectroscopy (EIS) were used to follow the corrosion process. Results have shown that under accelerated carbonation fly ash mortar shows higher corrosion rates. The chloride content in mortar exposed to accelerated carbonation increases with the amount of fly ash. However, under natural carbonation it decreases with the addition of fly ash.