A.M. Simões

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.

Semiconducting Properties of Passive Films Formed on Stainless Steels Influence of the Alloying Elements

Passive films formed on stainless steels in a borate buffer solution (pH 9.2) have been investigated by capacitance measurements and photoelectrochemistry. The study was carried out on films formed on AISI type 304 and 316 stainless steels and high purity alloys with differing chromium, nickel, and molybdenum contents. Complementary research by Auger analysis shows that the passive films are composed essentially of an inner chromium region in contact with the metallic substrate and an outer iron oxide region developed at the film/electrolyte interface. The semiconducting properties of the passive films are determined by those of the constituent chromium and iron oxides which are of p-type and n-type, respectively. Thus the influence of the alloying elements on the semiconducting properties of the passive films is explained by changes in the electronic structure of each of these two oxide regions.

Study of Passive Films Formed on AISI 304 Stainless Steel by Impedance Measurements and Photoelectrochemistry

Moss-Schottky plots and photoelectrochemical measurements were made on films formed at different potentials on AISI 304 stainless steel in a borate/boric acid solution, pH 9.2. The results allowed the determination of the semiconductive properties and band structure of the films, which account for the existence of two kinds of films depending on the formation potential. For potentials below 0 V (SCE), the results point out for a film with an inverse spinel structure constituted by Cr-substituted magnetite with two donor levels. Above 0 V only one donor level is detected, which should be Fe{sup 2 +} on tetrahedral sites.

Chemical Composition of Passive Films on AISI 304 Stainless Steel

Chemical characterization of passive films formed on AISI 304 austenitic stainless steel, in a borate/boric acid solution at pH 9.2, under various conditions of potential, temperature, and polarizations time, was made by Auger electron spectroscopy combined with ion sputtering, and x-ray photoelectron spectroscopy (XPS). The depth chemical composition, thickness, and duplex character of the passive layers were determined after processing AES sputter profiles by their quantitative approach based on the sequential layer sputtering model. Moreover, separated contributions of elements in their oxidized and unoxidized state could be disclosed from part to part of the oxide-alloy interface. The XPS study specified the chemical bondings which take placed inside the film, between Fe and oxygen (and water).

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.

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.

An impedance model for the estimation of water absorption in organic coatings. Part I: A linear dielectric mixture equation

A capacitance model for the estimation of water uptake in polymeric coatings is presented. The model is based upon a linear combination of the individual capacitances of the polymeric phase, the water and the air contained in the coating. Experimental comparison of the model with gravimetric data has revealed an improvement with respect to the classical equations.

E.I.S. evaluation of attached and free polymer films

Permeability of a PVC coating attached to a metal substrate and as a free film was assessed by electrochemical impedance spectroscopy. The results were compared with those from gravimetric tests. The impedance spectra of applied and free films have distinct behaviour. The concentration of the outer solution is determinant for the coating capacitance and for the water uptake of the film. The results revealed important differences between the two techniques.