M.G.S. Ferreira

Sol–gel coatings for corrosion protection of metals

Sol–gel derived films exhibit a high potential as substitutes for the environmentally unfriendly chromate metal-surface pre-treatment methods. Inorganic sol–gel derived films offer good adhesion between metals and organic paint. However, they cannot provide adequate corrosion protection due to their high crack-forming potential. Introduction of an organic component to an inorganic sol–gel system leads to the formation of thicker, more flexible and functionalized films with enhanced compatibility to different organic top coatings. Incorporation of nanoparticles in the hybrid sol–gel systems increases the corrosion protection properties due to lower porosity and lower cracking potential along with enhancement of the mechanical properties. Furthermore, the incorporation of inorganic nanoparticles can be a way to insert corrosion inhibitors, preparing inhibitor nanoreservoirs for “self-repairing” pre-treatments with controlled release properties.

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.

Enhancement of active corrosion protection via combination of inhibitor-loaded nanocontainers.

The present work reports the synthesis of layered double hydroxides (LDHs) nanocontainers loaded with different corrosion inhibitors (vanadate, phosphate, and 2-mercaptobenzothiazolate) and the characterization of the resulting pigments by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The anticorrosion activity of these nanocontainers with respect to aluminum alloy AA2024 was investigated by electrochemical impedance spectroscopy (EIS). The bare metallic substrates were immersed in dispersions of nanocontainers in sodium chloride solution and tested to understand the inhibition mechanisms and efficiency. The nanocontainers were also incorporated into commercial coatings used for aeronautical applications to study the active corrosion protection properties in systems of industrial relevance. The results show that an enhancement of the active protection effect can be reached when nanocontainers loaded with different inhibitors are combined in the same protective coating system.

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.

Novel Inorganic Host Layered Double Hydroxides Intercalated with Guest Organic Inhibitors for Anticorrosion Applications

Zn-Al and Mg-Al layered double hydroxides (LDHs) loaded with quinaldate and 2-mercaptobenzothiazolate anions were synthesized via anion-exchange reaction. The resulting compounds were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy/energy-dispersive X-ray spectroscopy. Spectrophotometric measurements demonstrated that the release of organic anions from these LDHs into the bulk solution is triggered by the presence of chloride anions, evidencing the anion-exchange nature of this process. The anticorrosion capabilities of LDHs loaded with organic inhibitors toward the AA2024 aluminum alloy were analyzed by electrochemical impedance spectroscopy. A significant reduction of the corrosion rate is observed when the LDH nanopigments are present in the corrosive media. The mechanism by which the inhibiting anions can be released from the LDHs underlines the versatility of these environmentally friendly structures and their potential application as nanocontainers in self-healing coatings.

Semiconducting properties of passive films formed on nickel–base alloys type Alloy 600: influence of the alloying elements

Abstract The semiconducting properties of passive films formed on nickel–base alloys type Alloy 600 in borate buffer solution were studied by capacitance measurements and photoelectrochemistry. The influence of the alloying elements (Fe, Ni, Cr) on the film properties was examined using pure metals and pure alloys. The results obtained show that the presence of both chromium and mixed nickel–iron oxides in the films revealed by quantitative analysis develops a p-n heterojunction that controls their electronic structure, in a similar manner to the case of stainless steels. The nickel oxide present in the films acts as a barrier layer that confers improved protection. The relationship between the electronic structure and the physicochemical properties of the passive film are discussed.

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.

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).