M. Da Cunha Belo

The electronic structure of passive films formed on stainless steels

Abstract The capacitance behaviour of passive films formed on austenitic type 304 stainless steel and Fe-Cr alloys (0 ≤ Cr ≤ 30%) was studied using the Mott-Schottky approach. The results obtained show that the films behave as n-type and p-type semiconductors in the potential range above and below the flat band potential, respectively. This behaviour is assumed to be the consequence of the semiconducting properties of the iron oxide and chromium oxide regions which compose the passive films. This interpretation is supported by Auger analysis which enables the establishment of a direct relationship between the chemical composition of the two oxide regions and the Mott-Schottky plots. Based on the results obtained a model of the electronic structure of the passive films formed on stainless steels is proposed.

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 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.

Composition, structure and properties of the oxide films formed on the stainless steel 316L in a primary type PWR environment

Abstract This paper reports a comprehensive study of the chemical composition, structure and electrochemical behaviour of oxide films developed on 316L stainless steel in a primary medium (1000 ppm B, 2ppm Li, 37ppm H 2 at 350 °C). The methods used are glow discharge optical spectfoscopy, grazing X-ray diffraction analysis, near field microscopy, transmission electron microscopy (TEM), Raman spectrometry and electrochemical measurements. According to the analytical and structural investigations, the outermost part of the film consists of the Ni 0.75 Fe 2.25 O 4 inverse spinel. In the intermediate part both Ni 0.75 Fe 2.25 O 4 and Fe 3 O 4 inverse spinels can be detected. Finally, the inner part of the film consists of mixed chromium oxides (Cr 2 O 3 + FeCr 2 O 4 ) and Fe 3 O 4 . The electrochemical study carried out by capacitance measurements (Mott-Schottky approach) shows that the inner and outer parts of the film manifest different semiconductivities. This is in agreement with the known semiconducting properties of the type of oxides concerned.

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).

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.

Anodising of Al 2024-T3 in a modified sulphuric acid/boric acid bath for aeronautical applications

A novel borate/boric/sulphuric acid anodising process is studied. The results show that the physical structure of the films is influenced not only by the bath used, but also and mainly by the substrate, i.e., Al 2024-T3 or Al. The corrosion resistance of the anodised specimens is satisfactory for practical applications and the fatigue resistance is not significantly different from that obtained with the traditional chromic acid anodising.