A series of in-door experiments was performed to get some insight into the corrosion behavior of a commercial alloy Fe-12% Cr (3CR12) exposed to imitated seawater. Applying different analytical methods, the main corrosion process was found to be the formation of flakes on the surface which, peel off after they have reached a certain size. Some Cr is dissolved in the solution, its relative concentration with respect to Fe is higher than in the bulk material. The flakes consist mainly of mixed oxihydroxides of the type FeOOH containing some Cr and Mg. The oxidic layer on the interface is very thin, behaves essentially stationary with a slight growth of about 0.05 nm/day. It consists of Cr oxide with some inclusions of Fe and Mg and is not of a chromite type. Immediately below this oxidic layer, the metallic substrate exhibits a thin layer depleted in Cr and behaving like a-Fe (bcc). As compared with stainless steel, potentiostatic current vs. time records at anodic potentials below the pitting potential indicate a very different stability of the surface films for 3CR12. The kinetics of the passive layer formation on 3CR12 was found to follow a parabolic law initially and to change later (after 10...100 seconds in deaerated solution and even earlier in aerated solution) to a linear law. After some time, pitting corrosion and/or cracks due to internal stresses play the dominant role. Cr does not form a protective oxidic layer. The surface morphology of samples exposed at -200 mV for 20 and 80 minutes has been studied by scanning electron microscopy and scanning Auger microprobe. The results reflect the competing formation of oxidic layers and pitting, the participation of Cr in the dissolution process. They also suggest that Mg, which is a component of the solution was incorporated into the rust and some Mg was also found on the metallic surface.

In this paper results are presented on the corrosion resistance against SO2-polluted atmospheres of ultrathin Fe layers (<2.5 nm) deposited on SiO2/Si wafers by two methods: vacuum evaporation and Langmuir–Blodgett (LB) technique. It is shown that the corrosion resistance of the evaporated films is similar to that shown by massive iron while the corrosion resistance of the LB films is remarkably higher. We also show that the corrosion resistance of iron evaporated films can be greatly enhanced by using TiN coatings. Results on the influence of the SO2 concentration, the atmospheric relative humidity and the exposure time on the corrosion behaviour of thin (30 nm) TiN layers deposited onto evaporated Fe films on SiO2/Si wafers are discussed.

Integral conversion electron Mossbauer spectroscopy (ICEMS), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and scanning electron microscopy (SEM) have been used to carry out a comparative study of the corrosion resistance against humid SO2-aggressive environments of ultrathin Fe-57 films, These films, having a thickness less than or equal to 2.5 mn, have been prepared by evaporation of Fe-57 under vacuum and by Fe-57 coating by a Langmuir-Blodgett (LB) technique on SiO2/Si wafers. The results have shown that the corrosion resistance of the evaporated hams is similar to that of massive Fe, However, the LB films show a remarkably higher corrosion resistance, Thus, although the Pe films prepared by evaporation have been completely corroded after 6 h of exposure to the deleterious environment, tate EB films remain unaltered even after 18 h of exposure to the same aggressive conditions. The higher corrosion resistance of the LB films appears to be related to the existence of a thin surface layer containing Si, Fe2+ and Fe3+ formed on top of the Fe metal him during the thermal treatments subsequent to the LB deposition. Copyright (C) 2000 John Wiley & Sons, Ltd.

The tin content of the bath side of float glass was studied by CEMS (conversion electron Mossbauer spectroscopy). The method provides a high surface sensitivity with an information depth of 1 mu m, and allows a distinction between the different valence states of tin. Depth profiles were measured by etching samples stepwise. The tin content decreases rapidly from the surface to the bulk and shows the typical hump between 5 and 10 mu m. Sn2+ dominates at the surface, but close to the hump, Sn4+ clearly prevails. Still deeper inside, the lower valence states become visible, until the tin concentration falls below the sensitivy limit.

The structural differences in the surface region between freshly fractured and leached silicate glasses containing 16.7, 18.8 and 44.4 mol% PbO, respectively, were investigated by photoelectron spectroscopy (XPS). The optical properties and the thicknesses of leached layers were determined by reflection measurements. The binding energies of the O1s signal components for untreated samples can be ascribed to non-bridging (NBO) and bridging (BO) oxygen and oxygen associated with lead as network former (OPb). The binding energy of OPb was found to be 529.1±0.2 eV. For quantitative conclusions, relative XPS sensitivity factors were determined for oxygen, silicon and lead in these glasses. The experimental NBO/BO/OPb ratio found is in good agreement with the NBO/BO/OPb ratio calculated by use of the `discrete bond model' (DBM). Leaching causes a removal of about 90% of the glass modifiers. The O1s signal of silanol groups and other O–H bonds formed appears at a higher binding energy as compared with the BO signal. Whereas the glasses with smaller Pb contents exhibit thin and well confined leached layers, on the glass with the larger Pb content a structured thick layer is formed. Its thickness growth follows a t-dependence expected for a diffusion controlled process. Pb at network forming sites is more stable with respect to leaching than network modifying Pb. After the removal of Pb from network modifying sites, in addition to the formation of silanol and other O–H bonds a rebinding between Si tetrahedra is found resulting in an increased density of the leached layer.

Demanding for a reduction in materials degradation is a serious problem all over the world. Mössbauer spectroscopy (MS) is, among others, a very valuable tool to follow many degradation processes. Evidently, Fe is the most important Mössbauer element considering the overall presence of iron in everyday life.

In this paper, time, potential and pH dependence of SO42- and HSO3- accumulations measured by an in-situ radiotracer method on surface oxide layers of low carbon steel are presented and discussed. This work is a continuation of a research program[ld] and deals with the characterization of the sorption behaviour of low carbon steel in solutions containing sulfate and bisulfite ions (as a possible model of acid rain). From the experimental results it can be stated that (i) the presence of bisulfite ions in the solution phase accelerates the corrosion of low carbon steel; (ii) at E > 0.40 V the electrooxidation of HSO3- ions presumably proceeds to yield a steel surface covered with oxidation products leg sulfate, dithionate[1]); (iii) surface excess values of SO42- and HSO3- ions (as well as the oxidized and reduced adspecies of bisulfite) are very high (up to Gamma = 6 x 10(-8) and Gamma = 1 x 10(-8) mol cm(-2), respectively) ie, a significant incorporation of aggressive ions into the surface oxide layers occurs; (iv) the strong embedding of anions studied is probably the result of some selective chemical interactions (chemisorption) of the above species with active sites involved in the steel surface undergoing structural transformation. Copyright (C) 1997 Elsevier Science Ltd.

Oxidic and metallic iron and iron/nickel multilayers, formed after thermal and chemical treatment of Langmuir-Blodgett films, were investigated. Reduced iron-stearate multilayers were compared with evaporated films concerning their phase composition and their lateral homogeneity. The sequence of metallic and oxidic iron in the films after reduction was determined. Oxidic iron/nickel mixed layers can be prepared from Langmuir-Blodgett films, in which a magnetically non-ordered Fe3+ phase exhibiting a distribution of the quadrupole splitting is observed. After reduction of such layers, two metallic iron phases appear dependent on the nickel concentration. The hyperfine field distribution of the magnetically ordered metallic phase can be evaluated using a method proposed by Rancourt.

By means of the Langmuir-Blodgett (LB) technique, mono- and multilayers of Ni2+ and Fe3+ containing fatty acid salts are transferred onto silicon wafers. After thermal treatment, oxidic layers can be produced.The absolute amounts of Fe3+ and Ni2+ ions per monolayer of LB film were measured and calculated by three different methods. In the case of nickel, the amount expected from theory and preparation conditions was obtained experimentally. In contrast, about twice the number of Fe3+ ions as expected is transferred. An explanation can be given by the presence of hydroxide groups bound to iron.The mean film thickness of the oxidic layers after thermal treatment was estimated where the thicknesses were found to be <1 nm.

By thermal and chemical treatment of Langmuir-Blodgett (LB) films, oxidic and metallic Fe and Ni layers can be prepared. The films are characterised, where differences e.g. in the lateral homogeneity are found. The metallic Fe containing samples show a higher corrosion stability as compared with films prepared by other coating techniques. Mixed Fe/Ni oxidic and metallic layers, respectively, are prepared, where the Ni concentration in the oxidic films can be determined as a function of the ratio of initially transferred numbers of LB layers. The phase composition of the metallic Fe changes systematically with the Ni concentration.

The passivation of low carbon steel was studied in aqueous solutions of 0.5 M Na2SO4 + 0.001 M NaHSO3 (pH 3.5, 6.5 and 8.5) which can be considered as a model of acid rain [1]. Electrochemical and conversion electron Mossbauer spectroscopy (CEMS) investigations proved that the sulfite ions induce pitting corrosion at pH 3.5 and 6.5, while the measurements showed suppressed pitting at pH 8.5. Only gamma-FeOOH was found as a corrosion product at pH 6.5 and 8.5, nevertheless, at pH 3.5 the Mossbauer lines belonging to Fe3C appear in the spectra, and also FeSO4 . H2O was detected after the shortest polarization time.

Integral electron Mössbauer spectroscopy (ICEMS) and additionally some electrochemical methods were used to characterize the passivation process of iron (low carbon steel) in sulfate, sulfate+sulfite (a possible model solution of acid rain) solutions and in phospate buffer. The phase compositions and thicknesses of the passive layers formed due to the electrochemical polarizations were analyzed in dependence on the duration of the anodic passivations and on the pH of the used electrolytes. The passive layer, as determined from the Mössbauer spectra, consists mainly of γ-FeOOH, however in sulfite containing sulfate aqueous solution at pH 3.5 Fe3C and despite ex-situ circumstances FeSO4·H2O was detected after the shortest polarization time. The film thickness, which was found to grow nearly linearly with polarization time in pure sulfate solution and in phospate buffer, reached a maximum of 60–160 nm (depending on pH) in sulfate+sulfite solution after a passivation time of about 4 hours. It has been proved, that HSO3−-ion, which is contained by acid rain, initiate pit formation under acid conditions and so enforces the corrosion of iron. The experimental results furthermore suggest, that not the whole oxidic layer is responsible for the passivity but only a very thin intermediate layer formed between an inner oxide layer of a cubic structure and the rhombic oxide (γ-FeOOH) cover.

Samples of α-(FexCr1−x)2O3 with various Fe/Cr ratios in the range of x = 0.2–0.8 and various particle sizes have been investigated by 57Fe Mössbauer spectroscopy in the temperature range 10–293 K. The dependence of the magnetic hyperfine field at the 57Fe nucleus on the composition of α-(FexCr1−x)2O3 and the influence of the particle size obtained by two different regimes of heat treatment of the samples are reported. The Mössbauer spectra do not show any superparamagnetic behaviour of the α-(FexCr1−x)2O3 samples, but a rearrangement of Fe3+ and Cr3+ ions at cationic lattice sites of the oxides is observed in the case of the heat treatment at higher temperatures.

The microscopic nature of the Mossbauer phase analysis has been found as being particularly valuable in testing assumptions suggested by TEM results regarding the microdomain structure and the population of different lattice sites by iron ions in the perovskite system (1 - y)La(2/3)rectangle(1/3)TiO(3).yLaFeO(3) with 0.04 < y < 0.25 (rectangle is an A-site vacancy). This system was found to contain only Fe3+-phases which are stable under normal conditions, but moderate heating in vacuo causes a partial reduction of ferric ions to ferrous ions.

The microscopic nature of the Mössbauer phase analysis has been found as being particularly valuable in testing assumptions suggested by TEM results regarding the microdomain structure and the population of different lattice sites by iron ions in the perovskite system (1-y)La2/3\(A\)1/3TiO3·yLaFeO3 with 0.04<y < 0.25 (\(A\) is an A-site vacancy). This system was found to contain only Fe3+-phases which are stable under normal conditions, but moderate heating in vacuo causes a partial reduction of ferric ions to ferrous ions.