I. Olefjord

The influence of nitrogen on the passivation of stainless steels

Abstract Two austenitic stainless steels, Fe-20Cr-20Ni-6Mo-0.011N and Fe-20Cr-20Ni-6Mo-0.2N were polarized in 0.1 M HCl + 0.4 M NaCl at 22 and 65 °C. Pitting is initiated on both steels. The high N-containing steel is repassivated while the pits are growing on the low N-containing steel at 65 °C. The composition and the thickness of the passive films (determined by ESCA) formed during polarization to the potentials −75, + 500 and + 800 mV (SCE) are independent of the N content of the steels. Nitrogen segregates anodically to the metal-oxide interface during passivation. A model for a synergistic effect between Mo and N is suggested.

Surface analysis of passive state

Abstract The principles of the ESCA and the Auger-techniques are described and a method for quantitative ESCA analysis of passive film is presented. The method is exemplified by an analysis of the passive film formed on stainless steel (20Cr18Ni6MoO.2N) during polarization in 0.1 M HCl + 0.4 M NaCl. The thicknesses of the passive films formed at −100 mV and 500 mV (SCE) are 13 A and 15.5 A respectively. The film consists of an inner FeCr oxide phase and an outer monolayer of Cr(OH) 3 . The average cation content of Cr 3+ in the inner oxide is about 50%. Variable angle analysis shows that the Cr 3+ content is higher at the metal interface than at the outer interface. Mo 6+ is enriched in the outer layer. The content of Mo 4+ in the oxide layer decreases with the potential. The Ni 2+ content in the passive film is low. Chloride ions are dissolved in the oxide as well as in the hydroxide layer. A stationary state is reached within 2 min.

Oxidation of tungsten and tungsten carbide in dry and humid atmospheres

Abstract Oxidation experiments were performed on pure tungsten and hot-pressed tungsten carbide. The chemical state and thickness of the oxide products were determined by ESCA. The oxidation of W and WC in dry atmosphere was performed in oxygen at temperatures ranging from 20 to 500 °C. The oxide formed is WO3. The thickness of the oxide layer increases slowly up to 200 °C, after which the oxide growth is rapid. The oxidation behaviour of W and WC in humid atmospheres was studied at room temperature in air at relative humidities of 60 and 95%. It was found that the thickness of the oxide layer increases with increased humidity. No formation of hydroxide was observed. Exposing W to water for one week results in a thick layer of WO2, WO3 and hydroxide. In the case of WC no oxide at all was visible after exposure to water. Furthermore, WC is resistant to further oxidation after exposure.

A Round Robin on combined electrochemical and AES/ESCA characterization of the passive films on FeCr and FeCrMo alloys

Abstract A Round Robin has been organized by the Working Party of the European Federation of Corrosion on Surface Science and the Mechanisms of Corrosion and Protection. Ten European laboratories participated in this Round Robin. The aim was to compare the results obtained by AES and ESCA analyses of passive films formed on FeCr and FeCrMo alloys in 0.5 M H2SO4. All participating laboratories followed a standard procedure that had been previously established by the Working Party. The recommended procedure defines the conditions for the surface preparation, the electrochemical treatment and the surface analyses by AES and ESCA. The following aspects have been investigated: (a) polarization curves of FeCr and FeCrMo alloys exposed to 0.5 M H2SO4: comparison of the corrosion potentials, passivation peak currents and potentials, and currents in the passive region, (b) thickness of the passive films determined by AES depth profiling and by ESCA analysis, (c) composition of the films and (d) chemical states of the elements in the films, binding energies and the chemical shifts measured by ESCA.

ESCA studies of the composition profile of low temperature oxide formed on chromium steels—II. Corrosion in oxygenated water

Abstract ESCA (Electron Spectroscopy for Chemical Analysis) has been used to analyse the chemical composition of passive films formed on chromium steels with 4–30% Cr at 25 and 70°C in oxygenated water for different times. The composition-depth profile of the films has been investigated by successive stripping of the film by ion etching. The composition of the film varies with exposure time: short time yields an FeCr ratio corresponding to that of the alloy; prolonged exposure produces increasing enrichment of chromium through the whole layer, with the highest concentration at the outer surface. The passive film generally seems to consist of two layers: outermost one of (Cr(OH) 3 ) and underneath one of iron-chromium oxide (Fe 1+ x Cr 2− x O 4 . The low alloy steels (3·9 and 7·8%Cr) were passive for a limited time during which the Cr enrichment was high (8 times); after breakdown of the chromium-rich passive layer, it was replaced by FeOOH. When a dry-formed oxide film, which consists mainly of iron oxide, is exposed to water, its composition slowly changes until that of a water-formed film is reached.

Selective dissolution and surface enrichment of alloy components of passivated Fe18Cr and Fe18Cr3Mo single crystals

Abstract The possible surface enrichment of chromium and molybdenum during dissolution of Fe18Cr (110) and Fe18Cr3Mo (110) alloys at constant potentials in the passive region is elucidated by taking into account quantitative information on partial dissolution rates of alloy components as measured by γ-spectrometry and on chemical composition of passivating films as measured by ESCA and AES. Combining results from all methods it is found that chromium under all conditions is enriched in the passivating films. An accumulation of chromium in the alloy is also indicated. For molybdenum, an enrichment in the passive film is observed at − 0.2 V (SCE), whereas in the potential interval 0.1–0.9 V, the molybdenum enrichment as determined by ESCA and AES is hardly significant. On the other hand, γ-spectrometry gives a clear indication of molybdenum enrichment in both potential regions.

Esca-studies of the composition profile of low temperature oxide formed on chromium steels—I. Oxydation in dry oxygen

ESCA (Electron Spectroscopy for Chemical Analysis) has been used to analyse the composition of oxide films formed on chromium steels with 6–20%Cr at room temperature in pure oxygen at 0·2 atm. during different times from 2 min to 46 h. Ion etching was used to clean the surface before oxidation and to investigate the composition profile by successive stripping of the oxide layer. It is demonstrated that the total chromium content in the oxide is proportional to the concentration in the metal, but that it varies with depth inside the film. At the oxide-gas boundary the chromium concentration is less than in the metal, while chromium is enriched at the metal phase boundary. It is suggested that the oxide consists of (Fe,Cr)2O3 in the outermost zone and FeCr2O4 at the inside, with the reservation that the method is not sensitive to deviations from stoichiometry. The thickness of the layer has been determined as 25 A from the intensity ratio of the deconvoluted ESCA peaks of the metallic and oxidized states. The thickness and the composition profile of the film do not vary noticeably with the oxidation time.

Surface reactions during pickling of an aluminium-magnesium-silicon alloy in phosphoric acid

Abstract The surface products formed during pickling of a modified AA 6000 alloy (0.70Mg1.52Si) in 0.1 M phosphoric acid were analysed using electron spectroscopy for chemical analysis, Auger electron spectroscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. Cold-rolled and heat-treated samples were compared. Pure Si, Mg2Si and AlFeSi particles are present in the material after the heat treatment. Pickling gives a chemical dissolution of the surface oxide and an anodic dissolution of the Mg2Si particles. The degree of dissolution increases with the pickling time. On the cathodic AlFeSi particles, aluminium phosphate is deposited. After prolonged pickling, micropits are visible on the surface. It is suggested that this is due to the dissolution of small Mg2Si particles precipitated during the annealing process.

The influence of sulphur on the dissolution and the passivation of a nickel-iron alloy—II. surface analysis by ESCA

Abstract Sulphur-containing (0.0082 at. %) single crystals of a Ni-25Fe alloy were anodically polarized in 0.05 M H 2 SO 4 in the potential range 300–1300 mV (SHE). Sulphur is enriched on the surface by selective dissolution of the metal elements. The passive film is not formed on the sulphur-covered surface. At the lowest potential, which corresponds to the potential at which a sulphur-free alloy begins to passivate, the surface of the alloy is covered by a monolayer of adsorbed sulphur. Then the nucleation and growth of a sulphide film take place. The sulphide grows to a continuous layer 25a˚A thick. The stoichiometry of the sulphide is close to Ni 2 . 5 Fe 0 . 5 S 2 . This sulphide has no passivating effect, instead the anodic current increases linearly with the potential. A model for the adsorption of sulphur followed by nucleation and growth of the sulphide is suggested from the comparison of the intensities of the ESCA signals with the total amounts of sulphur measured by the radiotracer method.

Oxidation behavior of Ni3Al and Fe3Al: I. XPS calibrations of pure compounds and quantification of the results

Pure oxides of Ni, Fe, and Al were synthesized and analyzed by X-ray photoelectron spectroscopy (XPS) for calibration purposes, as were the spinels NiAl2O4 and FeAl2O4. The quantitative XPS analysis used in further studies to calculate the oxide thickness and the distribution of oxides on the metal surface is described.