J.L. Grosseau-Poussard

Iron oxidation under the influence of phosphate thin films

Iron phosphate thin films have been obtained under controlled conditions by chemical conversion deposition. The films were formed on polycrystalline α-iron by immersion in acetone, as an organic solvent. After deposition, the films were investigated by x-ray photoelectron spectroscopy (XPS) and their influence on the thermal oxidation of α-iron was studied by means of in situ low incidence x-ray diffraction of synchrotron radiation. The study reveals interesting features related to the structure of both the phosphate and thermal oxide films. The XPS data suggest the iron phosphate to be constituted of long chains of phosphate groups PO43−; these groups being interconnected by Fe2+ and Fe3+ cations. X-ray diffraction measurements have shown a significant modification of the oxidation behavior of α-iron at 400u200a°C and atmospheric pressure, which is derived from the presence of the thin film: α-Fe2O3 formation is clearly enhanced to the detriment of Fe3O4, compared to the oxidation of pure iron in which Fe3O4...

Growth stresses in α-Cr2O3 thermal oxide films determined by in situ high temperature Raman spectroscopy

Growth stresses have been investigated in relation with the microstructure in the case of α-Cr2O3 growing oxide films on NiCr30 alloy. The equibiaxial growth stresses have been measured thanks to a technique coupling Raman spectroscopy and in situ high temperature oxidation of the NiCr30 alloy in the temperature range (700−900u2009°C). It is established that the compressive growth stress in such oxide films can reach more than 2 GPa, before additional thermal stress arises on cooling. Moreover, the growth stress kinetics—subsequent establishment and relaxation—are highly microstructure sensitive: in particular, as the oxidation temperature rises, the chromia mean grain size also increases, and it consequently retards the occurrence of the creep relaxation phenomena which needs an additional stress level to start.

Phosphating of bulk α-iron and its oxidation resistance at 400 °C

Fe bulk specimen have been treated with phosphoric acid in order to obtain a conversion surface layer and the oxidation resistance has been studied by means of in situ thermogravimetric experiments in artificial air (20% O2; 80% N2). For oxidation time of 48 h at 400 8C it shows an important decrease of the oxidation kinetic in comparison witha-Fe. Moreover, the kinetic can be described by two successive parabolic stages. This behaviour is respectively related to an X-ray photoelectron spectroscopy (XPS) analysis of the surfaces and to previous results obtained on the oxidation of a-iron. # 2002 Elsevier Science B.V. All rights reserved.

Role of a chromium ion implantation on the corrosion behaviour of nickel in artificial sea water

Ni bulk specimens have been implanted with Cr ions (60 keV, 4.1016 ions/cm2), and classical corrosion experiments performed in artificial sea water by means of potentiodynamics i(E) and potentiostatics i(t) studies. Both experiments show that an important decrease of the current density is obtained in comparison with pure nickel. Moreover, a significant extension of the passivation region can be observed, with mainly an increase of the pitting potential. This behaviour is respectively related to an X-ray photoelectron spectroscopy (XPS) analysis of the passive film formed on Ni (Ni, Cr and O environments), and to previous results obtained on similar alloys.

Influence of low energy–high flux nitrogen implantation on the oxidation behavior of AISI 304L austenitic stainless steel

Low energy–high flux (1.2 keV, 1 mA/cm2) nitriding of an austenitic AISI 304L stainless steel has been carried out by implanting a dose of 3.5×1019u2009ionsu200acm−2 at 400u200a°C for 1 h. An important increase in surface hardness has been found to occur as a result of the formation of the so-called expanded austenite γN phase. Transmission electron microscopy studies have also shown the existence of hexagonal Cr2N precipitates at the nitrided layer/matrix interfacial region. The oxidation behavior has been then studied in the temperature range between 400 and 550u200a°C for 24 h under synthetic air. It is shown that nitridation slightly modifies the oxidation kinetics as a result of the progressive transformation of the γN phase towards CrN precipitation and α-FeNi matrix formation. More chromium enriched scales are, anyhow, developed on the nitrided steel than in the untreated one. These results are discussed on the basis of the chromium diffusion in the nitrided layer and its preferential chemical bonding with nitroge...

Study of stress effects in the oxidation of phosphated α-iron: in situ measurement by diffraction of synchrotron radiation

Abstract In the present work the development of strains in the iron-oxide layers growing on phosphated α-Fe at 400xa0°C in artificial air at 1xa0atm was investigated by X-ray diffraction (XRD) of synchrotron radiation, both in situ during oxide growth and also at room temperature after cooling. This stress state may play an important role during the oxidation process, from the phases as well as from the kinetics point of view. The oxidation kinetics of α-Fe and phosphated α-Fe at 400xa0°C and the phases evolution during oxidation are first presented. Then a detailed study of the strains in the oxides layers is undertaken. Correlations between the stresses measurements and the successive parabolic oxidation stages for phosphated α-iron are established. It leads to a better understanding of the oxidation behaviour.

Structural characterisation of phosphated α-iron oxidised at 400 °C

α-Iron specimens were treated with phosphoric acid to obtain a conversion surface layer and the oxidation resistance of samples was studied at 400 °C in air. For oxidation time of 48 h, an important decrease in the oxidation kinetics was obtained in comparison with untreated α-Fe. The samples were characterised by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses of the surfaces and these results are related to previous data on the oxidation of α-iron.

Local stress determination in chromia-former thanks to micro-Raman spectroscopy: A way to investigate spontaneous delamination processes

Spontaneous delamination process for α-Cr2O3 thermal oxide films growing on NiCr-30 alloys is studied thanks to micro Raman spectroscopy. In particular, stress maps are performed through and around buckles developed on chromia films. Depending on the cooling rate at the end of the oxidation process, different buckle types appear which are investigated. Associated residual stress distribution clearly evidences the stress release field. In addition, geometrical features are determined for the different buckle types, and from comparison with modelling describing buckle formation and propagation, it is possible to get the interface toughness distribution.

Stress release phenomena in chromia scales formed on NiCr-30 alloys: Influence of metallurgical parameters

Stress release phenomena are studied for α-Cr2O3 thermal oxide films grown on NiCr-30 alloys. The influence of specific metallurgical parameters, such as cooling rate and initial surface roughness, is investigated thanks to Raman spectroscopy. Systematic correlations are established between the residual stress level in the scales and the damage rate resulting from a delamination process by buckling. Different buckling morphologies are characterized mainly according to the cooling rate range.

Substrate grain boundary effect on residual stress distribution at micrometer scale in chromia oxide thin films

Residual stresses in chromia oxide films formed at 700°C on NiCr30 have been investigated with confocal Raman spectroscopy. The high lateral resolution allows us to determine stress distribution in the oxide growing above both substrate grains and grain boundaries. It is demonstrated that the magnitude of the equibiaxial compressive stress is related to local oxide microstructure evolution: during the initial oxidation, stress is smaller in the chromia located above substrate grain boundaries as compared to that above substrate grains. Subsequently, this gap in stress magnitude progressively vanishes due to the occurrence of oxide grains forming bridges across depressions in the oxide scale above substrate grain boundaries.