Olivier Heintz

Influence of the Chemical Dissolution of MnS Inclusions on the Electrochemical Behavior of Stainless Steels

Immersion of stainless steel containing a well-controlled density of MnS inclusions in 1 M NaCI, pH 3 leads to the chemical dissolution of these heterogeneities. This process was studied using in situ atomic force microscopy and the dissolution rate of MnS inclusions was estimated between 0.01 and 0.19 μm 3 /min. The effects of MnS dissolution on the chemical composition and the local electrochemical behavior of the specimen surface were investigated using secondary ion mass spectroscopy, X-ray photoemission spectroscopy, and the electrochemical microcell technique. It was shown that stable CrS and unstable FeSO 4 were formed. The size of the areas around MnS inclusions affected by the presence of sulfur-containing species depends on the immersion time and the composition of the native film. The local electrochemical measurements reveal that the chemical dissolution of MnS inclusions promotes pitting corrosion in the surrounding grains at moderate applied potentials for short immersion time and general corrosion of the whole specimen surface for long immersion time.

Local Electrochemical Studies and Surface Analysis on Worn Surfaces

Specimens of duplex stainless steel UNS S32304 immersed in distilled water were worn under sliding friction conditions. The electrochemical microcell technique and local Auger spectroscopy were then applied to these heterogeneous surfaces to determine the influence of surface chemical composition and surface defects on the electrochemical behavior of each phase of the stainless steel in and out of the wear track. Experiments were performed both in 1 M NaCl, pH 3 (where the passive film is stable), and 0.5 M HCl (where an active peak is observed). In addition, the scanning vibrating electrode technique was combined with a modified microcell to study microgalvanic processes between the surrounding passive surface and the track at the OCP value.

Thermal effects on the growth by metal organic chemical vapour deposition of TiO2 thin films on (100) GaAs substrates

Abstract TiO 2 thin films were deposited on (100) GaAs substrates by LP-MOCVD with deposition temperatures ( T d ) ranking from 450 to 750 °C. The structure of these layers was studied by X-ray diffraction (XRD) and Raman spectroscopy. The growth of the TiO 2 anatase phase was observed for T d T d >600 °C. Finally, X-ray photoelectron spectrometry (XPS) and secondary ion mass spectroscopy (SIMS) experiments showed the presence of small quantities of Ga and As through the whole film thickness, slightly increasing at the surface of the layers. This result was related to the SEM observations and explained by considering the growth conditions.

Thermal Alumina Scales on FeCrAl: Characterization and Growth Mechanism

High temperature oxidation resistance of alumina-forming materials is connected to the growth of dense, stable and protective alumina scales. Depending on temperature, impurities present in the base alloys, presence of water vapour in the oxidizing atmosphere, the alumina scales are composed of alpha-alumina (which is the stable phase obtained for temperatures over 1000°C) or of transient alumina (γ,θ,δ obtained for lower temperatures). It is generally considered that γ- Al2O3 grows when T<850°C, that θ-Al2O3 is present for 850°C<T<1000°C and that α-Al2O3 is stable when T exceeds 1000°C. The exact role played by transient alumina formation and/or transformation on the high temperature performances of alumina-forming materials is not exactly defined. Many works proposed that transient alumina phases grew during the first steps of the oxidation process and transformed into the stable phase after further oxidation. The transformation of transient phases in the stable alphaphase is generally accompanied by a volume contraction of around 14 %. In order to get better oxidation resistance, the formation of transient alumina is not wished, because: 1) their growth rate is generally higher than that of alpha-alumina with, as a consequence, a huge Al consumption, detrimental for the material resistance after long exposures, 2) the change in volume during the transformation of transient phases into alpha-alumina can generate stresses in the oxide scale and can weaken its adherence to the underlying substrate, leading to massive spallation. The present study deals with the coupling of different characterization tools in order to precisely identify the transient phases grown on FeCrAl materials. The use of scanning electron microscope (SEM-FEG), transmission electron microscope (TEM), Photoluminscescence Spectroscopy(PLS), X-ray photoelectron spectrometry (XPS) and X-ray diffraction at different glancing angles (XRD) on model materials oxidized at two temperatures (850 and 1100°C) could help the identification of transient phases. These techniques gave a better understanding of the alumina scale growth mechanism.

Phthalocyanine–titanate nanotubes: a promising nanocarrier detectable by optical imaging in the so-called imaging window

TiONts–phthalocyanine nanohybrids combining an efficient optical probe and a promising nanovector have been developed in a step-by-step approach and were thoroughly characterized. Each 150 nm long TiONts–Pc bear ca. 450 Pc. Three nanohybrids were prepared including three different linkers in quest for the best stability.

Elaboration and characterization of barium silicate thin films

Room temperature depositions of barium on a thermal silicon oxide layer were performed in ultra high vacuum (UHV). In-situ X-ray photoelectron spectroscopy (XPS) analyses were carried out as well after exposure to air as after subsequent annealings. These analyses were ex-situ completed by secondary ion mass spectrometry (SIMS) profiles and transmission electron microscopy (TEM) cross-sectional images. The results showed that after air exposure, the barium went carbonated. Annealing at sufficient temperature permitted to decompose the carbonate to benefit of a barium silicate. The silicate layer was formed by interdiffusion of barium with the initial SiO2 layer.

Influence of modification time and high frequency ultrasound irradiation on self-assembling of alkylphosphonic acids on stainless steel: Electrochemical and spectroscopic studies.

Self-assembly of alkylphosphonic acids on stainless steel was investigated under different conditions. Four different alkylphosphonic acids exhibiting alkyl chain of various size were synthesized and studied: butylphosphonic acid (C4P), octylphosphonic acid (C8P), decylphosphonic acid (C10P), and hexadecylphosphonic acid (C16P). Electrochemistry experiments were extensively carried out in order to determine electrochemical surface blocking of adsorbed layers in function of grafting time. In term of surface blocking, an 8h modification time was optimal for all alkylphosphonic acids. Longer immersion times lead to degradation of adsorbed layers. For the first time, grafting of C16P was studied under high frequency ultrasound irradiation. Interestingly, grafting process is highly accelerated under sonication and well-covering C16P modified substrates are obtained after 1h of immersion under ultrasound irradiation. This would allow to elaborate high-quality alkylphosphonic acids modified samples within much shorter times. Water contact angles measurements and X-ray Photoelectrons Spectroscopy (XPS) confirmed presence of adsorbed alkylphosphonic acids on stainless steel surface. A very tight link between electrochemical blocking, surface hydrophobicity and species chemical grafting was established.

Structural and in depth characterization of newly designed conducting/insulating TiNxOy/TiO2 multilayers obtained by one step LP-MOCVD growth

Abstract TiNxOy/TiO2 multilayers have been grown by LP-MOCVD using titanium isopropoxide (TIP) precursor during the whole growth, but with an ammonia flow interrupted for the TiO2 layers. The one step growth process used to grow these structures allowed to stack the conducting and insulating layers without any growth breakdown. SIMS and TEM analyses showed the presence of an alternated insulating/conducting layers structure. Moreover, electrical measurements allowed to measure the dielectric part of insulating TiO2 stacked in these structures, whose permittivity was found to be about 80 for a MOS structure. Thus, such multilayers may lead to very promising applications in the microelectronics field.

Which tool to distinguish transient alumina from alpha alumina in thermally grown alumina scales

Abstract Alumina scales constitute excellent protective barriers when they form on alumina-forming steels. If they keep tightly adherent to the underlying substrate, they isolate it from the surrounding aggressive atmosphere at high temperature. The protectiveness of the alumina scale is highly dependant upon its growth mechanism. The nucleation and transformation of transient alumina (mainly γ-Al2O3 and θ-Al2O3) is known to play an important role on alumina scale formation. It is therefore fundamental to characterise these transient alumina especially during the early stages of the oxidation process. The morphology of the transient alumina was observed by scanning electron microscopy (SEM), their crystallographic phases determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). X-ray photoelectron spectrometry (XPS) analyses were performed on reference samples and then compared to the alloys oxidised 5 and 30 minutes at 850, 900 and 950°C.

Functionalized Fe3O4 nanoparticles: influence of ligand addition sequence and pH during their continuous hydrothermal synthesis

In this study we report various new efficient ways to synthesize and modify in situ magnetite (Fe3O4) iron oxide nanoparticles (NPs). Thanks to an apparatus especially developed for this new method of grafting, the NPs have been synthesized and functionalized by 3,4-dihydroxyhydrocinnamic acid (DHCA) or 3,4-dihydroxy-L-phenylalanine (LDOPA) in one step and under hydrothermal conditions using varying concentration ratios ([organic molecules]/[ferrous and ferric ions]). The organic molecules were added before or after the NP synthesis. The addition of these organic molecules modifies the structure, the morphology, the oxidation degree and the growth of the crystallites. Adding the organic molecules before the synthesis step and under acidic conditions increases the average crystallite size and prevents further oxidation whereas under basic conditions the growth is stopped but a partial oxidation of magnetite to maghemite NPs is observed. Adding DHCA or LDOPA after the synthesis step results in a modification of the lattice structure and oxidation degree of the NPs but does not change the average size. This study underlines the importance of the sequence of the addition of organic molecules on the synthesis of NPs.