Jean-Claude Colson

The effect of rare earths deposited on steel surfaces, by different processes (sol/gel, electrophoresis, OMCVD), on high temperature corrosion behaviour

Abstract In this paper, different methods of rare earth oxides deposition on the surface of AISI 304 steel are described: sol/gel, electrophoresis in aqueous or organic medium, OMCVD of rare earth β-diketonates. After the deposition, all the samples were reheated. This treatment was absolutely essential for a strengthening effect of the protective oxide scale. Oxidation of coated and non-coated samples was performed under isothermal and cyclic conditions. The effect of rare earth deposition is significant in isothermal conditions. However, in thermal cyclic conditions, the effect of a rare earth deposited on surface is spectacular. Scale spallation is then completely avoided under the chosen experimental conditions. The adherence of the protective scale could be due to an inversion of the defect structure oxide as it has been suggested in literature, but no experimental evidence to support this can be put forward from this work.

Mechanisms Involved by Reactive Elements upon High Temperature Chromia Scale Growth

The influence of Y 2 O 3 , Pr 2 O 3 , Nd 2 O 3 , Sm 2 O 3 and Yb 2 O 3 coatings on Fe-30Cr alloy oxidation behaviour was investigated at 1000°C in air under atmospheric pressure. Isothermal exposures indicated that the Y 2 O 3 coating was the most protective after 100 hours. Pr 2 O 3 , Nd 2 O 3 and Sm 2 O 3 coatings were less effective, but the less beneficial effect was observed when Yb 2 O 3 coating was applied onto the Fe-Cr alloy surface. Two-stage oxidation experiments in 16 O 2 and then 18 O 2 were performed to get information about the chromia growth phenomena with and without reactive elements. The 18 O-tracer distribution was determined by secondary ion mass spectrometry (SIMS) and sputtered neutral mass spectrometry (SNMS). The experiments performed on uncoated samples clearly demonstrated that chromia growth mechanism was controlled by chromium cationic diffusion, whereas on reactive element coated samples the external diffusion of chromium ions was not predominant.

Segregation of Neodymium in Chromia Grain-Boundaries during High-Temperature Oxidation of Neodymium Oxide-Coated Chromia-Forming Alloys

The influence of MOCVD reactive-element-oxide (REO) coatings (Nd2O3) onhigh-temperature, chromia-forming alloy oxidation was investigated. REOcoatings decreased steel oxidation rates and greatly enhanced oxide scaleadherence. Uncoated and coated F17Ti samples were oxidized over thetemperature range 700–1050°C in air at atmospheric pressure. SIMSexperiments were performed on oxidized-coated samples in order to determinethe RE distribution through the oxide scale. Nd was distributed across theoxide layers with a higher concentration in the outer part of thescale. Transmission-electron microscopy (TEM) investigations were performedto more precisely locate the RE through the scale. Transverse crosssections, prepared on oxidized Nd2O3-coatedFe–30Cr (model system), showed that Nd, associated with Cr and O,segregated at chromia grain boundaries. It is thought that this is the maincause of the beneficial effects usually ascribed to the RE inchromia-forming alloys. The effect of chromia grain-boundary segregation onchromia growth mechanism and its influence on the reactive-element effect(REE) are discussed.

The combined effect of refractory coatings containing reactive elements on high temperature oxidation behavior of chromia-forming alloys

The high temperature oxidation behaviors of chromia-forming alloys (F17Ti and Fe–30Cr alloys) have been studied at 1273 K under isothermal conditions and at 1223 K under cyclic conditions, in air under the atmospheric pressure. To extend the oxidation lifetime, coatings have been applied onto the alloy surfaces. Al2O3 and Cr2O3 films doped with Sm2O3 or Nd2O3 were prepared via the metal-organic chemical vapor deposition technique. Single Cr2O3, Al2O3, Nd2O3 and codeposited Cr2O3–Nd2O3, Al2O3–Nd2O3, Al2O3–Sm2O3 coatings drastically improved the chromia-forming alloy high temperature oxidation behavior, since they decreased the oxidation rate and enhanced the oxide scale adhesion. Results showed that a critical amount of reactive element (Nd or Sm) in chromia or alumina coatings (11–18 at.%) was needed to observe the most effective effect. The fast precipitation of NdCrO3 or NdTi21O38 and the segregation of reactive elements at the chromia grain boundaries slowing down outward cation transport and consequently blocking the chromia grain growth, was supposed to be the main reasons of the beneficial effect ascribed to the reactive elements in chromia scales.

The mechanism and kinetics of central cavity formation during the growth of thick layers of copper sulfides on the metal

A study of the sulfidation kinetics of thin copper wires and whiskers showed that cavity formation caused by metal consumption starts at the metal-sulfide interface and grows inwards as the reaction proceeds. The cavity does not start in the middle and expand outwards. An exterior honeycombed region is formed which is traversed by metal bridges and filaments linking the internal metal core to the sulfide. The bridges assure the metal supply and only disappear when conversion is virtually completed. The authors suggest a theoretical interpretation which accounts for these observations.

Effects of chromia coatings on the high-temperature behavior of F17Ti stainless steel in air: Analytical studies of the effect of rare-earth-element oxides

The MOCVD deposition of neodymium oxide and/orchromium oxide provided beneficial effects both onisothermal- and cyclic high-temperature behavior ofcommercial F17Ti stainless steel. Fracture crosssections provided information about the morphology ofthe oxide scales formed on bare steel and coatedspecimens. The chromia scales developed small equiaxedgrains on the Nd2O3-coated samplesand columnar grains on the uncoated ones. Neo dymium segregatedwithin a surface layer composed ofMn1.5Cr1.5O4 spineloxide. A complex phase (close to the structure ofCeTi21O38) was identified in thiszone. It could act as a source of neodymium ions, which couldsegregate to the grain boundaries of the chromia scale.Polished cross sections associated with X-ray mappingstudies confirmed the scale structure and the location of the rare-earth element in the outer part ofthe oxide layer.

A kinetic and morphological study of the sulphidation of Ni/23 Cr and Ni/33 Cr alloys in S2 vapour and hydrogen sulphide

The corrosion of Ni/23 Cr and Ni/33 Cr alloys has been studied in the presence of S2 vapour and H2S/H2 mixtures between 600 and 900°C. The reaction progress curves Δm/q = f(t) are parabolic and the transformed ones (Δm/q)2 = f(t) are linear. At low temperatures, the activation energy value is 92 < E < 260 kJ/mol depending on the nature of the sulphurizing agent and the composition of the alloy. The formation of two distinct sulphide layers is shown: the outer layer of N3S2, the underlying layer of Cr2S3. The formation of NiCr2S4 from both sulphides present is also observed. At higher temperatures, chromium sulphide alone develops and pure nickel blocks remain. The sulphidation rate does not obey the Arrhenius law.

Brass sulphidation by hydrogen sulphide and sulphur vapour at low pressure: Morphological and kinetic aspects

The effect of alloy composition on the sulphidation of brass in H2S/H2 mixtures and sulphur vapour is shown. In all cases the corrosion rate is determined by cationic diffusion through one of the sulphide layers formed. The different morphologies observed at the end of sulphidation are explained on the basis of zinc content of the alloys.

Morphological and kinetic study of copper corrosion when covered with digenite sulphide layer, in sulphur, under covellite formation conditions

Abstract The sulphurization of copper covered with a digenite protective layer, under covelliteformation conditions occurs in two stages: metal valence value change in the sulphide and then metal transformation. The first stage involves an external growth and covellite precipitation in the copper vacancies supersaturated in digenite, the second an external growth similar to that of metals alone.

Kinetic and morphological study of digenite sulfidation into covellite

Abstract A kinetic and morphological study of Cu 2−x S sulfidation into CuS shows that the metal valency change occurs according to two different mechanisms : “external” growth similar to that in metals and precipitation from the metal vacancies supersaturated substrate. The diffusion step of (E = 19,7 ± 1 kcal.M −1 ) within the covellite determines the transformation rate.