Valérie Parry

Optimisation of Metallic Interconnects for Hydrogen Production by High Temperature Water Vapour Electrolysis

For economical and environmental reasons, hydrogen is considered as a major energetic vector for the future. Hydrogen production via high temperature water vapour electrolysis (HTE) is a promising technology. A major technical difficulty related to high temperature water vapour electrolysis is the development of interconnects working efficiently for a long period. Working temperature of 800°C enables the use of metallic materials as interconnects. Chromia forming alloys are among the best candidates. The interconnect material chosen in the present study is a ferritic stainless steel with 18% chromium content. High temperature corrosion resistance and electrical conductivity of the alloy was tested in both cathode (H2/H2O) and anode (O2/H2O) atmospheres. Corrosion products were then characterized by SEM-EDX and XRD. Moreover chromium evaporation measurements were carried out under anode atmosphere.

Possible connection between nodule development and presence of niobium and/or titanium during short time thermal oxidation of AISI 441 stainless steel in wet atmosphere

Abstract AISI 441 ferritic stainless steel is a good candidate for metallic interconnects of solid oxide fuel cells (SOFCs). In this alloy, the minor elements Ti and Nb are used to stabilise the ferritic structure but their influence on steel durability is not well understood. This study focuses on the early stages of oxidation (24 h) at 800°C of AISI 441 under 5%H2O in O2 following the cathodic SOFCs conditions. The typical duplex oxide scale, composed of a (Mn,Cr)3O4 spinel top layer and a Cr2O3 rich sublayer is observed, with oxide nodules growing in places. These objects, in the micrometre range in size, are studied by FIB tomography. The analyses reveal a complex structure and a development strongly linked to the presence of niobium and/or titanium compound(s) in the subjacent substrate.

A Possible Mechanism for Protrusions Formation at the Metal/Oxide Interface During Short Time Oxidation of Ferritic Stainless Steel

High temperature oxidation of ferritic stainless steel for short durations leads to the formation of an original morphology at the metal/oxide interface. This interface is composed of metallic protrusions localized in a chromium-rich oxide layer through a discontinuous silica film. In this paper we propose a mechanism based on preferential diffusion paths for the oxygen through the oxide that are governed by the distribution of the hydrostatic pressure in this layer. We point out that the mechanical contrast between the oxide and the metal subjected to creep can be critical for the hydrostatic pressure gradient magnitude inside the oxide layer. This observation is likely to promote the formation of protrusions for specific conditions of temperature and time of exposure to oxidation.

Advanced Microstructural Investigations of AISI 441 Early Stage Oxidation in Wet Atmosphere

AISI 441 ferritic stainless steel is a good candidate for metallic interconnects in solid oxide fuel cells (SOFCs). The minor elements Ti and Nb are used to stabilize the ferritic matrix and also to reduce creep by a combination of solid solution strengthening and precipitation of intermetallic Laves phase particles along the grain boundaries. However their influence on the oxidation behavior is not well understood. This study focuses on the early stages oxidation (from 4 to 24 h) at 800 °C of AISI 441 under 5% H2O in O2. A relatively smooth micro-crystallized oxide scale and Ti, Nb containing nodules are observed. The internal microstructure of these objects is studied by FIB tomography which allows computing cross sectional views in any direction of interest. FIB study reveals a complex microstructure and a development strongly linked to the presence of niobium and/or titanium in the substrate.

Evolution of the metal–oxide interface during the initial stage of the high temperature oxidation of ferritic stainless steels

Abstract Two grades of ferritic stainless steel, a bi-stabilised Ti, Nb (AISI 441) and a stabilised Ti (AISI 439), were oxidised at 1060°C under the simulated process atmosphere for durations between 45 and 1800 s. Focused ion beam coupled with field emission gun and scanning electron microscopy was carried out to investigate the cross-section morphology of the oxide growing on ferritic stainless steels. Matrix protrusions localised at the metal – chromia interface through the silica layer are observed and the following mechanism for their formation is proposed. During the first step of oxidation, interface undulation, induced by growth stresses, in combination with silica precipitation at the metal – oxide interface lead to the formation of matrix protrusions in the chromia layer. For an increased oxidation time, due to the laterally silica growth matrix protrusions are trapped into the Cr2O3 layer as matrix inclusions.