Long-term behaviour of solid oxide fuel cell interconnect materials in contact with Ni-mesh during exposure in simulated anode gas at 700 and 800 °C

Highlights

• Material behaviour in a real SOFC stack is reported (exposure up to 3000 h).

• Interdiffusion between high Cr ferritic steel and Ni-mesh results in precipitation of σ-phase in the reaction zone.

• The mechanism of σ-phase precipitation is discussed in terms of alloy thermodynamics (CALPHAD approach).

• Effect of alloying elements, e.g. W and Nb, on the σ-phase stability is clarified.

Abstract

In the present study the long-term behaviour of two ferritic steels, Crofer 22 APU and Crofer 22H, in contact with a Ni-mesh during exposure in simulated anode gas, Ar–4%H₂–2%H₂O, at 700 and 800 °C for exposure times up to 3000 h was investigated. Ni diffusion from the Ni-mesh into the steel resulted in the formation of an austenitic zone whereas diffusion of iron and chromium from the steel into the Ni-mesh resulted in the formation of chromia base oxides in the Ni-mesh. Depending on the chemical composition of the steel, the temperature and the exposure time, interdiffusion processes between ferritic steel and Ni-mesh also resulted in σ-phase formation at the austenite–ferrite interface and in Laves-phase dissolution in the austenitic zone. The extent and morphology of the σ-phase formation are discussed on the basis of thermodynamic considerations, including reaction paths in the ternary alloy system Fe–Ni–Cr.

Introduction

Interconnect plates are used in planar solid oxide fuel cells (SOFC's) to separate the different atmospheres present at the cathode and the anode side, thus serving as current collector as well as electrical connectors. Chromia-forming alloys, such as ferritic steels or Cr-base ODS alloys, have been successfully used as interconnect materials since chromia has a reasonably good electronic conductivity at the high service temperatures [1], [2], [3], [4], [5], [6]. The overall electrical resistance of the interconnect material is directly related to the thickness of the chromia layer, which means that after long-term exposure at high temperatures, the electrical resistance values might reach unacceptably high levels for the proper operation of the cell [7], [8], [9], [10]. The overall performance of the cell can be enhanced by improving the electrical contact between the anode and the interconnector using an intermediate Ni-mesh between the anode and the interconnector. Nonetheless, the long-term interaction between the Ni-mesh and the interconnector material, involving interdiffusion of different alloying elements, may adversely affect the long-term performance of the cell.

The major concern of using a Ni-mesh in direct contact with a ferritic steel interconnect is that the presence of nickel results in the formation of an austenitic γ-FCC phase in the ferritic steel in the contact zone [11], [12], [13], [14], [15], [16], [17], [18]. This phase transformation is associated with a volume change and a change in the physical and mechanical properties of the interconnect material in the contact zone. Austenite possesses a substantially higher coefficient of thermal expansion than ferrite, which may result in initiation of mechanical stresses during thermal cycling [19], [20], [21], [22]. Additionally, the interdiffusion coefficients of chromium and nickel are approximately 30 times lower in austenite than in the ferrite [23], [24]. The slower chromium diffusion in the austenite may adversely affect the selective oxidation of Cr in the interconnect steel and thus, the formation and growth of a protective chromia base surface scale [19], [20], [21], [25]. The width of the austenitic zone is expected to increase with exposure time since Cr depletion as result of chromia scale growth implies that less Fe and Ni are needed to stabilize the γ-FCC phase.

Apart from austenite formation, interdiffusion between the interconnect material and the Ni-mesh are known to lead to phase transformations and microstructural changes in the interconnect material. Formation of σ-phase near the contact area between a Ni-mesh and the ferritic steel Crofer 22H has been observed after exposure in Ar–4%H₂–2%H₂O at 800 °C for 1000 h [18]. Niewolak et al. [12] found that σ-FeCr precipitates formed in the interdiffusion zone, i.e. at the interface between the original ferritic matrix and the austenite, when Crofer 22 APU electroplated with a 5–10 μm thick Ni layer was exposed in Ar–9.2%CO–3.7H₂–0.2%H₂O at 600 °C for 300 h. Presence of the brittle σ-phase may be a source for crack formation and therefore decrease the overall performance of the interconnect.

Also the properties of the Ni-mesh may be affected by interdiffusion processes. Nickel oxide is thermodynamically not stable at the equilibrium oxygen partial pressure prevailing in the anode gas. Nonetheless, Fe, Cr, and other alloying elements from the interconnector material may diffuse into the Ni-mesh driven by elemental activity gradients, and different types of oxides with low electrical conductivity might form on the Ni surface and thus reduce the cathode area [26].

The present study investigates the long-term behaviour of two commercial ferritic steels in contact with a Ni-mesh during exposure in simulated anode gas, Ar–4%H₂–2%H₂O, in the temperature range 700–800 °C for exposure times up to 3000 h. After exposure the specimens were characterized using light optical metallography scanning electron microscopy (SEM), energy and wavelength dispersive X-ray spectroscopy (EDX/WDX) and electron backscatter diffraction (EBSD). In the investigations main emphasis was put on the interdiffusion of alloying elements between the Ni-mesh and the interconnect materials, on surface oxide formation as well as on phase formation and dissolution in the interdiffusion zone.