Comparison of electroplating and sputtering Ni for Ni/NiFe₂ dual layer coating on ferritic stainless steel interconnect

Highlights

• Ni layer is electroplated and sputtered to prepare Ni/NiFe₂ dual layer coating.

• Cr out-migration was effectively prevented by the converted oxide coating.

• Electroplating Ni reduced oxidation resistance while sputtering Ni enhanced it.

• Surface scale of sputtered Ni sample exhibited a better electrical conductivity.

Abstract

Ni/NiFe₂ dual layer coating is fabricated to improve the performance of SUS 430 steel as solid oxide fuel cells (SOFCs) interconnects by a combined approach. Ni layer is deposited by electroplating or magnetron sputtering, and subsequently the NiFe₂ layer is sputtered on it. The coated steels are evaluated at 800 °C in air corresponding to SOFC cathode environment. It is found that the coating with electroplated Ni and sputtered NiFe₂ results in breakaway oxidation after 5 weeks. Preoxidation treatment in 900 °C air for electroplated Ni steel prior to depositing NiFe₂ layer improves oxidation resistance. The sputtered Ni and NiFe₂ sample exhibits superior oxidation resistance and inhibition against Cr outward migration while maintaining a low area-specific resistance (ASR) of surface scale. Comparative analyses of the coating preparation and resultant oxidation behavior with possible mechanisms and implications are discussed.

Introduction

Interconnect is a critical component of the planar solid oxide fuel cells (SOFCs) as it electrically connects the individual cells to stack and provides physical separation between air and fuel. Reduction of SOFC working temperature to 600–800 °C enables the use of high temperature metallic interconnects which show low fabrication cost and good electronic conductivity relative to traditional ceramic alternatives [1], [2]. Ferritic stainless steels (FSS) are currently among the most promising candidates owing to coefficient of thermal expansion (CTE) matching with other cell components and low cost [3]. During high-temperature exposure, Cr in the FSS undergoes selective oxidation to form a Cr₂O₃ scale that is sufficiently protective and conductive. However, the Cr₂O₃ scale reactively releases volatile Cr (VI) species in SOFC cathode environment, which causes electrochemical activity reduction of cathode. Moreover, the Cr₂O₃ scale grows gradually with time during long term exposure, leading to the increase of electrical resistance and even spallation of the oxide scale [4]. An effective and practical solution is applying diffusion barrier coatings on the steel interconnect. To date, rare earth perovskite and spinel type coatings are most extensively investigated [5]. They can effectively reduce Cr evaporation by several orders and improve the oxidation resistance [6], [7], [8], [9].

Although the coating could prevent Cr₂O₃ directly exposing to the high temperature air, Cr will migrate outwards to the scale surface gradually due to the inevitable interaction between native Cr₂O₃ and coatings [10], [11], [12], [13]. This not only impairs the protectiveness of the Cr₂O₃ layer, but also degrades the coating performance. For instance, the interdiffusion of Cr₂O₃ and La_0.6Sr_0.4CoO₃ peroskite coating leads to the formation of insulating SrCrO₄ phase [10]. Compared to peroskite, spinel coatings show better performance in reducing Cr outward diffusion due to its good Cr retention capability. Moreover, spinel coatings are more effective in reducing the inward diffusion of oxygen and the oxidation rate. Ferrite spinels have the closest match in thermal expansion to ferritic steels [14] and NiFe₂O₄ spinel has even been used as a diffusion barrier prior to depositing peroskite coating [15]. After long term exposure, the NiFe₂O₄ spinel coating is gradually converted into a mixed spinel containing Ni, Fe and Cr resulting from the interaction with Cr₂O₃ [16]. Although Cr evaporation rate of Cr-containing spinel is lower by more than an order of magnitude than that of Cr₂O₃, the incorporation of Cr into spinels will still cause a series of irreversible consequences, e.g. dramatic decrease of electrical conductivity and change in the CTE of the spinel [17], [18]. Therefore, the interaction between Cr₂O₃ and spinel coatings should be restrained to ensure the long-term stabilization of the interconnect performance.

NiO is also a potential coating for steel interconnects application and it exhibits much higher electrical conductivity compared to Cr₂O₃ [14]. And more importantly, the interfacial reaction between Cr₂O₃ and NiO is quite slow and thus NiO is effective in preventing Cr outward migration. Glazoff et al. [19] attribute this superior performance to the low solubility and mobility of Cr in NiO oxide matrix. However, NiO possesses a higher CTE (14–17 × 10⁻⁶ K⁻¹) [20] compared to Cr₂O₃ (9.6 × 10⁻⁶ K⁻¹) [11] and ferritic stainless steel substrate (11–12 × 10⁻⁶ K⁻¹), which induces micro cracks in the surface scale [21]. On this account, NiO could be used as a diffusion barrier against Cr outward migration combined with an outer spinel layer to develop a crack-free structure and long-term stability in coating performance. In our previous work [22], Ni/NiFe₂ dual layer coating was deposited on the steel to develop NiO/NiFe₂O₄ dual layer coating via magnetron sputtering. It is found that both Cr and Mn are well-confined behind the NiO layer and the outer NiFe₂O₄ spinel layer is constantly Cr-free after long term exposure, which thereby contributes to a lower oxidation rate compared to NiFe₂ coated steel. The thermal conversion method by depositing corresponding metallic or alloy coatings with subsequent oxidation is preferred for the preparation of NiO and NiFe₂O₄ coatings due to good adherence and compactness. Magnetron sputtering is a form of physical vapor deposition (PVD) that is widely applied to deposit a variety of coatings, many of which are of industrial importance due to the versatility of this technique as well as the ability to control composition and morphology. Additionally, electroplating is an alternative approach for preparation of metallic or alloy coatings. It has been widely known that electroplated coatings have more uniform microstructure and fewer defects compared to magnetron sputtered coatings. The difference in microstructures of the as-deposited alloy coatings via sputtering and electroplating, which closely correlate with the oxidation behavior and performance, is very likely to influence the conversion process and effectiveness in blocking Cr out migration and in improving oxidation resistance.

This work aims at comparative investigation of the Ni/NiFe₂ dual layer coatings on SUS 430 steel. The Ni layer was obtained by electroplating and magnetron sputtering, respectively, and the NiFe₂ layer was deposited by magnetron sputtering. To improve the oxidation resistance, some Ni coated steels by electroplating were preoxidized in air at 900 °C for 10 h prior to the NiFe₂ layer deposition. The coated samples were exposed to 800 °C air for up to 15 weeks to study the oxidation behavior and electrical property of the surface scales. The influences of the preparation processes of the Ni/NiFe₂ coatings on oxidation behavior were compared and discussed.