500 Hour Investigation of Electroless Ni-Plated AISI 441 Steel for Solid Oxide Fuel Cell Interconnect Applications Under Operating Conditions

Abstract

Solid Oxide Fuel Cells (SOFCs) are high-temperature (600-800°C) electrochemical devices that produce heat and electricity by oxidizing hydrogen or short chain hydrocarbon fuels. Due to their high efficiency, fuel flexibility, and low emissions, SOFCs have gained popularity as a method of both large- and small-scale power production. In planar SOFC technology, individual cells are stacked in electrical series using ferritic stainless steel interconnects (ICs), which separate the fuel (anode) and oxidant (cathode) chambers, facilitate gas distribution, and provide mechanical support. The ICs are simultaneously exposed to the fuel (H2) on one side and the oxidant (air) on the other, known as a “dual atmosphere exposure.” Recent advances in SOFC technology have allowed ferritic stainless steels (FSSs) to replace the cost-prohibitive ceramic IC components that were previously used. However, FSSs experience accelerated and/or anomalous corrosion in dual atmosphere environments (relative to single atmosphere exposure – e.g., air only) that leads to SOFC performance degradation. In this study, electroless Ni-plated FSS AISI 441 was evaluated under standard operating conditions, namely 700°C dual atmosphere with and without a current flow of 0.5 A for 500 hours. Morphology of the surfaces and cross sections of each sample were analyzed using a Field Emission Scanning Electron Microscope (FE-SEM) and the composition was assessed using Energy Dispersive X-Ray Spectroscopy (EDX). Anomalous and accelerated oxidation were observed with and without current flow involving massive outward iron diffusion phenomena, as well as inward diffusion of nickel plating. A layered oxide structure was found with and without current flow; however, substantial differences in elemental distribution throughout these layers were observed. These findings, along with mechanism hypotheses, will be presented and discussed in the context of developing durable interconnects for long-term high-performance SOFC systems. Figure 1: EDX linescan of the anode side (exposed to hydrogen) of Electroless Ni-plated AISI 441 cross sections after 500 hours of exposure at 700°C with current flow of 0.5 A (A) and without current flow (B) Figure 1