Surface Structure Analysis of Initial High-Temperature Oxidation of SS441 Stainless Steel.

Tung-Yuan Yung,Hui-Ping Tseng,Jeng-Shiung Chen,Wen-Feng Lu,Tien Shen,Kun-Chao Tsai,Hsin-Ming Cheng,Po-Tuan Chen

doi:10.3390/ma14206136

Abstract

Chromia-forming ferritic stainless steel (FSS) is a highly promising interconnect material for application in solid oxide fuel cells. In this study, initial oxidation of chromium oxides was performed at 500–800 °C to understand the evolution of materials at an early stage. The structural variations in oxide scales were analyzed through scanning electron microscopy, energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffractometry (XRD), laser confocal microscopy (LSCM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Surface electrochemical properties were investigated through electrochemical impedance spectroscopy to understand how the heat treatment temperature affected surface impedance. Treatment temperatures higher than 700 °C facilitate the diffusion of Cr and Mn, thus allowing ferritic spinels to form on the surface and leading to high electrical conductivity.

Highlights

Among currently available fuel cell types, solid oxide fuel cells (SOFCs) are preferred for stationary power generation and auxiliary power unit applications because they demonstrate high efficiency and fuel flexibility [1,2,3,4]
It was revealed that the oxygen content in the oxide scale compositions increased with increasing heat-treatment temperatures
The Electrochemical impedance spectroscopy (EIS) equivalent circuit was analyzed using Zview software; the results revealed that increasing heat-treatment temperatures directly increased electrochemical impedance

Summary

Introduction

Among currently available fuel cell types, solid oxide fuel cells (SOFCs) are preferred for stationary power generation and auxiliary power unit applications because they demonstrate high efficiency and fuel flexibility [1,2,3,4]. At high working temperatures (500–800 ◦C) and after long-term discharge operation, the Fe on the FSS surface forms harmful oxide layers, resulting in increases in resistance, spallation on the material surface, and a decline in fuel cell performance [8,9,10]. Prior formation of Cr oxide layers on the metallic interconnect surface can suppress the Fe oxide generated there and protect the alloys from corrosion [11,12]. Passive Cr oxide layers automatically grow on the surface of FSS, even before SOFC operation, because of the high Cr content in the ambient atmosphere. A moderately thick chromium oxide layer has satisfactory electrical conductivity; the chromium oxide scale must be maintained at below a certain thickness so as to maintain a low electrical resistance over the interconnects [13,14]. Cr oxides provide effective adherence to metal substrates and do not spall off with certain coating parameters [15]

Methods

Results

Conclusion