Effect of grain boundary characteristics on the oxidation behavior of ferritic stainless steel

doi:10.1016/j.corsci.2011.08.020

Corrosion Science

Volume 53, Issue 12, December 2011, Pages 4124-4130

https://doi.org/10.1016/j.corsci.2011.08.020 Get rights and content

Abstract

The effect of grain boundary characteristics on the oxidation of ferritic stainless steel was investigated using XRD, SEM, energy dispersive spectroscopy, and electron backscattered diffraction. Oxide crystals at special grain boundaries were compared with that of random boundaries. The spinel type oxide grew along the grain boundaries whereas both spinel and chromia grew at the grain interiors. Oxidation rate at special boundaries was slower than random boundaries. The resistance to oxidation of special boundaries is influenced by their angular deviation from the ideal coincidence site lattice (CSL) misorientation. The special boundaries near ideal CSL misorientation are more resistant to oxidation.

Highlights

► Oxidation behavior at grain boundaries of ferritic stainless steel was investigated. ► Oxidation behavior at CSL boundaries was compared with that of random boundaries. ► Coarse oxide forms at random boundaries and CSL boundaries are oxidation resistant. ► CSL boundaries near ideal CSL misorientation are more resistant to oxidation.

Introduction

Ferritic stainless steels are preferred for use as interconnects in solid oxide fuel cell (SOFC) because of their low cost, good formability, and similar thermal expansion coefficient to the other ceramic components of the fuel cell. The SOFCs operate in the temperature between 600 and 800 °C. At these temperatures these steels inevitably form a protective oxide layer. However, a thick oxide layer is detrimental to the contact resistance of the interconnect [1]. In order to increase the oxidation resistance of the steel interconnects, typically either the composition is modified [2], [3], [4] or the steel is given a surface treatment [5], [6]. Further improvements in oxidation resistance may be brought about by engineering the microstructure of the steel. In this regard understanding the oxidation behavior of the constituents of the microstructure, namely the grain and grain boundary, is necessary. It is known [7] that grain boundaries of a certain character have special properties. The grain boundary character is generally determined by coincidence site lattice (CSL) model and is described by the parameter Σ [8], [9].

Tan et al. [10] reported that in engineering the microstructure to increase the fraction of special boundaries (sigma ⩽ 29) in INCOLOY 800H, this increased the bulk oxidation resistance. Similarly, the resistance to intergranular oxidation of Ni–Fe alloy increased [11] upon increasing the fraction of special boundaries. Yamaura et al. [12] studied the extent of oxidation of individual Σ boundaries in Ni–Fe alloys based on morphological observations. They reported that Σ3, Σ11 and Σ19 were more resistant to oxidation than other Σ boundaries. Further, the boundaries with relative orientations closer to low Σ relationships were more resistant to oxidation than random boundaries [12]. Oxidation studies on single crystals of iron [13], copper [13], [14], Fe–Ni alloys [15] and Ferritic ODS alloys [16] have shown that the oxidation rate depends on the crystallographic orientation. There is no study, to the authors’ knowledge, on the influence of grain orientation and grain boundary characteristics on the elevated temperature oxidation behavior of polycrystalline ferritic stainless steels. In the present study, we investigated the relative size, structure, composition, and orientation dependence of the oxide that forms in the grain and along the grain boundaries during oxidation of the ferritic stainless steel Crofer 22APU.

Section snippets

Experimental

The ferritic stainless steel Crofer 22 APU, commercially available as 1.5 mm thick plate, was used in the present study. The composition in wt.% of Crofer 22 APU, determined using inductively coupled plasma spectrometer, is given in the Table 1. Samples of size 7 mm × 10 mm were cut from the plate for investigating oxidation behavior. The samples were ground with SiC abrasive papers up to 1200 grit and then electropolished using Struers Lectopol-5 electropolisher. Electropolishing was carried out at

Microstructural observations

The Brandon criterion [8], given by Eq. (1) below, was used to categorize the coincidence site lattice (CSL or Σ) boundaries. $θ_{m} = θ_{0} Σ^{- 1 / 2}$ where θ_m is the maximum deviation from the ideal misorientation angle for the CSL boundary, θ₀ is the angular limit for a low angle grain boundary namely 15°. The Tango software suite in Channel 5 software automatically identifies the CSL boundaries using the Brandon criterion. The Σ boundaries are indicated by colored lines in the band contrast image in Fig 1b.

Conclusions

Crofer22APU was oxidized in air at 650 °C for 130 h. The sample was characterized using SEM-EBSD to determine the microtexture, type and number of CSL boundaries and their angular deviation from the ideal CSL misorientation. Morphological observations of the oxide formed along the grain boundary and in the grain after different periods of oxidation were carried out using SEM, EDS, EBSD and XRD measurements to determine the composition, structure and phase of the oxides were carried out. The

Acknowledgment

This research was supported by Korea Institute of Science and Technology (Grant No. 2E21652).

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Citation Excerpt :
Also, they found that among low Σ CSLBs, the low deviation boundaries (θ/θm ≤ 0.5) were even more resistant to oxidation. Table 1 shows that the majority of the studies on the GBCD of BCC materials have followed the strain recrystallization route and have struggled to get significant improvement in GBCD [13–27]. So, in this work, the strain annealing route is employed to enhance the Σ3n CSLBs in IF steel.
In the last two decades, grain boundary engineering (GBE) study has neglected the high stacking fault energy BCC materials. Researchers have tried many single or multi-step strain recrystallization routes for improving the grain boundary character distribution (GBCD) in BCC materials, but the results have not been satisfactory. So, in this study, a single-step low deformation (5%, 10%) strain annealing route is employed for obtaining optimized GBCD and triple junction distribution (TJD) in interstitial free steel. This low deformation (5%) and low temperature (923 K) short annealing (10 min) treatment led to the evolution of 48.2% of Σ3, 54.3% of low Σ CSL boundaries, and 31.1% of special triple junctions (J2 and J3 type) in the microstructure. To the best of the authors' knowledge, these are the highest values reported for BCC material in literature. A good correlation is found between {111}〈110〉, {111}〈112〉 gamma fiber texture components and low deviation Σ3 boundaries. Moreover, a thorough analysis of average grain size, residual strain, GBCD, TJD, and grain orientation spread indicates that the regeneration mechanism (due to strain-induced boundary migration (SIBM)) is responsible for establishing a GBE microstructure in the thermomechanically processed sample (5%, 923 K, 10 min). The present work also explains the role of abnormal grain growth (AGG) on the deterioration of GBCD and TJD in the highly deformed (10%) samples.

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Effect of grain boundary characteristics on the oxidation behavior of ferritic stainless steel

Abstract

Highlights

Introduction

Section snippets

Experimental

Microstructural observations

Conclusions

Acknowledgment

Mater. Res. Bull.

J. Power Sources

Solid State Ionics

Acta Met.

J. Nucl. Mater.

Acta Mater.

Acta Met.

App. Surf. Sci.

Acta Mater.

Evaluation of several alloys for solid oxide fuel cell interconnect application

Scripta Mater.

Thermal growth and performance of manganese cobaltite spinel protection layers on ferritic stainless steel SOFC interconnects

J. Electrochem. Soc.

Improvement in oxidation resistance of ferritic stainless steel by carbon ion implantation

Electrochem. Solid-State Lett.