Elsevier

Materials Chemistry and Physics

Volume 186, 15 January 2017, Pages 534-540
Materials Chemistry and Physics

Microstructures and corrosion behaviors of FeCoNi and CrFeCoNi equimolar alloys

https://doi.org/10.1016/j.matchemphys.2016.11.033Get rights and content

Highlights

  • The microstructures of the alloys were identified.

  • The polarization behaviors of the alloys in the solutions were investigated.

  • The alloys show better corrosion resistance than commercial 304 stainless steel.

Abstract

This investigation studies the microstructures of FeCoNi and CrFeCoNi alloys, and their polarization behaviors in 1 M deaerated H2SO4 and NaCl solutions. The results revealed that both FeCoNi and CrFeCoNi alloys had an FCC granular structure, and their lattice constants were approximately 0.357 nm in as-cast and as-annealed states. However, many HCP precipitates were observed in the matrix of CrFeCoNi alloy in both as-cast and as-annealed states. The corrosion resistance of FeCoNi alloy in 1 M H2SO4 solution was better than that of CrFeCoNi alloy. The corrosion resistances of both FeCoNi and CrFeCoNi alloys in 1 M H2SO4 solution were better than that of 304 stainless steel. The corrosion resistance of FeCoNi alloy in 1 M NaCl solution was better than that of CrFeCoNi alloy. The activation energies of FeCoNi and CrFeCoNi alloys were calculated using the Arrhenius equation; in 1 M H2SO4 solution, they were 28.5 and 20.8 kJ/mol, respectively; and in 1 M NaCl solution, they were 24.6 and 89.1 kJ/mol, respectively.

Introduction

Iron, cobalt, nickel and chromium are very important transition metals which are widely used in many commercial alloys. Also, Fe, Co, Ni and Cr can be the major element of an alloys, or an additive element to improve some of its properties. For example, the addition of Cr can improve the corrosion resistance of Fe-50 wt%Ni alloy [1], and adding a little Fe enhances the corrosion resistance of Cu-30 at.%Ni alloy [2]. The coefficient of thermal expansion of Kovar alloy (Fe-Ni-Co, ASTM F-15 alloy) is close to that of borosilicate glass, and so the former has been commonly used in matched glass-to-metal sealing in microelectronic packages [3], [4]. However, 304 and 316 stainless steels are the most well-known alloys that contained the aforementioned elements.

An equimolar alloy, as described by Professor Yeh, was designed in this work. Professor Yeh introduced the concept of equimolar alloys, whose properties are not dominated by any one element [5], [6], [7]. FeCoNi equimolar alloy has a single FCC structure [8], and so contains no galvanic cell that can arise from differences between the potentials of adjacent phases [9]. Moreover, the fact that dense chromium oxide protects ferrous alloys, such as 304 stainless steel, is also well known [10]. Therefore, the microstructures of FeCoNi alloy and chromium modified FeCoNi alloy (CrFeCoNi) were investigated, as were their corrosion behaviors in 1 M sulfuric acid (H2SO4) and sodium chloride (NaCl) solutions. The results were compared with corresponding results for commercial 304 stainless steel.

Section snippets

Experimental

FeCoNi and CrFeCoNi equimolar alloys were prepared by arc melting using appropriate amounts of pure elements with purities higher than 99.9%. The alloys were made by combining the above elements and arc-melting them under a partial pressure of argon atmosphere (200 torrs). Before melting, the chamber was evacuated to at least 1 × 10−2 torr and then back-filled with argon to 200 torrs, this step was repeated 3–4 times to ensure the purity of atmosphere. The bottoms were remelted at least 4 times

Results and discussion

Fig. 1 displays OM micrographs of FeCoNi and CrFeCoNi alloys in both as-cast and as-annealed states. They all exhibit a granular structure with grain sizes of several hundred micro-meters. However, these two alloys differ in their precipitates: CrFeCoNi alloy contained many precipitates which were distributed in the grains in both as-cast and as-annealed states. Fig. 2 displays the XRD patterns of these two alloys under both as-cast and as-annealed states. The XRD patterns demonstrated that the

Conclusions

  • 1.

    Both FeCoNi and CrFeCoNi alloys had an FCC granular structure, with lattice constants of approximately 0.357 nm in as-cast and as-annealed states. Only a single FCC phase was observed in the FeCoNi alloy. Additionally, many HCP-structured Cr-rich precipitates were observed in the matrix of CrFeCoNi alloy.

  • 2.

    The corrosion resistance of FeCoNi alloy in 1 M H2SO4 solution was much better than those of CrFeCoNi alloy and 304SS. The activation energies of FeCoNi and CrFeCoNi alloys in 1 M H2SO4

Acknowledgement

The authors would like to thank the Ministry of Science and Technology of the Republic of China, Taiwan for financially supporting this research under Contract No. NSC 100-2221-E-034-009.

References (14)

  • L. Coutu et al.

    J. Magn. Magn. Mater.

    (2000)
  • J. Zhang et al.

    J. Alloy. Comp.

    (2010)
  • D.W. Luo et al.

    Acta Metall. Sin.

    (2008)
  • H. Asteman et al.

    Corros. Sci.

    (2007)
  • Y.Y. Chen et al.

    Corros. Sci.

    (2005)
  • A. Zanchetta et al.

    J. Eur. Ceram. Soc.

    (1995)
  • P.K. Huang et al.

    Adv. Eng. Mater

    (2004)
There are more references available in the full text version of this article.

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